Effect of SurfaceTreatment,Anodisation, and Inhibitors on ... · Aircraft CH004b.tex 21/11/2012 12:...

Aircraft CH004b.tex 21/11/2012 12: 18 47

SUBSECTION 4b

Effect of Surface Treatment, Anodisation,and Inhibitors on Corrosion of theAluminium Alloy AA2024

J.A. DeRose1, I. De Graeve2, A. Bałkowiec3, J. Michalski3,T. Suter1, K.J. Kurzydlowski3, P. Schmutz1, & H. Terryn2

1Laboratory for Joining Technologies and Corrosion, EMPA(Swiss Federal Institute of Materials Science & Technology), Switzerland2Research Group Electrochemical and Surface Engineering,Vrije Universiteit Brussel (VUB), Belgium3Faculty of Materials Science and Engineering (InMat),Warsaw University of Technology (WUT), Poland

Abstract

The impact of surface treatments, such as pretreatment (alkaline etching and acidpickling), anodisation (growth of thick oxide), and inhibitors (conversion coat-ings), on the corrosion properties of the Al 2024 alloy has been investigated. Theeffect of pretreatment and anodisation on the Al 2024 microstructure (presenceof intermetallic particles and defects) has been mainly investigated using electronmicroscopy in a manner similar to that described in Subsection 2b of this book.Microscale and macroscale polarisation curve measurements were made onAl 2024with chromate or cerium-based inhibitors in the electrolyte. The micropolarisationcurves allowed the site-specific effect of the inhibitors on the S (Al2CuMg), second(AlCuFeMnSi), and matrix phases of Al 2024 to be examined.

These curves were analysed to see if they could be used as experimental inputdata for the mass transport model MITReM, mentioned in Subsection 3a, in orderto determine whether the effect of the chromate or cerium-based inhibitors uponthe Al 2024 corrosion behaviour could be modelled.

1 Introduction

Aluminium alloys used for industrial applications are usually protected from cor-rosion by means of chemical pretreatment (alkaline etching and acid pickling),

www.witpress.com, ISSN 1755-8336 (on-line) WIT Transactions on State of the Art in Science and Engineering, Vol 6 , © 2012 WIT Press1

doi:10.2495/978-1-84564-752-0/009

Aircraft CH004b.tex 21/11/2012 12: 18 Page 148

148 ALUMINIUM ALLOY CORROSION OF AIRCRAFT STRUCTURES

anodisation (electrochemically induced growth of a thick oxide layer), inhibitorrelease from primer layers or conversion coatings (normally of chromate com-pounds), and diverse paints. The influence of pretreatment, anodisation, andinhibitors on theAl 2024 microstructure and corrosion behaviour has been observedand will be reported in this subsection.

The AA2024 alloy is particularly prone to corrosion; therefore, several studieshave been dedicated to its corrosion protection through electrochemical conversiontreatments [1–3], hybrid coatings [4], anodising [5–8], and inhibitors [4,9,10].

As already mentioned in Subsection 2b of this book, the S (Al2CuMg) and θ

(Al2Cu) phase intermetallic particles represent only a minority fraction (<30%)of the microscale particles of the non-pretreated and non-anodised Al 2024 sam-ples. The second phase (AlCuFeMnSi) intermetallic particles are clearly themost abundant (>70%) of the microscale particles; hence the focus was on thisphase when performing microstructural analysis of pretreated and/or anodised Al2024.

Corrosion management concepts utilising the application and integration ofcorrosion prediction tools for its occurrence and progression will be a drivingforce for advancement in the field of corrosion maintenance and development ofnew structural designs, materials, and processes for surface protection. Reliableprediction of corrosion imitation and propagation would be the basis for a cost-effective maintenance and repair strategy. SICOM will develop models that willbecome an essential part of future predictive maintenance strategies. Corrosionprediction models will reduce costs for aircraft development and use.

2 Experimental methods

2.1 Al 2024 sample preparation

Prior to pretreatment, anodisation, or experimental investigation, the Al 2024samples were mechanically ground with successively finer grades (1000 and 2500)of SiC paper which was lubricated with water. Afterwards, they were polishedwith 6 µm, 3 µm, 1 µm, and 0.25 µm diamond pastes on cloth polishing padsusing alcohol as the lubricant. In some cases of pretreatment and anodisation, theAl 2024 samples were not polished.

2.2 Microstructure analysis

Surface and cross-sectional imaging of the microstructure of anodised and untreatedAA2024 was done atVUB by means of field emission scanning electron microscopy(FE-SEM, JEOL JSM 7000F model) in combination with energy dispersive X-rayanalysis (EDX), operated at 10 to 20 keV.

Microstructural characterisation of the Al 2024 was done using a Hitachi SU70and/or S5500 ultra-high resolution field emission scanning electron microscope



EFFECT OF SURFACE TREATMENT, ANODISATION, AND INHIBITORS 149

(FE-SEM) at WUT. The Hitachi SEMs were operated at 5 kV to 15 kV for SEMimaging and EDX/EDS. Both secondary electron (SE) and backscattered elec-tron (BSE) detection methods were used to obtain SEM images. In addition,a less detailed sample characterisation was done using a LEO 1455 thermionicemission SEM, which was typically operated in the 5 kV to 20 kV range and inconjunction with an Oxford Instruments Inca 7353 energy dispersive X-ray spec-trometer (EDX/EDS). For EDX/EDS analysis with the LEO SEM, a large differencein the amount of Mg was the main criteria to distinguish S from θ phase anddetection of Fe, Mn, and possibly Si indicated second phase. More details aboutAl 2024 characterisation with electron microscopy can be found in Subsection 2bof this book.

2.3 Pretreatment and anodisation of Al 2024-T3

2.3.1 PretreatmentThe chemical compounds are home-made solutions or commercial cleaning andetching products. Pretreatment was done using three methods (dipping processwith Uniclean 150, a mild alkaline cleaner; Surtec 494, an acid pickling solution;and Nabuclean, a mild alkaline cleaner):

1. Method 1 (mild etching):

Uniclean 150 50 g/l, T = 60◦C, t = 600 s → rinse in de-ionised (DI)H2O → Surtec 494 4% vol, T = RT, t = 132 s → rinse in DI H2O

2. Method 2 (medium-to-strong etching with an average of 1 µm removal)

Nabuclean 50 g/l, T = 60◦C, t = 10 s → rinse in DI H2O → 5% NaOH,T = 60◦C, t = 10 s → rinse in DI H2O → 10% HNO3, T = 30◦C,t = 15 s → rinse in DI H2O

3. Method 3 (strong etching with 3–5 µm removal):

Ultrasonic cleaning in ethanol for 120 s, alkaline cleaning in P3-Almeco48 g/l, T = 70◦C, t = 10 min → rinse in DI H2O → alkaline etching in NaOH50 g/l, T = 70◦C, t = 40 s → rinse in DI H2O → acid pickling in HNO3

400 g/l, T = RT, t = 90 s → rinse in DI H2O

SEM and EDX/EDS data were obtained for Al 2024 samples which were pretreatedusing Methods 1 or 2 above, as these methods are designed not to etch the samplesurface too severely so that high-resolution analysis of the surface microstructureis still possible.

Macropolarisation curve data were obtained for non-polished Al 2024 sampleswhich were strongly etched using Method 3 in order to simulate the actual etchingconditions found in the aviation industry (Section 2.3.4 below).




2.3.2 AnodisationThe anodising procedures involved chromic acid and tartaric/sulphuric acid. Theyare defined as follows:

• Chromic acid anodising (CA):

◦ Aqueous solution of chromium (VI) trioxide, CrO3, 45 g/l, T = 40◦C ± 0.1◦C,t = 45 min → 0V to 40V in 10 min, constant 40V for 25 min, 40V to 50V in5 min, and constant 50V for 5 min.

• Tartaric/sulphuric acid anodising (TS):

◦ Aqueous solution of tartaric acid, HOOC(CHOH)2COOH, 80 g/l, withsulphuric acid (H2SO4), 22 g/l; T = 37◦C; t = 25 min → 0V to 14V in 5 min,and constant 14V for 20 min.

2.3.3 Combined pretreatment and anodisationThe following acronyms denote the various combinations of methods for pretreat-ment followed by anodisation of Al 2024:

• No pretreatment and no anodisation = NT• Pretreated and no anodisation = PT• No pretreatment + chromic acid anodised = CA• No pretreatment + tartaric/sulphuric acid anodised =TS• Pretreated + chromic acid anodised = PCA• Pretreated + tartaric/sulphuric acid anodised = PTS

2.3.4 Electrochemical measurementsMacroelectrochemical polarisation experiments were performed on non-pretreated (NT), pretreated (PT), non-pretreated + anodised (CA and TS), andpretreated + anodised (PCA and PTS) Al 2024 samples using the followingprocedure:

• 0.5 M NaCl, pH = 7, room temperature• Cathodic pre-polarisation at −700 or −600 mV (about 100 mV below open

circuit potential (OCP))• Polarisation in the anodic direction with a scan rate of 1 mV/s from about 100 mV

below OCP to about 10 mV above the pitting potential (PP)

2.4 Corrosion inhibitors

2.4.1 Sample preparationAllAl 2024 samples were polished before experimental investigation with inhibitors(refer to 2.1 above). A ternary AlCuMg alloy sample having large S phase pre-cipitates was also polished before investigation in the same manner as for Al2024.




2.4.2 Electrochemical measurementsThe inhibitors used were sodium dichromate (Na2Cr2O7) and cerium nitrate(Ce(NO3)3) purchased from Merck and Sigma-Aldrich, respectively. Solutionsof 0.5 M NaCl with 1 mM (10−3 M), 0.1 mM (10−4 M), or 0.01 mM (10−5 M)Na2Cr2O7 or 0.5 M NaCl with 0.1 mM (10−4 M), 0.01 mM (10−5 M), or 0.001 mM(10−6 M) Ce(NO3)3 were used as the electrolytes. The pH of all solutions wasadjusted to 7 by adding NaOH.

All polarisation curve measurements (microscopic and macroscopic) weremade on polished samples of Al 2024 sheet alloy or a ternary AlCuMg alloy withlarge S phases and done in the following way:

1. The rest or OCP was measured for 2 min prior to the start of polarisation.2. Polarisation in the anodic direction was made from OCP − 100 mV to

OCP − 50 mV until PP and polarisation in the cathodic direction was madefrom OCP + 100 mV to OCP + 50 mV until OCP − 300 mV.

3. A scan rate of 1 or 0.1 mV/s was used.

Micropolarisation curves were made with a microcapillary cell of 20–40 µmend diameter over large, isolated Al 2024 second (AlCuFeMnSi) phase intermetal-lic particles surrounded by matrix, Al 2024 matrix (AlCuMgMn) areas withoutmicroscale particles (but including nanoscale dispersoid and second phase parti-cles), andAlCuMg alloy S (Al2CuMg) phase intermetallic particles (due to the smallsize of S phase particles inAl 2024). Macropolarisation curves were made with a cellof 10 mm diameter (all Al 2024 phases included). A more in-depth description con-cerning Al 2024 characterisation and the micropolarisation and macropolarisationtechniques can be found also in Subsection 2b of this book.

3 Results and discussion

3.1 Pretreated Al 2024

Some of the microscale particles found in the Al 2024 alloy have a heterogeneouselemental composition, as can be observed in the SEM images of Figure 4b.1. Onthe left-hand side (Figure 4b.1a) is a SE detection SEM image of non-pretreatedAl 2024. On the right-hand side (Figure 4b.1b) is a BSE detection SEM image ofthe non-pretreated Al 2024 which shows several microscale particles with brightand dark zones where brighter regions in the core of the particle indicate the pres-ence of heavier elements, that is, Cu, Fe, and Mn. Hence, it can be expected thatpretreatment might result in a heterogeneous dissolution of such particles. Notealso that some round particles (most likely S phase particles) have been removeddue to the polishing leaving behind holes.

An SEM image of a polished Al 2024 sample after pretreatment with Method 1(Section 2.3.1 above) is shown in Figure 4b.2. The largest particle visible is secondphase (AlCuFeMnSi) and at the bottom of the image is a θ phase (Al2Cu) particle.




3000x 3000x1µm 1µm

(a) (b)

Figure 4b.1: SEM images of polished, non-pretreated Al 2024: (a) SE detectionand (b) BSE detection.

6400

AI

Cu

MgCu

Cu

keV

Cu

5600(b)

(d)

4800

4000

Cou

nts

Cou

nts

3200

2400

1600

800

01.5 3.0 4.5 6.0 7.5 9.0 10.5 12.0 13.5 15.0

keV

1.50

3.0 4.5 6.0 7.5 9.0 10.5 12.0 13.5 15.0

45,000

40,000

35,000

30,000

25,000

20,000

15,000

10,000

5000

AI

Cu

Cu Si MnFeFe Cu

Cu

10 m

004

014

Figure 4b.2: (a) SEM image of a polished Al 2024 sample after pretreatment withMethod 1 (Section 2.3.1 above), (b) EDX spectrum for point 004 of(a), (c) SE image of polished Al 2024 after pretreatment with Method1, and (d) EDS spectrum for point 014 of (c).

The second phase appears to have suffered a much more severe attack than theθ phase particle.

From Figures 4b.2 and 4b.3 it is clear that the second phase particles are affectedby the alkaline and acid pretreatment steps. The particle/matrix interface region has




(a) (b)

Figure 4b.3: (a) SE image (8000×) and (b) BSE image (5000×) of polishedAl 2024 after pretreatment with Method 2 (Section 2.3.1 above).

100,000x 100 nm

(a) (b)

100 nm

Figure 4b.4: SEM images ofAl 2024 anodised in tartaric/sulphuric acid (a) without(TS) and (b) with (PTS) prior pretreatment.

widened and the bulk of the particle is partially and heterogeneously disintegrated.The Cu-rich core regions are still present after etching (see the EDX/EDS spectraof Figure 4b.2). This localised dissolution and attack of the intermetallic particle,although not resulting in their removal, may facilitate it during subsequent anodisingsteps (Section 2.3.2). A second major benefit of pretreatment is that the outer surfaceof the material becomes more uniform, which is especially visible after anodisation.The SEM images of Figure 4b.4 clearly illustrate this point.

Note that, as expected, Method 2 (Figure 4b.3) results in stronger chemicaldisintegration of the intermetallic particles than Method 1 (Figure 4b.2).

The shape, size, distribution, and alloy surface area fraction of the microscaleS and θ phase particles observed in pretreated Al 2024 are very similar to those ofnon-pretreated Al 2024. For second phase particles in pretreated Al 2024, the shaperemain the same, but the size, distribution, and surface area fraction decrease byabout 10% compared to non-pretreated Al 2024.




3.2 Pretreated and anodised Al 2024

Individual microscale particles, as well as clusters of them, in non-pretreatedAl 2024 have already been discussed in detail in Subsection 2b of this book. Asshown in Section 3.1 above, mainly the second phase intermetallics are affected bythe pretreatment; they show chemical attack but their removal is dependent on theseverity of the etching conditions used. If the sample is anodised after pretreatment,there is evidence that the microscale particles which were originally at the outersurface of the material are dissolving in the acid anodising solutions leaving largeholes as defects in the layer formed by anodisation (called the anodic layer). Pre-sumably the particles are first incorporated into the oxide layer that grows aroundthem, but because of contact with the acid anodising solution for a long period oftime, they slowly dissolve and/or fall out of the anodic film due to a lack of chemi-cal bonding at the particle/oxide interface. This phenomenon is illustrated for bothanodising procedures in Figure 4b.5. Note, however, that the higher magnifica-tion SEM images (Figures 4b.5a–4b.5c) of these “fingerprint” defect zones showthat oxide has formed at the bottom of these holes (the oxide formation continuedbeneath the incorporated particle), shielding the metal from the environment and,thus, protecting it against corrosion.

1 µm1 µm

100 nm 100 nm

1 µm10,000x

50,000x 50,000x

20,000x

(a) (b) (c)

(d) (e)

20,000x

Figure 4b.5: SEM images: (a) microscale particle clusters (second, S, and θ phase)which were originally at the Al 2024 outer surface and then removedduring anodisation; (b) PCA and (c) PTS leaving “fingerprint” holesin the anodic layers; (d) CA anodised Al 2024; and (e) TS anodisedAl 2024 where a particle dissolved or fell out, but oxide formed at thebottom of the remaining holes.




The microscale particles that are below the original outerAl 2024 surface beforethe onset of anodisation will encounter the growing anodic layer when the regressingmetal/oxide interface reaches their location in the Al 2024 bulk. At this moment,these particles become encapsulated by the oxide film and, depending upon theamount of access the acid has to them, either remain fairly intact in the layer ordissolve partially or completely in the acidic electrolyte leaving holes in the oxide.This result is illustrated in Figures 4b.5d–4b.5e. During the oxide formation process,a cluster of particles may be encountered below the surface (as seen in Figure 4b.6),causing the oxide layer to be highly deformed and damaged at these positions. Suchlayer defects are not necessarily visible at the outer surface of the anodic film. Also,note that the oxide layer pore direction sometimes changes around these particles,with open pore mouths becoming visible in a cross-sectional view of the layer, thusindicating the loss of a regular and parallel shaped porous structure.

Thus, it can be concluded that anodising with both the chromic acid andtartaric/sulphuric acid procedures shields or removes particles from the outer sur-face of the Al 2024 and covers the surface completely with oxide. However, theoxide layer does not have a regular anodic film structure at places where microscaleparticles are encapsulated or removed. The film shows large holes (in the bulk orat the outer surface where particles dropped out or dissolved) and has regions withdistorted pore directions.

10,000x 8000x

(a) (b)

(c) (d)

15,000x20,000x

Figure 4b.6: BSE-mode SEM images of an anodised Al 2024 (PCA) cross-sectionshowing particles and holes (formed due to particle dissolution) in theanodic layer.




Table 4b.1: Density of defects per unit area (D)on anodised surfaces of Al 2024.

Anodised Al 2024 D (1/mm2)

TS 8700

PTS 10,000

CA 9565

PCA 8043

9.00kX

15.0kX 22.0kX

15.0kX

2% Mn, 4% Fe,1% Cu

(a) (b)

(c) (d)

Second phase (3% Mn10% Fe,10% Cu)

Figure 4b.7: SEM images of cross-sections of CA (a and b) and PCA (c andd) anodised Al 2024 samples where pores, cavities, and cracks arevisible.

A quantitative analysis of defects appearing in the oxide layer on pretreated plusanodised (PCA and PTS) and non-pretreated plus anodised (CA and TS) Al 2024samples was done. The number of observed defects on the surface with diametersgreater than 0.5 µm was counted in relation to surface area. The defect area density(number of defects per unit surface area), D, was calculated from the observationof 4600 µm2 of the pretreated Al 2024 surface. Results are shown in Table 4b.1. Inthe case of TS and PTS, the pretreated Al 2024 has more surface defects than thenon-pretreated one. In case of CA and PCA, the pretreated Al 2024 has less surfacedefects than the non-pretreated one. For all samples, the defect area density falls inthe range of 8000/mm2 to 10,000/mm2.

To better understand the defect morphology, a cross-section of each samplewas prepared and observed by SEM. Defects in the anodic layer of CA and PCAanodised Al 2024 are shown in Figure 4b.7.




(a) (b)

(c)

x7.00k

x8.00k x22.0k

5.00µm

5.00µm 2.00µm

5.00µmx9.00k

(d)

Figure 4b.8: SEM images of cross-sections of TS (a and b) and PTS (c andd) anodised Al 2024 where pores, cavities, cracks, and embeddedintermetallic particles are visible.

SEM images of CA anodised Al 2024 reveal that remnants of second phaseparticles can remain in the layer. No S or θ phase particles were observed though.The particle remnant is depleted in Cu and always surrounded by a cavity. Othercases of second phase particle remnants have shown that the particle’s surface isalso covered with a porous oxide layer, but the pores have a different size than theoxide layer formed within the matrix.

Defects observed for PTS anodised Al 2024 were mainly cavities and Cu-richremnants of second phase particles (Figure 4b.8). In the PT case (no anodisation),second phase remnants (which were not Cu rich), cavities, and cracks were alsopresent. Statistical analysis was done to determine the frequency of defects alongthe cross-section length. The results are shown in Table 4b.2.

For CA anodised Al 2024, the most common defects are cracks, while for TSanodised Al 2024 they are cavities. PCA anodised Al 2024 has slightly more cracks(6.85/mm) than CA anodised Al 2024 (6.15/mm). The same trend was seen forTS and PTS anodised Al 2024, where the PTS case showed cracks but none wereobserved for the TS case. The pretreatment has a strong influence on the number ofcavities in the layer–substrate interface for pretreated and anodised Al 2024. For thePCA case, the number of cavities is almost double that of the CA case (4.23/mm vs2.46/mm). For the PTS case, there are also cavities at the interlayer, but for theTS case, none were found. The large difference can be related to the influenceof pretreatment on the particles which results in Cu-rich particle remnants. The




Table 4b.2: Number of defects per mm, NL, along the analysed cross-section length. Layer refers to the anodic layer, substrateto the remaining Al 2024, and external layer to an anodiclayer at the sample surface.

NL (1/mm) PTS TS PCA CA

Cracks 0.40 0.00 6.85 6.15

Cavities at layer–substrate interface 5.73 0.00 4.23 2.46

Cavities in layer 10.60 12.27 2.85 4.00

Cavities in external layer 3.33 3.55 2.31 3.46

Particle remnants in layer 0.07 2.27 0.69 0.00

100%

80%

17%

53%

28%

68%

20%

13%4%

14%

17%

25%

40% 38%

15%

25%

22%Particles built into the layer

Cavities in the external layer

Cavities in the layer

Cracks

Cavities at the layer–substrateinterface

2%

60%

40%

20%

0%PTS TS PCA CA

Figure 4b.9: Distribution of defects for each of the Al 2024 anodisation processes.Layer refers to the anodic layer, substrate to the remaining Al 2024,and external layer to an anodic layer at the sample surface.

Cu-rich remnants can induce oxygen (O2) evolution during the anodising process,which can induce cavity formation. This conclusion is also supported by the factthat the PTS and PCA cases have fewer cavities inside the layer. For the TS andPTS cases, the number of cavities in the external layer is approximately the same(3.55/mm and 3.33/mm). However, there is a large difference between the TS andPTS cases concerning particle remnants embedded in the layer. The TS case has2.27 remnants/mm, while the PTS case has only 0.07/mm. However, the CA andPCA cases are similar with respect to particle remnants (0.69/mm for PCA and nonefor TS).

The distribution of defect types versus anodisation process is shown inFigure 4b.9. As clearly seen, the most common defect type for the TS case iscavities at the bottom of the surface, whereas for the CA case it is mainly cracks.




3.3 Effect of pretreatment and anodisation on corrosionbehaviour of Al 2024

The corrosion behaviour of pretreated and anodised Al 2024 has been studiedvia potentiodynamic and potentiostatic macropolarisation curves and microscopy.Results for pretreated (but not anodised) Al 2024 clearly show evidence that pittingis possible (Figures 4b.10a and 4b.10b). Some representative macropolarisationcurves for pretreated and anodised Al 2024 are shown in Figure 4b.10c. The non-pretreated and pretreated Al 2024 show a small shift in the curve (small differencein OCP, although this is not very reproducible) but essentially the same shape.Conversely, anodised Al 2024 block the anodic current and no polarisation effectsare observed with post-mortem SEM analysis (no evidence of crystallographic pit-ting). There is no clear influence of the pretreatment (before anodising) on thepolarisation behaviour.

The reproducibility of the polarisation behaviour is poor (refer to Figure 4b.10d).Sometimes anodised samples do show a small anodic current increase, which ispresumably due to the anodic (oxide) film breaking down at weak spots; however,this breakdown must be very localised because no evidence of it can be foundwith SEM.

3.4 Inhibitors

Microscale and macroscale polarisation curve measurements were made on Al2024 and a ternary AlCuMg alloy having large S phases with the inhibitorNa2Cr2O7 or Ce(NO3)3 present in the NaCl solution at three different concentrations(Section 2.4.2). Micropolarisation curves measured over second and matrix phaseand macropolarisation curves measured over all phases were examined for trendsto see if they could be used as input data for modelling (refer to Subsection 3a ofthis book for details about modelling).

Examples of micropolarisation curves for Al 2024 second phase and matrix,as well as, an AlCuMg alloy S phase, are shown in Figure 4b.11. As has beenmentioned already in Subsection 2b of this book, there is a large variation for OCPand PP values between the measurements, which is commonly seen for microscaleelectrochemical measurements on Al alloys. In order to evaluate the ability ofNa2Cr2O7 and Ce(NO3)3 to inhibit corrosion, electrochemical current vales weremeasured at 200 mV above the OCP (OCP + 200 mV) for curves polarised in theanodic direction or 200 mV below the OCP (OCP − 200 mV) for curves polarisedin the cathodic direction.

Electrochemical current data were extracted from curves for second, S, andmatrix phases, such as those shown in Figure 4b.11, at the potential OCP + 200 mV(anodic polarisation direction) or OCP − 200 mV (cathodic polarisation direction).Upon examining the current data, it becomes clear there is no discernible trend withrespect to the concentration of the Ce inhibitor for any of the phases. However, a lin-ear trend is seen for theAl 2024 second phase and Cr inhibitor (refer to Figure 4b.12).




0 NT/PT (c)

(d)

NT bPT aCA eTS bPCA cPTS d1

TS/PTS

CA/PCA

0

Potential (V) vs SCE

log 1

0(cu

rren

t)

0.5 1.0 1.5

0


log 1

0(cu

rren

t)

0.5 1.0 1.5

PTS aPTS a1PTS bPTS b1PTS cPTS c1PTS dPTS d1PTS e

(a)

(b)

Figure 4b.10: (a) SEM image of pretreated Al 2024 after a macropolarisationcurve measurement showing evidence of crystallographic pitting,(b) zoom-in of the area marked in (a), (c) macropolarisation curvesfor pretreated and anodised Al 2024 (NT, PT, CA, PCA, TS, andPTS), and (d) overlay of macropolarisation curves for PTS anodisedAl 2024 showing the extent of reproducibility.




Second phase

abs(

I) (

A/m

2 )

E vs SCE (mV)

(a)

Matrix anodic

Matrix cathodic

abs(

I) (

A/m

2 )

E vs SCE (mV)

(b)

abs(

I) (

A/m

2 )

E vs SCE (mV)

(c)

S phase

abs(

I) (

A/m

2 )

E vs SCE (mV)

(d)

0.1 mM Cr

0.01 mM Cr

0.001 mM Cr

1 mM Ce

0.1 mM Ce

0.01 mM Ce

No Cr, Ce

Figure 4b.11: Micropolarisation curves for Al 2024 phases with the inhibitorNa2Cr2O7 (Cr) or Ce(NO3)3 (Ce) present in the NaCl solution atthree different concentrations (Section 2.4.2 above): (a) second phaseparticles polarised in the cathodic direction, (b) matrix polarised inthe anodic direction, (c) matrix polarised in the cathodic direction,and (d) S phase particles polarised in the anodic direction.


Aircraft CH004b.tex 21/11/2012 12: 18 2


Second Phase I cathodic

0.01 mM Ce

0 mM Cr or Ce (a)

(b)

1 mM Ce

0.1 mM Ce

0.001 mM Cr

0.01 mM Cr

0.1 mM Cr

Electrochemical current value (nA)

Cr

or C

e co

ncen

trat

ion

(mM

)

0.0

0 mM Cr or Ce

1 mM Ce (off-scale)

0.01 mM Ce (off-scale)

0.1 mM Ce

0.001 mM Cr

0.01 mM Cr

0.1 mM Cr

Electrochemical current value (nA)

Cr

or C

e co

ncen

trat

ion

(mM

)

0.00

Figure 4b.12: (a) Electrochemical current data (at OCP − 200 mV) extracted frommicropolarisation curves (cathodic direction) for Al 2024 secondphase exposed to three different Cr or Ce inhibitor concentrationsand (b) data of (a) on a smaller scale.

Macropolarisation curve measurements, both potentiostatic and potentiody-namic, for Al 2024 are shown in Figure 4b.13. These results show that the Ceinhibitor performs less efficiently than the Cr inhibitor, as the currents measuredfor the Cr inhibitor are much smaller than those measured for the Ce inhibitor inboth set of curves.

Results show that Na2Cr2O7 is a better inhibitor than Ce(NO3)3 based uponthe polarisation curve data. A previous study found that the chemical kinetics forcerium nitrate are much slower than those for sodium dichromate [9]. In fact, somedays are required before Ce(NO3)3 is able to form a stable conversion coating onAl 2024. However, another cerium-based inhibitor, which is not readily available,has been shown to have significantly better corrosion inhibiting properties [10] anddeserves further consideration.




150 450 750

Time (s)

(a)

0.1 mM Ce #1

0.1 mM Ce #2

1 mM Ce #1

1 mM Ce #4

0.1 mM Cr #1

0.1 mM Cr #5

No Cr, Ce #1

No Cr, Ce #3

Log

(I)

1050 1350 1650


(b)

0.1 mM Ce #1

0.1 mM Ce #2

1 mM Ce #1

1 mM Ce #2

0.1 mM Cr #1

0.1 mM Cr #3

No Cr, Ce #1

No Cr, Ce #2

Log

(I)

Figure 4b.13: (a) Cathodic potentiostatic macropolarisation curves of Al 2024 for1500 s at −750 mV; comparison of curves measured for 0.5 M NaClwithout inhibitor, with 1 mM and 0.1 mM Ce inhibitor, and with0.1 mM Cr inhibitor. (b) Cathodic potentiodynamic macropolarisa-tion curve of Al 2024 from −850 mV to OCP with a scan rate of0.1 mV/s; comparison of curves measured for 0.5 M NaCl withoutinhibitor, with 1 mM and 0.1 mM Ce inhibitor, and with 0.1 mM Cr.inhibitor.

Microscale mass transport modelling of corrosion inhibitors using MITReMexploits microscale and macroscale polarisation curve input data, which can beused to determine the reduction in anodic and cathodic electrochemical kineticsdue to the formation of a conversion coating on the Al 2024 surface. However, for




the data above, a trend was only seen for Al 2024 second phase exposed to sodiumdichromate; thus modelling the corrosion inhibiting process with the input dataavailable was not possible. With appropriate and sufficient input data, essentiallythe inhibiting effect of any corrosion inhibitor can be modelled.

4 Conclusions

The effect of pretreatment and anodisation on the microstructure and corrosionbehaviour of Al 2024 has been studied. Three different pretreatment and twodifferent anodisation (chromic and tartaric/sulphuric acid) methods were used. Pre-treatment attacks mainly the second phase particles in Al 2024, while anodisationtends to remove all particles, although Cu-rich remnants of intermetallic particlescan be found in the anodically grown oxide layer. The defects present in oxidelayers grown on Al 2024 by both chromic acid and tartaric/sulphuric acid anodisa-tion were determined to be particle remnants and cavities. Polarisation curve datashowed that both the non-pretreated and pretreated Al 2024 were susceptible tolocalised corrosion, while anodised Al 2024 was not.

Based upon the results shown, cerium nitrate (Ce(NO3)3) may not make asuitable inhibitor to replace sodium dichromate (Na2Cr2O7) or other chromate-based inhibitors. However, there are other cerium-based inhibitors, such as ceriumdibutylphosphate (Ce(dbp)3), which have shown more promising results in otherstudies and deserve further consideration [10].

Microscale mass transport modelling of corrosion inhibitors using MITReMexploits microscale and macroscale polarisation curve input data which can beused to determine the reduction in electrochemical kinetics due to formation of aconversion coating. However, electrochemical current data extracted from micropo-larisation curves measured on second, S, or matrix phase with an inhibitor present insolution show only a trend forAl 2024 second phase exposed to sodium dichromate.These results make modelling the effect of inhibitors quite difficult. However, nor-mally with appropriate and sufficient input data, the inhibiting effect of a corrosioninhibitor can be modelled.

Acknowledgements

This work was funded by the EU Commission 6th Framework Programme forthe project called “SICOM” (AST5-CT-2006-030804). We would like to thankH. Lohner, Airbus Deustchland, Bremen, for supplying the AA2024-T351 alloy.

References

[1] Campestrini, P., Terryn, H., Hovestad, A., & de Wit, J.H.W., Formation of a cerium-based conversion coating on AA2024: relationship with the microstructure. Surface &Coatings Technology, 176(3), pp. 365–381, 2004.

[2] Campestrini, P., Microstructure-related Quality of Conversion Coatings on AluminiumAlloys, PhD thesis, TU-Delft, DUP Science, ISBN 90-407-2342-7, 2002.




[3] Andreatta, F., Paussa, L., & Aldighieri, P., Water-based ZrO2 pretreatment for AA2024aluminum alloy. Surface and Interface Analysis, 42(4), pp. 293–298, 2010.

[4] Raps, D., Hack, T., & Wehr, J., Electrochemical study of inhibitor-containing organic–inorganic hybrid coatings onAA2024. Corrosion Science, 51(5), pp. 1012–1021, 2009.

[5] de Miera, M.S., Curioni, M., Skeldon, P., & Thompson, G.E., Preferentialanodic oxidation of second phase constituents during anodising of AA2024-T3 andAA7075-T6 alloys. Surface and Interface Analysis, 42(4), pp. 241–246, 2010.

[6] Curioni, M., Saenz de Miera, M., Skeldon, P., Thompson, G.E., & Ferguson,J., Macroscopic and local filming behavior of AA2024 T3 aluminum alloy duringanodizing in sulfuric acid electrolyte. Journal of the Electrochemical Society, 155(8),pp. C387–C395, 2008.

[7] Boisier, G., Pebere, N., & Druez, C., FESEM and EIS study of sealed AA2024T3 anodized in sulfuric acid electrolytes: influence of tartaric acid. Journal of theElectrochemical Society, 155(11), pp. C521–C529, 2008.

[8] Marzocchi, V., Iglesias-Rubianes, L., & Thompson, G.E., The influence of tartaric acidadditions on the anodizing behaviour of AA2024-T3 alloy in sulphuric acid. CorrosionReviews, 2(3–4), pp. 461–473, 2007.

[9] Buchheit, R.G., Mamidipally, S.B., Schmutz, P., & Guan, H., Active corrosion pro-tection in Ce-modified hydrotalcite conversion coatings. Corrosion, 58(1), pp. 3–14,2002.

[10] García, S.J., Muster, T.H., Özkanat, Ö., Sherman, N., Hughes, A.E., Terryn, H.,de Wit, J.H.W., & Mol, J.M.C., The influence of pH on corrosion inhibitor selectionfor 2024-T3 aluminium alloy assessed by high-throughput multielectrode andpotentiodynamic testing. Electrochimica Acta, 55, pp. 2457–2465, 2010.


Effect of SurfaceTreatment,Anodisation, and Inhibitors on ... · Aircraft CH004b.tex 21/11/2012 12:...

Documents

Transcript of Effect of SurfaceTreatment,Anodisation, and Inhibitors on ... · Aircraft CH004b.tex 21/11/2012 12:...