Caroline Martin
email: caromartin@numericable.fr

Benoit Bideau

Pierre Touzard

Richard Kulpa

& R Kulpa

Identification of serve

Keywords: professional players, performance, fatigue, serve velocity

published or not. The documents may come

INTRODUCTION

The activity pattern of tennis match play is intermittent with players switching between short bouts of high intensity effort (< 10 seconds), short recovery (10-20 seconds) and rest periods of longer duration (90 -120 seconds) [START_REF] Martin | Effects of playing surface (hard and clay courts) on heart rate and blood lactate during tennis matches played by high-level players[END_REF] . A typical match duration is between 1 and 2 hours but this duration can be prolonged during five set matches (from 3 to 6 hours) [START_REF] Reid | The development of fatigue during match-play tennis[END_REF] . Throughout an extreme 5-set match, players can hit around 500-1000 groundstrokes and 200-400 explosive serves [START_REF] Reid | The development of fatigue during match-play tennis[END_REF] . The margin between winning and losing a 5-set match is small and may be related to serve performance. The ability to produce high ball velocity and to reach high 1 st serve percentage is a key element of successful play, because it puts the opponent under stress and may hinder its return [START_REF] Johnson | Performance demands of professional male tennis players[END_REF] . Indeed, the number of good returns decreases and the number of aces and 1 st serve points won increase as serve velocity increases over 44 m•s -1 4 . Moreover, the percentages of 1 st serves, 1 st serve points won and 2 nd serve points won impact the final match outcome [START_REF] Berendijaš | Comparison of game characteristics of male and female tennis players at Roland Garros 2005[END_REF] .

Serve velocity and accuracy (measured by a scoring system taking account target areas in the serve box) decline from the beginning to the end of a prolonged match or a strenuous training session on hard courts 6 7 . These decreases are generally attributed to fatigue, since studies have reported muscular force deficits in upper and lower limbs at the end of a prolonged tennis match 8 9 6 7 . Moreover, potential causes of fatigue during tennis match play are multiple: limitations in energy supply (phosphocreatine), intramuscular accumulation of metabolic products (lactate, H + , inorganic phosphate) [START_REF] Mendez-Villanueva | Exercise-induced homeostatic perturbations provoked by singles tennis match play with reference to development of fatigue[END_REF] , muscle damage [START_REF] Gomes | Muscle damage after a tennis match in young players[END_REF] , sweat loss and thermal stress [START_REF] Périard | Competitive match-play tennis under heat stress: a challenge for all players[END_REF] , central activation failure and alterations in excitation-contraction coupling [START_REF] Girard | Neuromuscular fatigue during a prolonged intermittent exercise: Application to tennis[END_REF] . Alternatively, different studies observed no decreases in serve velocity between the beginning and the end of a prolonged match or competition 14 15 16 . Maquirriain et al. (2016) analyzed serve statistics from 15 five-set male professional tennis matches. Their results showed no statistical difference of serve velocity between the first and the fifth set on grass courts. One may hypothesize that the playing surface could influence the evolution of serve velocity during prolonged tennis matches.

During the 5-set Wimbledon semi-final in 2013 between Djokovic and del Potro that lasted 4h43, del Potro's first serve velocity decreased by 2 m•s -1 between the first and the fifth set, whereas Djokovic's first serve velocity increased by 1.5 m•s -1 during the same time. Consequently, some theories suggest that performance variations could be the result of conscious or unconscious pacing strategies to preserve physical condition, prevent injuries and enable successful completion of the match [START_REF] Reid | The development of fatigue during match-play tennis[END_REF] . Although this is an interesting hypothesis, no data exist in the literature to support or reject such a statement. Yet, fatigue and pacing in scientific literature about intermittent sport have become increasingly popular in recent years 17 18 . Pacing is described as the distribution of energy resources that optimize match performance whereas fatigue is considered as an unidirectional construct that relates to eventual reduction in performance compared with baseline values [START_REF] Waldron | Fatigue and pacing in high-intensity intermittent team sport: an update[END_REF] . Several studies in intermittent team sports quantified the evolution of sport performance during a match to determine if players fatigue or modulate their effort according to a pacing strategy 17 18 . By analyzing athletic performances, researchers have observed "negative-split", "all-out", "positive", "constant", "parabolic-shaped" and "variable" pacing strategies (defined in the methods section) in different sports such as running, swimming, cycling, soccer, and rugby [START_REF] Abbiss | Describing and understanding pacing strategies during athletic competition[END_REF] . However, no similar studies have been applied on tennis serve performance throughout prolonged (e.g. 5 sets) matches. Yet, analysis of pacing strategies employed by successful professional players might lend insight into the most desirable pacing strategy for a 5-set match in Grand Slam tournaments. Indeed, in order to structure efficient and productive training programs, coaches must have a solid knowledge of their players' physical responses [START_REF] Kovacs | Tennis physiology: training the competitive athlete[END_REF] . Serve performance statistics and pacing strategies are thus very useful for coaches because they give information about match period from or during which the player is more or less efficient.

The aim of this study is to investigate the prominence of different pacing strategies adopted by professional tennis players and their relationship with match outcome, ranking and Grand Slam tournament.

METHODS

Experimental approach of the problem

This study analyzed fifty main draw five-set men's singles matches from the 2014 Grand Slam tournaments (12 matches at Australian Open, 12 matches at French Open, 12 matches Wimbledon and 14 matches at US Open) by using the statistics published on official websites of Grand Slam tournaments. Serve performance parameters of professional players (ATP ranking from 1 to 406) were analyzed.

Mean values of 1 st and 2 nd serve velocities, 1 st serve in and 1 st and 2 nd serve points won were collected for each of 5 sets by an IBM radar gun. Abbiss and Laursen (2008) defined main pacing strategies for athletic events. The authors of the current study have adapted them for tennis competition (Table 1). According to fluctuations of mean 1 st serve velocity for each of the 5 sets, players were classified into five types of pacing strategies: "variable", "parabolic", "constant", "all-out" and "negative-split" [START_REF] Abbiss | Describing and understanding pacing strategies during athletic competition[END_REF] (Table 1).

Pacing strategy

Athletic competition [START_REF] Abbiss | Describing and understanding pacing strategies during athletic competition[END_REF] Tennis competition This strategy concerns a player who temporarily reduces its first serve velocity during a match's period (1, 2 or 3 sets) but increases it during the latter part of match. This tactic ultimately results in U, J or reverse J-shaped behavior Negative-split strategy An event is considered to have been performed with a negativesplit, or through use of a negative pacing strategy, when there is an increase in speed observed over the duration of the event.

There is a gradual increase in 1 st serve velocity observed set after set Table 1. Definition of the pacing strategies in athletic events according to Abbiss and Laursen (2008) and their adaptation for tennis competition Separately for pacing strategies, players were sorted into the following groups for comparison: (a) "winners" versus "losers" according to match's outcome, (b) "< Top 20" versus "> Top 20" according to player's Association of Tennis Professionals (ATP) ranking at the beginning of tournament. "<Top 20" means players that have a ranking number lower than 20, indicating a better player than the 20 th ranked player. Similarly, ">Top 20" means players who have a ranking number greater than 20, indicating a worse player than the 20 th ranked player.

Statistical analysis

An analysis of variance with repeated measures and a Student-Newman-Keuls post hoc test were used to determine significant differences in serve performance between sets for all 5-set tennis matches. To investigate data differences between winners and losers, two-way mixed analyses of variance with repeated measures (match outcome x set) and Student-Newman-Keuls post hoc tests were used. A two-way mixed analyses of variance with repeated measures (tournaments x set) was used to investigate data differences between tournaments (Australian Open, French Open, Wimbledon, US Open).

Fisher exact tests were used to analyze:

-effect of pacing strategies on match outcome -effect of players' ATP ranking on pacing strategies -effect of Grand Slam tournament on pacing strategies Statistical significance was accepted as P <0.05. Effect size (ES) (Cohen's d) was calculated to document the size of statistical effects observed and defined as small for ES >0.1, medium for ES >0.3, and large for ES >0.5 [START_REF] Cohen | Statistical Power Analysis for Behavioral Sciences[END_REF] . Cramer's V was computed to complement the Fisher's exact test results.

RESULTS

Mean 1 st serve velocity was significantly higher for the 1 st set of match than for the 3 rd , the 4 th and the 5 th sets (main effect: P =0.020, 1 st and 3 rd sets comparison: P= 0.006, ES =0.27; 1 st and 4 th sets comparison: P =0.008, ES =0.26; 1 st and 5 th sets comparison: P=0.032, ES=0.21) (Table 2). On average, players lost 0.6 m•s -1 between the 1 st and the 5 th set. Conversely, set duration (main effect: P =0.449), 2 nd serve velocity (main effect: P =0.083), 1 st serve in (main effect: P =0.365), 1 st serve points won (main effect: P =0.533) and 2 nd serve points won (main effect: P =0.758) were not significantly different from one set to another. "Variable" pacing strategy was incorporated the most (45%), followed by "parabolic" (20%), "constant" (18%), and "all-out" (15%) strategies, which were closely distributed (Figure 1). Finally, "negative-split" strategy was infrequently used among the 64 players observed (2%).

Whole

Differences between serve performance of winners and losers during 5-set matches

Winners' 1 st serve velocity did not significantly differ across the five sets (main effect: P =0.355) while losers' 1 st serve velocity changed significantly between sets (main effect: P =0.023). 1 st serve velocity significantly decreased for losers between 1 st and 3 rd sets (P =0.002, ES =0.15), a further decrease at the 4 th (P =0.002, ES =0.24) and 5 th sets (P =0.002, ES =0.28), and between 2 nd and 5 th sets (P =0.013, ES =0.14) (Figure 2(a)). On average, losers lost 1.2 m•s -1 between 1 st and 5 th sets. Moreover, for the 5 th set, mean 1 st serve velocity was significantly higher (+ 1.4 m•s -1 ) for winners than for losers (P =0.021, ES =0.22). Winners' 2 nd serve velocity did not significantly differ across the five sets (main effect: P =0.943), while 2 nd serve velocity significantly decreased during match for losers (main effect: P =0.023, comparison of 1 st and 5 th sets: P =0.003, ES =0.41) (Figure 2(b)). Concerning percentage of 1 st serve points won, results significantly decreased for losers during the match (main effect: P =0.013, comparison of 1 st and 5 th sets: P =0.013, ES =0.32, comparison of 2 nd and 5 th sets: P <0.001, ES =0.53, comparison of 3 rd and 5 th sets: P =0.007, ES =0.37), while results significantly increased for winners during the match (main effect: P =0.016, comparison of 1 st and 5 th sets: P =0.002, ES =0.39, comparison of 3 rd and 5 th sets: P =0.002, ES =0.42). Furthermore, percentage of 1 st serve points won was significantly higher for winners (+13%) than for losers for the 5 th set (P <0.001, ES =0.35). In the 5 th set, winners were able to significantly increase their percentage of 2 nd serve points won in comparison with all of other sets (main effect: P <0.001, 1 st and 5 th sets comparison: P <0.001, ES =0.62; 2 nd and 5 th sets comparison: P <0.001, ES =0.52; 3 rd and 5 th sets comparison: P =0.001, ES =0.44; 4 th and 5 th sets comparison: P <0.001, ES =0.63). For losers, the percentage of 2 nd serve points won in the 5 th set was significantly lower than in all other sets (main effect: P =0.006, 1 st and 5 th sets comparison: P <0.06, ES =0.48; 2 nd and 5 th sets comparison: P <0.001, ES =0.51; 3 rd and 5 th sets comparison: P =0.019, ES =0.33; 4 th and 5 th sets comparison: P =0.004, ES =0.39). During the 5 th set, winners won significantly more points (+ 27%) on their 2 nd serve than losers (P <0.001, ES =0.55).

Comparison of pacing strategies used by winners and losers during 5-set matches

Fisher exact test (pacing strategy x match outcome) demonstrates a trend towards an association between pacing strategy and match outcome (Fisher exact test =7.99, P =0.074, ES =0.29). Winners and losers use "variable" (50 vs. 40%), "parabolic" (22 vs. 18%) and "constant" (16 vs. 20%) in similar proportions. "All-out" strategy was twice as common for losers (22%) than for winners (8%). "Negative-split" was only observed in winners (4%). When players used "variable" or "parabolic" strategies, they won match in 55 and 56% of the cases. But, when they used "all-out" strategies, they lost match in 73% of the cases (Figure 3a).

Comparison of pacing strategies according to players' ATP ranking

There was no significant association between players' ATP ranking and type of pacing strategy used (Fisher exact test =4.15, P =0.384, ES =0.04). "Variable" (42 vs. 46%), "parabolic" (18 vs. 19%), and "negative split" (3 vs. 1.5%) pacing strategies were used in similar proportions in < Top 20 and > Top 20 players (Figure 3b). "All-out" strategy was used 2 times more by > Top 20 (18%) than by < Top 20 players (9%). "Constant" strategy was more frequently observed in < Top 20 (27%) than in > Top 20 players (15%).

Differences of serve performance between Grand Slam tournaments during 5-set matches

There was a statistically significant interaction between tournaments and sets concerning 1 st serve velocity (main effect: P =0.008) (Table 3). Results show no significant differences for 2 nd serve velocities and for 2 nd serve points won between tournaments (main effect: P =0.262 for 2 nd serve velocity and P =0.261 for 2 nd serve points won). Conversely, 1 st serve in and 1 st serve points won were significantly different between tournaments (main effect: P <0.001). There was not significant interaction between tournaments and sets for 1 st serve in (P =0.160) and 1 st serve points won (P =0.388). In Wimbledon, 1 st serve in was significantly higher in comparison with all other Grand Slam tournaments (Australian Open P =0.031, Roland Garros P =0.006, and US Open P <0.001). In Roland Garros, 1 st serve points won was significantly lower in comparison with all other Grand Slam tournaments (Australian Open P =0.017, Wimbledon P =0.029, and US Open P <0.001). Significantly different from Wimbledon (@ P<0.05; @@ P<0.01; @@@ P<0.001).

Whole

Significantly different from Roland Garros (#P<0.05; ### P<0.001).

Comparison of pacing strategies according to Grand Slam tournaments

There was no significant association between Grand Slam tournaments and type of pacing strategy used (P =0.875). "Variable" (around 45%), "all-out" (between 12.5 and 16.7%), and "negative split" (between 0 and 4.2%) pacing strategies were used in similar proportions all Grand Slam tournaments (Figure 4). "Parabolic" strategy was used 2 times more in Roland Garros (29%) than in Wimbledon (12%). "Constant" strategy was 3 times more used in Wimbledon (27%) than in Roland Garros (8%).

DISCUSSION

This study was conducted to establish the prominence of different serve pacing strategies in professional tennis players and their relationship with performance outcome by retrospectively analyzing 5-set matches. The main result was that Fisher exact test (pacing strategy x match outcome) demonstrates a trend towards an association between pacing strategy used and match outcome. There is no significant association between players' ATP ranking and type of pacing strategy used. Serve velocity and serve points won are decreased in losers while they are increased or kept constant in winners during the 5 th set of match.

There was no significant difference across the five sets in percentage of "1 st serve in" between sets for winners and losers. However, our findings show significant differences in percentages of 1 st and 2 nd serve points won for winners and losers. During the 5 th set, winners won significantly more points after their 1 st (+ 13%) and 2 nd serves (+ 27%) than losers. This may relate to serve velocity, given that, 1 st serve velocity is significantly higher (+ 1.4 m•s -1 ) for winners than for losers for the 5 th set. Moreover, between the 1 st and last sets, mean 2 nd serve velocity for losers shows a 0.9 m•s -1 decrease. These results are in line with previous findings who noted a significant relationship between serve velocity and probability of winning the point [START_REF] O' Donoghue | The impact of speed of serve in Grand Slam singles tennis[END_REF] . By executing faster serves, winners are logically placing greater time constraints on their opponent's return, potentially affording them an advantage for winning the point [START_REF] Gillet | A notational analysis of elite tennis serve and serve-return strategies on slow surface[END_REF] . However, the higher percentages of points won after 1 st and 2 nd serves in winners reported in this study were probably caused not only by faster serves but also by the interaction between serve velocity and spin rate or serve location 24 25 .

Repartition of different serve pacing strategies is meaningful for tennis players and coaches. Muscle activation and exercise intensity are centrally regulated in response to intrinsic (i.e. physiological, biomechanical and cognitive) and extrinsic sensory feedbacks necessary to maintain physiological homeostasis 26 27 . Based on the "central governor hypothesis", it seems logical that the uppermost players select "variable" strategy to adjust their serve velocity to match circumstances (score evolution, match duration, power struggle, fatigue feelings). Indeed, whatever the Grand Slam tournament and the playing surface, players mainly used "variable" pacing strategy (45%) and with this strategy, they won the match in 56% of the cases. A lot of questions and/or feelings cross players' minds between points and during change-overs throughout a long tennis match [START_REF] Crespo | Tennis psychology: practical drills and the latest research[END_REF] . According to Noakes (2009), athletes could regulate their effort due to afferent sensory feedbacks and questions such as: "Have I sufficient energy reserves to finish the match?", "Am I able to win a 5-set match, have I done this before?", "Will my muscles be damaged?", "How much effort do I still have to do to win the match?", etc. [START_REF] Noakes | The central governor model: evolution and update[END_REF] . Consequently, the variable strategy appears effective since it could allow tennis players to continuously adapt and regulate their serve velocity based on this complex process involving peripheral sensory feedbacks and anticipated workload remaining. "Constant" pacing strategy is mainly used when exercise duration is unknown [START_REF] Baden | Effect of anticipation during unknown or unexpected exercise duration on rating of perceived exertion, affect, and physiological function[END_REF] . In our study, players frequently use this strategy (18%). The percentage of this strategy increases to 27% in best-ranked players. Those results are the consequence that tennis match duration is hard to predict. Under stable external conditions, a constant pace is "optimal" for prolonged events such as running, swimming, rowing, and cycling [START_REF] Abbiss | Describing and understanding pacing strategies during athletic competition[END_REF] because metabolic resources are conserved during exercise to improve athletes' energy efficiency. In rugby, "constant" strategy has been observed in studies that reported no evident deterioration in high-intensity activity performed during match 31 32 . Moreover, more successful Canadian national and international caliber pursuit and track cyclists used more constant pace race profiles, whereas less successful riders did not [START_REF] Wilberg | A survey of the race profiles of cyclists in the pursuit and kilo track events[END_REF] . Our results are in accordance with this previous study since the "constant" strategy is more frequently observed in best-ranked players (< Top 20: 27%) than in others (> Top 20: 15%). Moreover, 1 st and 2 nd serve velocities do not significantly change between sets for winners. All these findings suggest that best tennis players are able to spread their energy to conserve a constant 1 st serve velocity throughout the match. One may also hypothesize that the best-ranked players have a better experience of 5-set matches that helps them to more frequently choose constant strategy. Indeed, experience has been suggested to influence pacing strategy of athletes [START_REF] Micklewright | Previous experience influences pacing during 20 km time trial cycling[END_REF] . "Constant" strategy greatly varies from one Grand Slam tournament to the other (only 8% in Roland Garros vs. 27% in Wimbledon). Different factors may explain this result. Firstly, rallies and match duration are usually shorter in Wimbledon than in Roland Garros [START_REF] O' Donoghue | A notational analysis of elite tennis strategy[END_REF] . Moreover, it has been shown that the serve has a greater importance for winning a match on grass courts than on clay 36 37 . Consequently, it appears easier and much more crucial for tennis players to maintain constant their serve performance in Wimbledon than in Roland Garros.

"All-out" strategy is twice as common in > Top 20 (18%) than by < Top 20 players (9%) and twice as common for losers (22%) than for winners (8%). When players used "all-out" strategies, they lost the match in 73% of cases. It is difficult to know if players voluntarily chose this kind of strategy before match according to situational influences (i.e. level of opponent, number of consecutives matches already played in tournament for example) or if "all-out" strategy is primarily a consequence of fatigue generated by the current 5-set match. That risk-taking could be a resolute strategy for players with reduced physical abilities caused by previous strenuous matches or injuries. Since they know that their energy stocks are limited, they try to shorten match duration. This behavior could also be a conscious strategy chosen by players who think that their best chance to win the match needs a relatively fast starting serve pacing strategy and to maintain that level performance as long as possible to put pressure on their opponent. It seems that low-ranked players (> Top 20) give priority to this strategy when they compete against best-ranked players (< Top 20), maybe because they underestimate their chance to win the match if its duration is prolonged. Adoption of this tactic has been seen during a number of high-level competition events. For example, cyclists are often seen attempting to break away from the main group of riders during numerous road cycling events (such as Tour de France), presumably for the purpose of winning the race stage [START_REF] Abbiss | Describing and understanding pacing strategies during athletic competition[END_REF] . However, such a tactic is seldom successful (only 27% players in this study won their match with this strategy) and instead often results in a progressive decrease in exercise intensity (from the 2 nd set to the last one for 1 st serve ball velocity), maybe due to disturbances in physiological homeostasis. This progressive decrease in ball velocity observed with "all-out" strategy is meaningful for coaches because it may indicate player's physical limitation, his inability to repeat powerful serves during a prolonged period. "All-out" strategy does not seem influenced by the playing surface, since it varies between 12 and 17% of all strategies used in the different Grand Slam tournaments.

1 st and 2 nd serve velocities significantly decrease in the last sets of match for losers. These results are in line with previous findings reporting increases in RPE, decreases in EMG activity and modifications in serve biomechanics responsible for velocity decline during prolonged tennis events 8 13 9 6 7 . Moreover, Gomes et al. (2014) reported an increase in muscle soreness and muscle damage appearance that impairs players' performance after a 3-hour tennis match [START_REF] Gomes | Muscle damage after a tennis match in young players[END_REF] . None of the previous studies cited about fatigue influence on serve performance compared winners and losers' data. Consequently, we can only express a hypothesis about the fact that 5-set matches did not affect winners' serve velocities in this study. We may suppose that winners were physically stronger and better prepared for prolonged matches than losers. Consequently, they could better delay the onset of fatigue and thus its appearance. One may also suppose that winners managed their effort in a better way during 5-set matches than losers.

"Parabolic" strategy concerns 20% of players and 22% of winners. This strategy is two times more used in Roland Garros (29%) than in Wimbledon (12%). In tennis, one may suppose that this strategy reflects the concept of "transient" or "temporary" fatigue previously observed in soccer [START_REF] Bradley | Match running performance fluctuations in elite soccer: indicative of fatigue, pacing or situational influences?[END_REF] . This term refers to a period of deliberately reduced intensity in performance after the most intense period of sport matches [START_REF] Mohr | Match performance of high-standard soccer players with special reference to development of fatigue[END_REF] . During that reduced intensity period, a player may down-regulate energy output for serve during the 3 rd and/or 4 th sets to preserve energy for later and crucial match periods (5 th set for example). One of the best examples of this strategy is the success of Santoro against Safin during Roland Garros 2001. After the match, Santoro admitted that he has voluntarily decreased his effort during the 4th set to recover and be ready to win the 5th set. He said "at the end of the third set I felt I had to drop the fourth. I know it was a risk but I needed a rest. After that it was a flawless performance." (http://www.rediff.com/sports/2001/jun/02safin.htm). According to the flush model [START_REF] Millet | Can neuromuscular fatigue explain running strategies and performance in ultra-marathons?: the flush model[END_REF] , there is always a reserve for muscle recruitment (the security reserve) that can be used for the so called "end-spurt" when the athlete is at his highest level of peripheral fatigue. One may suppose that an increased motivation in the 5 th set counteracts fatigue sensations and allows players to dip into his security reserve to increase serve velocity.

"Negative split" strategy concerns only 4% of players for which an increase in 1 st serve velocity is observed over the duration of match, set after set. However, all players who adopt this strategy won their match. Indeed, adoption of such a pacing strategy is thought to be efficient in prolonged exercise performance by reducing rate of carbohydrate depletion [START_REF] Abbiss | Models to explain fatigue during prolonged endurance cycling[END_REF] , lowering excessive oxygen consumption [START_REF] Sandals | Influence of pacing strategy on oxygen uptake during treadmill middle-distance running[END_REF] and/or limiting accumulation of fatigue-related metabolites (i.e. inorganic phosphate, potassium and hydrogen ions) early on in the exercise task 40 42 . It is believed that this strategy may be the result of an increase in motor unit recruitment [START_REF] Tucker | Impaired exercise performance in the heat is associated with an anticipatory reduction in skeletal muscle recruitment[END_REF] and the use of the anaerobic energy reserve [START_REF] Foster | Effect of competitive distance on energy expenditure during simulated competition[END_REF] . However, it is curious that relatively few players used "negative split" strategy, given the attention this type of pacing has received in previous literature for different sports [START_REF] Abbiss | Describing and understanding pacing strategies during athletic competition[END_REF] . Further studies are necessary to confirm our results about the use of "negative split" strategy in tennis players and its influence on success.

Our results reveal differences of serve performance between Grand Slam tournaments. In Wimbledon, 1 st serve in was significantly higher in comparison with all other Grand Slam tournaments. In Roland Garros, 1 st serve points won was significantly lower in comparison with all other Grand Slam tournaments. Those results are similar to previous studies about serve performance parameters of professional players in different playing surfaces 45 46 . Indeed, the speed of playing surfaces influences the players' tactical behavior. For example, at Wimbledon, increasing the percentage of 1 st serve in is a key element of successful play on grass, because it puts the opponent under stress and may hinder its return. A more aggressive and attacking game is commonly associated with faster surface, such as grass [START_REF] Hughes | Surface effect on elite tennis strategy[END_REF] . Conversely, the clay of Roland Garros slows the ball down and reduces the advantage of the server.

This study presents limitations. First, our results reveal only a trend towards an association between pacing strategy used and match outcome. It is well known that p values depend upon sample size but our study provides a retrospective analysis of pacing profiles in professional tennis players only for data limited to a single year. As a consequence, some flexibility is desirable to interpret our p value. Moreover, the comparison of pacing strategies between the intermittent tennis activity and the continuous effort of other sports (cycling, rowing, running) may be delicate. No intervention was performed, and no physiological, biomechanical or psychological data were collected; therefore, mechanisms underpinning the observed results are hypothetical. More research is required to establish if perturbations in serving performance are primarily a consequence of fatigue, pacing or tactical and situational influences. Further studies focusing on the relationships between pacing strategies, court surfaces and match outcome are necessary.

Conclusion

This is the first study that provides an insight into pacing behavior in professional tennis players during 5-set Grand Slam matches. For players and coaches, pacing strategy may be a variable to take into account during pre-match strategy or post-match analyses. Authors are conscious that it is really difficult for a tennis player to know if the match will last 3, 4 or 5 sets. However, in some cases, players may anticipate duration and difficulty of match according to their opponent's level, their head-to-head opponent. For example, if ATP number 1 is drawn to play a qualifier in first round of a Grand Slam tournament, he should choose a pacing strategy that suits a three-set match but if he is going to play against one player member of the ATP top 5 in a semi-final for example, he may expect a tough and long match. Consequently, he could adopt a pacing strategy that suits a five-set match. In this case, defining strategic recommendations is instructive for tennis players. The current results can provide guidelines for coaches and competitors to follow when they expect a long tennis match, since pacing strategy used by players tends to exert an influence on 5-set match outcome. It appears that players should favor 'negative-split', 'variable' and 'parabolic' strategies in order to win 5-set matches and avoid 'all-out' strategy. Indeed, according to the data of this study, when players used 'variable' and 'parabolic' or 'negative-split' strategies; they respectively win the match in 56 -100 % of cases. But, when they used 'all-out' strategy, they lost the match in 73 % of cases. Consequently, 'all-out' strategy appears inefficient. Finally, data show that serve velocity and serve points won are decreased for losers while they are increased or kept constant for winners during the 5 th set of the match. Post-match analysis of serve statistics (serve velocity, percentage of serve in, points won) during 5-set matches is meaningful for tennis coaches because it can provide insight and information about player's physical fitness level. For example, an acute decrease in 1 st serve velocity during the 5 th set may reveal a player's physical weaknesses. In this way, pacing strategy used should be a key factor for consideration when coaches determine specific training programs to prepare high level tennis players for 5-set tennis matches [START_REF] Kovacs | Applied physiology of tennis performance[END_REF] . A conditioning program should focus on power endurance development to delay fatigue effects [START_REF] Verstegen | Developing strength. In: Strength and Conditioning for Tennis[END_REF] and avoid decreases in serve velocity during the 5 th set. In this way, explosive tennis-specific strength training with medicine balls and dumb-bells are recommended with emphasis on leg drive, trunk and shoulder rotations [START_REF] Verstegen | Developing strength. In: Strength and Conditioning for Tennis[END_REF] , since they are important contributors to serve velocity 50 51 . Moreover, motor imagery can be an interesting tool for helping players to maintain their serve performance. Indeed, it has been shown that motor imagery improves serve performance during high intensity intermittent training in young tennis players [START_REF] Guillot | Motor imagery and tennis serve performance: the external focus efficacy[END_REF] .

Figure 1 .

 1 Figure 1. Proportion of pacing strategies about 1 st serve velocity for all players

Figure 2 .

 2 Figure 2. Evolution of 1 st serve (a) and 2 nd serve (b) velocities in winners and losers throughout the 5-set matches. Significantly different from 1 st set (** P<0.01; *** P<0.001). Significantly different from 2 nd set (£ P<0.05). Significantly different from losers ($ P<0.05). Data are presented as means and standard error of the mean.

Figure 3 .

 3 Figure 3. Proportions of pacing strategies about 1 st serve velocity used according to the match outcome (a) and the ATP ranking of the players (b)

Figure 4 .

 4 Figure 4. Proportions of pacing strategies about 1 st serve velocity used according to the Grand Slam tournaments

match 1 st set 2 nd set 3 rd set 4 th set 5 th set

	Whole players	71 ± 8	71 ± 12	74 ± 13	72 ± 11	71 ± 16	± 14
	Winners		71 ± 11 $$	74 ± 12	72 ± 10 $$	73 ± 15	± 12£££
	Losers		71 ± 13 $	75 ± 13 $$$	72 ± 12 $$	69 ± 16	± 14
			2 nd serve points won (%)
	Whole players	51 ± 9	51 ± 16	53 ± 16	51 ± 18	49 ± 17	± 21
	Winners		50 ± 14 $$$	52 ± 17 $$$	54 ± 17 $$	49 ± 17 $$$	± 15£££
	Losers		52 ± 18 $	54 ± 15 $$$	48 ± 19 $	49 ± 16 $$	± 18
				Duration (min)
	Whole players	212 ± 27	39 ± 12	43 ± 10	43 ± 11	40 ± 11	47 ± 22
				1 st serve velocity (m•s -1 )
	Whole players	50.5 ± 2.7 51.1 ± 3.0	50.7 ± 3.0	50.5 ± 2.7**	50.4 ± 2.7** 50.5 ± 3.0*
			2 nd serve velocity (m•s -1 )
	Whole players	41.2 ± 2.5 41.4 ± 2.6	41.4 ± 2.5	41.3 ± 2.9	41.3 ± 2.9	40.9 ± 2.8
				1st serve in (%)
	Whole players	62 ± 7	62 ± 11	62 ± 10	63 ± 10	61 ± 11	64 ± 11
	Winners		62 ± 12	62 ± 9	63 ± 9	61 ± 9	65 ± 11
	Losers		62 ± 10	61 ± 10	62 ± 11	61 ± 13	63 ± 10
				1 st serve points won (%)

Table 2

 2

. Duration and serve performance parameters for all 5-set matches analyzed (n=50). Data are presented as means and standard deviation of the mean. Significantly different from 1 st set (*P <0.05; **P <0.01). Significantly different from 5 th set ($ P <0.05; $$$ P <0.001), £££: significantly different from losers (P <0.001)

match 1 st set 2 nd set 3 rd set 4 th set 5 th set 1 st serve velocity (m•s -1 )

	Australian	62.4 ± 7.5 @	66.6 ± 10.7 61.0 ± 8.7	61.0 ± 10.4	61.8 ± 11.0	62.2 ± 8.8
	Open
	Roland Garros 61.4 ± 6.7 @@	59.0 ± 10.6 60.5 ± 9.9	61.5 ± 10.1	55.8 ± 13.7	61.6 ± 11.4
	Wimbledon	65.7 ± 4.8	66.2 ± 10.4 67.4 ± 9.0	66.3 ± 8.6	65.2 ± 8.9	67.2 ± 10.3
	US Open	59.7 ± 5.7 @@@ 55.7 ± 11.1 57.0 ± 9.0	60.3 ± 9.1	59.8 ± 10.5	62.9 ± 10.7
			1 st serve points won (%)
	Australian	72.7 ± 8.6 #	73.8 ± 10.0 77.4 ± 11.7	73.6 ± 11.0	68.7 ± 16.6	70.9 ± 10.7
	Open
	Roland Garros 66.4 ± 7.6	67.0 ± 13.0 68.3 ± 11.6	67.4 ± 11.3	66.0 ± 13.3	71.3 ± 16.1
	Wimbledon	72.5 ± 6.7 #	71.9 ± 13.9 70.8 ± 13.7	71.9 ± 9.2	70.3 ± 20.5	73.8 ± 15.5
	US Open	72.7 ± 8.5 ###	71.4 ± 13.4 81.6 ± 9.4	72.0 ± 14.0	76.8 ± 10.4	72.8 ± 12.9
			2 nd serve points won (%)
	Australian	50.9 ± 10.6	50.5 ± 14.6 51.2 ± 15.2	47.2 ± 18.8	50.3 ± 9.9	51.0 ± 18.3
	Open
	Australian Roland Garros 49.6 ± 7.9 50.5 ± 2.5	50.6 ± 2.6 50.4 ± 20.6 51.9 ± 17.6 50.8 ± 2.7	50.0 ± 2.3 50.9 ± 20.3	50.5 ± 2.5 45.4 ± 20.1	50.6 ± 2.4 52.5 ± 22.0
	Open
	Wimbledon	52.8 ± 6.6	54.2 ± 17.2 56.3 ± 17.5	54.4 ± 15.3	49.1 ± 19.5	53.8 ± 21.7
	Roland Garros 49.8 ± 3.2	50.0 ± 3.2** 49.4 ± 3.4**	49.9 ± 2.8	49.7 ± 3.0	49.8 ± 3.6
	US Open	51.8 ± 11.4	52.3 ± 13.1 51.0 ± 13.3	53.6 ± 17.4	52.3 ± 14.9	54.1 ± 22.4
	Wimbledon	50.7 ± 2.8	50.9 ± 2.9	50.6 ± 2.7	50.5 ± 2.9	50.7 ± 2.7	50.8 ± 2.8
	US Open	51.4 ± 2.9	52.5 ± 2.8	51.8 ± 2.9	51.4 ± 2.7$$	50.8 ±	50.6 ±
						2.6$$$€€	3.3$$$€€
			2 nd serve velocity (m•s -1 )
	Australian	41.0 ± 2.8	41.1 ± 2.6	41.2 ± 2.9	41.3 ± 3.0	40.8 ± 2.7	40.8 ± 2.7
	Open
	Roland Garros 40.6 ± 3.2	40.7 ± 2.4	40.5 ± 2.2	40.5 ± 2.3	40.6 ± 2.7	40.5 ± 2.6
	Wimbledon	41.7 ± 3.0	41.8 ± 2.6	42.0 ± 2.3	41.5 ± 3.2	41.9 ± 3.2	41.3 ± 3.6
	US Open	41.7 ± 2.6	42.1 ± 2.5	41.7 ± 2.6	41.9 ± 3.0	41.8 ± 2.8	41.2 ± 2.3
				1 st serve in (%)

Table 3 .

 3 Serve performance parameters for all Grand Slam tournaments. Data are presented as means and standard deviation of the mean. Significantly different from 1

st set ($P <0.05; $$P <0.01; $$$P <0.001). Significantly different from 2 nd set (€ P <0.05; €€ P<0.01; €€€ P <0.001). Significantly different from US Open (**P <0.01).

Funding statement

The authors received no financial support for the research, authorship, and/or publication of this article.

Conflict of interest statement

The authors declare that they have no conflict of interest.


Martien Duvall

Deffo Ayagou

C Mendibide

Claude Duret-Thual

Jean Kittel
email: jean.kittel@ifpen.fr.

Khawla Belkhadiri

Thi Tuyet

Mai Tran

Eliane Sutter

Bernard Tribollet

Nicolas Ferrando

Corrosion and Hydrogen Permeation in H 2 S Environments with O 2 Contamination, 1: Tests on Pure Iron at High H 2 S Concentration

Keywords: hydrogen permeation, hydrogen sulfide, hydrogen-induced cracking, sulfide stress cracking

Materials selection in the oil and gas industry relies on engineering standards, such as NACE TM0177 and NACE TM0284, which stipulate that oxygen pollution should be avoided during materials testing in H 2 S-containing media. In this paper, we explore the manner in which traces of oxygen can modify the test solution chemistry and the corrosion of/ hydrogen permeation across iron membranes in H 2 S-containing solutions. Oxygen pollution is shown to strongly influence solution chemistry, through the introduction of sulfur-oxygen reaction products resulting in bulk acidification. Weight loss, electrochemical methods, and solution chemistry measurements conclude that iron corrosion rates in the presence of oxygen pollution are doubled, when compared against the control system (without oxygen pollution). Unexpectedly, despite a lower pH and higher corrosion rates in the oxygen-polluted H 2 S-containing solutions, the hydrogen permeation rate decreases monotonically, relative to the control. We discuss how this observation is most likely related to a disruption of sulfur adsorbates involved in hydrogen entry promotion.

INTRODUCTION

Materials used in oil and gas industries can be exposed to sour environments containing hydrogen sulfide (H 2 S), which is corrosive and known to promote hydrogen entry into steels. This may lead to several types of failures such as hydrogeninduced cracking, sulfide stress cracking, or stress-oriented hydrogen-induced cracking. Corrosion and hydrogen embrittlement of steels in H 2 S-containing environments has been studied for several decades. Standard test methods have been developed for the selection and the qualification of steels for use in H 2 S-containing environments, such as NACE TM 0177 and TM 0284. [START_REF] Nace Tm | Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H 2 S Environments[END_REF][START_REF] Nace Tm | Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking[END_REF] These standards strongly recommend to avoid any oxygen ingress in test environments. For instance, it is stated that "obtaining and maintaining an environment with minimum dissolved O 2 contamination is considered very important." It is also mentioned that, "O 2 contamination may induce an increase of the corrosion rate and reduce hydrogen evolution and hydrogen entry into the steel", although "systematic studies of the parameters affecting these phenomena have not been reported in the literature."

There are many different ways by which oxygen can pollute a test medium, such as an incorrect or insufficient deaeration of the initial test solution, a poor sealing of test reactors, the use of plastic tubing permeable to oxygen, and a multitude of operations during the test (solution sampling, pH adjustment, etc.). For the present study, we have investigated the impact of a continuous ingress of small amounts of gaseous O 2 in a test solution, during a corrosion test with aqueous H 2 S. Song, et al., [START_REF] Song | [END_REF] have reported that a solution continuously bubbled with a mixture of H 2 S and O 2 continuously acidifies and does not rise back to neutrality even after purging out H 2 S with an inert gas (argon, nitrogen). Such an experimental finding is in good agreement with the reaction pathway proposed by Crolet, et al., [START_REF] Crolet | The Role of Trace Amounts of Oxygen on the Corrosivity of H 2 S Media[END_REF] showing that O 2 and H 2 S can react to form dissolved thiosulfate or tetrathionate. In both cases, H + ions are also produced, resulting in a decrease in pH. The produced sulfur-oxygen (S-O) compounds could, then, modify the kinetics and corrosion mechanisms. This was confirmed by Kuo, et al., who showed that the presence of thiosulfate in chlorinated H 2 S medium significantly increased the corrosion rate of immersed steels. 5 These authors used anodic polarization experiments and microscopic observations to conclude that thiosulfate participates in electrochemical reactions. It is also well known that H 2 S promotes hydrogen entry in steels via sulfur adsorbates. [6][7][8] Metal surface reactions with O 2 or with O 2 -H 2 S reaction products could interact with the hydrogen entry process. Some authors have proposed to use thiosulfate solutions to replace H 2 S in cracking experiments. [9][10] Indeed, they suggest that thiosulfate reduction at the metal surface could result in the formation of H 2 S, resulting in an increase of hydrogen permeation.

In summary, oxygen traces in H 2 S-containing environment could modify the system in three distinct areas:

 changing the composition of the corrosive environment;  modifying the corrosion rate and the nature of corrosion deposits;  interacting with H 2 S adsorbates and changing the efficiency of hydrogen entry. This series of papers aims at improving the understanding of corrosion mechanisms and hydrogen entry into steels in H 2 Scontaining environment polluted by traces of oxygen. In this first part, a model system is used. It consists of pure iron in the annealed condition, exposed to 35 g/L NaCl saturated with H 2 S at a partial pressure close to 0.1 MPa. A gas supply system permits well-controlled O 2 entry in the test cell, at a partial pressure of 1 kPa to 1.5 kPa, thus corresponding to 500 ppbw (parts per billion by weight) dissolved O 2 at equilibrium. Both corrosion and hydrogen uptake tests are conducted using a classic two-chamber hydrogen permeation cell. At the entry side, electrochemical impedance spectroscopy (EIS) is used to evaluate the corrosion behavior of the iron membrane in the H 2 S medium. At the exit side, hydrogen permeation is measured by anodic extraction. Chemical analysis of the test solution is also performed in order to identify reaction products between O 2 and H 2 S.

EXPERIMENTAL PROCEDURE

Test Solution

In order to keep the system as simple as possible, tests were performed in solutions containing 35 g/L NaCl without added acids or buffering species. Continuous bubbling of gases into the solution was maintained during all experiments. In order to control the quantity of oxygen introduced in the test cell, two separate gas lines with mass flow controllers were used. The first gas line consists of a pure H 2 S or H 2 S/N 2 mix, with a typical flow rate of 17 mL/min to 20 mL/min. The second gas line is fed with O 2 /N 2 at a typical O 2 concentration of 10%, and is used at a flow rate comprised between 0 mL/min and 3 mL/min. Gas lines and several valves (fabricated from 316L stainless steel) manage the gas supply into the cell. Gases are homogenized in a spiral gas line of approximately1 m before bubbling into the test solution and the oxygen content in the gas line immediately before entering into the test cell is confirmed using an HACH 410 Orbisphere™ probe. With this system, O 2 partial pressure in the gas entering into the test cell can then be controlled precisely between zero and a few kilopascals.

The theoretical amounts of dissolved O 2 and H 2 S are given by Henry's law, i.e., their concentration (C i ) is equal to their partial pressure (P i ) times the Henry's constant (K i ):

C i = K i P i (1)
The Henry's constant values for H 2 S and O 2 at 24°C are, respectively, 1 mol/L/Mpa and 1.3  10 -2 mol/L/MPa. [11][12] The range of P O2 , comprised between 0 and a few kilopascals, thus corresponds approximately to 0 ppbw to 500 ppbw dissolved O 2 at saturation. In this paper, two gas compositions were used: pure H 2 S and a mix of H 2 S (87.5%), N 2 (11.25%), and O 2 (1.25%). 1.25% O 2 at 24°C corresponds to 500 ppbw dissolved at saturation. Because the recent revisions of NACE TM0177 and NACE TM0284 mention some O 2 limits expressed in mass ppb, this unit will be used in the paper, though the parameter that was indeed controlled was O 2 partial pressure.

All experiments were performed at 24°C  2°C.

Electrochemical Measurements

Permeation measurements were performed using the electrochemical technique with a Devanathan-Stachurski-type jacketed cell. 13 The experimental setup was made of twin cells separated by the iron membrane, with one side exposed to the corrosive H 2 S medium (entry side) and the other side (covered by Pd) exposed to deaerated 0.1 M NaOH (exit side). We highlight that the 35 g/L test solution, for the entry side, was first purged with argon gas (16 h to 20 h), before a 6 h purging of the H 2 S or H 2 S-O 2 gas mixture. The cells' jackets permitted the circulation of thermostatically controlled water, and thus maintained the solution temperature at 24°C  2°C throughout the testing period.

The membrane was 99.5% iron, with composition given in Table 1. After machining, the permeation membranes were heat treated under vacuum at 900°C for 30 min, in order to reduce metallurgical defects such as dislocations. The sample membrane thickness was 0.5 mm and the exposed surface was 16 cm² on both sides. The exit face was covered by palladium following well-established procedures. 14 Under such conditions of iron membrane thickness and environment, it is considered that permeation is mostly governed by reactions at the entry face, minimizing the impact of bulk diffusion of hydrogen. 7,15 The charging side of the iron membrane was used as a working electrode during EIS measurements, with an Ag/AgCl reference electrode and a platinum mesh as auxiliary electrode. A perturbation of 10 mV amplitude around the corrosion potential was applied with a range of frequency of 10 kHz to 1 mHz. Each EIS measurement was preceded by an open-circuit potential (OCP) measurement of about 2 h. OCP and EIS measurements were repeated over the total experiment period, which lasted between 3 and 4 weeks. The exit surface of the membrane was held in a deoxygenated 0.1 mol/L NaOH solution and, as a working electrode, was polarized to +250 mV Hg/HgO (1 M KOH) reference electrode.

With the three-electrode system completed with an immersed platinum mesh, the resulting current at the exit surface provided a direct measurement of the hydrogen flux across the iron membrane. Specific precautions (instrument ground in floating mode) were taken to avoid interferences between electrochemical measurements performed simultaneously on both sides of the iron membrane. In addition to the permeation membrane, weight-loss coupons (0.9 cm  0.9 cm  0.05 cm) were also introduced into the entry side chamber. These coupons were used for weight-loss corrosion rate evaluation, as well as surface analysis by scanning electronic microscopy (SEM), and x-ray diffraction (XRD). Considering the surface areas of the permeation membrane and iron coupons, the ratio between test solution volume and exposed iron surface area was approximately 20 mL/cm².

During each test, the charging solution was periodically sampled and analyzed by ionic chromatography (IC) to determine the S-O species concentrations. Sampling was performed through a septum with a syringe equipped with a filter, in order to remove FeS particles. Argon purging was then immediately applied to water aliquots in order to remove dissolved H 2 S and to retain only nonvolatile sulfur-containing compounds. The removal of H 2 S from these aliquots was required prior to IC measurement. The total volume of solution sampled represents less than 20 mL compared to 400 mL in total. Solution replenishment was not considered necessary. At the end of the test, the total sulfur content (exclusive of H 2 S) in the charging solution was analyzed by inductively coupled plasma emission spectroscopy (ICPES).

RESULTS

Control of O 2 Partial Pressure

Preliminary tests were performed using CO 2 instead of H 2 S in the acid gas line, and N 2 /O 2 in the oxygen line, in order to check that dissolved O 2 could be controlled at various levels between a few tens of parts per billion by weight and 500 ppbw. Such verification could not be performed with H 2 S, because reactions between O 2 and H 2 S are expected, resulting in an O 2 concentration that would be far less than its solubility equilibrium. Different levels of O 2 partial pressure were then obtained by selecting different flow rates for both gas lines. Dissolved O 2 concentration was measured with the O 2 probe and compared with theoretical value at equilibrium. The results illustrated in Figure 1 show that a stable O 2 content can be achieved and maintained. Experimental values were very close to theoretical values calculated from mass flow rates settings and Henry's law, also shown in Figure 1.

From these tests, our gas mixing system allowed a control of oxygen content at least up to 1.

Impact of Oxygen on Corrosive Medium

In order to determine the impact of O 2 contamination on the test solution, several tests were conducted with continuous pH measurement and periodic sampling of test solutions used for chemical analysis. The pH trends of test solutions with or without oxygen are shown in Figure 2. These pH measurements were performed during a permeation experiment, i.e., with iron coupons in the test solution.

In the beginning of both tests, pH is close to 3.9, corresponding to H 2 S-saturated water at 0. Over a long time period, this cumulative acidification can have a considerable effect on the pH of the solution.

In addition to in situ pH measurements, periodic sampling of the test solutions was performed for chemical analysis by IC.

The main results are plotted in Figure 3. Before analysis, residual dissolved H 2 S was eliminated by argon stripping in the water aliquot. Eliminating dissolved H 2 S before the IC analysis was required because H 2 S was found to be highly reactive with the column and might lead to fast deterioration of the equipment. The main dissolved sulfur compound detected was sulfate. Sulfite and thiosulfate were also found at lower concentrations.

In the test solution with oxygen, the sulfate concentration increases gradually and reaches 100 mg/L at the end of the test after 4-weeks exposure. Sulfites and thiosulfates are also detected at trace levels. A yellowish elemental sulfur deposit was also observed in the corresponding test cell. These results are in good agreement with Reactions (2) through (4).

According to these reactions, thiosulfate is an expected reaction product, although it is known to be unstable and can oxidize to sulfate and elemental sulfur in an oxygen-containing environment. ICPES results showed that H 2 S solution treated with oxygen consisted of 45 ppm of total sulfur. Hence, approximately 70% of the sulfur compounds detected by IC comprise dissolved sulfur. On the contrary, tests conducted without added oxygen did not show any elemental sulfur precipitation and no significant increase of sulfate, sulfite, or thiosulfate content.

At the end of the test, the dissolved iron content for the oxygen-free test is about 0.35 mg/L, and about 38 mg/L for the test in the presence of oxygen. This higher value of the dissolved iron content in the presence of oxygen is probably the result of the acidification of the solution, which increases the solubility of iron. At the same time, bulk precipitation consisting mainly of elemental sulfur was also observed.

Impact of Oxygen on Corrosion

The results showing the effect of oxygen on weight-loss corrosion rate are presented in In addition to weight-loss coupons, corrosion rates were also evaluated through electrochemical measurements at the entry face of the permeation membrane. Time evolution of corrosion potentials (E oc ) is plotted in Figure 4. Without O 2 , E oc remains stable during the entire test period, at -0.69 V Ag/AgCl  0.01 V Ag/AgCl . On the contrary, O 2 pollution induces a slight, but continuous increase of E oc . The amplitude of E oc increase at the end of the test (+30 mV) is well correlated with the pH evolution (+1 pH unit), considering Tafel slopes of 40 mV and 120 mV for the anodic and cathodic reactions, respectively.

Typical impedance diagrams obtained after various exposure times with or without O 2 pollution are presented in Figure 5.

EIS analysis required the development of a specific model, with all details provided in a previous work. 16 This model takes into account the build-up of a highly porous and conductive iron sulfide layer, represented by Z film . As a result of a distribution of pore sizes, the impedance of this film presents a dual behavior with two parallel contributions: (i) corresponding to a plane electrode (Equation [5]) and (ii) corresponding to a porous electrode (Equation [6]). In these expressions, Z eq defines the interfacial impedance at the surface of the conductive film inside the pores, and n, r, and l are the number of pores, their radius, and their length, respectively. The square root term of the porous electrode contribution explains the 45° inclination of the impedance at long exposure times observed in pure H 2 S without O 2 pollution. The resulting equivalent circuit is given in Figure 6, where Z film corresponds to the contribution of the corrosion product scale given by Equation ( 7), R t is the charge transfer resistance associated with the double layer capacitance constant phase element (CPE), CPE dl , and R a and CPE a are associated with the adsorption-desorption step of anodic dissolution. CPEs were used instead of pure capacitance as a result of time-constant distribution associated with surface heterogeneities. At very short immersion times, when the corrosion product scale is very thin, the contribution of Z film is negligible, and the equivalent circuit is then similar to that used by Ma, et al., in H 2 S solutions at lower pH. [17][18] (5)

(6) (7) rl n 2 Z n / Z Z eq pore plane , film    2 3 eq porous , film r n 2 Z Z    1 porous , film 1 plane , film 1 film Z Z Z     
All EIS spectra were analyzed with this equivalent circuit. The main difference between both cases becomes apparent at long exposure times. For the tests in pure H 2 S solution (Figure 5[a]), impedance diagrams tend to incline at 45°, but the second capacitive loop associated with the adsorption step maintains a high diameter. On the contrary, in presence of O 2 , the resistance of the first capacitive loop (charge transfer) decreases with time, together with a much stronger decrease of the low frequency contribution associated with the adsorption step. The values of the equivalent circuit components corresponding to the charge transfer loop for EIS diagrams of Figure 5 are given in Table 3.

From EIS analysis, corrosion current densities (J corr ) were calculated from R t using the Stern and Geary relationship. As shown in a previous paper, EIS diagrams are dominated by the impedance of anodic reactions. 16 As a consequence, a modified version of the Stern and Geary relationship was used, considering only the anodic Tafel coefficient (b a ). A value of 40 mV was used, as usually observed in H 2 S environments: [START_REF] Zheng | Electrochemical Mechanism and Model of H 2 S Corrosion of Carbon Steel[END_REF] (8)

The evolution of corrosion current densities with exposure time are compared in Figure 7 for the experiments conducted with and without O 2 pollution. As already shown by weight-loss test results, the acceleration of corrosion rates by O 2 pollution is confirmed. Through EIS monitoring, it is also shown that the acceleration is not immediate. During the first days, corrosion rates are similar in both experiments, starting close to 0.8 mm/y, and quickly decreasing to 0.5 mm/y to 0.6 mm/y after 1 d or 2 d. However, although the iron corrosion rate remained stable for the remaining 3 weeks in the reference situation, it started to increase after approximately 300 h in presence of O 2 pollution. At the end of the test, the corrosion rate was close to 1.8 mm/y. This increase is well correlated with the pH evolution shown in Figure 2. For both test solutions, an excellent agreement was found between the average corrosion rate calculated from electrochemical measurements (530 µm/y and 905 µm/y without or with O 2 pollution, respectively) and weight-loss measurements (490 µm/y and 976 µm/y without or with O 2 pollution, respectively). Surface analysis of the coupons was also performed. Results of XRD analysis showed that mackinawite (FeS) was the dominant phase, with some traces of greigite (Fe 3 S 4 ). No iron oxides or hydroxides were detected for the tests conducted with O 2 pollution. Although our XRD methodology did not identify any notable differences between scale compositions formed at the iron surface in either solution type, the same cannot be said for SEM (Figure 8). For oxygen-free tests, a rather homogeneous surface is observed, mainly composed of iron and sulfur as shown by electron-dispersive spectroscopy (EDS) mapping. On the other hand, for iron samples exposed to aqueous oxygen, EDS mapping shows differences in the deposits. Surfaces appear inhomogeneous, and cracked, with some zones confirmed to be rich in oxygen and others rich in sulfur. In addition, in the tests performed in the presence of oxygen, extensive precipitation was also observed on the walls of the test cells. XRD analysis was performed on these deposits, confirmed to consist mainly of elemental sulfur and marcasite. Although not detected at the surfaces of the corroded iron coupons, elemental sulfur might have been deposited and contributed to an increase of corrosion rate. From our point of view, the increase in the quantity of bulk precipitation in the presence of O 2 was unexpected. Indeed, the lowering of pH should favor acid dissolution of iron sulfide minerals. However, it appears that O 2 ingress compensates this phenomenon by promoting other forms of precipitates. We accept that an analysis of surface and bulk precipitates was not fully addressed in the current work, in order to elucidate these antagonistic effects.

Impact of O 2 on Hydrogen Permeation

In order to verify the reproducibility of electrochemical permeation measurements, five series of tests were performed in the model solution (35 g/L NaCl solution) under 0.1 MPa H 2 S. The results obtained are shown in Figure 9. It is observed on permeation transients that after 40 h of test, permeation current densities of between 41 μA/cm 2 and 48 μA/cm 2 are obtained, i.e., a variation of less than 10% around the average. In presence of oxygen, the interval between the highest and lowest values of five replicates is slightly higher, and it reaches approximately 20% after 600 h.

t a corr R 3 . 2 b J 
The effects of oxygen on permeation through a pure iron membrane in NaCl-saturated solution with 0.1 MPa H 2 S are illustrated in Figure 10. It must be noted that impedance measurements of the entry face were performed periodically during all of the tests, leading to small fluctuations of the permeation current. This explains the visual impression of noise on these permeation transients. No particular effect of oxygen is observed on the permeation current during the first 100 h of the test. However, after 200 h, a monotonic decrease in the hydrogen permeation current is measured in the presence of O 2 . After 620 h of tests, the permeation current density was approximately19 µA/cm 2 and 39 µA/cm 2 in the solution with and without oxygen, respectively. This trend is reproducible for the four sets of tests that have been done under these conditions. Such a decrease in the permeation current is rather unexpected, considering that the pH decreases with time down to 3.2 (with oxygen), while it stayed close to 4.4 (without oxygen). Indeed, increasing the acidity should normally have the opposite effect, with an increase of permeation rate. 7,[START_REF] Kittel | [END_REF] Comparing corrosion and permeation rates also gives an interesting perspective. The impedance measurements permit the evaluation of corrosion current densities of the iron membrane surface as a function of time over the test period (Figure 7), whereas the hydrogen permeation rates provide an indication of the hydrogen flux through the same membrane. This comparison is presented in Figure 11. For both tests, the permeation efficiency (E perm ) was also plotted. This parameter is calculated from the ratio between the permeation current density and the corrosion current density, expressed in percentage. For the test conducted without O 2 , there is a remarkable equality between the corrosion current density and the hydrogen permeation current density (Figure 11). This result is a clear evidence that the cathodic reaction involved in pure H 2 S systems consists only of proton reduction (via dissociated H + and direct H 2 S reduction), and also that the permeation efficiency is 100%, suggesting that 100% of hydrogen produced by the cathodic reaction enters the iron membrane. Such behavior is observed in particular hydrogen permeation systems, provided that there is no impediment to hydrogen entry, that no hydrogen diffusion limitation in the metal takes place (i.e., thin membrane, low trapping), and that the hydrogen extraction is efficient. 7,15 On the contrary, the tests with O 2 pollution depict a clear deviation between corrosion and permeation rates after a few days exposure (Figure 12). Although the permeation efficiency was close to 100% at the beginning of the test, it decreases to 12% after 26 d. At this point in the discussion, we identify three hypotheses to explain the decrease of permeation efficiency in the O 2polluted system. (1) Considering the role of the corrosion products formed in the presence of dissolved H 2 S-O 2 , the higher quantity of surface and bulk precipitates were expected to provide a blocking effect, reducing electrochemical reactions at the metal surface. However, such a hypothesis is contradicted by the fact that O 2 pollution also resulted in doubling the corrosion rate. Because H-entry into metals in H 2 S systems is understood to be a result of corrosion, an electrode blocking effect would lower both the corrosion rate and the permeation current. However, this is not observed. (2) The second hypothesis considers that dissolved O 2 , or more likely the reaction products of dissolved O 2 and H 2 S, have a significant contribution to the cathodic reaction electrochemistry. We reiterate that thiosulfates have been identified as a candidate reaction product, possibly leading to the following electrochemical contribution: 21

S 2 O 3 2-+ 6 H + + 4 e - 2 S + 3 H 2 O (9)
The direct reduction of oxygen can also be considered. The common feature of these reactions is that they do not produce "atomic hydrogen," i.e., they do not contribute to hydrogen entry in the steel, although promoting an increase in the corrosion rate.

(3) The third hypothesis to explain the decrease of hydrogen permeation current density over time is to consider a decrease of hydrogen entry promotion at the metal surface, i.e., a modification of the catalytic activity. Because hydrogen entry is suggested to be promoted by the action of specific adsorbates (H 2 S, HS -), which play the role of intermediate reactants during proton reduction, 6,8 those present in an O 2 -polluted solution may be less efficient than those observed in pure H 2 S media. A theoretical analysis proposed by Marcus and Protopopoff compared the adsorption of oxygen or sulfur atoms on iron, in various environments containing H 2 S and thiosulfates. 22 According to these authors, the presence of thiosulfate could modify the ratio between O ads and S ads , with potential consequences on the anodic reaction and on the enhancement of H entry into the metal.

In order to discriminate between bulk (Hypothesis 2) and surface (Hypothesis 3) reactions to explain the decrease of hydrogen permeation, two additional experiments were performed. The first one consisted of preparing a test solution by exposing iron to an O 2 -H 2 S solution for 3 weeks. This "aged" test solution, already polluted by O 2 -H 2 S reaction products and with a pH of 3.6, was then used for a permeation test on a fresh iron membrane. The resulting hydrogen permeation transient is presented in Figure 13, where it is compared with a transient obtained with a fresh solution. Both permeation transients are within the dispersion band of 10%, suggesting that the decrease of permeation efficiency is perhaps not linked to bulk solution composition.

Finally, in order to differentiate between bulk solution chemistry (Hypothesis 2) and surface reaction chemistry (Hypothesis 3) another type of test was performed. This experiment commenced like all previous tests with O 2 pollution, with hydrogen permeation being measured for 15 d (Figure 14). After this period, the permeation current density had decreased down to 35 µA/cm², and test solution pH was 3.5. Subsequently, taking all necessary precautions to avoid accidental oxygen entry, the test solution was drained; it was immediately replaced with a fresh solution saturated with H 2 S (containing oxygen) at a pH of 4. As Figure 14 confirms, the permeation current density remained unchanged after replacement of the test solution, suggesting that the contributions of the bulk solution chemistry reaction products are not highly relevant toward a diminution of the rate of hydrogen uptake. By a process of elimination, this result strongly indicates that the decrease of permeation efficiency is controlled by the changes to the nature and catalytic activity at the surface of the iron electrode, rather than by impurities in the test solution.

CONCLUSIONS AND FUTURE WORK

The effect of traces of oxygen on the corrosion of, and hydrogen permeation across, pure iron was studied in a 35 g/L NaCl test solution saturated with 0.  Corrosion is strongly accelerated in the presence of O 2 , on average by a factor of about 2, during exposures lasting 3 to 4 weeks. EIS evaluations at various exposure times show that the corrosion rate acceleration is not immediate and appears to follow the pH drift resulting from O 2 pollution. After 1 month exposure, the instantaneous corrosion rate was close to 0.5 mm/y in the reference system (pH 4.4), while it was in the order of 1.8 mm/y in presence of O 2 , with a pH close to 3.2. Differences are also observed in the morphology and homogeneity of corrosion deposits, with more oxides and elemental sulfur present in the case of O 2 -polluted systems. In addition, in both cases (with and without O 2 ), XRD analyses show that crystalline mackinawite is the majority compound. Test solution acidification is thought to be the major reason for enhancing the corrosion rate.

 As EIS measurements were being performed on the entry face of the thin membrane, hydrogen permeation was measured on the opposite face. In the reference situation without O 2 pollution, corrosion current densities and hydrogen permeation current present similar values. This permeation efficiency of 100% was expected, because the system presents an efficient hydrogen entry promoter (i.e., a high H 2 S concentration) and a thin membrane. Unexpectedly, hydrogen permeation does not follow the same trends in the presence of O 2 pollution. Whereas corrosion rate increased as a result of test solution acidification, permeation efficiency continuously decreased. Through additional tests, we show that this decrease was mainly associated with changes to the surface reaction rates of hydrogen uptake.

In the next parts of this series of papers, similar experiments will be performed at various H 2 S concentrations. In addition, in order to obtain a more direct comparison of NACE standard test conditions (e.g., as per NACE TM0284 or NACE TM0177) some tests will be conducted in buffered solutions and with steel grades used for oil and gas production.

FIGURE 1 .

 1 FIGURE 1. Verification of O 2 control at various levels in the test medium (a) and comparison with theoretical values (b).

FIGURE 2 .

 2 FIGURE 2. pH evolution in the corrosion test solution composed of 35 g/L NaCl with 0.1 MPa H 2 S, including the impact of a continuous contamination of 500 ppb O 2 .

FIGURE 3 .FIGURE 4 .

 34 FIGURE 3. Chemical analysis by ionic chromatography of S-O species in test solutions composed of 35 g/L NaCl with 0.1 MPa H 2 S, without (a) and with (b) continuous O 2 contamination at 500 ppbw

FIGURE 5 .

 5 FIGURE 5. Typical impedance spectra of pure iron in NaCl 35 g/L solution saturated with 0.1 MPa H 2 S at various exposure times, without (a) and with (b) O 2 pollution.

FIGURE 6 .

 6 FIGURE 6. Equivalent circuit for the corrosion of pure iron in H 2 S-containing environment with the formation of a conductive and porous FeS layer.16

FIGURE 7 .

 7 FIGURE 7. Evolution of corrosion current densities obtained from electrochemical impedance spectroscopy measurements, with and without O 2 pollution.

FIGURE 8 .

 8 FIGURE 8. Electron-dispersive spectroscopy mapping for corrosion products at the metal surface after tests (a) without oxygen (b) with oxygen (same magnification for both pictures).

FIGURE 9 .

 9 FIGURE 9. Reproducibility of permeation measurements in 35 g/L NaCl solution under 0.1 MPa H 2 S.

FIGURE 10 .

 10 FIGURE 10. Permeation curves in 35g/L NaCl under 0.1 MPa H 2 S, with and without oxygen.

FIGURE 11 .

 11 FIGURE 11. Comparisons between J corr and J perm, and evolution of the permeation efficiency, as a function of time for the test without O 2 pollution.

FIGURE 12 .

 12 FIGURE 12. Comparisons between J corr and J perm , and evolution of the permeation efficiency, as a function of time for the test with O 2 pollution.

FIGURE 13 .FIGURE 14 .

 1314 FIGURE 13. Hydrogen permeation transient with a test solution obtained after 3 weeks ageing with H 2 S and O 2 bubbling in presence of iron. Comparisons with a reference test with a fresh solution are included.

  S, the residual O 2 concentration was well below 5 ppbw. Hence, test conditions with O 2 pollution correspond to a wellcontrolled PO 2 in the gas phase bubbling into the test solution, although the effective value of dissolved O 2 is not known, as a result of H 2 S-O 2 reactions. One must then keep in mind that O 2 values in parts per billion by weight given in this paper thus correspond to what would be observed in the test solution in the absence of reactive species (e.g., H 2 S, but also dissolved iron that can precipitate with O 2 ). The same remark holds for NACE TM0284 and NACE TM0177, where maximum O 2 content is given in parts per billion by weight.

3 kPa, corresponding to approximately 500 ppbw dissolved O 2 . This system was then used with H 2 S. No additional O 2 measurements were performed with H 2 S, because O 2 -H 2 S reactions were likely to occur quickly in water. In addition, H 2 S could also deteriorate the oxygen probe. It was also carefully checked that, for reference corrosion tests without oxygen, i.e., pure H 2

  1 MPa. As expected, the pH then increases up to 4.3 with time, corresponding to FeS saturation. Once saturation is reached, no change of pH is observed for the test without O 2 (pH at end = 4.4). The test solution treated with both H 2 S and O 2 experiences a short initial rise in pH, followed by a continuous decrease in pH, by an order of magnitude, until the end of the test (pH at end = 3.2). It must be mentioned that for this test, measurements of O 2 concentration in the gas phase at the outlet of the test cell were periodically performed. We note that O 2 partial pressure was of the same order as it was at the inlet of the cell, suggesting that only a small fraction of O 2 has reacted in the test solution. This result illustrates that, during the first hours of the test in both solutions, the major reaction is the corrosion of iron, which produces alkalinity and dissolved iron and

	following reactions: 3-4
	2 H 2 S + O 2  2 S +2 H 2 O	(2)
	2 H 2 S + 2 O 2  S 2 O 3 2-+ H 2 O + 2 H +	(3)
	4 H 2 S + 9/2 O 2  S 4 O 6	2-+ 3 H 2 O + 2 H + (4)

consequently increases the pH. Once iron sulfide saturation is reached, the pH stops increasing. Particularly, in the test solution exposed to O 2 pollution, there are additional reactions between oxygen and H 2 S-producing S-O species that acidify the medium. This is in agreement with separate reports from Song, et al., and Crolet, et al., who proposed the

Table 2 .

 2 Four series of tests were performed in parallel, with and without oxygen.For the four replicate tests, the average corrosion rate in H 2 S-containing test solutions without O 2 pollution is 489 µm/y  68 µm/y. Conversely, oxygen pollution results in a strong increase in the corrosion rate, which is 976 µm/y  248µm/y. This increase in corrosion rate can be associated with the continuous decrease of pH due to O 2 -H 2 S reactions, which is expected to enhance the cathodic reaction. It is also possible that thiosulfate, or other unstable S-O species that may have formed, contribute directly to the cathodic current density and increase the corrosion rate. The higher dispersion of corrosion rates for the tests with O 2 might result from a less homogeneous solution, especially with regards to O 2 -H 2 S reaction products. One observation worth noting was that weight-loss coupons placed closer to the gas frit exhibited more severe corrosion. Unfortunately, magnetic bar stirring was not possible during our tests, because it resulted in significant disturbance of permeation measurements. Taking into account the test durations and the ratio between the surface of metal and the volume of test solution, these corrosion rates correspond to the release of 1.3 g (without O 2 ) or 2.5 g (with O 2 ) of iron per liter of test solution. Comparison with dissolved iron measured at the end of the tests (0.35 mg/L and 38 mg/L, respectively) confirms massive precipitation of FeS in solution.

  1 MPa H 2 S, as a function of time over 3 to 4 weeks. O 2 pollution was introduced in a continuous manner in the test gas, at a partial pressure of 1.3 kPa corresponding to 500 massppb dissolved O 2 at equilibrium. The following conclusions were drawn from this study:  O 2 pollution in an H 2 S-saturated solution has strong influence on, and modifies, the test solution chemistry. Although the main O 2 -H 2 S soluble reaction product is sulfate, the presence of the unstable intermediate species sulfite and thiosulfate are also detected; these are suspected of reacting with the metal before the more stable sulfate form is reached. Indeed, in the presence of O 2 , elemental sulfur precipitates are also detected. As expected, such solution chemistry reaction pathways lead to a strong acidification of the test medium, dropping by 1 pH unit over a period of 1 month.

TABLE CAPTIONS TABLE 1 .

 CAPTIONS1 Chemical Composition of Test Material (ppmw)

	Al	Cr	Cu	Mn	Ni	Si	Ti	C	P	S	Fe
	107	133	55	240	159	100	5	18	60	48	> 99500

TABLE 2 .

 2 Corrosion rates (µm/y) of Iron Coupons Exposed to Test Solution Composed of 35 g/L NaCl with 0.1 MPa H 2 S, with and Without O 2 Pollution

		H 2 S (a)	H 2 S + O 2 (b)	(b-a)/a (%)
	Test 1	480	1,025	114

TABLE 3 .

 3 Values of EC Components Related to the Charge Transfer Loop Obtained from the Analysis of Electrochemical Impedance Spectroscopy Data

			H 2 S			H 2 S + O 2
		2 h	47 h	572 h	2 h	47 h	572 h
	R e (Ωcm²)	143	140	136	136	135	132
	R t (Ω cm²)	276	381	458	254	336	168
	Q dl (mFs (α-1) /cm 2 )	3	13	44	3	12	14
	α dl	0.89	0.90	0.88	0.85	0.86	0.78

ACKNOWLEDGMENTS

The authors express their gratitude to Alexandre Bonneau for his active participation in the implementation of the experimental setup and Remy Mingant, for EIS analysis. The contribution of IFPEN analysis department in the surface characterization and solution analysis is also greatly acknowledged. Special thanks are also addressed to G. Joshi for the final proofreading.