Critical influence of chemical pretreatments on the deposition and effectiveness of Zr-based conversion coating on AA2024-T3 aluminum alloy

The critical role of chemical pretreatments on the deposition and effectiveness of Zr-based conversion coatings to protect high strength aluminum alloy is presented. Several pretreatments were tested, including mechanical grinding, alkaline etching, acid desmutting, and combinations therein. Zr-based conversion coatings were deposited on aluminum (AA2024-T3) from a solution of 0.01 M Zr ⁴⁺. The corrosion behavior of the pre-treated and coated alloys was examined using potentiodynamic polarization in sodium chloride solution. The results showed that acid desmutting in nitric acid can cause localized corrosion in the form of selective magnesium dealloying and alkaline trenching around the S-Al ₂CuMg particles that continued during deposition and can compromise the corrosion protection. Alkaline etching in a sodium hydroxide solution removed some of the larger intermetallic particles and increased the deposition rate but resulted in a cracked coating with inadequate adhesion. Desmutting in weak acid did not cause any trenching during the pretreatment and deposition. However, uneven and cracked deposition of the coating on intermetallic particles compromised the corrosion protection. These results show the benefits and limitations of common pretreatments which can lead to the development of improved pretreatment procedures and corrosion protection.