ESA STM-276

February 2008

Assessment of ChemicalConversion Coatings for theProtection of Aluminium Alloys

A Comparison of Alodine 1200with Chromium-Free ConversionCoatings

A.M. Pereira, G. PimentaInstituto de Soldadura e Oualidade (ISO), Oeiras, Portugal

B.D. DunnProduct Assurance and Safety Department, ESA/ESTEC,The Netherlands

'uropean Spate AgentyAgenle spatiale europeenne

2 ESA STM-276

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Assessment of Chemical Conversion Coatings for theProtection of Aluminium Alloys (ESA STM-276,February 2008)

K. Fletcher

ESA Communication Production OfficeESTEC, Noordwijk, The Netherlands

Tel: +31 71 565-3408Fax: +31 71 5655433

The Netherlands

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978-92-9221-897-2

0379-4067(Q2008 European Space Agency

ESA STM-276 3

Contents

2

Introduction 51.1 Project Background and Objectives 51.2 State of the art 6

1.2.1 Commercially Available Chemical ConversionCoatings ... 6

1.2.2 Review of Published Papers 9

ExperimentalPhase ... 172.1 Materials, preparation of specimens, and treatments 172.2 Evaluation of the aluminium alloys samples 17

2.2.1 Surface roughness 172.2.2 Surface electrical resistivity 182.2.3 Thermal cycling tests 182.2.4 Salt spray tests ... ... 182.2.5 Scanning electron microscopy and energy dispersive

X-ray spectrometry (SEM/EDS) 192.2.6 Chemical conversion coatings procedure 19

Results / Discussion... 213.1 Visual inspection 21

3.1.1 'As received' ... 213.1.2 'As produced' 213.1.3 After thermal cycling tests 213.1.4 After salt spray tests 213.1.5 After thermalcycling+ salt spray tests , 21Surface roughness. 29Surface electrical resistivity measurements 30Surface morphology analysis - Scanning electron microscopy.. 313.4.1 After cleaning with Novaclean AI86 313.4.2 After chemical conversion coating treatments 323.4.3 After thermal cycling tests 393.4.4 After salt spray tests 433.4.5 After thermal cycling plus salt spray tests 473.4.6 Comparison and correlation between Aluminium

alloys and techniques performed 51

3

3.23.33.4

4

5

Conclusions 57

References 59

4 ESA STM-276

AbstractThe most common conversion coating (CCC) currently used for improvingthe corrosion resistance of aluminium and its alloys that are used in theconstruction of ESA spacecraft is Alodine 1200 manufactured by Henkel.This commercial product is a chromium (VI)-based chemical conversioncoating. However, due to the environmental impact of hexavalent chromate,and recent EU regulations enforcing prohibition of the use of certainhazardous substances in electrical and electronic equipment, there is a greatneed for chrome (VI)-free anti-corrosion coatings.

The aim ofthis work was the study of commercially-available alternatives tochromium (V I)-based treatments. They should assure the same level ofperformance in terms of corrosion resistance, thermal and electricalproperties. The objective of this study was to perform an assessment ofcommercially existing chemical conversion coatings for the protection ofaluminium alloys that are to be used in the hardware of ESA space vehicles.

In this study, the aluminium alloys A17075, Al22l9 and Al5083 were usedas substrates for evaluation of the chemical conversion coatings.

Among various chemical conversion coating systems, three alternativeconversion coating pretreatments were selected for evaluation: Alodine5700, Nabutan STI/310 and Iridite NCP. The chromium (VI) conversioncoating Alodine 1200 was used as the reference.

The quality assessment of conversion coatings was evaluated by optical andscanning electron microscopy. Coated samples were characterised by saltspray exposure in order to simulate ground storage conditions; thermalcycling under vacuum, which simulates an in-orbit space environment; andelectrical resistivity, as spacecraft sub-systems are electrically grounded toone another.

ESA STM-276 5

1 Introduction

1.1 Project Background and Objectives

Aluminium and its alloys are largely used on board space vehicles due to agood combination of specific weight and mechanical properties. Althoughspace hardware for spacecraft use is usually confined to controlledenvironments, in emergency conditions, the pre-launching phase, orprolonged storage for reusable spacecraft, it may be exposed to harshenvironments, such as the marine atmosphere that exists at coastal launchsites. This requires that all aluminium alloys must be protected by a properanti-corrosion treatment. It is usual for all aluminium alloys to receive a pre-treatment and a coating scheme. The most successful pre-treatment for Aland Al alloys used so far are Chromium (VI) based chemical conversioncoatings. A notable representative of this is a product commercially availableunder the brand name Alodine 1200.

Recent EU legislative restrictions enforced strong limitations to the use ofchromate conversion coatings. Member states should ensure, from 1 July2006, new electrical and electronic equipment put on the market shall notcontain hexavalent chromium [1]. Exempted are materials and componentsin which design changes are technically, or scientifically, impracticable, orwhere the negative environmental, health and/or consumer safety impactscaused by the substitution are likely to outweigh the environmental, healthand/or consumer safety benefits.

Although the aerospace and aeronautic industries have been exempted fromcompliance with the EU regulations, ESA is preparing itself to comply withthe ROSH directive [1] and simultaneously to keep the lead in the search foran environmentally cleaner, socially responsible industry.

With this goal in mind, ESTEC contracted the Instituto de Soldadura eQualidade, ISQ, to produce a benchmarking study on the viable alternativesfor Cr (VI) based coatings. The main objective of this applied research workis to assess existing commercially-available chemical conversion coatings tobe used as pre-treatments coating on aluminium alloys. Candidate pre-treatments for this study must:

be industrially available for ready application;

ensure at least the performance level achieved by current Cr (VI) pre-treatments

include one coating having a European origin.

Further, technical performance requirements for viable alternative chemicalconversion coatings in this study, are:

good corrosion resistance to tropical environments: the Europeanspacecraft launching base is located in the tropics, a regioncharacterised by high humidity, high temperature and salty atmosphere;

Good electrical properties: many Al alloys are used in the spacecraft asstructural components. However, besides adequate mechanicalproperties, they must ensure proper electrical resistance in order toobtain an equipotential surface;

Thermal properties: extreme temperatures can be achieved either inspace (::I::100°C) or in prolonged storage condition, on Earth, often intropical climates (+50°C and very high humidity).

Aluminium alloy: Composition

AI7075 5.5% Zn; 2.5% Mg; 1.5% Cu; 0.3% Cr

AI2219 6.3% Cu; 0.3% Mn

AI5083 4.5% Mg; 0.7% Mn

6 ESA STM-276

In this study, Alodine 1200 (Henkel) will be used as the state-of-the-art

chemical conversion coating.

Table I illustrates the aluminium alloys compositions used in this study.

Table 1 - Aluminium alloys, chemical composition [2].

It should be noted that the low strength aluminium alloys (5000 and 6000series) are permitted to have a final finish of chromate conversion coating.However, ESA prescribes in ECSS-Q-70-71 ('Data for the selection of spacematerials and processes'), that the high strength alloys (2000 and 7000series) shall have a further painted finish applied to the chromate pre-treatment, or alternatively be anodised.

1.2 State of the art

In this section a literature survey of recently published studies related tochromate-free coatings has been performed.

1.2.1 Commercially Available Chemical Conversion Coatings

A study has been conducted in the USA under contract by the DeputyUndersecretary of Defense for Environmental Security. Its overall objectivewas to validate and implement multiple CHROMATE-free aluminium pre-treatment alternatives for a broad range of applications [4]. Alternativesstudied were all aqueous solutions designed to obtain a thin coating on thesubstrate that would enhance paint adhesion and corrosion protection. Someof the alternatives provide unpainted corrosion protection as well aselectrical conductivity in corrosive environments, especially in the 2xxxseries alloys. The best results have been obtained with a Cr (III) basedcomposition. The second best alternative, and the best of the chrome freealternatives, was Alodine 5200/5700, although it did not perfonn as well inunpainted applications as Navair Trivalent Chromium Pretreatment (TPC)did. Alodine 5200/5700 did not perform well against filiform corrosion, butwas found to be process flexible and could be applied similarly to chromateconversion coatings.

NASA tested three alternative pre-treatments: Alodine 5700 (Henkel Corp.),Okemcoat 4500 (Oakite Products, Inc.) and Chemidize 727ND (MacDermidInc.). Okemcoat 4500 was dropped from qualification due to poorperformance during Bend Strength - Flexibility and Water Resistancequalification tests. The other two were found to be acceptable for flight.Alodine 5700 was classified as having flexible processing parameters andwas recommended for implementation as a pre-treatment alternative. NASAchanged to Alodine 5700, as part of their new Hentzen system(05510WEPX10551I CEH-X primer and 4636 WUX-3/4600CHA-SGtopcoat) from Hentzen Coatings Inc. 1 Alodine 5700 in June 2002, and thefirst hardware flew in late 2002.

At the ASST 2006 Conference [5] a session was dedicated to Alpretreatments. This session started with an overview from Alcan R&D

ESA STM-276 7

concerning new alloys for aircraft application, namely AI-Cu-Li alloys.Some emphasis was also given to joining techniques with potential forapplication, namely laser beam and friction stir welding of panels.Furthermore, some studies with sol-gel coatings were presented [6-8].Sepulveda et al. [5] implemented a sol-gel conversion coating process toproduce a zirconium oxide coating on A12024- T3 alloy, that wascharacterised morphologically and by electrochemical techniques. Thecoatings produced presented a uniform film, with no evidence of cracking ordelamination. Polarisation studies revealed no breakdown potential up to+0.5 V (SCE), indicating an ennoblement of the pitting potential of over+ 1.0 V, at a low scan rate, and the passive region exhibited variations in thecurrent density that suggest a self-healing capacity ofthe coating. This studyallowed establishing conditions for optimisation of zirconia sol-gel coatingsfor structural aluminium alloy, using electrochemical impedancespectroscopy. Hughes et. al. [7] studied the performance of a ceriumtridibutylphosphate-based inhibitor for corrosion of aluminium alloys.Raman spectroscopy and SEM/EDS were used to characterise the reactionsof A12024- T3 with Ce( dbp)3 solutions in the presence ofNaCI.

Raps et al. [8] tested alternative systems based on permanganate, cerates,zirconate and trivalent chromium, which form a chemically-growing oxidefilm in a traditional tank process. Also investigated was a different approachbased on organic-inorganic hybrid sol-gel coatings and silane films, whichcan develop an oxide layer on the natural aluminium oxide by formation ofcovalent AI-O-Si bonds. Thus, those coatings offer the possibility to link themetal oxide and the organic coating (primer). The film morphology andcorrosion protection properties were studied by SEM, salt spray test, filiformcorrosion test and electrochemical test methods. They concluded that thetrivalent chromium-based conversion coating and the sol-gel coatings areable to close the gap between barrier functionality and adhesion promotion.The chromium (III) process was reported to have the most promisingperformance of all the chemical conversion coatings tested, being the bestalternative and thus the most effective substitute for Cr (VI) basedconversion systems.

Under this present ISQ-ESA research contract, contacts with differentsuppliers (HENKEL, ATOTECH, CHEMO-PHOS, SURTEC, MacDermid,Kluthe and Enthone, Alcan) were undertaken for an update on alternativeindustrial chemical conversion pretreatments. However, only some of thesecompanies sent pre-treatment solution samples and product information,namely Henkel (by Henkel Iberica Surface Technologies), CHEMO-PHOS,SURTEC and MacDermid (through its sales representative in Portugal,Artegalva Lda.).

An overview ofthe alternative chemical conversion pretreatments, as well ofthe conventional chromium based coatings is described.

a) Alodine 12005 (Reference) [9J

Alodine 1200S (Henkel Surface Technologies) is a hexavalent chromiumbased chemical conversion coating. In this study it will be the reference forthis pretreatment evaluation. Alodine 1200S treated surfaces have anexcellent corrosion resistance, and an increase in the paint adherence. Theimmersion bath has a long term stability and does not often need to bereplenished. The solution can be used by immersion, spray or wipeprocesses. The immersion bath has a long term stability and does not oftenneed to be replenished. The pretreatment by immersion is performed at

8 ESA STM-276

temperatures in the range of 25-35°C for 15 seconds to 3 minutes, adjusting

the pH 1.3 to 1.8.

Furthermore, ALODINE 1200S treated surfaces have colours ranging from alight iridescent golden to tan, depending on alloy and coating thickness.

b) Henkel A Iodine 5200/5700 [4, IOJ

Alodine 5700 (Henkel Surface Technologies) is a chromium-free conversioncoating, formulated as a ready-to-use material for spray or immersionapplication, and is a diluted form of the Alodine 5200. The solution is usedin a similar fashion to conventional chromate pretreaments and is performedat a temperature of 25-35°C for 1-5 minutes. Alodine 5200/5700 is anorganometallic zirconate complex based pretreatment. Deposited coatingshave a light colour ranging from blue to tan, depending on the alloy [4].

c) Sanchem Safeguard 7000 with seal [4}

It is based on potassium permanganate, with a complex sealant composed ofpolyacrylic acid polypropylene glycol and fatty acid esters. It is processed asa two-component product (one solution potassium permanganate, and oneseal). The pretreatment is performed at room temperature; however, itrequires high temperatures for seal cure (about 92°C, 200°F). Application isperformed by immersion, spray, and wiping. It induces a bronze gold colourcoating.

d) Surtec Chromital TPC [11/

On a benchmarking performed by Surtec, using Alodine 1200S as reference,Surtec 650 pretreatment presented very similar performance characteristicsto the control product. Surface preparation is dependent on the siliconcontent of the alloy: for Al alloys containing less than 1% Si, surfacepreparation includes alkaline degreasing, alkaline etching and deoxidation;for Al alloys containing more than 1% Si, surface preparation does notrequire the alkaline etching step.

In application by immersion, Surtec 650 is to be applied in a 20%concentration in deionised water. The solution pH must be adjusted to 3.9(3.7 - 3.95), with sulphuricacid or sodiumhydroxide.The pretreatmentis tobe performed in the range 30-40°C. This temperature will influence thebathing time. Pre-treatment times range from 0.5 to 4 minutes.

e) Nabutan STl/3IO [12J

Nabutan STI/310 is a chrome-free, no-rinse, mixture of inorganic andorganic acids composed fundamentally of fluorotitanic acid and can beapplied to the surface treatment of aluminum alloys. The corrosion resistanceand the adherence of subsequent paint coats to the treated surface will beimproved. This coating is applied preferentially by dipping and spraying.

J) lridite NCP [13}

The Iridite NCP is a chromium-free passivation treatment designedspecifically for aluminium and its alloys; it provides both high adhesionvalues for paints and powder coatings, and excellent corrosion resistance.Iridite NCP also has the ability to withstand high temperatures, so it issuitable for use on items that require high operating temperatures. Thisunique property, not found on chromates, also allows applicators to increase

Conversion i ChemistryI

Application. Advantages

I

Disadvantagescoating

I

(from MSDSs) ~Methods

Alodine l200S Chromic acid, Contains hexavalentReference complex

Immersion, Easy to use;standard chromium

(Henkel, Germany) fluoridesspray, wIpe

OrganometallicEasy to use; Minimal corrosion

Alodine 5700 Immersion, drop-in inhibition;zirconate

(Henkel, Germany) complexspray, wIpe replacement for unperceivable colour

chromates change

Nabutan STI/31 0 Minimal corrosion

(Chemo- Phos,Fluorotitanic Immersion, Easy to use; inhibition;acid spray chromium- free

Germany) Colourless

Minimal corrosion

Iridite NCP ProbablyImmersion, Easy to use;

inhibition;

fluorotitanate Colour depends on the(MacDermid, USA) spray chromium- free

based chemical processingconditions and alloy

ESA STM-276 9

the temperature of their dry-off ovens prior to painting. Unlike traditionalchromate treatments that require 24 h to cure to a hard film, lridite providesa hard amorphous crystalline coating as formed. The IRIDITE NCP coatingis normally clear at low coating weights to a light blue colour at highestcoating weights. There are times when the coating is iridescent due torefraction of light by the conversion coating. The colour is very muchdependent on the chemical processing conditions, substrate alloy and surfacecondition being processed. Because of this, coating quality cannot beassessed by judging the colour of the coating [14].

Based on the properties of each of the commercially-available chemicalconversion coatings mentioned above, ESA and ISQ selected three of themfor evaluation.

The chromium (VI) conversion coating Alodine 1200 will be used asreference. The characteristics of these chemical conversion coatings aresummarised in Table 2 [9,10,12 and 13].

Table 2 - Summary of chromate and chromium-free conversion coatings.

1.2.2 Review of Published Papers

Chromate conversion coatings (CCC) have been intensively studied over thepast decades due to the effective corrosion protection they confer on Alalloys and the desire to develop an effective and environment-friendlyreplacement. The composition and structure of CCCs formed on pure Al andAl alloys have been investigated using a variety of analytical tools [15].

The influence of surface preparation on performance of chromate conversioncoatings on Alclad 2024 aluminium alloy was investigated by Campestrini etal. [16, 17]. Since the parameters of each pretreatment step, performedbefore the chromating process, give rise to important modifications of themorphology and microstructure of the aluminium surface, they stronglyinfluence the nucleation and growth of the chromate layers, and therefore,their final properties. Researchers studied the influence of both theimmersion in a nitric-hydrofluoric acid solution, carried out between the acidpickling step (immersion in sulfuric-phosphoric acid solutiQn) and thechromating step, and the duration of the acid pickling step on the surface

10 ESA STM-276

properties of Alclad 2024- T3 aluminium alloy was studied. Scanning

Electron Microscopy (SEM), Atomic Force Microscopy (AFM) andelectrochemical investigations highlighted the fact that this intermediate stepin the pretreatment route gave rise to a more etched but less reactive surface,due to the removal of a large number of cathodic sites, mainly related to AI-Fe-Si phases present in the clad layer. A similar effect, but on a smallerscale, was caused also by the increase of immersion time in sulphuric-phosphoric acid solution. The decrease of galvanic coupling gives rise to amore homogeneous nucleation of the chromate conversion coating on thesurface of the Alclad 2024- T3. Moreover, the chromate film tends to bedenser and with fewer defects. In fact, removal of galvanic coupling fromthe aluminium surface appears to be a requirement for the formation of achromate layer with good corrosion resistance [17]. Besides, the surfaceproperties of the aluminium substrate largely influence the morphology andcomposition, not only of the chromate film but also of the corrosion productsformed at weak spots, which seem to play an important role in the level ofprotection provided by the chromate conversion coating.

Waldrop and Kendig [18] studied the formation of CCC on AA2024- T3using AFM. The CCC on the Al matrix was found to nucleate and grow veryfast in the form of nodules. Nucleation and growth of the coating were fasteron AI-Fe-Cu-Mn particles than on AI-Cu-Mg. A similar AFM study ofnucleation and growth of CCC formed on AA2024- T3 in combination withFE-SEM and TEM, was conducted by Brown and Kobayashi [19]. Theyconcluded that CCC formation and growth on intermetallics stronglydepended on the size, shape and composition of intermetallics.

Sun et at. [20] used SEM, transmission election microscopy (TEM), and X-ray photoelectron Spectroscopy (XPS) to examine chromate coatings formedon 2024- T3 samples, to which different pre-treatments had been given.These samples have also been evaluated for their corrosion behaviour.Although a previous work has shown that different pre-treatments candirectly influence the compositions at surface regions of Al alloys, this workhighlights the fact that this initial enrichment can affect the composition andproperties of a subsequent conversion coating. The present study forchromate coating provides a good reference for comparison with parallelwork being done on alternative non-chromated coatings.

Treacy et al. [21] carried out several surface analysis studies on chromateconversion coated 2014- T6 aluminium alloy prior to, and after exposure to aneutral sodium chloride salt fog environment. The development of darkstains on surfaces after 48 h of salt spray fog exposure coincided with theloss of chromium, indicative of the coating failure. This fact was observedby surface analytical techniques: XPS, AES (Auger electron spectroscopy)and LIMA (Laser induced mass analysis. From LIMA analysis it wouldappear that these regions are associated with copper-containingintermetallics, suggesting that chromate coating deterioration during salt fogexposure occurs at highly active copper-containing intermetallic sites.

Lunder et al. [22] investigated the role of microstructure in the formation ofa chromate conversion coating on extruded AA6060. The surfaces weregiven a conventional surface treatment, involving alkaline etching anddeoxidation prior to treatment in a commercial chromate solution (AlodineC6100). The coated surfaces were examined by surface analytical techniquesand electrochemical techniques. A non-uniform growth of the chromateconversion coating (CCC) on AA6060- T6 resulted in a porous morphology,with cracks extending down to the base metal. Poor coverage wasparticularly observed at the grain boundaries.

ESA STM-276 11

It is generally accepted that CCCs form via a redox reaction between Al inthe alloy and Cr (VI) species in the chromating solution. They areamorphous and mainly composed of hydrated mixed Cr (III) /Cr (VI) oxide.The Cr (VI) species can be stored in CCC and released as soluble chromatespecies when CCC is exposed to solution. This characteristic of CCC isbelieved to lead to the important 'self-healing' ability of CCC-treated alloys.Chromate conversion coatings (Alodine 1200S) on AA7075-T6 werecharacterised by Meng et at. [23] using SEM, focused ion beam sectioningand s-TEM with nanoelectron dispersive spectroscopy line profiling. TheCCC formed on AA 7075 is heterogeneous and the coating formed on the Almatrix was much thicker than that formed on the coarse AI-Cu-Mg, AI-Fe-Cu and Mg-Si intermetallic particles. The coating formation on theintermetallics has been shown to be dependent on the electrochemicalreactivity of the intermetallics, the local pH change and the interaction of theintermetallics with HF and ferricyanide.

A review by Kending et at. [15] of studies carried out on aluminium andaluminium alloys, showed that the oxoanions of hexavalent chromiumuniquely inhibit the corrosion of many metals and alloys. They areparticularly useful for protecting the high strength aluminium alloys againstcorrosion. The inhibition of Al corrosion is derived from both inhibition ofoxygen reduction and inhibition of metal dissolution reactions, mainly due toa delay in the onset of pitting. Inhibition of corrosion by chromates appearsto be closely linked to their ability to irreversibly adsorb on to metal andoxide surfaces.

In order to characterise filiform corrosion on a commercial AA2024- T351aluminium alloy, a detailed microscopic study using SEM and EDX wasperformed by Mol et al. [24]. Some of the aluminium alloy samples werealkaline-cleaned, deoxidised, and coated with a chromate conversion andothers were only alkaline-cleaned before coating. Both samples weresimilarly spray-coated with a 42 /lm clear polyurethane topcoat. Crosssection and top views examined by SEM revealed varying degrees of attackranging from generalised etching without local attack, to severe local attackin the form of pitting, resulting in grain etch out, grain boundary attack andsubsurface etchout. EDS revealed the presence of chloride inside the pits,and subsurface etch out.

Characterisation of the metal-coating interface is crucial to theunderstanding, and prediction of the corrosion protective-coatingsperformance. Bierwagen et at. [25] reported the use of different methods toexamine the interface between steel and marine coatings and to image thesurface of a) untreated and b) chromate/phosphate treated aircraft aluminiumalloys. Electrochemical noise (EN), EIS and Prohesion TM testing wereperformed in parallel with AFM/SEM measurements. They proved to bebeneficial in the characterisation of surface morphology, corrosion resistanceproperties of coatings and possibly for determining mechanisms ofcorrosion. The e-coat systems displayed resistance values higher than thespray coat systems and their delamination areas were lower, indicating theyare better for corrosion control for undamaged systems.

Fedrizzi et al. [26] studied the effect of chemical cleaning (alkaline, acid,combination of the two) and environmentally-friendly and chromate pre-treatment baths on the filiform corrosion behaviour of aluminium AA6060alloys. They concluded that the poor behaviour of samples cleaned in analkaline bath can be related to a remarkable chemical surface modificationinduced by the chemical etching. In fact, after this alkaline etching, thealuminium surface was found to be highly enriched in elements like copper,zinc or magnesium, which has been shown to affect the filiform corrosion

12 ESA STM-276

resistance to varying extents. The use of acid cleaning after the alkalineetching allowed a significant improvement of the aluminium surfacechemistry, and consequently the filiform corrosion resistance. The bestbehaviour was obtained by chromate or fluorotitanate conversion coatingsafter an alkaline plus acid chemical cleaning.

Zhao et at. [27] studied the effects of chromate conversion coatings (CCC)and chromate in solution on the corrosion behaviour of AA2024-T3, andpure AI. The influence of the chromate on pit growth was studied by the thinfilm pit-growth technique. The release and migration of chromate fromCCCs and the comparison of the CCCs chemistry with various syntheticmixed oxides composed of Al (III), Cr (III) and Cr (VI) species, wasinvestigated by Raman spectroscopy. The effect of released chromate wasalso studied by electrochemical methods. Using an artificial scratch cell,chromate released from CCCs was seen to migrate to, and protect, a nearbyuncoated area. However, experiments investigating the effect of chromate insolution on the anodic dissolution kinetics under potentiostatic controlindicated that large chromate concentrations were needed in order that aneffect can be seen.

John Bibber [28] published an overview of non-hexavalent chromiumconversion coatings that could provide excellent corrosion resistance andpaint adhesion characteristics when used with aluminium and its alloys,compared with the chromium-based conversion treatment. The four maintypes of conversion coatings in common use are based upon:

I) The formation of a chromium hydroxide and/or chromium oxide film;

2) The production of a precipitated heavy metal phosphate or oxide film;

3) The use of various synthetic polymers, with or without heavy metalphosphates or oxides; and

4) The formation of a manganese oxide/aluminium oxide film by use ofpermanganates.

The author concluded that the manganese oxides produced by theheptavalent-manganese-based system are by far the most closely related tothe chromium oxides and hydroxides in terms of their respective chemistries.Thus, the aluminium oxide/manganese oxide film, as produced by theheptavalent-manganese conversion coating system, is the alternative pre-treatment most closely matching the actual CCC in terms of actual chemistryand performance.

NCAP ('Non-Chromate Aluminum Pretreatment Project') [4] hasundertaken a project to validate, and implement, multiple chromate-freealuminium pretreatment alternatives. The researchers concluded that allalternatives tested showed acceptable performance. From the evaluatedchemical coatings, the only pretreatments that came close to matching thetechnical performance, cost and flexibility of chromates are those based ontrivalent chromium (Navair TCP). Furthermore, the Alodine 5200/5700pretreatment performed similarly to TCP in most painted systems, but not inunpainted applications. In general, although none of the other selected pre-treatments can compete with the overall performance of these twopretreatments, many could be selected for specific coating systemapplications, and achieve the desired performance.

Cerium-based Al and AI-alloy pre-treatments have been proven as efficient,environmentally-clean alternatives to the environmentally-undesirableCr (VI) compounds. Thus, full immersion in cerium salt solutions allows anidentical corrosion protection level to that of chromate-based treatments. A

ESA STM-276 13

major drawback is that treatment time is too long for industrial applications[29].

Bethencourt et al. [30] proposed accelerated methods for obtaining cerium-rich conversion coatings on aluminium-magnesium alloys (AI5083). Thefilms developed have been characterised by SEM and EDS. These studiesrevealed that the coatings have a mixed or heterogeneous nature, beingcomposed of a layer of alumina covering the matrix, together with islands ofcerium formed over the cathodic intermetallics present on the alloy surface.Furthermore, electrochemical studies indicate that the protection levelprovided by these coatings is several orders of magnitude higher thanachieved with other treatments, such as chromate-based treatment.

Work by Arnott et at. [3 I] has shown that cerium-based coating depositionrelies on electrochemical interactions between the aluminium matrix andintermetallic inclusions that make up structural aluminium alloys.

Fahrenholtz et al. [32] studied the performance of a cerium-based conversioncoating formed by spontaneous reaction between a water-based solutioncontaining CeCh and aluminium alloy 7075-T6 substrates. This study hasshown that the cerium conversion coating morphology and salt fogperformance were affected by the type of pre-treatment of the panel prior tocoating. The best pre-treatment consisted of desmutting, degreasing, andacid activation.

Rivera et al. [33] developed an improved process for the spontaneousdeposition of cerium oxide conversion coatings for corrosion protection ofaluminium alloy 7075- T6, by a spontaneous dip-immersion process thatresulted in improved salt fog corrosion performance. The role of substratesurface preparation and post-treatment was discussed in terms of the surfacemorphology, thickness and performance of coatings on aluminium alloy7075-T6.

Decroly et al. [34] investigated a chromate conversion coating alternativetreatment (cerium salt-based conversion) that could meet all therequirements for industrial application. The corrosion protection propertiesof this Ce conversion coating were investigated by cathodic and anodicpolarisation measurements and by electrochemical impedance spectroscopy(EIS). However, although the coatings formed presented interestingproperties, their performances did not match those of chromate conversioncoatings.

The cerium-based chemical conversion coating is an effective method toimprove the resistance to localised corrosion of the aluminium boratewhisker reinforced AA6061 composite (AI1sB4033w/606IAI). Hu et al. [35,36] reported a cerium conversion coating formed by an immersion processthat resulted in improved salt spray corrosion performance. The CeChconcentration in the test solution has been shown to have a significantinfluence on the corrosion behaviour, and surface morphologies of thecoated samples.

The electrochemical behaviour of a Ce conversion coating formed on Alalloy 2024- T3 covered with a Cu-rich smut has been studied by Palomino etal. [37]. The microstructural characterisation has shown that the precipitationof the Ce conversion layer seems to occur homogeneously in the presence ofthe Cu-rich smut originating a uniform coating. Moreover, it has beenconfirmed that the dry-mud structure of this kind of conversion layer is just asuperficial phenomenon, with only a few cracks attaining the metallicsubstrate) most likely due to stress relieving. Electrochemical results and

SEM/EDS characterisation have evidenced the self-healing properties of the

14 ESA STM-276

Ce conversion layers, which allow ions released from the coating toprecipitate above active sites during the immersion of the coated metal in anaggressive electrolyte.

The same researchers investigated the corrosion behaviour of a bi-Iayercerium-silane pretreatment on Al 2024- T3 in 0.1 M NaCI [38], byelectrochemical techniques (anodic polarisation curves and electrochemicalimpedance spectroscopy (EIS)). EIS has shown a continuous increase of theimpedance response during the whole test period, which was interpreted onthe basis of a pore-blocking mechanism. This theory was supported by SEMobservation and equivalent circuit fitting. On the other hand, mechanicaltests have evidenced the good adhesion of the silane layer to the Ceconversion layer, which has been interpreted as a better linking between thesilane molecules and the cerium bottom layer.

Hughes et at. [39] studied filiform attack on a cerated AA2024- T351aluminium alloy with a polyurethane topcoat by SEM/EDS. Theobservations revealed that filiform corrosion results were similar to thoseobtained on chromated and coated surfaces. This conclusion led todevelopment of a filiform corrosion model, attributing the main cause ofdelamination to the volume expansion of the corrosion product.

Guan et at. [40] published an overview describing the formation, chemistry,morphology, and corrosion protection of a new type of inorganic conversioncoating. This coating, referred to as a vanadate conversion coating (YCC),forms on aluminium alloy substrates in a matter of minutes by deeping thesubstrate in aqueous vanadate-based solutions at ambient temperatures.Electrochemical testing was used to confirm earlier reports of Al corrosioninhibition by soluble vanadates, which showed that in near-neutral solutions,vanadate increases the pitting potential, and decreases the oxygen reductionreaction rate on AA2024-T3. Impedance experiments suggest that slow filmformation may be occurring on Al in contact with vanadate-bearingsolutions.

Khoabaib et at. [4 I] evaluated the corrosion resistance behaviour of aproprietary sol-gel based coating system, as replacement for Alodine 1200,on AI2024 alloys. The sol-gel method is also being applied to obtain a thinprotective coating of cerate, molybdate and other rare-earth compounds onthe aluminium substrate through environmentally-friendly processes. Theperformance of this coating evaluated by electrochemical techniquesconcluded that the sol-gel treated E-coat provided an acceptable corrosionprotection level, similar to that of the conventional chromate system.

Yang et at. [42] investigated the behaviour of a Si02 and Zr02 sol-gelconversion coating, both alone and in combination with a polyurethaneunicoat (TT-P-2756, self-priming topcoat) on Al 2024-T3 alloy. Theevolution of the coating system under immersion was followed by AFM,SEM, EIS, and XPS. Although after two days of immersion, pittingcorrosion and degradation products on the sol-gel single coating surfacewere observed, further pitting corrosion ceased after a few days ofimmersion. Undercoating blisters in the sol-gel plus polyurethane topcoatsystem were found at 4 weeks of immersion, after which no further increasein blister size was observed. Results are explained based on a theory thatafter initial corrosion, aluminium and silicon oxides may form a stablemixed-oxide barrier layer at the interface hindering further pitting corrosion.

Parkhill et at. [43] demonstrated that an epoxide modified silicate sol-gelfilm spray coated on aluminium alloy 2024-T3 panels provides exceptionalbarrier and corrosion protection when compared to chromate-based surfacetreatments (Alodine 1200). The ormosil film was characterised using

ESA STM-276 15

profilometry, optical spectroscopy, and electrochemical analysis. Theauthors concluded that a spray-coated water-based environmentally-compliant sol-gel coating exhibits great potential for the replacement of thechromate-based conversion coatings presently used in the corrosion-prevention scheme of aircraft aluminium.

CCC on Magnesium Alloys

Zhao et at. [44] reported on the chromium-free conversion coatingperformance for application on magnesium alloys. In one of these works theresearchers studied the coating performance of phosphate-permanganatetreatment that adopted phosphate, incorporated with di-potassium hydrogenphosphate and permanganate, on magnesium alloys. They concluded thatthis treatment is capable of producing uniform coatings and with reasonableadhesion to the substrate. Furthermore, the results of the corrosion testingshowed that the coating corrosion resistance was equivalent to the protectionafforded by the traditional chromate treatment.

The same authors [45] developed a chromium-free multi-element complexcoating (MEEC), that was obtained using a new kind of chemical conversionprocess and an environmentally-friendly treatment technique. Thecomposition and microstructure of the coating layer formed on magnesiumalloys were characterised and showed good corrosion resistance.

The growth process of the same multi-element complex coating (MECC) onmagnesium alloy was also studied [46]. The morphological and structuralcomposition ofthe coatings studied by SEM, EDX, XRD and AFM, allowedthe conclusion that coating formation is a three-stage process: initial stagewith formation of a compact, amorphous layer; metaphase stage where theintermediate layer consists of a mixture of amorphous and crystallinematerial, and a final stage, in which the crystallinity increases and thecoating surface becomes smooth and compact.

The degradation process of a chromium-free conversion coating on Mgalloys was investigated in NaCI solutions [47], by EIS, and a mathematicalmodel for the degradation mechanism was proposed.

The development of a potassium permanganate bath chemical conversioncoating for magnesium alloys was investigated by Umehara et at. [48]. Thecoating formed consisted of an amorphous film composed mainly ofmanganese oxides, magnesium oxide or hydroxide.

The corrosion protection supplied by an aluminium arc-spray coating, post-treated by hot pressing and anodising on an AZ31 magnesium alloy, hasbeen studied by Chiu et al. [49] by using electrochemical polarisation andEIS. In this study, the Al arc-spray coating as well as a post-hot pressing andanodising treatment were carried out on an AZ31 Mg alloy, and the effect ofthe processing on the corrosion resistance was evaluated. The corrosion inthe AI-sprayed AZ31 specimen was enhanced by a galvanic cell occurring atthe interface of the Al coating and the AZ31 substrate, under thecircumstances when electrolyte was supplied through the porous Al coatingto the interface region.

Montemor et at. [50] investigated the composition and corrosion resistanceof cerium conversion films on the AZ31 magnesium alloy and its relation tothe salt anion. For that purpose, pretreatments based on different cerium (III)salts were performed by immersion of Mg alloy in a solution of ceriumchloride, cerium nitrate, cerium sulphate and cerium phosphate. Thechemical composition of the treated surfaces was investigated by X-rayphotoelectron spectroscopy and Auger electron. spectroscopy, whereas thecorrosion behaviour of the pretreated AZ31 substrates was investigated in

16 ESA STM-276

0.005 M NaCI solutions, using potentiodynamic polarisation and open-circuit potential monitoring. The results showed that the chemicalcomposition and the corrosion protection ability are dependent on the ceriumsalt used for the deposition of the conversion layers. Conversion layersobtained by immersion in CeCh are the thickest and the most protective. Theelectrochemical results show that all the conversion pre-treatments reducedthe corrosion activity of the AZ31 Mg alloy substrates in the presence ofchloride ions. The corrosion protection efficiency is related to the anionpresent in the cerium salt.

In previous work performed by the same authors [50] it was demonstratedthat conversion pre-treatments based on cerium nitrate or lanthanum nitratecould protect the AZ31 Mg alloy, the effectiveness of the conversion layerbeing dependent on the treatment time. Electrochemical and analyticaltechniques were combined to study the rare-earth conversion films formedon the AZ31 Mg alloys and their ability to provide corrosion protection.Cerium and lanthanum conversion layers provide corrosion protection on theAZ31 Mg alloy. The corrosion protection seems to be independent of thechemical composition of the conversion layer, however it does depend onthe thickness.

Magnesium-rich coatings present a new and challenging field ofdevelopment for the corrosion protection of aluminium structures [52].These coatings are capable of sacrificial protection, but assessment of theirefficiency and durability can be strongly affected by the testing environment.The corrosion behaviour of two aluminium alloys (A12024 and A17075)coated with a magnesium-rich coating, also of pure magnesium and of thebare aluminium substrates, was assessed in the two solutions (0.1 % NaCIand dilute Harrison solution, DHS) using electrochemical techniques (OCPand EIS). The corrosion rate of pure magnesium was higher in DHS,although the dissolution rate of the magnesium embedded in the polymermatrix was not significantly affected. The impedance spectra of the scribedsamples resembled that of the bare substrates in NaCI solution but not inDHS.

Alloys AI Cu Mg Mn Fe Zn Cr Si Ti

AI7075 Bal. 1.2-2.0 2.1-2.9 0-0.3 0-0.5 5.1-6.1 0.2-0.4 0-0.4 0-0.2

AI2219 Bal. 5.8-6.8 0-0.02 0.2-0.4 0-0.3 0-0.1 0-0.2 0.02-0.1

AI5083 Bal. 0-0.1 4.0-4.9 0.4-1.0 0-0.4 0-0.25 0.05-0.25 0-0.4 0-0.15

'As'As After thermal After 55

received''Cleaned' produced' cycling

(coated)exposure

Surface roughness X X X X -

Surface electrical resistivity - - X X -

Thermal cycling* - - X -

Salt spray (ASTM B 117/07) X - X X -

Scanning electron microscopyX X X X X

(SEM)

* The thermal profile corresponds to a standardised profile of -50°C and +1 OO°C,with dwell times of 15 min,at cooling/heating rates of 10°C/min;

ESA STM-276 17

2 Experimental Phase

2.1 Materials, preparation of specimens, and treatments

Specimens of three aluminium alloys (Table 3) were mechanically cut in4 x 4 cm dimensions and used as received for coating with the chemicalconversion coating treatments selected for evaluation. For each pair,chemical conversion coating / aluminium alloy triplicates were prepared.

Table 3 - Chemical composition of aluminium alloys selected (wt %).

To assure that the metal surface was free from oil, grease and other foreignmatter before the treatment, an appropriate cleaner was used. Because of itsavailability, Novaclean A186 was used. This is a cold degreaser foraluminium used in immersion processes, combining degreasing anddeoxidising characteristics.

2.2 Evaluation of the aluminium alloys samples

The following tests were performed on the samples.

Table 4 - Tests performed on aluminium alloys samples.

2.2.1 Surface roughness

The surface roughness of the test panels was measured using a MitutoyoSurftest 201. A stylus attached to the detector unit of the SJ-20 1P isdisplaced at constant speed across the sample's surface, tracing surfaceirregularities. The vertical stylus displacement produced during tracing ofthe sample's surface is converted into electrical signals. The parameter usedfor expressing roughness was Ra, which is the arithmetic mean of theabsolute values of the profile deviation from the centre line within theevaluation length [53].

18 ESA STM-276

2.2.2 Surface electrical resistivity

Surface resistivity could be defined as the material's inherent surfaceresistance to current flow multiplied by the ratio of the specimen's surfacedimensions (width of electrodes divided by the distance between electrodes),which transforms the measured resistance to that obtained if the electrodeshad formed the opposite sides of a square [54]. Surface resistivity does notdepend on the physical dimensions of the material.

The surface electrical resistivity of the test panels was carried out with aMicroohmimeter Schuetz Messtechnik, model MR 300C, based on the 4point method. The resistance measured is then calculated through theequipment by applying Ohm's Law. The measured electrical resistance isproportional to the length of the sample and to the surface resistivity, andinversely proportional to the cross-sectional area of the sample:

p = (R x S) /l

where:

p = surface electrical resistivity (Q m);

R = electrical resistance of a uniform specimen of the material (Q);

S = cross-sectional area of the specimen (m2);

I = length of the specimen (m).

2.2.3 Thermal cycling tests

Thermal cycling is a test consisting of cycling a sample between twoextreme temperatures which are held for a certain amount of time. Thetemperature change is performed at a constant rate. This is done for aspecified number of cycles. As samples heat up and cool down they expandand contract, possibly causing fatigue or adhesion failure of the coating overtime [55].

The thermal cycling tests were performed in air using a thermal chamberARALAB, model Fitoterm 300. The thermal profile corresponds to astandardised profile of -50°C and + 1OO°C, with dwell times of 15 min., atcooling/heating rates of 10°C/min. The 500 cycles test was interrupted at the200th cycle, for resistivity measurements.

2.2.4 Salt spray tests

Neutral salt spray testing was carried out according to ASTM B 117 [56]with a 5% NaCI solution, at 35°C. The panels were placed in the cabinetbetween 15 - 300 from the vertical, for 7 days. Photographic recording wasperformed each 24 h. The test panels were not allowed to contact othersurfaces in the chamber and condensed or corrosion products on theirsurfaces were not permitted to cross-contaminate each other. After exposure,test panels were rinsed in deionised water to remove salt deposits from theirsurface and then immediately dried.

The salt spray exposure was carried out in a salt spray chamber ERICHSEN,model 606/400 L.

The samples were subsequently stored in a desiccator.

ESA STM-276 19

2.2.5 Scanning electron microscopy and energy dispersive X-rayspectrometry (SEM/EDS)

Surface morphology and chemical composition of the samples wasperformed on a scanning electron microscope JEOL-JSM model 6700,equipped with an Oxford energy-dispersive X-ray spectrometer (EDS),operated at 20 keY.

2.2.6 Chemical conversion coatings procedure

Figure 1 summarises treatment steps to which samples were submitted. Thechemical conversion coatings were treated according to the respectivetechnical data supplied (see Annexes).

Ultrasonic cleaning in acetone (10 min.)

Drying

Treating withAlodJne 12005

conversion coating,3 min. (350C)

Treating withAlodJne 5700

conversion coating,3 min. (35°C)

Treating withIridite NCP

conversion coating,5 min. (35°C)

Treating withNabutan 5TI/310conversion coating,

2 min. (35°C)

Figure 1 - Cleaning and treatment processes scheme.

ESA STM-276 21

3 Results / Discussion

3.1 Visual inspection

3.1.1 'As received'

The Al2219 test specimen' As received' presented a visually rougher surfacecompared to the Al7075 and Al5083 aluminium alloys, as shown in Figure2.

3.1.2 'As produced'

Figure 3 shows the test panels surfaces after application of the chemicalconversion coatings:

Alodine 1200S tested specimen surface presented an iridescent goldencolour for all the alloys; .

Alodine 5700 provided a light golden appearance brighter on Al 2219;

Iridite NCP gave colours ranging from light blue (AI 2219 and AI 7075)to yellowish (AI 5083) over the samples' surfaces;

Nabutan coated samples kept the initial metallic aspect, and appearedcolourless.

All pre-treatment coatings applied on Al2219 kept a mirror-like aspect ofthesurface. This can perhaps be interpreted as the deposited films in this alloynot being as thick as the other substrates.

3.1.3 After thermal cycling tests

The thermal cycling test did not produce visible changes in the samples(Figure 4). No cracks, delamination, or other form of degradation afterthermal cycling exposure were observed.

The only visible effect of thermal cycling on the treated test panels was todarken the coating colours, probably as a result of dehydration caused by thecombined effects of thermal contraction and expansion due to t~etemperature changes applied during the tests.

3.1.4 After salt spray tests

Figure 6 illustrates the coated test panel surfaces after the exposure to saltspray test:

There is no evidence of corrosion on the three types of aluminium testpanel coated with the Alodine 1200S, after 7 days of exposure;

Alodine 5700 and Nabutan coatings showed surface corrosion within 24h on AI7075 and A12219, with formation of pits;

Iridite NCP presents some corrosion on AI7075 and Al2219 after 48 hof exposure;

There is no evidence of corrosion on the Al5083 coated samples after 7days of exposure, for each of the treaments studied.

3.1.5 After thermal cycling + salt spray tests

Figure 7 shows the coated test panel surfaces subjected to thermal cyclingmeasurements after the exposure to salt spray test. The results are verysimilar to those obtained for the samples after salt spray test:

22 ESA STM-276

There is no evidence of corrosion on A17075 and A15083 test panelscoated with the Alodine l200S, after 7 days of exposure;

A122l9 samples coated with Alodine l200S present some evidence ofcorrosion after 7 days of exposure;

Alodine 5700 and Nabutan coatings presented surface corrosion onA17075 and A122l9 alloys, within 24 h of exposure; formation of pits;

Iridite NCP shows some corrosion on A17075 and A12219 after 48 h ofexposure with formation of pits;

There is no evidence of corrosion on the A15083 coated samples after 7days of exposure, for each of the treaments studied.

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3.2 Surface roughness

Test specimen roughness was measured on samples at each step of theprocess: 'As received', cleaned with the N ovaclean Al86 agent, coated, afterthermal cycling tests, after salt spray tests, and after thermal cycling plus saltspray tests. For each test specimen, three measurements were carried out intwo perpendicular directions (longitudinal and transverse) and the averageresults are shown in Figures 8 and 9.

In general, roughness measurements can be summarised as follows:

a)

b)

c)

'as received' roughness is higher for A12219;Alodine 1200 and lridite NCP provide a higher average surfaceroughness for Alloys 2xxx and 5xxx. On the contrary, for Al 7xxxhigher roughness was obtained for Alodine 5700 and Nabutan;

A decrease in roughness was seen for all substrates after thermalcycling, as compared to the 'as coated' condition measurements. Thisevidence was probably related to the fact that after thermal cyclingmeasurements the coated specimens exhibit a more compact anduniform surface.

Effect on Ra (LD)2,5 ----------

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AI7075

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30 ESA STM-276

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For the Al7075 alloy, there is a decrease in roughness for all treatments;there is a decrease in roughness with the coating application as well as withthe thermal cycling.

For the Al 5083 alloy the influence of the different surface conditions is thesmallest, but it follows the same tendency as the AL 7075 alloy: a decreasetendency in roughness with coating application and thermal cycling.

The Al 2219 alloy presents a decrease of roughness with the chemicalcleaning but the coating deposition strongly increases the surface roughness,which decreases again with the thermal cycling.

In summary, thermal cycling always decreases surface roughness. Thisfeature can be related to a dehydration of the surface layer. Treatmentapplication also decreases roughness, except for A12219, for which thistreatment makes roughness values higher than those obtained in the 'asreceived' condition.

The behaviour described is essentially the same for the two perpendiculardirections.

In general, the Al2219 test panels show a higher surface roughness than theother two aluminium alloys. It should be noted that the conversion coatingtreatments Alodine 1200 and Iridite NCP provide a higher average surfaceroughness, except for the Al7075 samples, where those coated with Alodine5700 and Nabutan showed higher values.

The surface roughness measurements after the salt spray tests were onlyperformed on the test panels treated with the Alodine 1200S and on samplesof A15083, since that in general the surface of samples presented visibleevidence of corrosion to allow the measurement of the roughness. In thosesamples measured it can be seen that there is no significant differencesbetween the roughness values obtained after salt spray tests.

3.3 Surface electrical resistivity measurements

The surface resistivity of the test specimens was measured for the samples'as produced', at 200 cycles of the thermal cycling test. Once again, thesemeasurements were not performed on test panels after salt spray exposure,since in general the surface was attacked. For each test specimen /

Surface Resistivity0 as produced

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ESA STM-276 31

conversion coating treatment, 10 measurements were carried out. Figure 10summarises the average results obtained.

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Figure 10 - Surface electrical resistivity measurements of test panels.

In general, the surface electrical resistivity of all test panels 'as produced' isvery similar for all the chemical conversion treatments under evaluation anddid not suffer significant change after the thermal cycling tests, if theexperimental standard deviation is considered.

3.4 Surface morphology analysis - Scanning electronmicroscopy

3.4.1 After cleaning with Novaclean Al86

Figure 11 illustrates the SEM micrographs of Al2219 and Al7075specimens, after cleaning with Novaclean A186. Samples presented higherroughness before the cleaning step, and some dirt was eliminated in this step.

The remaining particles on the surface were secondary phases from thecorresponding aluminium alloy. This was confirmed by chemical analysis bySEM/EDX. Some holes can also be seen on the surface; this is attributed tothe mechanical removal of the secondary phases, either during materialprocessing or selective leaching of secondary phases by the cleaning agent.

No cleaning test was performed on the Al5083 samples, due to insufficientmaterial quantities.

32 ESA STM-276

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3.4.2 After chemical conversion coating treatments

Figures 12 to 14 show the secondary electron photomicrographs of thecoated test panels. The EDS spectra correspondent for each chemicalconversion coating were stored.

Apart from the Alodine 5700, SEM observation showed relatively uniform,closed coatings for all the treatments tested. Discontinuities found on thesurface can be related to previous surface defects or microstructuralheterogeneities induced by the secondary phases. In fact, it is expected thatvarying substrate composition and reactivity presented by the three alloys(AI7075, AI2219 and A15083) will influence the formation of the surfacefilms.

EDS analysis revealed that these morphological heterogeneities are alsocoated, making the coating a continuous film over the complete alloyssurfaces.

The Alodine 5700 coating peeled off in significant areas of the A17075, astronger attachment being observed on AI2219 surfaces. This peeling off canbe interpreted by a poor adhesion to the substrate or internal stresses of thecoating or a combination of both of these effects. In the Al5083 alloy, on thecontrary, this treatment formed a compact film (Figure 13). EDS spectrarevealed the presence of F, Ti, Zr, 0 and C, along with the alloying elementscharacteristic of the alloys under study (all results and images have beenstored electronically for further reference).

ESA STM-276 33

In an attempt to overcome the poor adhesion of Alodine 5700 on AI7075 andAI2219 alloys, coating tests were performed with varying immersion times(30 seconds, 1, 3 and 5 minutes), with all other parameters kept constant.The corresponding morphologies are illustrated in Figures 15 to 16. For theAI2219 alloy, the best result was obtained for 3 minutes immersion, whilefor the 7075 alloy the best result was obtained for I minute immersion.Nevertheless, the surfaces were very similar to those obtained previously, interms of morphology and composition. Furthermore, it was possible toobserve that the surface was probably composed of two coating layers: onemore superficial and the other beneath.

SEM observation shows that films formed on the test panels are probablythin. In fact, it must be emphasised that some defects were present incoatings, probably related with the secondary phase distribution over thesurface.

SEM images of the test panels coated with the Alodine 1200S treatmentshow a typical mud-cracked surface morphology and are very similar for allthe aluminium alloys. It was possible to identify some black spots over thesurface, which indicates that the coating did not cover the whole surface ofthe sample. These sites are supposed to be related to intermetallic particlespresented on the surface of these alloys, suggesting that the thickness of thecoating is less on top of these particles. The same mud-crack surfacemorphology has been observed by other authors [23], in a study performedon chromium conversion coating AI7075 alloys.

The semi-quantitative analysis of the chemical elements reveals the presenceof some constituents of the aluminium alloys and elements that proceed fromthe conversion chemical coating, mainly: Cr, F, Zr and O. However, due tothe limitations of this technique, namely the inability to analyse lightelements, the percentage of 0 cannot be consider in a reliable quantitativeway.

The surface morphology of test panels coated with the Nabutan and lriditeNCP showed a uniform surface, nevertheless some black holes existed onthe surface, suggesting that the coating did not cover the whole surface ofthe samples. The EDS analysis identified that the treated layers are formedmainly by F, Ti, 0 for the Nabutan treatment. F, P, Ti and 0 plus theelements of each alloy were associated with the Iridite NCP treatment. Onceagain, the percentage of 0 cannot be considered in a quantitative form.

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ESA STM-276 39

3.4.3 After thermal cycling tests

SEM observation after the thermal cycling tests revealed minor changes inthe morphology of all coated surfaces except for the Al7075 and Al2219(Figures 17 to 19). This was more evident in test panels coated with Alodine1200 and Alodine 5700. In effect, the comparison of SEM images before andafter thermal cycling shows clearly that, after thermal cycling, the coatedspecimens exhibit a more compact and uniform surface. For Al7075 andA12219, the non (poor)-adherent bits of the coatings observed in the 'asdeposited' condition were absent after thermal cycling. This is coherent withlack of adherence to the substrate, and the thermal stress induced by thetemperature cycling promoted the complete peel-off of the non-adherentfilms parts.

All these results support surface roughness measurements, which revealed adecrease in Ra after the thermal cycling test.

The representative EDS spectra have been retained at ISQ.

The dark areas observed on the coated samples surfaces are related with thecoating layer formed, while the white area corresponds to the surface wherethe coating treatment had poor adhesion to the substrate, which is confirmedby energy dispersive spectroscopy (EDS) analysis. This fact was moreevident on samples A17075 and Al2219 submitted to the Alodine 5700treatment. The test panels treated with Nabutan and lridite NCP wererelatively unaffected by the thermal cycling tests.

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3.4.4 After salt spray tests

The evaluation of samples after salt spray exposure by SEM revealed thatAlodine 1200S coated samples had a similar surface morphology to the 'Asproduced' samples. Nevertheless, the mud-cracked surface appearance wasnot so evident, except for samples from A12219. The semi-quantitativeanalysis of the chemical elements identified the constituents of thealuminium alloys, as well as elements that proceed from the conversionchemical coating and a small percentage ofNa.

In general, SEM observation of the test panels AI7075 and AI2219 treatedwith the three chemical conversion coatings under study, showed that thesurface morphology is formed by a 'white' corrosion product, mainlycomposed of AI, 0, Na and the elements of each alloy, indicating Alsubstrate degradation and sodium chloride residues.

The AI5083 test panels exposed to salt spray did not present evidence ofcorrosion, even after 7 days of exposure. SEM images of these test panelsshowed an unaffected surface morphology, although a small amount of whitecorrosion product could be observed. The EDS analysis identified theelements revealed before the exposure, with an increase in 0 percentage andthe presence of a small percentage ofNa.

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ESA STM-276 47

3.4.5 After thermal cycling plus salt spray tests

On the whole, SEM observation of the test panels that were submitted tothermal cycling followed by salt spray exposure brought out changes in themorphology of all coated surfaces quite similar to those obtained after thesalt spray exposure.

Once again, AI5083 samples showed no signs of corrosion for all treatmentsstudied.

The surface morphology of test panels from the other two treated aluminiumalloys revealed an equivalent morphology to those obtained before. Thesurfaces were covered with a layer of white corrosion products, formedmainly of AI, 0, Na and other constituents of the aluminium substrates.

In summary, sample exposure to thermal cycling did not introducesignificant changes on the corrosion behaviour.

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ESA STM-276 51

3.4.6 Comparison and correlation between Aluminium alloysand techniques performed

Tables 5 to 7 summarise the results obtained for each aluminium alloy andcharacterisation performed.

Al7075

Generally, the surface roughness of test panels treated with the fourconversion chemical coatings was very similar. However, for Alodine 5700and Nabutan coated samples, the values were slightly higher. It should benoted that there was a decrease in roughness values for all treatments aftercoating application, as well as after the thermal cycling. This feature can berelated, on one side to the chemical etching performed by the cleaning agentand coating deposition and on the other side, to the fact that after thermalcycling the coated specimens exhibited a more compact and uniform surface,probably due to surface layer dehydration and spalling of poor-adherentcoated areas.

The surface electrical resistivity of all test panels 'as produced' was verysimilar for all the chemical conversion treatments under evaluation, and didnot suffer significant changes after thermal cycling.

Alodine 1200S treated samples showed a typical mud-cracked surfacemorphology, and semi-quantitative analysis of the chemical elementsrevealed the constituents of the aluminium alloy, as well as elements thatproceed from the conversion chemical coating. The Alodine 5700 coatingmorphology developed on the surface test panel exhibited a surface wherethe coated layer is not continuous, indicating a poor substrate adhesion. TheEDS spectra on the coated layer revealed the presence of elementscharacteristic of the chemical coating and Al7075 alloy. The surfacemorphologies of the Nabutan and Iridite NCP coated test panels showeduniform surfaces.

After thermal cycling, the coated specimens display a compacter and moreuniform surface, and the morphology in terms of the elements constituents ofthe coating layer appears to be quite similar. This was more evident inAlodine 5700 coated test panels since the thermal stress induced by thetemperature cycling promoted the complete peel-off of the non-adherentfilms areas.

After the salt spray tests, except for Alodine 1200S treated samples, thesurface morphologies are covered by a layer of 'white' corrosion products.

Al2219

Al2219 test panels present higher initial surface roughness, a roughnessdecrease could be observed after chemical cleaning. However, the coatingdeposition strongly increases the surface roughness, which decreases againafter thermal cycling, probably related to surface layer dehydration. TheAlodine 1200 and lridite NCP conversion coating treatments provide higheraverage surface roughness.

The electrical surface resistivity of all 'as produced' test panels were verysimilar for all the chemical conversion treatments under evaluation, and didnot suffer significant changes after the thermal cycling.

Alodine 1200S coated samples present a typical mud-cracked surfacemorphology, and the surface is mainly composed of elements from the alloyand from the chemical conversion treatment. The Alodine 5700 coatingpeeled off in significant areas of the Al2219 surface, which can be

52 ESA STM-276

interpreted as a poor adhesion to the substrate or internal stresses of thecoating, or a combination of both of these effects. On the other hand, thesurface morphology of test panels coated with the Nabutan and Iridite NCPshowed a uniform surface.

After thermal cycling, the coated samples showed a compacter and moreuniform surface, and the morphology of the coating layer in terms of theelements constituents was very similar. This was more clearly seen inAlodine 5700 coated test panels since the thermal stress induced by thetemperature cycling promoted the complete peel-off ofthe non-adherent filmareas. In Nabutan and lridite NCP treated test panels, it could be seen thatthey were roughly unaffected by the thermal cycling.

After the salt spray tests, except for Alodine 12008 coated test panels,surfaces exhibit a white corrosion product layer.

AI5083

A15083 alloy showed the same influence of different surface conditions, butit followed the same tendency as the Al7075 alloy: roughness decrease withcoating application and thermal cycling.

In general, surface roughness and surface electrical resistivity of all testpanels 'as produced' were very similar for all the chemical conversiontreatments under evaluation, and did not suffer significant changes afterthermal cycling.

Coated specimen surface morphologies displayed a compacter and moreuniform coating. Furthermore, Alodine 5700 treated samples did not presentthe lack of substrate adhesion observed on samples of Al7075 and AI2219alloys.

Thermal cycling did not significantly affect sample surface morphologies;however they exhibited a more compact surface after cycling. The coatinglayers were composed essentially of the same elements: element constituentsof the aluminium alloy and those that provided for the chemical conversiontreatment.

After the salt spray tests, there was no evidence of corrosion on the AI5083coated samples after 7 days of exposure, for each of the treatments studied.

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ESA STM-276 57

4 ConclusionsChemical cleaning of the as-received sheet samples in Novaclean Al86has been seen to decrease the surface roughness of all alloys (AI7075,Al2219 and AI5083).

The coating supplier recommended processes were applied. OnlyAlodine 1200S was observed to be defect-free after its application. Theother coatings contained some defects. For Alodine 5700, evenimmersion time variations did not exclude defects (except for the 30-second immersion).Only the Alodine 1200S samples (for all alloys) survived thermalcycling; the other conversion coatings were seen to partially spall-off.

The electrical resistivities of all samples were very similar, even afterthermal cycling.Only the Alodine 1200S samples (for all alloys) can be considered tohave survived the salt spray test. The other coatings (A Iodine 5700,lridite NCP and Nabutan STi/310) did not provide adequate corrosionprotection.

For A15083, chromium free coatings seem to have promising corrosionbehaviour.Only Alodine 1200S provided a compact finish. The other coatingstended to spall-off and are considered to be a severe source forcontamination in any given spacecraft application.

It is recommended that corrosion protection schemes for ESAspacecraft continue to utilise Alodine 1200S, as the other (hexavalentchromium-free) coatings did not provide suitable protection.

ESA STM-276 59

5[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

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Y. Sepulveda, C.M. Rangel, M.A Paez, P. Skeldon, and G.E.Tompson, "ASST 2006 - Aluminium Surface Science andTechnology", Abstract 76, 4th Int. Symp. (2006)

AE. Hughes, M. Forsyth, N. Dubrule, T. Markley, B. Hinton, andS.A Furman, "ASST 2006 - Aluminium Surface Science andTechnology"; Abstract 55, 4th Int. Symp. (2006)

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