Trivalent Chromium Process Coatings Research Articles

Influence of trivalent chromium process (TCP) concentration on the anti-corrosion protection of 2024-T3 aluminium alloy

ABSTRACT In the present work, the surface of 2024-T3 aluminium alloys was coated with different concentrations (25, 35, and 50 wt.%) of the trivalent chromium process (TCP), and they were evaluated by comparing them to hexavalent chromium coatings (HCC). The findings show that TCP coating had a porous surface regardless of the concentration of TCP coating while the HCC exhibited a cracked pattern. The TCP coating increased the surface roughness as well as the wettability compared to HCC. The immersion test in 3.5 wt. % NaCl solution revealed that the open circuit potential (Eocp) of coated samples was more positive than that of the uncoated sample. However, the HCC coating was slightly more anodic after half of the immersion. The potentiodynamic test confirmed the formation of a passive region for all samples. Furthermore, the corrosion potential (Ecorr) and the critical pitting potential (Epit) were more positive for the lowest concentration coating. The polarisation resistance of the samples suggested that the HCC coating was superior to TCP coatings. In general, the low concentration of TCP coating improved the corrosion protection of 2024-T3 but not better than HCC coating due, probably, to the porous surface morphology.

Comparative analysis of the anticorrosive properties of trivalent chromium conversion coatings formed on 2024-T3 and 2024-T351 aluminium alloys

The anticorrosive properties of the trivalent chromium process (TCP) coatings were studied for a 2024 aluminium alloy (AA2024) in both T3 and T351 metallurgical states. Better corrosion resistance was measured for the TCP coated AA2024-T3 compared to AA2024-T351, which was clearly related to the surface copper coverage measured after the pre-treatments for the different samples. The differences were explained considering the reactivity of both the S-phase coarse intermetallics (IMCs) and intergranular Cu-rich precipitates during deoxidation. Large S-phase IMCs and numerous intergranular Cu-rich precipitates constituted critical metallurgical parameters for the anticorrosive properties of the TCP coatings.

Influence of post-treatment time of trivalent chromium protection coating on aluminium alloy 2024-T3 on improved corrosion resistance

Low corrosion protection performances of Trivalent Chromium Process (TCP) coatings with reference to Chromium Conversion Coatings (CCC) deposited on aluminium alloys can be overcome by application of post-treatment processes. This work shows the effect of post-treatment bath (containing hydrogen peroxide and lanthanum salt) on the chemical composition, structure and the corrosion performances of TCP coating deposited on AA 2024-T3 aluminium alloy. Different times of post-treatment bath were applied on the TCP coating and the samples were analyzed by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), X-ray Photoelectron Spectroscopy (XPS) and Glow Discharge-Optical Emission Spectrometry (GD-OES). The ToF-SIMS and GD-OES depth profiles show that the post-treatment time has no influence on the thickness of the TCP coating and on its bi-layered structure. It is composed of an outer layer, rich in zirconium and chromium oxides and an inner layer rich in aluminium oxides and oxy-fluorides. 3D ToF-SIMS images reveal an enrichment of Cr and Zr-like species over the surface of intermetallic particles with reference to alloy matrix, whereas a homogenous distribution of La is observed. The analysis of in-depth distribution shows that La is present principally in the outer part of the TCP coating. The La concentration increases with increasing post-treatment time. The effect of post-treatment time on protection was evaluated by Electrochemical Impedance Spectroscopy (EIS) and Linear Sweep Voltammetry (LSV) in a 0.001 M NaCl + 0.1 M Na2SO4 electrolyte. The increased post-treatment time enhances the cathodic inhibition against oxygen reduction. The post-treatment provides improved coating homogeneity and sealing properties.

Detection of H2O2 from the Reduction of Dissolved Oxygen on TCP-Coated AA2024-T3 : Impact on the Transient Formation of Cr(VI)

The detection of H2O2 formed in naturally-aerated electrolyte solution contacting uncoated and TCP-coated AA20243-T3 alloy surfaces is reported on. This molecule is produced from the reduction of dissolved oxygen and it has been implicated in the mechanism of transient formation of Cr(VI) in trivalent chromium process (TCP) conversion coatings. Linear sweep voltammetry and a spectrophotometric assay involving 2’,7’-dichloro-dihydrofluorescein diacetate (DCFH-DA) were used to detect H2O2 produced in 0.5 M Na2SO4. The activation of DCFH-DA to form the redox probe, DCFH, and its oxidation by H2O2 to the fluorescent dichlorofluorescein was used for detection. Immersion tests were performed in naturally-aerated 0.5 M Na2SO4 + 0.1 mM DCFH-DA for periods of time up to 96 h under open circuit conditions. The H2O2 concentration was greater in solution exposed to uncoated than to TCP-coated AA2024-T3 for equivalent times due to the inhibition of the oxygen reduction reaction kinetics by the conversion coating. With this direct evidence of H2O2 production, the following mechanism for transient formation of Cr(VI) in TCP coatings is confirmed: (i) dissolved oxygen is reduced to H2O2 on the alloy surface presumably at cathodic intermetallic sites, (ii) the H2O2 then diffuses to nearby coating sites to oxidize insoluble Cr(OH)3 to soluble Cr(VI) species (e.g., CrO42−) and (iii) the transiently formed Cr(VI) species diffuses to nearby corroding sites on the alloy where the inhibitor gets reduced back to passivating Cr(OH)3.

Evaluation of a Trivalent Chromium Process (TCP) Conversion Coating on AA2024-T3 That Requires No Surface Pretreatment

We report on the study of a new commercial trivalent chromium process (TCP) conversion coating, TCP-1, that requires no deoxidation or etching surface preparation of the aluminum alloy for optimum performance. The corrosion resistance provided by TCP-1 and a comparable conversion coating, TCP-2, both formed by immersion on AA2024-T3, were evaluated using scanning electron microscopy and energy dispersive X-ray analysis, digital microscopy, XPS depth profiling, electrochemical impedance spectroscopy and potentiodynamic polarization techniques. Accelerated degradation testing was also performed to determine how the laboratory electrochemical data correlate with the coating's corrosion protection during neutral salt-spray exposure and full-immersion testing. The results indicate that TCP-1 produces a protective coating on the metal that minimizes corrosion to an equivalent degree as TCP-2; a conversion coating that requires a conventional deoxidation step prior to application. Electrochemical data revealed that both TCP coatings increase the polarization resistance by ∼100x and suppress anodic and cathodic currents by ∼10–100x, as compared to the uncoated alloy. Specimens coated with either TCP coating exhibited little degradation, discoloration or corrosion during a 14-day neutral salt-spray (ASTM B117) exposure and a 14-day full immersion in 3.5 wt.% NaCl at 55°C. In summary, the stand-alone corrosion protection provided by TCP-1 to this alloy is as good as the protection provided by a standard TCP coating.

Influence of the microstructure on the corrosion behaviour of 2024 aluminium alloy coated with a trivalent chromium conversion layer

The influence of 2024 aluminium alloy microstructure on the protective properties of trivalent chromium process (TCP) coatings was studied: samples aged at 190 °C were analysed by comparison to a T3 state. The surface Cu coverage after deoxidation was higher for long-aged samples because nanometer scale Cu-rich precipitates formed during ageing behaved as distinct electrochemical entities and contributed to the formation of copper deposits. The growth mechanisms and kinetics of TCP coatings were directly related to the surface Cu coverage: thicker but more defective TCP coatings formed on long-aged samples leading to the lowest anti-corrosion properties.

Cross comparison of TCP conversion coating performance on aluminum alloys during neutral salt-spray and thin-layer mist accelerated degradation testing

The stand-alone corrosion resistance imparted to aluminum alloy 7075-T6 by different commercial trivalent chromium process (TCP) conversion coatings was evaluated after exposure to 7-day (i) neutral salt-spray (NSS) and (ii) thin-layer mist (TLM) tests. Comparison studies were performed with a non-chromium conversion coating (NCP). Quantitative assessment of the corrosion damage was made using scanning contact profilometry, weight loss and digital microscopy image analysis data. Analysis of the inorganic chemical composition in the coating baths was performed to correlate the Zr, Cr and F levels with the corrosion resistance. All the TCP and NCP conversion coatings provided an equivalent level of corrosion protection to this alloy during the 7-day NSS test. In contrast, greater differences in coating performance were observed after the 7-day TLM test. In terms of average pit depth, surface roughness, weight loss, pit area and pit density, one TCP coating outperformed the other TCP and NCP coatings. This coating bath had the highest Cr concentration (416 ppm) and the highest Zr/Cr ratio (2.1). The results demonstrate that differences in the stand-alone TCP coating performance can be observed but this depends on the nature of the accelerated degradation test used.

Influence of the alloy microstructure and surface state on the protective properties of trivalent chromium coatings grown on a 2024 aluminium alloy

The protective properties of trivalent chromium process (TCP) coatings grown on a 2024-T3 aluminium alloy were studied on the basis of electrochemical measurements performed both in sulphate and chloride solutions and neutral salt spray tests. The influence of the alloy microstructure and surface state was studied: two batches, each one characterized by its own coarse intermetallic particle distribution, and two surface states, i.e. laminated and polished, were considered. Results showed that in 0.1 M Na2SO4, the protective properties of the TCP coatings decreased when the roughness of the initial surface increased. Furthermore, improved protective properties were observed for a TCP coating grown on a surface containing a lower amount of Al-Cu-Mg IMCs in the initial microstructure. The most plausible explanation is that a fast kinetics of coating growth, either associated to strong initial roughness or a great surface copper coverage, was detrimental for the protective properties of the coatings. In more aggressive solutions, i.e. 0.5 M NaCl solution or for neutral salt spray tests, the differences are not significant. The findings are highly relevant for industrial applications: the results showed that variations in batches, for a same type of alloy, or in initial surface state should not be detrimental for the corrosion resistance of the TCP coated samples. However, the conversion process had to be adapted for different types of alloys, characterized by their own microstructure.

Electrochemical Characterization of Different Variants of a Commercial Trivalent Chromium Process (TCP) Coating on Aluminum Alloy 7075-T6

The electrochemical properties (open-circuit potentials, anodic and cathodic polarization curve currents, and polarization resistances) were evaluated for AA7075-T6 alloys coated with three variants of a commercial trivalent chromium process (TCP) pretreatment coating. The coatings were formed on degreased and deoxidized aluminum alloy specimens. Measurements were made in oxygenated 0.5 M Na2SO4 and 3.5% NaCl. Comparison of coatings formed by immersion and spray was undertaken. The three coating variants were 650 chromitAL, versions E, V, and C. Similar concentrations of Cr were in all three coating baths but there were differences in Zr, Zn, S (likely as sulfate), and Fe among the three. TCP coatings formed by immersion exhibited electrochemical properties similar to those formed by spray. Overall, the greatest level of corrosion protection was provided by 650 E based on electrochemical data and results from a 14 d thin-layer mist (3.5% NaCl, 55°C) accelerated degradation test. The coating provides both anodic and cathodic protection in low-chloride electrolytes and functions as more of a cathodic inhibitor in high-chloride electrolytes. Rotating disk voltammetric data revealed the coating inhibits the reduction of dissolved oxygen by providing a diffusional barrier and possibly blocking sites for O2 chemisorption on the cathodically-active intermetallic phases.

The Role of Chromium (III) in the Corrosion Inhibition of AA2024-T3 By Trivalent Chromium Process Coatings

High strength aluminum alloys (e.g., AA2024-T3) are widely used as aircraft structural components. The inclusion of two major alloying elements, Cu and Fe, make the metal more susceptible to localized damage in the form of pitting and intergranular corrosion. The susceptibility of the alloy to localized corrosion is due to the fact that galvanic cells form between the Cu- and Fe-rich intermetallic phases, sites that support relatively rapid oxygen reduction reaction kinetics, and the aluminum, which undergoes dissolution. The intermetallic phases are more noble than the surrounding aluminum matrix leading to the localized pitting or dissolution of the aluminum near the intermetallics. Inhibiting this localized corrosion is essential for achieving long performance lifetime of these structural components. Conversion pretreatment coatings are one component of a multilayer coating system that is typically used to protect aerospace aluminum alloys from corrosion. The multi-layer coating system consists of a conversion coating, a primer and an organic topcoat. The focus of current research in our group is on the application of alternate, more environmentally-friendly conversion coatings that serve as suitable replacements for the currently-used chromate conversion coatings (CCC). The corrosion inhibition provided by the multilayer coating system depends on the chemical composition, physical structure and thickness and uniformity of the conversion coating. The need for some level of corrosion inhibition by the conversion coating arises when there is coating breakthrough, in other words a scratch through the coating system that exposes some bare metal. Conversion coating thicknesses are generally in the 50-200 nm range and they provide corrosion inhibition, at least in part, by providing a barrier to oxygen and water/electrolyte accessibility to the metal surface. The trivalent chromium process (TCP) conversion coating is an environmentally-friendly potential replacement for the currently used chromate conversion coating. TCP is regarded as one of the leading drop-in replacements for CCC. Prior work has shown that the TCP coating inhibits corrosion on aluminum alloys, in part, by serving as a barrier layer. A key question with regard to the mechanism of corrosion inhibition is the role of Cr(III) or the transiently formed Cr(VI) that has been shown to form in the coating. We conducted studies using a trivalent chromium conversion coating from Henkel Corp., named Alodine® T5900™ , and a formulation of the same coating devoid of Cr(III). The formation, structure and corrosion inhibition of the coatings was investigated on AA2024-T3. The coating was formed by immersion on a polished, degreased and deoxidized specimen. These conversion coatings are about 100 nm thick when formed by immersion and consist of a biphasic structure with a hydrated zirconia (ZrO2·nH2O) top layer and a fluoroaluminate (e.g., KxAlF3+x) interfacial layer. Electrochemical methods were used to assess the degree of corrosion inhibition. Despite having the same physical structure, the conversion coating containing the Cr(III) exhibited a polarization resistance 10x greater than that for the coating devoid of it. Polarization curves revealed that the TCP coating provided cathodic inhibition. Rotating disk voltammetry was to assess the effect of the coating on the oxygen reduction reaction (ORR) kinetics. The results demonstrated that the TCP coating, as formed, inhibits the ORR. The reduced rate of oxygen reduction is attributed to (i) the formation of a poorly conducting layer on the Cu-rich intermetallics that blocks sites for O2 chemisorption and serves as a barrier to electron transfer and (ii) the formation of diffisusion barrier layer for O2 to the reaction sites.

Effects of Pretreatments on the Adhesion of Acetoacetate to AA2024-T3 Using the Blister Test

The blister test was used to assess the effect of various pre-treatments, including chromate conversion coatings, non-chromium process conversion coatings, and trivalent chromium process coatings, ...

Effects of Aging Temperature and Time On the Corrosion Protection Provided By Trivalent Chromium Process Coatings On AA2024-T3

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Pretreatment Effects On the Corrosion Protection Provided By Trivalent Chromium Process Coatings On AA2024-T3

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Formation and Structure of Trivalent Chromium Process Coatings on Aluminum Alloys 6061 and 7075

The formation mechanism and resulting structure of trivalent chromium process (TCP) conversion coatings on AA6061-T6 (UNS A96061) and AA7075-T6 (UNS A97075) were investigated. The formation of TCP on both alloys is driven by an increase in the interfacial pH caused by the dissolution of the passivating oxide layer, which leads to an elevated rate of proton-consuming cathodic reactions under open-circuit conditions. These reactions cause the interfacial pH to increase. This pH increase drives the hydrolysis of the fluorometalate precursors in the bath and results in the precipitation of a hydrated metal oxide coating on the surface. The coating appears to have a biphasic structure consisting of a hydrated zirconia (ZrO2·nH2O) and chromium hydroxide (Cr[OH]3) outer layer, and a fluoroaluminate (e.g., KxAlF3+x) interfacial layer. The coating thicknesses on both alloys are in the range of 80 nm to 100 nm. The TCP coating exhibited good stability on both alloys during full immersion testing in both naturally aerated sodium sulfate (Na2SO4) and sodium chloride (NaCl) electrolyte solutions. This was evidenced by no pits forming during immersion in chloride solution. The coating provided corrosion resistance to both alloys as polarization resistance (Rp) increased by about 100X in both Na2SO4 and the Na2SO4 + NaCl. Transient formation of Cr(VI) was detected in the coating on both alloys using Raman spectroscopy after immersion in air-saturated solutions.

Effects of Aging Temperature and Time on the Corrosion Protection Provided by Trivalent Chromium Process Coatings on AA2024-T3

The effects of aging temperature and time on the physical structure of and corrosion protection provided by trivalent chromium process (TCP) coatings on AA2024-T3 are reported. The TCP coating forms a partially blocking barrier layer on the alloy surface that consists of hydrated channels and or defects. It is through these channels and defects that ions and dissolved O2 can be transported to small areas of the underlying alloy. Reactions initiate at these sites, which can ultimately lead to undercutting of the coating and localized corrosion. We tested the hypothesis that collapsing the channels and or reducing the number of defects in the coating might be possible through post-deposition heat treatment, and that this would enhance the corrosion protection provided by the coating. This was tested by aging the TCP-coated AA2024 alloys in air overnight at room temperature (RT), 55, 100, or 150 °C. The TCP coating became dehydrated and thinner at the high temperatures (55 and 100 °C). This improved the corrosion protection as evidenced by a 2× increase in the charge transfer resistance. Aging at 150 °C caused excessive coating dehydration and shrinkage. This led to severe cracking and detachment of the coating from the surface. The TCP-coated AA2024 samples were also aged in air at RT from 1 to 7 days. There was no thinning of the coating, but the corrosion protection was enhanced with a longer aging period as evidenced by a 4× increase in the charge transfer resistance. The coating became more hydrophobic after aging at elevated temperature (up to 100 °C) and with aging time at RT as evidenced by an increased water contact angle from 7 to 100 °C.

Characterization of trivalent chromium process coating on AA2024-T3

Trivalent Chromium Process (TCP) coatings, which have been used recently as an alternative to chromate conversion coatings (CCCs), have been characterized. The TCP coating was found to be a dense layer consisting of particles, hundreds of nm in size on the coating surface, which is similar to CCCs. The TCP coating has a two layered structure, with zirconium-chromium mixed oxide in the outer layer and aluminum oxide or oxyfluoride at the metal/coating interface. No Cr(VI) was found, which supports its use as an environmentally friendly replacement for CCC. The TCP thickness was in the range of 40–120nm, considerably thicker than the zirconium based coating without chromium species. The high vacuum condition in the SEM dehydrates the coating quickly resulting in the formation of mud-crack artifacts. The TCP coating provides corrosion protection to the AA2024-T3 through suppressing the oxygen reduction reaction on aluminum alloy surfaces by acting as a protective barrier layer. Two pretreatments were investigated but found to result in little difference in the resulting TCP formed on AA2024-T3.