Marie Candito

Mark Liberman

Keywords: Annotated corpora, Resources for NLP, Linguistic resources MOTS-CLÉS : Annotation de corpus, Ressources pour le

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction to the special issue on annotated corpora Marie Candito * -Mark Liberman ** 1. From corpus-based theology, anthropology, and lexicography to modern NLP Long before the invention of digital computers, the collection and annotation of corpora began as efforts to support the interpretation of culturally important texts, and continued as efforts to document language use. We'll introduce this special issue with a brief survey of some of this history, before providing some historical notes on annotated corpora for NLP in section 1.1, and for research in linguistics in section 1.2. One well-known example of text augmented with interpretation comments is the Talmud, which, as [START_REF] Mielziner | Introduction to the Talmud[END_REF] explains, "consists of two distinct works, the Mishna, as the text, and the Gemara as a voluminous collection of commentaries and discussions of that text". A different sort of religiously-motivated example is [START_REF] Strong | The Exhaustive Concordance of the Bible[END_REF], which assigns a number to each of 8,674 Hebrew "root words" (what we would call "lemmas") used in the Old Testament, and similarly to each of 5,624 Greek lemmas in the New Testament, and annotates each relevant word of the English King James Version with the number of the source-language word. This allows someone to read the original texts, in some sense, even if they have little or no knowledge of the source languages. And because the whole thing is indexed as a concordance, they can find and compare all of the passages that use a given English, Hebrew, or Greek word.

An approach more similar in design to contemporary corpus annotation is the tradition of interlinear text, represented in a fully mature form in [START_REF] Müller | The First Book of the Hitopadeśa containing the Sanskrit text, with interlinear transliteration, grammatical analysis, and English translation[END_REF]. As that work's title states, it presents three layers of annotation for each word of the Sanskrit text of the Hitopadeśa: "transliteration, grammatical analysis, and English translation". As with the Talmud and Strong's concordance, Müller's work was based on a text that had existed for hundreds of years. In the late 19th and early 20th century, interlinear annotations of newly-collected texts became a standard tool in the development of linguistic anthropology and linguistic documentation. Franz Boas and his many followers saw the collection and annotation of texts as central to the understanding of languages and cultures [START_REF] Boas | Kwakiutl texts[END_REF][START_REF] Epps | A holistic humanities of speaking: Franz Boas and the continuing centrality of texts[END_REF]. Roberto Busa's 1946dissertation (Busa, 1949) dealt with the concept of "presence" in the thought of Thomas Aquinas. Busa came to believe that he needed to understand the shades of meaning associated with Aquinas's use of the Latin preposition in, and based his dissertation on 10,000 examples of this usage, collected and written by hand on file cards. As he completed that work, Busa began to dream of a set of machinereadable cards a thousand times larger, from which automatic sorting could create unlimited opportunities for such corpus-based theology. In 1949, even before the first primitive digital computers became generally available, Busa persuaded IBM to support the creation of the Index Thomisticus, a complete lemmatized concordance to everything Aquinas wrote [START_REF] Jones | the emergence of humanities computing: the priest and the punched cards[END_REF][START_REF] Winter | and the invention of the machine-generated concordance[END_REF]. The millions of punched cards constituting this corpus were completed in 1967 [START_REF] Busa | Index Thomisticus Sancti Thomae Aquinatis Operum Omnium Indices Et Concordantiae in Quibus Verborum Omnium Et Singulorum Formae Et Lemmata Cum Suis Frequentiis Et Contextibus Variis Modis Referuntur[END_REF].

A similar transition from file cards to punched cards (and digital tape and onwards) took place in the field of lexicography. Starting in 1879, James Murray's "reading programme" turned mountains of books into thousands of "slips" documenting word usage, eventually organized into the Oxford English Dictionary [START_REF] Winchester | The Professor and the Madman[END_REF].

A similar program was carried out by the Merriam-Webster company in creating their series of dictionaries. In the early 1960s, Kučera and Francis created the Brown corpus [START_REF] Kučera | Computational analysis of present-day American English[END_REF], a million-word balanced corpus of written American English meant to support "computer-based research in the English language"; and just a few years later, Houghton-Mifflin used the Brown corpus as the citation base for the first edition of the American Heritage Dictionary (Morris, 1969).

The idea of basing dictionaries and other forms of language documentation on large representative digital text corpora was soon adopted widely, since it was essentially a computational enhancement of methods that had long been in use. In Sweden, the pioneering work of Sture Allén at the University of Gothenburg in the 1960s led to Press-65, an electronic text corpus of one million words of newspaper text [START_REF] Allén | Report on work in computational linguistics at the University of Göteborg[END_REF], and the establishment of Språkbanken (the Swedish Language Bank) in 1975. Also in the 1970s, researchers at Lund University created Talbanken, a syntactically-annotated corpus of Swedish (Einarsson, 1976a;Einarsson, 1976b). The Czech Academic Corpus project started in the 1970s at the Institute of the Czech Language 1 . The primary goal of this project was to create a corpus that would contain manual annotation of morphology and syntax of Czech, as a base for building a frequency dictionary. In the UK, the Collins Birmingham University International Language Database (COBUILD) project, begun in 1980 and funded by the Collins publishing company, formed the basis for the Collins COBUILD Dictionary, first published in 1987, and for a series of other reference works [START_REF] Sinclair | Looking up: An account of the COBUILD project in lexical computing and the development of the Collins COBUILD English language dictionary[END_REF][START_REF] Moon | Words, grammar[END_REF]. And, beginning in 1991, Oxford University Press led a consortium of dictionary publishers, academic research centers, and government organizations in creating the British National Corpus (BNC), an open corpus of 100 million words from sources meant to be representative of spoken and written English in Great Britain in the last decade of the 20th century [START_REF] Aston | The BNC handbook: exploring the British National Corpus with SARA[END_REF]. Part-of-speech and sense tagging were involved in the BNC project from the beginning [START_REF] Atkins | Tools for computer-aided corpus lexicography: the Hector project[END_REF][START_REF] Leech | CLAWS4: the tagging of the British National Corpus[END_REF][START_REF] Kilgarriff | Gold standard datasets for evaluating word sense disambiguation programs[END_REF].

In France, the Trésor de la Langue Française ("French language treasury", hereafter "TLF") project started in the 1960s to study French vocabulary usage, at the INALF laboratory in Nancy. As part of this project, the Frantext textual database of literary and technical texts was set up in order to provide examples for the TLF dictionary. After dictionary completion (the 16 volumes were released between 1971 and 1994, and are now an online resource, the "TLFi"), access was given to the textual database, first within the lab via a search engine in 1985, and later via online access in 1998 (according to Montémont [2008]). The current version contains over 5,000 automatically-parsed texts, totaling 250 million words 2 . And the MULTEXT project, funded by the European Commission in the early 1990s [START_REF] Ide | MULTEXT: Multilingual text tools and corpora[END_REF], aimed to provide "generally usable software tools to manipulate and analyse text corpora and to create multilingual text corpora with structural and linguistic markup."

The MULTEXT plan included standards for encoding linguistic annotation, including morphology, syntax, parallel text alignment, and prosody.

Corpora for Human Language Technology

Turning to the use of corpora for human language technology, research on the statistics of messages, by engineers tasked with sending them efficiently turned to have a major impact -perhaps the most important one for the modern use of corpora in NLP, since it has resulted in a flowering of diverse approaches to the annotation of large linguistic datasets, used to train computer systems for tasks far beyond the problems of message encoding and transmission. [START_REF] Shannon | A mathematical theory of communication[END_REF] laid out a theory of optimal transmission over noisy channels, and showed that a simple empirical model could provide an arbitrarily close approximation to the relevant statistics of message sequences. [START_REF] Baum | A maximization technique occurring in the statistical analysis of probabilistic functions of Markov chains[END_REF] and others provided computationally-efficient extensions of this model to cases where we can observe only a stochastic function of the hypothesized underlying text -and, over the course of the next few decades, such methods were applied to problems of speech recognition [START_REF] Baker | Stochastic modeling as a means of automatic speech recognition[END_REF], machine translation [START_REF] Brown | A statistical approach to machine translation[END_REF], part-of-speech tagging [START_REF] Church | A stochastic parts program and noun phrase parser for unrestricted text[END_REF], parsing (Lari and [START_REF] Young | The estimation of stochastic context-free grammars using the insideoutside algorithm[END_REF], and many other forms of speech and language analysis.

Starting in the mid-1980s, the US Defense Advanced Research Projects Agency (DARPA) began promoting research in this area, using what has been called the "Common Task method" [START_REF] Liberman | Human Language Technology[END_REF] or "Shared Task method". This research management technique begins with a shared training dataset and an automated quantitative performance metric, with periodic competitive evaluations on test data withheld for the purpose. Because the datasets and evaluation software were published, and because the method succeeded in fostering gradual improvements in the targeted technologies, the Common Task approach has been widely adopted.

And large annotated corpora have been at the center of the process, since the dominant paradigm has been supervised machine learning, in which a system is trained and tested on a body of text or speech in which the desired analysis is explicitly presented [START_REF] Liberman | The trend towards statistical models in natural language processing[END_REF]. Such analyses include morphosyntactic categories and relations in text, word senses, references to semantically-defined entities, textual evidence for adding information to a "knowledge graph", and many other things. The effectiveness of supervised models crucially depends on the availability of a large volume of annotated corpora. We focus below on the early days of two famous types of annotation, POS-annotated corpora and syntactic treebanks, whose development has been parallel to statistical taggers and parsers.

Perhaps the earliest example of digital corpus annotation was the part-of-speech (POS) tagging of the previously-mentioned Brown Corpus, which was motivated by language documentation and lexicography, and obtained by manually correcting the output of hand-written rules [START_REF] Greene | Automatic Grammatical Tagging of English[END_REF]. POS taggers based on statisti-cal machine learning [START_REF] Garside | The Computational Analysis of English: A Corpus-Based Approach[END_REF][START_REF] Church | A stochastic parts program and noun phrase parser for unrestricted text[END_REF] were developed in the 1980s and early 1990s, as were broad-coverage statistically-trained parsers [START_REF] Magerman | Parsing a Natural Language Using Mutual Information Statistics[END_REF][START_REF] De Marcken | Parsing the LOB corpus[END_REF]. These systems were trained on text collections provided with part-of-speech tags and syntactic analyses -and the same systems were also used in the creation of these treebanks to improve the productivity of human annotators by providing them with an automatically-created draft to correct. Morphosyntactically annotated corpora followed for a wide variety of typologically diverse languages.

The most well known and widely used syntactically-analyzed corpus has been the Penn Treebank [START_REF] Marcus | Building a Large Annotated Corpus of English: The Penn Treebank[END_REF], whose creation started in the late 1980s. The resulting dataset has been used for thousands of works on English syntax and systems for analyzing it -Google Scholar lists about 19,000 works that reference it. A crucial reason for this popularity (besides the economic importance of the English language) is that the Penn Treebank was made easily available to researchers all over the world, as the Brown corpus had been, in contrast to some other early collections such as the Swedish treebanks, which were tightly held by their creators. Similar treebanks were subsequently built for many typologically diverse languages.

A parallel strand of corpus creation has used syntactic dependencies [START_REF] Tesnière | Éléments de syntaxe structurale[END_REF] rather than syntactic constituents. From a formal point of view, these two representations are essentially equivalent, but the non-nested syntactic relations that are common in free-word-order languages are more easily represented by (crossing) dependencies than by (discontinuous) constituents. The Prague Dependency Treebank (with a first release in 1998 [Hajič, 1998]) was an important influence on the history of this approach, which has culminated in the Universal Dependencies project3 [START_REF] Nivre | Universal Dependencies v1: A Multilingual Treebank Collection[END_REF], an attempt to provide cross-linguistically consistent dependency annotation. This open community effort with over 200 contributors has led to 146 treebanks covering 83 languages (as of version 2.4). Despite the inevitable approximations of the cross-lingual scheme, the resource is abundantly used for typological quantitative research and for cross-lingual parsing.

Over the past decade, so-called "deep learning" methods have achieved better performance than statistical machine-learning methods on most NLP tasks. But these methods are even hungrier for training data, and so the need for large annotated corpora to support Human Language Technology has only increased.

Corpora for scientific and scholarly research

The Child Language Data Exchange System (CHILDES), begun in 1984, established a repository for sharing records of child language acquisition [START_REF] Macwhinney | The child language data exchange system[END_REF][START_REF] Macwhinney | The CHILDES project: Tools for analyzing talk[END_REF], based on a system for discourse notation and coding called CHAT, and including transcripts of caregiver-child interactions expressed in that form, in some cases with audio and/or video recordings. The accumulation of shared material in CHILDES continues, with more than 130 different sources in 26 languages now available. The CHAT format allows for (but does not require) annotation of time codes, pronunciations, dysfluencies and speech errors, prosody, and speech act categories, among other things.

The study of historical syntax has always been corpus based, since historical texts provide the only concrete evidence of how language was used in earlier periods. Over the past twenty years, researchers at universities in the US, the UK, Finland, and Norway have created a series of parsed corpora totaling about 9 million words and covering more than a thousand years of linguistic history [START_REF] Taylor | Treebanks in Historical Syntax[END_REF]. Similar efforts are underway for French, Portuguese, and Spanish.

Since the 1960s, research in quantitative sociolinguistics has been based on statistical modeling of annotation of the speech patterns of people varying in gender, age, location, socio-economic status, and communicative context. In earlier times, recordings and annotations were closely (and informally) held by the researchers that collected them, but, more recently, the culture of the discipline has begun changing in the direction of digital archiving and sharing of research datasets [START_REF] Kendall | On the history and future of sociolinguistic data[END_REF]. A similar trend has resulted in the digitization and (partial) publication of the archives of many dialect atlas projects from the past century [START_REF] Nerbonne | Data-driven dialectology[END_REF].

Thanks to the influence of psychology, as well as results within theoretical linguistics itself, such as [START_REF] Bresnan | Is syntactic knowledge probabilistic?[END_REF], statistical information is increasingly viewed as part of the competence of speakers rather than a mere artefact of linguistic performance. The statistical exploration of corpora thus serves to validate linguistic hypotheses and even to discover new patterns. In such studies, sophisticated linguistic annotation may be needed in order to compile the needed counts.

Forward-looking perspective

Looking forward to the future of annotated corpora, we can first comment on some "sociological" aspects, both within the linguistics and NLP research communities. Annotation projects are costly and time-consuming, meaning reusability is crucial. NLP researchers tend to reuse already existing datasets, not only to enable comparison between systems but simply because it is easier. This might explain the emergence of annotation scheme standards, often stemming from English-centered projects, initiated by major American NLP players. Resources in other languages either use the same schemes, even if at a smaller scale, or an original annotation scheme, at the risk of lacking international visibility. Interestingly, the Universal Dependencies project, although initially driven by such players (Stanford, through the Stanford dependencies, and Google, through the universal POS tagset), has fostered a global reflection on an annotation scheme with a multilingual vocation.

From a practical point of view of easing linguistic resource production, there are currently several approaches, in particular using more efficient tools for experts, or us-ing non-expert annotators. The possibility to leverage a potentially worldwide workforce via crowd-sourcing platforms has modified the economics of resource production. Note though that inequalities between languages are reinforced. In 2014, out of 100 languages, 13 only were considered to have an adequate workforce among the turkers [START_REF] Pavlick | The Language Demographics of Amazon Mechanical Turk[END_REF]. Ethical concerns are now part of major NLP conferences and the focus of specific workshops or special issues [START_REF] Fort | Éthique et traitement automatique des langues et de la parole : entre truismes et tabous[END_REF][START_REF] Hovy | Proceedings of the First ACL Workshop on Ethics in Natural Language Processing[END_REF]. Games with a purpose are an alternative used for varied linguistic resources including annotated corpora, for instance for coreference annotation [START_REF] Chamberlain | Using Games to Create Language Resources[END_REF] or dependency syntax [START_REF] Guillaume | Crowdsourcing Complex Language Resources[END_REF].

Another research lead is to make better use of the existing resources, in particular with more efficient learning from small datasets. Multilingual learning leverages data in several languages to better model the very same languages or some related ones.

Coping with multilinguality for resource production is indeed a major challenge, both practical and scientific. As already noted, the Universal Dependencies project succeeded in having hundreds of contributors collaborate. The PARSEME project has produced guidelines and data for 20 languages, for verbal multi-word expressions. Finally, annotating multimodal data is another challenge, with the necessity to annotate interconnections between different modalities (speech, gesture, emotional states...).

The framework of annotating corpora using analytic categories (for whatever kind of linguistic concept) is itself challenged, in particular given the current use of continuous representations in neural models. Frontiers between linguistic categories are often difficult to draw sharply (examples of well-known difficulties are the argument/adjunct distinction in syntax, or the adjective versus participle distinction in many languages, including French or German). This inevitably impacts the annotation process. For instance, [START_REF] Plank | Linguistically debatable or just plain wrong?[END_REF] show that disagreements on POS annotation concern debatable linguistic points rather than random errors, and should be used in the learning phase. These difficulties question the theory, but also the current annotation methodology, in which the resolution of annotation conflicts is the most time-consuming phase.

From the theoretical point of view, this indeterminacy of analytic category frontiers might be justified. Firstly, abandoning rigid frontiers between categories is empirically validated by the success of using continuous representations in neural NLP models, for all kinds of discrete symbols, enabling various mathematical representations of category combinations. Secondly, this can also lead to the idea of abandoning analytic categories altogether. The current trend in NLP is to overcome the need for annotated data, which is insufficient for most languages and tasks. Learning from raw texts and end-to-end models benefit from the enormous amount of raw texts, and challenge supervised NLP models trained on scarse but sophisticated symbolic annotations. But symbols do come back by the window: current research efforts on interpretability of neural models show that NLP without meta-linguistic explicitation is not satisfying. A promising research perspective is to integrate top-down (formalization to data) and bottom-up (from data to neural nets parameters) approaches.

Content of the special issue

The special issue contains three papers, two concerning syntactic treebanks and one concerning the evaluation of the quality of annotations. It is striking that the first treebank paper (Abeillé, Clément and Liégeois, Un corpus arboré pour le français: le French Treebank) provides an overview and some feedback on the first treebank project for French, namely the French Treebank, which has nourished NLP research for the French language, and whose development spans over the last twenty years. On top of an overview of the major linguistic choices underlying the annotation scheme, this paper is the occasion to provide some feedback on what could have been made differently, in the light of the various uses of the corpus in the last years. The paper presents the first full release of the corpus, namely the complete annotation of the whole corpus, namely meta-data concerning the author and domain of the articles, and, for each sentence, the annotation of multi-word expressions, parts-of-speech, morphological features, syntactic constituents and grammatical functions. Finally, a partial evaluation of the annotation quality is provided for the first time.

Interestingly, the other treebanking paper (Guillaume, de Marneffe and Perrier, Conversion et amélioration de corpus du français annotés en Universal Dependencies) focuses on how to have various treebanks converge within the multilingual annotation scheme of the Universal Dependencies. The difficulties of retaining linguistic description accuracy while using a scheme meant to be multilingual are described and illustrated with the French case. The paper presents a few annotation choices for cases not fully specified in the UD guidelines. A methodology for improving the quality of the resulting corpora is described, namely the double conversion method (converting into one annotation scheme, and converting back), thanks to the use of graph-rewriting rules. Differences with the original annotation signal errors either in the conversion rules, or in the original annotation.

The paper by Brégeon, Antoine, Villaneau and Halftmeyer, Redonner du sens à l'accord inter-annotateur : vers une interprétation des mesures d'accord en termes de reproductibilité des annotations focuses on annotation quality evaluation. Such an evaluation is essential to enable annotated corpora to serve as basis for valuable scientific findings. More precisely, the paper concerns the evaluation of a categorization task, in the multi-annotator setting, in which case the quality evaluation focuses on the inter-annotator agreement. The authors consider that popular chance-corrected measures such as Cohen's kappa and Krippendorff's alpha scores are difficult to interpret, and point out the arbitrary nature of the usual interpretation scale of the kappa. To get a more interpretable measure, the authors propose to evaluate the stability of the reference annotation. This is achieved by considering an average variation of the reference that would be obtained when taking a majority vote on subsets of annotators instead of on all annotators (note that this entails that at least three annotators per item are required in order to take subsets).

Experiments conducted both on real and simulated data show a correlation between the multi-annotator kappa score and the proposed reproductibility score, which the authors consider more interpretable. On a closer look though, the proposed metric varies, for the same kappa value, according to the distribution of divergences, i.e. according to whether the divergences are concentrated on a few items or scattered on many of them. On the former case, the variation rate tends to augment. So, despite its sensitivity to the number of classes and number of annotators to consider in subsets, the metric is proved to provide additional information with respect to the kappa.

https://universaldependencies.org/.

TAL. Volume 60 -n • 2/2019

Acknowledgements

We are very grateful to the editorial board and to the reviewing committee of the TAL journal, with special thanks to Emmanuel Morin.

We would also like to warmly thank the members of this issue's specific reviewing committee: Pascal Amsili (LLF, Université Paris Diderot), Farah Benamara (IRIT, Université Toulouse III -Paul Sabatier), Christophe Benzitoun (ATILF, Université de Lorraine), Delphine Bernhard (LiLPa, Université de Strasbourg), Kim Gerdes (ILPGA, Université Sorbonne Nouvelle), Marie-Catherine de Marneffe (Ohio State University), Paola Merlo (Université de Genève), Thomas François (Université catholique de Louvain), Carlos Ramisch (LIS, Aix-Marseille Université), Benoît Sagot (Almanach, INRIA), Agata Savary (LIFAT, Université de Tours), Djamé Seddah (Almanach, INRIA), Marie Tahon (LIUM, Université du Mans).


Xavier Verdalet-Guardiola

Benoit Fori

Jean-Pierre Bonino

Sandrine Duluard

Christine Blanc

• Clrima

Coatlng

Nucleation and growth mechanis ms of trivalent chromium conversion coatings on 2024-T3 aluminium alloy

A Alumlnium alloy A Copper B. EIS B.XPS C. Trivalent chromlum

Introduction

Due to their high specific modulus, aluminium alloys are widely used in the aeronautical domain [START_REF] International | Properties and Selection[END_REF]. The addition of specific alloying elements increases the mechanical propa-ties of aluminium, but has a detrimental effect on its corrosion resistance [START_REF] Muster | Copper Distribution in Al Alloys[END_REF][START_REF] Hughes | High Strength Al-alloys: Microstructure, Corrosion and Principles of Protection[END_REF]. As a consequence, for most of their applications, aluminium alloys cannot be used without surface treatment. Hexavalent chromium conversion coatings are Jar gely used to prevent corrosion of aluminium alloys. However, in 2013, the European Regulation REACh adopted a ban on the use of ercvn [ 4).

For the aeronautical field, this restriction will become effective in 2024 so that a lot of research studies are focused on the development of Cr (VI) free coatings, for example the trivalent chromium process (TCP) that is already commercially available [5 14). The anticorrosive properties of TCP coatings have already been studied extensively and the results showed that this new type of conversion coating is pro mising; the optimisation of their performance is still possible and should make the TCP prooess a very powerful solution [8,9,11 20). For this purpose, a better insight on the nucleation and growth mechanisms of TCP coatings should be helpful [12,21 23). Different scenarios and hypotheses are proposed in the literature. Unlike chromate coatings, for which a redox reaction based formation mechanism is proposed, TCP coatings are formed via a pH assisted precipitation [START_REF] Guo | Characterization of trivalent chromium process coating on AA2024-T3[END_REF][START_REF] Dardona | In situ spectroscopic ellipsometry studies of trivalent chromium coating on aluminum[END_REF][START_REF] Li | In situ pH measurement during the formation of conversion coatings on an aluminum alloy (AA2024)[END_REF]. lt is as sumed that, in a first step, the native aluminium oxide thickness • Corresponding author. decreases and metallic aluminium can be oxidised. This reaction con sumes protons and leads to a pH rise near the surface so that chromium and zirconium hydroxides precipitate at the surface. The reaction rate decreases as the surface becomes fully covered by the coating. This general process is generally admitted and supported by several stud ies. Dardona et al. [START_REF] Dardona | In situ spectroscopic ellipsometry studies of trivalent chromium coating on aluminum[END_REF] studied the growth of a TCP coating on pure alu minium by in situ spectroscopie ellipsometry. They proposed a three step mechanism: (i) an induction pa-iod with no growth of the coating and dissolution of the native oxide by F-ions; (il) rapid growth of the coating with a thickness vs. time linear law; (ili) low i.e. a logarithmic law, or no growth of the coating. Similar mechanisms were reported by Qi et al. [START_REF] Qi | Trivalent chromium conversion coating formation on aluminium[END_REF] based on the open circuit potential (OCP) variation of pure aluminium when immersed in the TCP solution. However, the studies do not detail the origin of the duplex layer microstructure of the coating. Li et al. [START_REF] Li | The formation, structure, electrochemical properties and stability of trivalent chrome process (TCP) coatings on AA2024[END_REF] suggested that, once the native oxide layer was dissolved, the aluminium was oxidised by fluoride ions and formed an inner layer of fluoroaluminates. Chromium and zirconium hydroxides precipitated at the surface of the fluoroal uminates to form the outer layer. They also suggested that intermetallic particles catalysed the pH increase by reducing the incubation pa-iod before the TCP coating growth. Qi et al. [START_REF] Qi | Formation of a trivalent chromium conversion coating on AA2024-T351 alloy[END_REF] measurements supported this last hypothesis. Indeed, using energy dispersive spectroscopy (EDS) analyses, the au thors detected zirconium after only 15 s of exposure to the TCP solution near a S phase particle and after 120s on the aluminium matrix.

E-mail address: christine.blanc@ensiacetfr ( C . Blanc).

https://doi.org/10.1016/j.corsci.2019.04.035

prepared with SEM FIB under the conditions described above. GD OES analyses were performed with a HORIBA Jobin Yvon GD Profiler 2 at the platform of micro characterisation Raimond Castaing (Toulouse, France). A copper anode of 4 mm diameter was used with an argon pressure of 630 Pa, a power of 30 W and radiofrequency of 13.56 MHz. A flush time of 120 s was operated before sampling. Samples were covered by a gold sputtered layer in order to prevent instability of the plasma initiation. The acquisition interval was set to 0.01 s. The emission lines used were: H 121.567 nm, O 130.217 nm, C 165.701 nm, Cu 324.754 nm, Zr 339.198 nm, Al 396.152 nm, Cr 425.433 nm and Au 242.795 nm. GD OES profiling was used to de termine in depth variations of the element concentration for the TCP coatings formed for different conversion times.

XPS profiles were performed with a ThermoFisher K alpha spec trometer at Aquitaine Platform of Material Characterisation (PLACA MAT, Bordeaux France). Base pressure was fixed to 10 -9 mbar. Al Kα monochromated radiation (1486.6 eV) was used as X ray source. Measured spot diameter was fixed to 200 μm. Sputtering was performed with a Ionic Ar canon (500 eV) and a spot diameter of 500 μm. These conditions give a sputtering of 0.3 nm.s -1 on SiO 2 .

Electrochemical measurements

OCP measurements were performed in the TCP solution with a VSP 150 potentiostat (Bio Logic). A two electrode cell set up composed by a saturated calomel electrode (SCE) as reference electrode and the alu minium alloy substrate as working electrode was used.

Local pH measurements were performed during the TCP coating growth in the TCP solution by recording the electrical potential of a tungsten microelectrode (W microelectrode). The W microelectrode was prepared as described in Guo [START_REF] Guo | pH response behaviors and mechanisms of different tungsten/tungsten oxide electrodes for long-term monitoring[END_REF] and Li [START_REF] Li | In situ pH measurement during the formation of conversion coatings on an aluminum alloy (AA2024)[END_REF] previous works. A pH potential calibration curve was plotted in TCP solution allowing to correlate the potential of the W microelectrode to the pH around it. The W microelectrode potential was measured with a VSP 150 potentiostat (Bio Logic); before each measurement, it was allowed to stabilise during 5 min in a TCP solution placed in a beaker. For the local pH measurements, the W microelectrode was positioned by using an in termittent contact scanning electrochemical microscope IC SECM M470 (Bio Logic). The distance between the alloy surface and the W micro electrode was estimated around 100 μm. When the W microelectrode and the degreased and desmutted sample were well positioned in the IC SECM set up, the TCP solution was added and the potential mea surement started. The pH measurement was performed in a TCP solu tion at room temperature (22 ± 1 °C).

Electrochemical Impedance Spectroscopy (EIS) measurements were performed in naturally aerated 0.5 M NaCl solution with a three elec trode cell (SCE reference electrode and graphitic rod as counter elec trode). Before each measurement, OCP was recorded for 1 h. Measurements were performed at OCP for different immersion times in the NaCl solution: 1 h, 3, 7, 14 days. For more clarity, only the results at 1 h and 14 days are shown in the paper. Frequency range was set to 6 10 4 to 5 10 -3 Hz with a potential amplitude of 10 mV (rms). Z view software (version 3.5d, Scribner Associates) was used to fit the ex perimental data.

All electrochemical measurements were performed in a laboratory room with controlled (22 ± 1 °C) temperature and repeated at least three times for reproducibility.

Results and discussion

Surface reactivity in the TCP solution

Variation of the OCP in TCP solution for the degreased and des mutted alloy is reported in Fig. 1. The shape of the curve was similar to the OCP described in the literature [13, 25 27]. An initial potential decrease was measured before a fast OCP increase followed by a To the authors' knowledge, there is no direct evidence of this three step mechanism. Therefore, to go further in the description of the TCP coating nucleation and growth mechanisms, the present work focusses on the evolution of the coating morphology and chemistry after dif ferent immersion times in the TCP solution, considering in particular very short immersion times to detect the very first stages of nucleation and growth. Scanning electron microscope (SEM) and transmission electron microscope (TEM) observations, X ray photoelectron spectro scopy (XPS) and Glow Discharge Optical Emission Spectroscopy (GD OES) analyses were performed to identify and characterise the TCP coatings formed after different immersion times. Then, electrochemical impedance spectroscopy (EIS) measurements were performed in order to evaluate the evolution of the corrosion properties of the samples in relation with the coating morphology.

Materials and methods

Material and sample preparation

Samples were removed from cold rolled sheets (1 mm thick) of aluminium alloy 2024 (AA 2024) thermally treated T3 and provided by Kaiser Aluminium. The chemical composition of the alloy (wt. %, Al balance) was: Cu 4.5, Mg 1.4, Mn 0.51, Fe 0.15, Si 0.08, Zn 0.13, Cr 0.01, Ti 0.02, V 0.01, Zr 0.01, others 0.05. The samples were 12 × 8 x 0.1 cm 3 parallelepipeds with the TCP coating formed on the rolling plane. Before the surface treatments, the sample surface was abraded using SiC papers down to P grade 2400 and then polished to a 1 μm diamond paste, in order to have a controlled surface. Then, the TCP coating was formed on the sample surface using a multi step process. Firstly, in order to remove inks and other surface pollutants, samples were rinsed with acetone prior to surface preparation, as done during the industrial process. Then, they were degreased in an alkaline bath (40 g•L -1 sodium tripolyphosphate, 40 g•L -1 borax and 5 mL•L -1 Turco 4215 additive, pH = 9) at 60 °C for 1200s. The next step corresponded to an immersion in a desmutting bath, i.e. a pH = 1 sulfo nitro ferric solution, at room temperature for 300 s in order to remove coarse in termetallic particles and native oxide. The TCP coating was then formed by immersion of the sample in a 32% v/v Socosurf TCS (Socomore, France) solution at 40 °C for various duration times from 1 s to 600 s. The pH of the solution was set at 3.9 ± 0.1 with a sulfuric acid solu tion. Finally, the samples were dried with dried air stream. For each step, the solutions were prepared using industrial grade chemical re actants, and the samples were rinsed in deionised water following each step.

Morphological and chemical characterisations

The surface morphology of the TCP coatings was characterised by using a SEM equipped with a field emission gun (SEM FEG), JEOL JSM 7800 F Prime, operating at 5 kV in secondary electrons mode (platform of micro characterisation Raimond Castaing, Toulouse, France). The coating thickness was measured on cross sections prepared with a SEM focused ion beam (SEM FIB) FEI HELIOS 600i equipped with a gallium Ionic Canon (platform of Micro characterisation Raimond Castaing, Toulouse, France). Gallium canon operated at 30 kV and 47 nA to 80 pA during cutting. Before cutting, samples were covered by an electro nically deposited carbon layer (0.5 μm in thickness, operating at 5 kV and 2.5 nA, precursor: naphthalene (C 10 H 8 )) and a 3 μm thick ionically deposited platinum layer (operating at 30 kV and 0.43 nA, precursor: methylcyclopentadieny(trimethyl)platinum (C 5 H 4 CH 3 Pt(CH 3 ) 3 )). Because the contrast between TCP coating and carbon layer was low, a gold coating was sputtered by physical vapour deposition on top of the conversion coating to increase the contrast.

TEM observations combined with EDS analyses were performed with a JEOL JEM 2100 F (platform of micro characterisation Raimond Castaing, Toulouse, France) operating at 200 kV. TEM samples were plateau. Such changes in OCP were attributed to changes in reactivity of the alloy surface. Indeed, in the literature, the OCP decrease was as sociated to the dissolution of the air formed oxide on the alloy surface and the OCP increase to a rapid growth of the conversion layer. The OCP plateau was correlated to a complete coverage of the alloy by the TCP coating. Andreatta et al. [START_REF] Andreatta | SKPFM and SEM study of the deposition mechanism of Zr/Ti based pre-treatment on AA6016 aluminum alloy[END_REF] suggested that further immersion in the conversion bath could lead to an increase of the thickness layer without OCP variation.

To go further in the explanation of the OCP curve and description of the nucleation and growth mechanisms of the TCP coating, the coating morphology, composition and anticorrosion properties were studied throughout the conversion coating formation. Cross marks plotted in Fig. 1 indicate the samples that were further studied: samples immersed in the TCP solution for different duration times, i.e. between 1 600 s, were studied.

Local pH variations during TCP coating growth

Local pH variations measured near the AA 2024 surface during the conversion treatment in the TCP solution are plotted in Fig. 2; the OCP variation is also reported for comparison. Immediately after immersion, a rapid increase of the pH was observed, from 3.86 to a maximum value of 4.5 reached after 30 s. Then, the pH decreased slowly to reach a value of 3.9 after 300 s in the TCP solution; it remained relatively stable around 3.9 4 during the following 300 s.

As discussed in the literature, the TCP coating growth is based on the precipitation of oxy hydroxide due to the pH increase [START_REF] Dardona | In situ spectroscopic ellipsometry studies of trivalent chromium coating on aluminum[END_REF][START_REF] Li | In situ pH measurement during the formation of conversion coatings on an aluminum alloy (AA2024)[END_REF]. Comparison of the OCP and pH curves showed that the minimum OCP and maximum pH values were not reached exactly at the same time. However, the position in time of the maximum pH was not very accu rate due to the fact that the W microelectrode was relatively far from the sample surface (around 100 μm). Therefore, it was assumed that, as indicated in the literature, the TCP coating growth was controlled by the pH. Moreover, after 300 s, the pH near the surface was similar to the bulk solution. Such a decrease of the pH gradient indicated a slowdown or a stop of the coating growth, as reported in the literature [START_REF] Dardona | In situ spectroscopic ellipsometry studies of trivalent chromium coating on aluminum[END_REF][START_REF] Dong | Structure and composition of trivalent chromium process (TCP) films on Al alloy[END_REF].

Evolution of the TCP coating morphology and thickness during the conversion treatment

Surface morphology of the TCP coating

SEM FEG micrographs of the sample surface before immersion in the TCP solution and for different immersion times in the TCP solution are reported in Fig. 3. Firstly, after surface preparation, i.e. after de greasing and desmutting, alloy surface exhibited a typical scalloped morphology (Fig. 3a) [START_REF] Verdalet-Guardiola | Influence of the alloy microstructure and surface state on the protective properties of trivalent chromium coatings grown on a 2024 aluminium alloy[END_REF]. Results also showed that the scalloped morphology of the as prepared substrate was maintained in the first period of the coating formation, i.e. after only 1 s of immersion in the TCP solution, but small nodules were observed on the ridges of the scallops (Fig. 3b). The nodules were assumed to correspond to the first evidence of the coating growth. After 3 s of immersion in the TCP so lution (Fig. 3c), the alloy surface was fully covered by the nodules but pores were observed. After 13 s (Fig. 3d), i.e. when the minimum OCP value was reached (Fig. 1), pores were not detected any more. Then, the coating surface morphology remained similar during the rest of the chemical conversion treatment (Fig. 3e to h) which was coherent with the literature [START_REF] Li | The formation, structure, electrochemical properties and stability of trivalent chrome process (TCP) coatings on AA2024[END_REF][START_REF] Guo | Characterization of trivalent chromium process coating on AA2024-T3[END_REF][START_REF] Rangel | Conversion coating growth on 2024-T3 Al alloy. The effect of pre-treatments[END_REF]. However, observations showed that the nodule size was increasing as a function of the exposure time to the conversion solution. In Fig. 4, average values of the nodule size are given for the different exposure times; they result from at least 30 measurements performed on 10 different zones of the sample, with 3 measurements per zone. This method was used to evaluate whether an influence of the grain orientation of the substrate on the nodule size could be detected but this was not shown. Fig. 4 shows that the nodule diameter was around 30 nm from 1 to 45 s of immersion in the TCP solution. Therefore, during the first 45 s, no significant growth of the nodules was observed, but, during the OCP plateau (Fig. 1), the nodule diameter increased and reached 70 nm for 300 s of exposure which could indicate a modification of the growth mechanism. For a 600 s immersion, the nodule diameter could not be measured accurately.

Thickness of the TCP coating

SEM FEG micrographs of FIB cross sections prepared for the TCP coatings formed after different immersion times in the TCP solution are shown in Fig. 5. After only 1 s of exposure to the TCP solution, it was difficult to observe the TCP coating because it corresponded only to small nodules on the ridges of the scallops (Fig. 3b), but the TCP coating was clearly observed for immersion times as short as 3 s. Moreover, independent on the immersion times (equal and/or higher than 3 s), the coating thickness was enough to lead to an apparent complete covering of the substrate roughness (which did not mean that pores were not observed). Fig. 4 showed that the coating thickness strongly increased at the beginning of the TCP treatment (12 nm after 3 s, 42 nm after 45 s) and went on increasing even during the OCP plateau (Fig. 1) to reach 85 nm after 300 s of exposure to the TCP solution. Results also showed surprisingly that the TCP coating was less thick after 600 s of exposure to the TCP solution than after 300 s. Considering that the TCP coating thickness was calculated on the basis of ten measurements performed for three different samples in each case, it was assumed that the thin ning of the TCP coating for long immersion times in the TCP solution had to be considered. Moreover, after 600 s of exposure to the TCP solution, the TCP coating was about 60 nm thick, which was in ac cordance with the 50 100 nm thicknesses reported in the literature [7,8,13,31 33]. A TEM micrograph of the TCP coating formed on AA 2024 T3 after a 3 s immersion in the TCP solution is reported in Fig. 6 including cor responding EDS mapping scans. In Fig. 6, the different zones have been labelled, especially the platinum deposit; indeed, when platinum is present in very large content, its signal distorts that of zirconium as seen for the zirconium map at the bottom of the map. TEM micrograph seemed to show a single layer coating, unlike usual TCP coatings de scribed in the literature and characterised by a duplex structure [START_REF] Guo | Characterization of trivalent chromium process coating on AA2024-T3[END_REF]. The coating showed a non homogeneous thickness, between 8 and 15 nm, which was not inconsistent with previous SEM measurements, especially by considering that TEM observations were very local whereas the thickness measured by SEM was calculated on the basis of ten measurements done on different sites. EDS mapping scans showed that the chromium, zirconium and fluorine maps could be superimposed which might confirm the single layer structure of the coating formed after 3 s of exposure to the TCP solution. Therefore, all the results suggested that the coating formed after very short immersion times in the TCP solution should have to be considered as a TCP pre cursor coating. Nevertheless, it remains possible that the TEM/EDS mapping resolution equal to a few nanometres does not allow to detect a more complex microstructure, and perhaps a two layer structure of the TCP coating formed for short exposure was missed. Furthermore, local variation in the thickness of TEM sample could disrupt nano scale microscopy observations.

Chemical composition profiles

GD OES elemental depth profiles plotted for AA 2024 samples im mersed in the TCP solution for different immersion times are shown in Fig. 7. No result was for fluorine because GD OES analyses were performed under an argon pressure which does not allow to detect this element [START_REF] Wagatsuma | Detection of fluorine emission lines from Grimm-type glow-discharge plasmasuse of neon as the plasma gas[END_REF]. Copper signal was used to localise the position of the TCP coating/AA 2024 interface on the basis of the copper enrichment generated by surface preparation treatment as shown in previous work [START_REF] Qi | Comparative studies of thin film growth on aluminium by AFM, TEM and GDOES characterization[END_REF]; this interface is indicated by the vertical dash lines in Fig. 7. It is of interest to note that the intensity of the copper peak linked to copper enrichment at the alloy surface was similar independently of the immersion time in the TCP solution and was similar also to that ob served for the desmutted substrate (not shown here). This suggested a low influence of the TCP conversion on the copper enriched layer. Results showed that the TCP coating/alloy interface was reached after 0.4, 0.72, 1.06, 1.17, 1.9, 2.18 s of GD OES sputtering for samples ex posed to the TCP solution for 3, 13, 15, 45, 300 and 600 s, respectively, which could be related to an increase of the TCP coating thickness when the immersion time in the TCP solution increased. Moreover, for a 3 s immersion in the TCP solution (Fig. 7a), only fine peaks of chromium and zirconium were observed in agreement with the presence on the sample surface of a fine TCP coating that was supposed to be a single layer coating on the basis of TEM observations (Fig. 6). Considering the GD OES sputtering area (around 4 mm), the small thickness and the nodular structure of the coating, associated with strong roughness, for this short duration time of exposure to the TCP conversion solution, it was difficult to conclude on the coating structure, i.e. single layer or two layer coating. On the contrary, when the immersion time in the TCP solution increased (Fig. 7b f), the width of the chromium and zirconium signals increased and two layers were distinguishable in the TCP coating for immersion times in the TCP solution longer than 13 s: an outer layer mainly composed of chromium and zirconium and an inner layer containing mainly aluminium. The literature also men tioned a two layer structure with oxide and hydroxide of chromium and zirconium in the upper part (i.e., outer layer) and fluoroaluminates in the inner one (i.e., inner layer) [START_REF] Guo | Characterization of trivalent chromium process coating on AA2024-T3[END_REF][START_REF] Qi | Trivalent chromium conversion coating formation on aluminium[END_REF]. Further, except for a 600 s long TCP treatment, concerning the outer layer, the results also showed that the chromium signal had a relatively constant intensity whereas the intensity of the zirconium signal increased with sputtering time which could indicate differential precipitation kinetics of those elements during the conversion treatment. After 600 s of immersion in the TCP solution, the intensity of the zirconium signal was more stable during sputtering which suggested that different mechanisms could occur be tween 300 and 600 s of immersion in the TCP solution. The evolution of the growth mechanisms could lead to a modification of the TCP coating structure, e.g. its compacity; those changes could be associated to dif ferent sputtering rates of the coating as suggested by Qi et al. [START_REF] Qi | Trivalent chromium conversion coating formation on aluminium[END_REF]. These observations could explain why, with GD OES measurements, the TCP coating thickness seemed to increase even between 300 and 600 s, whereas SEM FEG observations (Fig. 5) showed a thinning of the TCP coating after 300 s of immersion in the TCP solution.

Because GD OES measurements are sensitive to the differential sputtering capabilities of the elements and do not allow to detect fluorine in the experimental conditions used (argon gas), XPS con centration profiles were plotted to complete the study (Fig. 8). Due to the carbon surface pollution (> 40% at ), the first measuring point was not considered (grey zone in Fig. 8). After 3 s of immersion in the TCP solution (Fig. 8a), low concentrations of chromium and zirconium were detected (0.5 and 3% at. respectively) on the sample surface. A low content of fluorine was also measured around 2% at . This corroborated the previous results (TEM EDS mapping and GD OES analyses) that showed the presence of the TCP coating even after only a 3 s exposure to the TCP solution. After 13 s of immersion in the TCP solution (Fig. 8b), chromium, zirconium and fluorine concentrations increased on the sample surface; moreover, the maximum signals for the three elements were observed after similar sputtering times; this suggested a single layer structure for the coating, in agreement with the hypothesis proposed above. Moreover, the analysis of the XPS profiles for longer exposure to the TCP solution confirmed the validity of this hypothesis. Indeed, after 15 s (Fig. 8c), chromium and zirconium surface con centrations went on increasing. For the fluorine, the global concentra tion increased also but the concentration profiles clearly showed that the maximum fluorine concentration was observed inside the coating whereas the chromium and zirconium signals were maximal in the extreme surface. Further, after 45 s (Fig. 8d), chromium and zirconium surface concentrations reached values equal to 3% at and 22% at , re spectively, i.e. similar values to those reported in the literature for TCP coatings [START_REF] Li | The formation, structure, electrochemical properties and stability of trivalent chrome process (TCP) coatings on AA2024[END_REF][START_REF] Schram | Study of the composition of zirconium based chromium free conversion layers on aluminium[END_REF]. For this immersion time in the TCP solution, the maximum chromium and zirconium concentrations were still measured at the extreme surface with concentrations decreasing inside the coating. By comparison to Fig. 8c, the maximum signal for the fluorine was even more shifted inside the coating. Therefore, the results showed that, for immersion times in the TCP solution equal to or higher than 15 s, a duplex structure was observed for the TCP coating and the chromium and zirconium rich outer layer was clearly defined. After 300 s of exposure to the TCP solution (Fig. 8e) and for a complete treatment (600 s, Fig. 8f), a plateau was observed for both chromium and zirconium signals during the first 100 150 s of sputtering. The concentrations on the plateau were equal to 4.5 and 23% at for chro mium and zirconium, respectively, similar to those measured at the extreme surface after 45 s of exposure to the TCP solution. Near the TCP coating/substrate interface, chromium and zirconium concentrations decreased significantly whereas the fluorine signal was maximum showing clearly the presence of the fluorine rich inner layer. Results also showed that the fluorine concentration in the inner layer increased as the immersion time increased between 300 and 600 s. This result indicated the permeability of the outer layer to the fluorine rich elec trolyte as observed by EIS in previous work [START_REF] Verdalet-Guardiola | Influence of the alloy microstructure and surface state on the protective properties of trivalent chromium coatings grown on a 2024 aluminium alloy[END_REF].

To sum up, the results showed that, for very short immersion times in the TCP solution, only a TCP precursor layer could be observed. Indeed, TEM images associated with EDS mapping had suggested a single layer structure for short exposure to the TCP solution, which did not correspond to the two layer structure commonly observed for TCP coating. But, the possibility that this two layer structure had been missed could not be neglected. Then, the sample roughness associated to the low lateral resolution of GD OES and XPS could lead to some uncertainty on the conclusions. However, the methodology used for XPS analyses (with an accurate study of the evolution of the con centration profiles with increasing exposure to the TCP solution) led us to assume that, for short immersion time, the superimposition of the maximum signals for chromium, zirconium and fluorine was not an artefact and showed that the TCP coating had a single layer structure whereas it evolved towards a two layer structure during its growth leading to a gradual shift of the maximum signal of fluorine compared to chromium and zirconium signals. Indeed, when the immersion time increased, a duplex structure was observed progressively: the pre cipitation of chromium and zirconium oxide occurred leading to the formation of an outer layer permeable to fluoride ions, which thus al lowed the formation of the fluorine rich inner layer. Moreover, a sig nificant enlargement of the chromium and zirconium plateaus was observed between 300 and 600 s of exposure to the TCP solution sug gesting a thickening of the TCP coating as observed with GD OES analyses (Fig. 7). This observation was inconsistent with the FIB mea surements (Fig. 5) which indicated a thickness decrease between 300 and 600 s of exposure to the TCP solution. However, as previously in dicated, GD OES analyses suggested that chemical and/or morpholo gical changes of the TCP coating could explain significant variations in sputtering rate. As an example, a densification of the outer layer could generate significant sputtering rate variations.

Evolution of the anticorrosive properties of the TCP coating as a function of the immersion time in the TCP solution

EIS spectra of TCP coatings formed on AA 2024 T3 for different immersion times in the TCP solution are plotted in Fig. 9. After 1 h of exposure to the 0.5 M NaCl solution, all the coated samples exhibited higher impedance modulus at 10 -2 Hz than the degreased and des mutted sample. Moreover, the EIS spectra for the degreased desmutted sample showed two well defined time constants, the one at 10 -1 Hz being related to the oxygen diffusion on the sample surface. For the 3 s coated sample, the two time constants were not clearly seen but a shoulder on the phase was noticeable. For all others samples, only one time constant, with high phase angles on a large frequency domain, was distinguished. As shown in a previous paper [START_REF] Verdalet-Guardiola | Influence of the alloy microstructure and surface state on the protective properties of trivalent chromium coatings grown on a 2024 aluminium alloy[END_REF], high impedance modulus and a large frequency range of high phase angles could be related to improved anticorrosion properties for the TCP coating [START_REF] Qi | An optimized trivalent chromium conversion coating process for AA2024-T351 alloy[END_REF][START_REF] Qi | Trivalent chromium conversion coating formation on aluminium[END_REF][START_REF] Verdalet-Guardiola | Influence of the alloy microstructure and surface state on the protective properties of trivalent chromium coatings grown on a 2024 aluminium alloy[END_REF][START_REF] Matzdorf | Evaluation of modified zirconium/ trivalent chromium conversion coatings by accelerated corrosion and electrochemical techniques[END_REF]. After 14 days of immersion in NaCl, an enlargement of the frequency domain were high phase angles were observed and an increase by one or two orders of magnitude of the impedance modulus at 10 -2 Hz were noticeable for all samples except for the degreased desmutted and 3 s TCP coated samples that showed similar EIS spectra. The 600 s TCP coated sample showed the largest frequency domain with high phase angles which suggested that this coating had the highest anticorrosive properties after 14 days of immersion in NaCl. On the basis of these results, the EIS spectra plotted for the degreased desmutted (1 h and 14 days) and 3 s TCP coated (14 days) samples were analysed with the same equivalent circuit (Fig. 10a). It comprised a parallel combination of the charge transfer resistance R ct and a constant phase element Q dl associated with another combination including the resistance linked to the oxygen reduction R difO2 and a constant phase element Q difO2 . In accordance with Qi et al. [START_REF] Qi | Trivalent chromium conversion coating formation on aluminium[END_REF] and previous work [START_REF] Verdalet-Guardiola | Influence of the alloy microstructure and surface state on the protective properties of trivalent chromium coatings grown on a 2024 aluminium alloy[END_REF], another equivalent circuit was used (Fig. 10b) to analyse the EIS spectra plotted for the other coated samples after 1 h and 14 days in NaCl solution (and for the 3 s coated sample after 1 h in 0.5 M NaCl). For this circuit, the second parallel combination was replaced by a coating resistance R coat and a constant phase element Q coat . EIS para meters extracted from spectra plotted after 1 h and 14 days of immer sion in NaCl are reported in Tables 1 and2, respectively.

At the beginning of the immersion in NaCl solution (1 h immersion), R coat values were similar for all TCP coated samples, even for the short exposure duration time to the TCP solution (3 s). For all samples, R coat values were in the same order of magnitude than the electrolyte re sistance (R e ) which indicated a permeability of the coating allowing access of the electrolyte to the alloy as shown by Dong et al. [START_REF] Dong | In situ evolution of trivalent chromium process passive film on Al in a corrosive aqueous environment[END_REF]. Further, for all TCP coated samples, R ct values increased by one order of magnitude by comparison to the degreased desmutted sample which demonstrated a significant improvement of the corrosion behaviour of the TCP coated samples even for short duration times in the TCP so lutions. Concerning the CPE coating parameters, low α coat and high Q coat are associated to strong heterogeneity of the conversion coating [START_REF] Qi | Trivalent chromium conversion coating formation on aluminium[END_REF][START_REF] Li | Electrochemical characterization of trivalent chromium process (TCP) coatings on aluminum alloys 6061 and 7075[END_REF][START_REF] Qi | Comparative studies of thin film growth on aluminium by AFM, TEM and GDOES characterization[END_REF]. The results showed no significant differences of those parameters as a function of the immersion time in the TCP solution which indicated a good homogeneity of all the TCP coatings. The result was consistent with the SEM micrographs (Fig. 3) for the samples ex posed to the TCP solution for duration times equal or higher than 13 s, i.e. after a full surface covering. It was more surprising for the 3 s coated sample for which numerous pores were observed on the surface of the coated sample (Fig. 3c). Besides, the evolution of the EIS spectrum after 14 days of immersion in NaCl solution for this sample was quite con sistent with the SEM (Fig. 3c) and TEM (Fig. 6) observations. Indeed, after a long time of exposure to the NaCl solution, the 3 s coated sample behave similarly to the degreased desmutted sample and similar R ct values were measured for both samples, which corresponded to a de crease of this parameter for the 3 s coated sample when the immersion time in the NaCl solution increased. This could be explained as a de crease of the corrosion resistance for the 3 s TCP coated sample that could be related to the porous surface morphology observed in Fig. 3c. The pores represented preferential ways towards the aluminium surface for chloride ions and oxygen and led to a similar corrosion behaviour as for the degreased desmutted sample. More globally, the results could led us to assume that the single layer structure of the 3 s TCP coating, considered as a precursor of the TCP coating, was not an optimised structure for anticorrosion properties. On the contrary, the other sam ples showed a significant increase in R ct values between 1 h and 14 days of immersion in NaCl solution whereas R coat , Q coat and α coat values remained similar. As shown in previous paper [START_REF] Verdalet-Guardiola | Influence of the alloy microstructure and surface state on the protective properties of trivalent chromium coatings grown on a 2024 aluminium alloy[END_REF], the results showed that the anticorrosive properties of the TCP coating were mainly related to the inner layer; moreover, the improvement of the anticorrosion properties of those TCP coatings when the immersion time in NaCl increased could be explained by a self healing mechanism linked to the hydration of the TCP coating [START_REF] Verdalet-Guardiola | Influence of the alloy microstructure and surface state on the protective properties of trivalent chromium coatings grown on a 2024 aluminium alloy[END_REF]. Nevertheless, the main result was that substantial anticorrosive properties were associated to a duplex structure of the TCP coating.

Finally, EIS measurements showed that the TCP coatings exhibited anticorrosive properties even for short immersion times in the TCP solution but the stability of the TCP coating in aggressive solution was only obtained for long conversion durations, when the TCP coating showed a duplex structure. However, the effect of the conversion duration on anticorrosive properties for exposure to the TCP solution longer than 13 s, was not clearly detected with the EIS experiments performed here, even though the anticorrosive properties of the 600 s coated samples seemed to be the highest.

Nucleation and growth mechanism

From the results presented above, a nucleation and growth me chanism for TCP coating can be proposed. Three steps, each one being associated to one of the three zones distinguished on the OCP curves plotted during the TCP treatment, can be defined: the first step is re lated to the initial potential decrease, the second one to the rapid po tential increase and the third corresponds to the processes occurring when the OCP is stabilised at the end of the TCP treatment.

3.6.1.

Step 1: Initial potential decrease (0 13 s)

After surface preparation, the alloy surface is characterised by a scalloped surface with ridges composed of copper and other non dis solved impurities and troughs [START_REF] Verdalet-Guardiola | Influence of the alloy microstructure and surface state on the protective properties of trivalent chromium coatings grown on a 2024 aluminium alloy[END_REF][START_REF] Caicedo-Martinez | Surface nanotextures on aluminium[END_REF][START_REF] Caicedo-Martinez | Behavior of impurity and minor alloying elements during surface treatments of aluminum[END_REF]. During step 1, the native oxide due to the activation of substrate surface by the fluoride ions (Eq. 1) in agreement with the mechanism proposed in the literature [7,22,39 41]. The role of fluorine is to undermine the air formed film to provide the appreciable electron tunnelling for the solution/alumi nium reactions. Therefore, competition between aluminium dissolution and precipitation reactions takes place as described by the following equations (Eqs. 1, 2 and 3) leading, in particular, to the formation of fluoroaluminates (Eqs. 1 and 3):

Al 2 O 3 + 2xF -+ 6H + → 2AlF x (3 x) + 3H 2 O (1)
+ → + + + - Al H O Al OH H 2 6 2 6 3 s ( ) 2 3 2
(2)

+ → + - Al F AlF 3 s 3 3( ) (3)
As a consequence, a pH gradient takes place near the alloy surface, as described in the literature and measured here by using a local pH probe (Fig. 2). The local pH increase, due to protons consumption, al lows the precipitation of zirconium and chromium oxides and hydro xides, in agreement with the large amount of oxygen measured with the different techniques used (Eqs. 4 and 5]:

+ → + - - - ZrF OH ZrO H O F 4 . 2 6 6 2 2 2
(4) Fig. 10. Equivalent circuit used to fit the EIS spectra plotted for (a) degreased-desmutted and 3 s TCP (14 days) coated samples, (b) all other TCP coated samples.

Table 1

Parameters extracted from EIS spectra by using equivalent circuits shown in Fig. 10 for TCP coated samples after different immersion times in the TCP solution. The EIS spectra were plotted after 1 h in 0.5 M NaCl. Results obtained for the degreased and desmutted sample (Deg-des.) are also given for comparison.

All those reactions lead to the formation of small nodules, first on the ridges of the scalloped surface, that constitute preferential and re active sites on which reactions are promoted. When these sites are fully covered, the precipitation occurs on the whole sample surface. This first step ends with a complete surface covering by a precursor coating containing small quantities of fluorine, chromium and zirconium. At the end of this step, the precursor coating is formed; it corresponds to a single layer coating that exhibits anticorrosive properties as measured by EIS in NaCl solution (Fig. 9), but a rapid degradation of the coated sample is observed when the exposure to the NaCl solution is extended. A schematic diagram of the first step is given in Fig. 11.

Step 2: Rapid increase of the potential (15 s 45 s)

At the end of the first step, the native oxide is dissolved and the aluminium surface is almost completely covered by the precursor coating. Therefore, it can be assumed that the amount of aluminium ions near the sample surface decreases so that the formation of fluor oaluminates is not promoted on the surface. Because the dissolution of aluminium is slowed down, the pH gradient decreases but it still exists as shown by Fig. 2. Therefore, chromium and zirconium oxide pre cipitation reactions go on and correspond then to the major reactions, leading to the enrichment in chromium and zirconium at the surface: these processes correspond to the outer layer nucleation (schematic diagram in Fig. 12). Because the intensity of the pH gradient has decreased, the growth kinetics of the coating is slowed down as shown in Fig. 4. In parallel, the chemical composition of the precursor coating previously formed evolves gradually and this coating becomes enriched in fluorine due to the permeation of this element through the chromium and zirconium rich outer layer in formation. Therefore, the precursor coating gradually evolves and its composition tends to be similar to that of the inner layer commonly observed for TCP coating. At the end of this step, the sample surface is covered by a coating composed of two layers with improved anticorrosion properties. Between 15 and 45 s, due to the evolution of the TCP coating from a precursor to a two layer coating, a gradual increase of the anticorrosion properties of the coating was observed leading to a gradual increase in the OCP measured in the TCP solution. The second step corresponds to the nucleation of the outer layer and the transition of the precursor coating to the inner layer. During the last step (Fig. 13), the pH gradient goes on to decrease slowly to reach gradually the bulk solution pH so that the growing rate of the coating still decreases. However, the precipitation of chromium and zirconium goes on slowly leading to the growth of the outer layer. In parallel, the fluorine goes on diffusing through the outer layer leading to the growth of the inner layer. In agreement with literature [START_REF] Qi | Trivalent chromium conversion coating formation on aluminium[END_REF][START_REF] Verdalet-Guardiola | Influence of the alloy microstructure and surface state on the protective properties of trivalent chromium coatings grown on a 2024 aluminium alloy[END_REF], the EIS results showed that the TCP coating presented a permeability to an electrolyte through pores and defects. All these reactions are assumed to maintain Table 2 Parameters extracted from EIS spectra by using equivalent circuits shown in Fig. 10 for TCP coated samples after different immersion times in the TCP solution. The EIS spectra were plotted after 14 days in 0.5 M NaCl. Results obtained for the degreased and desmutted sample (Deg-des.) are also given for comparison. a pH gradient at the sample surface, which is very low but sufficient to allow the TCP coating to go on growing. After 300 s, the coating thickness is found to decrease. The characterisations performed in this work are not sufficient to explain this evolution, but different hy potheses can however be proposed. The near surface pH being similar to the bulk pH after 300 s, it could lead to the dissolution of the coating; an unidentified densification mechanism of the coating could also occur. Nevertheless, at the end of step 3, the duplex TCP coating is formed and shows long lasting anticorrosive properties.

Conclusions

The main conclusions of this work are given in the following:

1) A precursor layer of TCP coating was formed immediately after immersion in the TCP solution. This precursor layer exhibited a single layer structure and was composed of chromium, zirconium, fluorine and aluminium. It was considered as a precursor coating of the final duplex structure. 2) For immersion times in the TCP solution longer than 13 s (minimum potential on the OCP curve), the coating exhibited the duplex structure commonly observed and composed by an aluminium and fluorine rich inner layer, and a zirconium and chromium rich outer layer. When the exposure to the TCP solution was extended, i.e. until 600 s, the chemical composition of both the outer and inner layers evolved.

3) The TCP coating exhibited anticorrosive properties (measured by EIS) even for immersion time in the TCP solution as short as 3 s which meant that even the precursor layer exhibited anticorrosive properties. However, during immersion in a NaCl solution, the de gradation of the precursor layer occurred rapidly whereas the TCP coating with the duplex structure showed significant anticorrosive properties even after 14 days of immersion in a NaCl solution. The best anticorrosive properties were obtained after 600 s of exposure to the TCP solution which showed that the chemical and structural changes that had occurred between 13 and 600 s were of major interest for the anticorrosive properties of the TCP coating. 4) A nucleation and growth mechanism of TCP coating was proposed on the basis of the electrochemical, chemical and morphological characterisations performed. It could be described as a three step process; the first step corresponded to the dissolution of the native oxide and the formation of the precursor layer, the second one to the nucleation of the outer layer combined with an enrichment in fluorine of the precursor layer to reach the composition commonly measured for the inner layer. Finally, the third step corresponded to the growth of both the outer and inner layers of the TCP coating.

The results therefore showed that, with the conditions applied for the TCP coating nucleation and growth, a duration time of 600 s was requested to obtain substantial anticorrosive properties. This corre sponded to a specific chemical composition and structure of the TCP coating. From an industrial point of view, it could be interesting to find a way to accelerate the chemical reactions leading to this optimised composition and structure of the TCP coating in order to reduce the process duration.

Fig. 1 .

 1 Fig. 1. OCP versus immersion time in the conversion solution (TCP solution = Socosurf TCS) for the pre-treated AA 2024-T3. Crosses indicate the duration time of the TCP treatments for the different samples analysed in the following.

Fig. 2 .

 2 Fig. 2. Local pH and OCP versus immersion time in the conversion solution (TCP solution = Socosurf TCS) for the pre-treated AA 2024-T3.

  3.4. Evolution of the chemical composition of the TCP coating during the conversion treatment 3.4.1. Proof of the presence of the TCP coating on AA 2024 surface after very short immersion times in the TCP solution

Fig. 3 .

 3 Fig. 3. SEM-FEG micrographs of the AA 2024-T3 surface (a) before conversion and after different immersion times in the Socosurf TCS solution: (b) 1 s, (c) 3 s, (d) 13 s, (e) 15 s, (f) 45 s, (g) 300 s and (h) 600 s.

Fig. 4 .

 4 Fig. 4. Nodules diameter and TCP coating thickness (measured on micrographs shown in Fig. 5) versus immersion time in TCP solution (Socosurf TCS) for AA 2024-T3 samples.

Fig. 5 .

 5 Fig. 5. SEM-FEG micrographs (secondary electron images) of cross-sections of AA 2024-T3 samples after Focus Ion Beam cutting for treatment durations in the TCP solution equal to (a) 3 s, (b) 13 s, (c) 15 s, (d) 45 s, (e) 300 s and (f) 600 s. Vertical measurements of the coating thickness need to be corrected by cos(38) due to the tilt (52°) imposed by the ionic column.

Fig. 6 .

 6 Fig.6. TEM micrograph and EDS mapping scans of a coating formed on a AA 2024-T3 after 3 s in the TCP solution (Socosurf TCS).

Fig. 7 .

 7 Fig. 7. GD-OES profiles plotted for TCP samples coated during (a) 3 s, (b) 13 s, (c) 15 s, (d) 45 s, (e) 300 s and (f) 600 s. Dash lines indicate the coating/aluminium substrate interface characterised by a copper enrichment layer.

Fig. 8 .

 8 Fig. 8. XPS profiles plotted for TCP samples coated during (a) 3 s, (b) 13 s, (c) 15 s, (d) 45 s, (e) 300 s and (f) 600 s. The grey zones correspond to analyses performed in the carbon-polluted thickness of the coating.

Fig. 9 .

 9 Fig.9. EIS spectra plotted for TCP coated samples after 1 h and 14 days of immersion in 0.5 M NaCl.

3. 6 . 3 .

 63 Step 3: Stabilisation of the potential (after 45 s of immersion in the TCP solution)

Fig. 11 .

 11 Fig. 11. Schematic describing the formation of the TCP coating during the initial potential drop (Step 1).

Fig. 12 .

 12 Fig. 12. Schematic describing the formation of the TCP coating during the rapid potential increase (step 2).

Fig. 13 .

 13 Fig.[START_REF] Qi | Trivalent chromium conversion coating formation on aluminium[END_REF]. Schematic describing the formation of the TCP coating during the period with stabilised potential (step 3).

Acknowledgments

The authors thank Mecaprotec and the ANRT for their financial support. The technical support of the team of the micro caracterisation platform Raimond Castaing and PLACAMAT was also greatly appre ciated. Thanks to R. SAILLARD (CIRIMAT) for fruitful discussion.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.