Study of Trivalent Chromium Conversion Coating Formation at Solution—Metal Interface

Abstract

The present study employed a tungsten pH microelectrode to study the formation of trivalent chromium conversion (TCC) coatings on Al, AA 2024-T3 aluminum alloy and AZ91D magnesium alloy in SurTec ChromitAL solutions. The tungsten microelectrode had a sensitivity of –60 mV/pH in the SurTec ChromitAL solutions. The peak and final pH values for pre-treated Al, AA 2024 alloy and AZ91D alloy were 4.9 and 3.5, 4.3 and 4.1, 4.7 and 3.5, respectively. The initial pH value is of great importance in influencing coating deposition-driven force and final coating thickness. In addition, the time to attain the peak values was 10, 100 and 20 s for Al, AA 2024 alloy and AZ91D alloy, respectively. The interfacial pH evolution is associated with the hydrolysis process of the weak acid nature of zirconium and chromium salts as the main components of TCC coating solutions. Raman spectra revealed the presence of chromium and zirconium oxides and sulphate in the solution deposits from in-house Cr/Zr and SurTec ChromitAL solutions.

Keywords

• trivalent chromium
• conversion coating
• interface
• tungsten microelectrode
• pH
• magnesium