An Optimized Trivalent Chromium Conversion Coating Process for AA2024-T351 Alloy

In this work, a trivalent chromium conversion coating applied on AA2024-T351 alloy has been optimized for corrosion protection in sodium chloride solutions. Scanning electron microscopy, energy dispersive X-ray spectroscopy and electrochemical measurements were employed to characterize the coating. An immersion post-treatment of the coated alloy in 40 ◦ C deionized water for 120 s considerablyenhancedthecorrosionprotectionproperties,comparedwithaposttreatmentat20 ◦ C,ortheabsenceofapost-treatment. Electrochemical noise measurements, combined with real time imaging, and potentiodynamic polarization experiments indicated a conversion treatment for 300 or 600 s provides optimal corrosion protection. Relatively long conversion treatments decreased the corrosion protection due to an increase in coating defects and cracks, especially around the second-phase particles. ©

Trivalent chromium conversion (TCC) coating processes are promising replacements for chromate conversion coating processes, which are highly regulated because of the toxicity and carcinogenic risks associated with chromate species. 1-3 TCC coating formation is a pH-driven process and the coating formed on AA2024 alloy has a twolayer coating structure, with an outer Zr-/Cr-containing oxide layer over a fluoroaluminate interfacial layer. [4][5][6] With respect to the electrochemical behavior, Li et al. 7 used electrochemical impedance spectroscopy (EIS) to reveal substrate-dependent corrosion resistances of TCC coatings formed for 600 s in 0.5 M Na 2 SO 4 solutions. The difference of charge-transfer resistances between bare and coated specimens decreased in the following order: AA6061 > AA7075 > AA2024 alloys. In comparison with AA6061, copper-rich particles present in AA2024 alloys encourage the widespread formation of coating cracks, leading to a greater coating heterogeneity. 6 As a consequence, a lower corrosion resistance and a higher porosity of the TCC coatings were revealed on AA2024 alloys. 7 Therefore, surface pre-treatments of AA2024 alloys that remove the copper-rich particles are of great importance to improve the surface microstructure, coating formation and corrosion resistance. [8][9][10] A pre-treatment of alkaline etching and nitric acid desmutting (HNO 3desmuttering method) is generally used to clean surface contaminations. However, it may not be fully effective in removing surface particles, as evidenced by the protruding particles which may remain after this pre-treatment. In contrast to the use of HNO 3 , an acid deoxidizer with addition of oxidants can significantly enhance deoxidation effect. 8,9 In this regard, an Fe(III)-desmutting method (Oxidite D-30) has been reported to be more effective in removing etching products and surface particles. 9,10 The improvement is attributed to the presence of Fe(III), which acts as an oxidizing agent that promotes the dissolution of nobler copper-containing precipitates. 8 The resultant TCC coating provided a significant improvement in the corrosion protection properties in 0.05 M NaCl solutions. 10 Thus, the present work applied this Fe(III)-desmutting method to AA2024-T351 alloys in an investigation of the TCC coating process, with consideration of the influence of the conversion treatment time and post-treatment procedures. The electrochemical behavior of TCC coated alloy was investigated using EIS, potentiodynamic polarization (PDP) and electrochemical noise analysis (ENA). Furthermore, high-resolution scanning electron microscopy (SEM), associated with energy dispersive X-ray spectroscopy (EDS), was used to understand the effects of the Figure 1. EC models used for fitting EIS data of the coated specimens, including electrolyte resistance (R e ), coating resistance (R coat ), Warburg resistance (W s ) and coating capacitance (C coat ), charge-transfer resistance (R p ) and capacitance of the coating/substrate interface (C dl ). EC abbreviation is (a) R(Q(R(QR))) and (b) R(Q(R(Q(WR)))) respectively.
(OCP) for 30 min to reach a steady value, (ii) EIS scan from 10 5 to 10 −2 Hz by applying a sinusoidal potential waveform with an amplitude of 10 mV about the OCP, (iii) OCP measurement for 30 min to stabilize the coated surface and (iv) PDP scan at 1 mV/s from −1.0 V SCE to 1.0 V SCE . The reproducibility of the results was confirmed by triplicated measurements under the same conditions. In order to quantify EIS data, they were fitted using equivalent circuit (EC) models by Zview software (version 3.1, Scribner Associates, Inc.). Figures 1a and 1b display EC models represented for TCC coatings without and with diffusion of species at the coating/substrate interface respectively. [14][15][16] The diffusion process in the latter EC model is due to the presence of coating cracks located at the boundary of coated intermetallic particles. The EC models included the following components: electrolyte resistance (R e ), coating resistance (R coat ), Warburg resistance (W s ), coating capacitance (C coat ), charge-transfer resistance (R p ) and capacitance of the coating/substrate interface (C dl ).
The real time corrosion performance of the coated specimens was investigated by image-assisted electrochemical noise analysis (ENA)   in naturally-aerated 0.5 M NaCl solution. For ENA measurements, two identical specimens with exposed areas of ∼2.25 cm 2 , coupled by a 10 kohm resistor, were used, as described in detail in Ref. 17. A NI-SUB6009 (National Instrument) analogue-to-digital converter was employed to record the potential of the specimens relative to a SCE. The potential signals, with 1023 Hz segments of 1000 points at each iteration, were recorded by an in-house developed software, and averages of the 1000 values were used as a single value of potential acquired between the iterations. The final dataset of detected potential values was spaced by 1 ± 0.05 s in time. Assuming that negligible noise was present above 1023 Hz and below 0.5 Hz, ENA experiments between 0.5 and 1023 Hz can provide accurate potential signals at the frequencies of interest. In-house LabVIEW software was used to determine the time evolution of the low frequency noise resistance (NR), according to the mathematic procedure described by Curioni et al. 17,18 In addition, real time imaging of the corroding surfaces during immersion in the corrosive electrolyte was carried out every 10 minutes using Maplin USB microscopes with an in-house developed software controller.

Results and Discussion
Effect of the temperature of post-treatment in the deionized water bath.- Figure 2 shows the influence of the DI water post-treatment Table I. Parameters of TCC coatings/alloy systems obtained from fitting EIS data using a R(Q(R(QR))) equivalent circuit for the coated alloy after conversion treatment for 300 s. The posttreatment of freshly-formed specimens included that without water immersion (no water), with cold water immersion (20 • C) and with warm water immersion (40 • C). The chi-squared errors were all less than 5 × 10 −3 .

Specimens
No water on (a) the impedance modulus-frequency and (b) the phase anglefrequency plots of the alloy, after conversion treatment for 300 s, during the immersion in naturally-aerated 0.05 M NaCl solution. The freshly-formed specimens were post-treated with or without water immersion at 20 or 40 • C, and all were dried in a cool-air stream and aged overnight in the laboratory air. The plots of the coatings with a standard post-treatment were presented in a previous paper in Ref. 10. The steady OCPs of all specimens prior to the EIS measurements were around −0.52 V SCE . In Fig. 2, two time constants were evident at the frequencies of 10 0 and 10 2 Hz; the phase angles were less than 90 • . Constant phase elements (CPEs) were used in the EC modelling to represent the capacitance property on the basis of the depressed semicircles of Nyquist plots (not shown here); CPEs have been used to model similar coating systems in Ref. 7,19. Post-coating treatment of immersion in DI water at 40 • C resulted in the largest phase angle of ∼75 • at 1 Hz. This is suggested to be due to the stabilized coating composition and improved structure after water immersion post-treatment at 40 • C, which is indicated by the findings from the fittings of EIS data presented in Table I. The coating subjected to the standard post-treatment revealed the best corrosion protection property; the charge-transfer resistance (R p ) increased in order: specimens without water immersion (∼0.10 M cm 2 ) < specimens with water immersion at 20 • C (∼0.17 M cm 2 ) < specimens with water posttreatment at 40 • C (∼0.20 M cm 2 ). Furthermore, the 40 • C water treatment led to the maximum coating resistance (8230 cm 2 ) and a lower coating capacitance (Q coat ) in comparison with the first two cases. A higher temperature of DI water (40 • C) has been previously associated with a reduced amount of fluorine species in the coating and an increased amount of the oxides/hydroxides. 20 Figure 3 displays PDP curves of the bare and coated alloy, prepared similarly to the specimens used in Fig. 2, after immersion in 0.05 M NaCl. The curve for the standard post-treatment has been presented in a previous paper in Ref. 10. The cathodic current density was reduced in order by the application of the coatings: e.g. at −0.6 V SCE , 1.0 × 10 −6 A/cm 2 for the bare alloy > 1.9 × 10 −7 A/cm 2 for the coated alloy with on post-treatment > 1.4 × 10 −7 A/cm 2 for post-treatment at 20 • C > 8.4 × 10 −8 A/cm 2 for the standard post-treatment. The suppressed current density is associated with a barrier role of TCC coatings in inhibiting the oxygen reduction reaction (ORR) on the alloy substrate after oxygen diffusion through the porous coatings. 4 In terms of anodic branches, the curves were only slightly different for the bare and coated alloys: e.g. at −0.2 V SCE , the current densities for the coated alloys were around 3.3 × 10 −4 A/cm 2 , compared with ∼6 × 10 −4 A/cm 2 for the bare alloy. This suggested an insufficient barrier to penetration of the coating by chloride ions and that low concentrations of chromate species formed in the coatings offered negligible anodic inhibition. 10 The findings from the PDP measurements are consistent with results from previous polarization tests in 0.05 wt% NaCl + 0.35 wt% (NH 4 ) 2 SO 4 solutions 4 and 0.5 M Na 2 SO 4 and 0.5 M Na 2 SO 4 + 0.05 M NaCl solutions. 7

Effect of the time of conversion treatment on electrochemical
behavior.-In order to determine the preferred time of conversion treatment of the alloy prior to the immersion step in DI water, EIS measurements were carried out in 0.05 M NaCl solution on specimens treated for 600, 900, or 1800 s followed by the standard post-treatment. Figures 4a and 4b show the resulting impedance modulus-frequency and phase angle-frequency respectively. The steady OCP values for specimens coated for 600, 900 and 1800 s were −0.54, −0.55 and −0.7 V SCE , respectively. The more negative OCP of the specimen treated for 1800 s suggested a less protective coating, which will be discussed later. A similar OCP was recorded before the PDP scan for this coating condition, suggesting a stable solution/coating system during EIS and PDP measurements. TCC coatings formed for 1800 s led to the lowest impedance modulus at 10 −2 Hz (Fig. 4a). Correspondingly, the frequency-dependent phase angles displayed a single time constant (10 2 Hz, Fig. 4b). Furthermore, the frequency width of the phase angle peak of the specimens treated for 1800 s was the smallest, which also suggested reduced corrosion protection of the coating. 21 In contrast, a higher impedance modulus at 10 −2 Hz and the two time constants (10 0 and 10 2 Hz) resulted with the coatings formed for 600 and 900 s; the data were fitted using the EC model of Figure 1a. However, it was necessary to include a Warburg resistance in the EC to fit the impedance data (Fig. 1b) for the specimens treated for the longest time to account for the effect of a greater number of defects in the coating formed for 1800 s. The results of fitting the data are presented in Table II. R coat values increased in order: ∼418 cm 2 for a treatment of 1800 s < ∼2327 cm 2 for a treatment of 900 s < ∼14336 cm 2 for a treatment of 600 s. The lowest charge-transfer resistance, R p , was revealed in the specimen coated for 1800 s (∼6.3 × 10 3 cm 2 ), which was reduced by × 50 relative to the specimen coated for 600 s (∼3.3 × 10 5 cm 2 ). It is concluded that increasing conversion treatment times can lead to a deterioration of the corrosion protection, especially for the specimen coated for 1800 s with an enlarged Q dl and smaller exponent n dl . 22 In addition, the presence of Warburg resistance of ∼6328 cm 2 in the coatings formed for 1800 s is attributed to diffusion of oxygen and metal ions through the defects in the coating, such as the coating cracks around the particles. Figure 5 shows PDP scans from −1.0 to +1.0 V SCE in 0.05 M NaCl solutions of the specimens coated similarly to those used in Fig. 4. In agreement with findings from EIS, PDP curvers of coatings formed for 1800 s displayed a reduced corrosion protection, evident from an increased current density in the anodic and cathodic branches of the curve. The occurrence of a peak was due to oxidation of copper (∼−0.2 V SCE ). 7 The corrosion in the specimens coated for 1800 s are  possibly associated with accumulation of fluorine at the coating/substrate interface and local detachment of the coated intermetallic particles. Figures 6a and 6b show scanning electron micrographs of specimens after conversion treatment for 600 and 1800 s respectively, followed by the standard post-treatment. Unlike for the shorter treatment, more numerous cracks in the coatings were evident in the specimen coated for 1800. Figures 6c and 6d display SEM/EDS point analyses of TCC coatings formed for 300, 600 and 1800 s above the matrix and intermetallic particles respectively. The intensity of the zirconium peak can be used as an indicator of coating thickness. 6 Increased zirconium intensities on the matrix and particles were revealed with increasing conversion treatment times up to 1800 s. 6,23 The coating formed above the matrix for 300 and 600 s yielded similar zirconium intensities; the intensities were also similar above the particles, but increased relative to the matrix, due to the locally increased pH. The ratio of the zirconium intensity above the matrix in the coating formed for 1800 s relative to that formed for 600 s was ∼1.8; the corresponding ratio for the coatings above the particles was ∼1.1. Transport of fluorine species can proceed through the coating to the alloy substrate. 6,24 In previous work of the authors, time-dependent fluorine accumulation was revealed by ion beam analysis, and localized corrosion events occurred which appeared to be associated with enrichment of both fluorine and copper at the coating base after conversion treatment for 300 s. 6,25 In this regard, increasing the time of Table II. Parameters of TCC coatings/alloy systems obtained from fitting EIS data. An EC model of R(Q(R(QR))) was used for the specimens coated for 600 and 900 s, while the coatings formed for 1800 s were represented by an EC model of R(Q(R(Q(WR)))). All specimens were given after the standard post-treatment. The chi-squared errors were all less than 4 × 10 −3 . conversion treatment to 1800 s can encourage further fluorine enrichment at the coating/substrate interface, which can promote substrate dissolution around the cathodic particles.

Specimens
Image-assisted electrochemical noise analysis.-In order to assess the longer-term corrosion protection performance of TCC coatings in naturally-aerated 0.5 M NaCl solution, ENA measurements were undertaken, combined with in-situ imaging. Owing to the decreased corrosion resistances of specimens coated for 900 and 1800 s in the EIS and PDP tests, measurements were restricted to specimens treated for 300 and 600 s. The time evolutions of the low-frequency noise resistances (NRs) of the bare alloy and TCC coated alloy with the standard post-treatment are shown in Fig. 7a. Similar values of the NRs were found in the cases of the coatings formed for 300 and 600 s after exposure for 240 h, suggesting similar corrosion protection properties. The TCC coated specimens showed a × 6 increase in the NR relative to that of the bare alloy. Figure 7b displays the optical macrographs of the bare and coated alloys obtained during corrosion tests. On the bare alloy, corrosion products were generated after only 1 h (not shown here). Corrosion subsequently propagated to generate the dark pits after exposure for 60 h whereas no corrosion products were observed on specimens with treated for 300 and 600 s. However, bubbles formed on both coating surfaces after immersion for 10 min and their population increased to reach a stable level after 5 h. A greater number of bubbles was present on the coatings formed   for 600 s relative to that formed for 300 s, as shown in Fig. 7b. The bubbles would be attributed to hydrogen gas generated during corrosion events at coating defects, possibly indicated more localized corrosion. The greater number of bubble on the specimens treated for 600 s suggests a lower corrosion protection from the coating in the longer-term corrosion performance test. Figures 8a and 8b show scanning electron micrographs of the corroded surface of the coatings formed for 300 s, with a standard posttreatment, at the end of ENA measurements at 240 h. Low-resolution imaging in Fig. 8a shows the presence of crystalline NaCl deposits (as labelled by "1" and "2") and regions of partially-exposed substrates (as labelled by "3"), with compositions having been analyzed by SEM/EDS (not shown here). Figure 8b shows chloride-containing corrosion products accumulated around the regions of partially-exposed substrate of about 0.1 mm in diameter, which are about the size of hydrogen bubbles observed using the optical camera (Fig. 7b). With regard to the regions of exposed substrate, SEM/EDS point analyses shown in Fig. 8c revealed a large variation in the zirconium intensities between five examined spots (yellow stars, Fig. 8b). Some points indicated little or no coating remaining.
In summary, a standard immersion post-treatment in a DI water bath at 40 • C for 120 s can enhance corrosion protection properties of TCC coatings formed on Fe(III)-treated AA2024 alloys, consistent with the literature. 20 With regard to the effects of the conversion treatment time, increasing conversion treatment time up to 1800 s resulted in a reduction in the corrosion protection properties as evident from the results of EIS and PDP in 0.05 M NaCl. This phenomenon is attributed to the increased number of defects, e.g. cracks, in the coating, especially around the cathodic second phase particles. Simultaneously, fluorine accumulation in the coatings and copper enrichment at the coating base proceed with increasing TCC coating treatment duration. 6 As a consequence, localized corrosion events   Figure 8. (a,b) Scanning electron micrographs of TCC coatings formed for 300 s with the standard post-treatment after immersion in air-aerated 0.5 M NaCl solutions for 240 h, the NaCl deposits were labelled by "1" and "2" and the exposed substrate is labelled "3"; (c) SEM/EDS point analyses of zirconium on five spots (yellow stars, Fig. 8b) in the circular region of degraded coating.
occur at the alloy substrate, especially beneath the copper-rich particles, to cause detachments of coated particles, which is in agreement with the literature. 24,26 In contrast, EIS and PDP measurements concluded that TCC coatings formed for 600 s with the standard post-treatment provided the best corrosion protection after exposure in dilute 0.05 M NaCl solutions. Furthermore, ENA measurements in the 0.5 M NaCl suggested corrosion protection was maintained in the longer term. Although similar noise resistances were recorded for specimens treated for 300 and 600 s, a larger population of hydrogen bubbles was observed on the surface coated for 600 s, which were associated with the local corrosion events. In this regard, the preferred coating process consists of surface pre-treatment (NaOH etching and D-30 desmutting), conversion treatment for 300 s and the standard posttreatment in the DI water at 40 • C, cool-air drying and overnight air ageing.

Conclusions
1. An immersion post-treatment in a DI water bath at 40 • C for 120 s can considerably enhance the coating corrosion protection properties of TCC coatings on AA2024-T351 alloy compared with the absence of a post-treatment or post-treatment at 20 • C. 2. Coatings formed for 900 and 1800 s revealed worse corrosion protection properties than coatings formed for 300 and 600 s due to an increase in the number of defects, such as cracks, especially around intermetallic particles. 3. Electrochemical impedance and potentiodynamic polarization measurements reveal that TCC coatings formed for 300 or 600 s with a standard post-treatment offer the best corrosion protection properties compared to the coatings formed for 900 and 1800 s. 4. Real time imaging of the coated surfaces reveals the presence of hydrogen bubbles due to the local corrosion events that appeared to be more numerous for the latter coating.