Effects of aging on Chromate Conversion Coatings Formed on AA2024-T3 and Intermatallic Constituents of AA2024-T3.

 

An explanation for the CCC aging phenomenon, known to delay commercial coating production, was sought by a combination of XANES, SIRS, and EXAFS analyses. Chromate reduction was not a significant factor to CCC aging over a 24-hr period, but surface dehydration and structural change were correlated and causative. A reduction in surface Cr(VI)/ Cr(III) ratio was observed over a longer aging period (35 days).

The influence of the structure of CCC formed on AA2024-T3 and on IMC constituents of AA2024-T3 as a function of aging was studied. EXAFS, XANES and contact angle measurements were used to monitor the structural changes and surface wetness respectively of the CCC with time. It was observed that the different IMCs have little effect on the structural changes that occur in the CCC, though in all cases, a slight decrease in the Cr(VI) to Cr(III) ratio occurred during the first 24 hours following coating. EXAFS data in all cases indicated that the CCC is similar to the Cr mixed oxide compound proposed by McCreery et al. as shown on Figure 1 and that there is an increase in ordering in the CCC structure with time (Figure 2). In addition, contact angle measurements show an increase in hydrophobicity of CCC on intermetallic thin films within 24 hours of aging as observed by contact angle measurements. The CCC structure seems to have a similar behavior when formed on both individual and galvanic coupled IMCs.

 

References:
  1. "Hydrophobicity and Structural Stability During Aging of Chromate Conversion Coatings on Al Alloys", C. R. Clayton, G. P. Halada, M. J. Vasquez, J. R. Kearns, D. Chidambaram, and R.D. Granata, Passivity and Localized Corrosion - An International Symposium in Honor of Professor Norio Sato, M. Seo, B.MacDougall, H. Takahashi and R.G. Kelly (Eds.) Proceedings Volume 99-27, p.290, The Electrochemical Society, Pennington, NJ (1999).

  2. "Effects of Aging on Chromate Conversion Coatings Formed on AA2024-T3 and Some Intermetallic Constituents", M. J. Vasquez, G. P. Halada, C. R. Clayton and J. P. Longtin, in Corrosion and Corrosion Prevention of Low Density Metals and Alloys, B.A. Shaw and R.G. Buchheit, Eds, PV 2000-23, p101, The ECS Proceedings Series, Pennington, NJ (2001).

  3. "Studies on the Structure and Formation of Chromate Conversion Coatings on Thin Films of Copper-Containing Intermermetallic Constituents in AA2024-T3", M. J. Vasquez, G. P. Halada, C. R. Clayton", M. J. Vasquez, G. P. Halada and C. R. Clayton, in State-of-the-Art Application of Surface and Interface Analysis Methods to Environmental Material, Interactions: In Honor of James E. Castle's 65th Year, D.R. Baer, C.R. Clayton, G.P. Halada, and G.D. Davis, Eds, PV 2001-01, The ECS Proceedings Series, Pennington, NJ (2001). Accepted for publication.

  4. "The Application of Synchrotron-Based Spectroscopic Techniques to the Study of Chromate Conversion Coatings", M. Jaime Vasquez, G. P. Halada and C. R. Clayton, Electrochimica Acta (2002). (Accepted).

 



 

Materials Science Department June-2005