A Study of Trivalent Chromium Conversion Coatings on Aluminum Alloy 2024-T3
Date Issued
2015
Date
2015
Author(s)
Yi, Han-Bang
Abstract
Aluminum alloy 2024-T3 has widely been used in the aircraft industry. To avoid an air crash caused by corrosion of the alloy, conversion coatings have commonly conducted on aluminum alloy 2024-T3. Hexavalent chromium conversion coatings provide excellent anti-corrosion properties; however, hexavalent chromium is a toxic element and harmful to species and environment. International directives have strictly banned the use of hexavalent chromium. Therefore, many efforts have been made to find the process alternative to hexavalent chromium conversion process. Trivalent chromium conversion coating is a potential alternative process to hexavalent chromium conversion coating. However, the oxidizing capability of trivalent chromium is inferior to hexavalent chromium. Moreover, trivalent chromium conversion coating does not have self-healing capability. Modifications of the processing procedure are thus essential to improve the anti-corrosion properties of trivalent chromium conversion coating. The trivalent chromium conversion solution we now use in Taiwan is imported from foreign manufacturers and the modifications we can make are limited by aerospace specifications. Under that premise, this study is to understand the effect of two different acidic pretreatments, nitric acid and Deoxidizer 6, on the microstructure and properties of the subsequent trivalent chromium conversion coating on aluminum alloy 2024-T3. The results show that the coating treated with nitric acid pretreatment performs better in potentiodynamic polarization test and are able to pass 168-hr of the salt spray test. Pits attacked by F ions become larger and then coalescence with each other, resulting in a rough surface after pretreatment in Deoxidizer 6. The uneven surface may also influence the uniformity of the trivalent chromium coating. However, there are only a few pits on the surface pretreated in nitric acid and the surface seems smoother than that pretreated in Deoxidizer 6. Cross-sectional characterization reveals the presence of trivalent chromium conversion coating on aluminum alloy 2024-T3 after immersion in Metalast TCP-HF regardless of the acidic pretreatments. The trivalent chromium conversion coating on aluminum alloy 2024-T3 pretreated in nitric acid is denser and thicker compared to that on aluminum alloy 2024-T3 pretreated in Deoxidizer 6. A two-layered structure is clearly observed, where Cr(III) and Zr content in the outer part of the coating is higher than that in the inner part. On the contrary, the inner part of the coating is rich in Al. The trivalent chromium conversion coating on aluminum alloy 2024-T3 is composed of oxide and hydroxide of Cr(III), Zr, and Al, as revealed by XPS. Due to the presence of chromate ions in Deoxidizer 6, there is already a thin film containing Cr(III) on the surface after the acidic pretreatment. This thin film serving as a passivation layer deteriorates the reactivity during the following TCP. With respect to the uniformity of the coating and reactivity of conversion reaction, the pretreatment in nitric acid is more suitable for the trivalent chromium conversion process using the Metalast TCP-HF solution.
Subjects
Aluminum alloy 2024-T3
trivalent chromium conversion coating
acidic pretreatment
corrosion resistance
Type
thesis
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