Influence of surface pretreatments on the quality of trivalent chromium process coatings on aluminum alloy

• Published in: Applied surface science Vol. 423; pp. 927 - 938
• Main Authors: Viroulaud, Rémi, Światowska, Jolanta, Seyeux, Antoine, Zanna, Sandrine, Tardelli, Joffrey, Marcus, Philippe
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30.11.2017
• Subjects: Aluminum alloy AA2024; Coating coverage; Corrosion; Pretreatments; TCP conversion coatings; Trivalent chromium processes (TCP); Coating coverage; TCP conversion coatings; Pretreatments; Corrosion; Aluminum alloy AA2024; Trivalent chromium processes (TCP)
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2017.06.246

[Display omitted] •Significant dependence of surface pretreatments on the TCP coating homogeneity.•Lower TCP coverage on pure Al and AA2024 alloy after pickling step pretreatment.•Strong Cu enrichment on AA2024 alloy not detrimental for TCP quality.•Thicker TCP coating and/or presence of Al oxy-fluorides leading to coating failure. The effects of surface pretreatments (degreasing and pickling) on the characteristics of the Trivalent Chromium Process (TCP) coating on pure aluminum and on AA2024-T351 aluminum alloy were investigated here by means of surface sensitive techniques: X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The XPS and ToF-SIMS results evidence that the TCP coating homogeneity is strongly dependent on the pretreatment process used. The TCP coverage factor, calculated from XPS results, is significantly lower, on both pure aluminum and AA2024-T351 alloy surface, when a pickling step is applied. One of the main effects of pickling pretreatment is strong metallic copper enrichment at the surface of the 2024 alloy, associated with chemical dissolution of Al-Cu intermetallic particles. However, it is evidenced here, that the copper enrichment is not detrimental for the quality of the TCP coating. The coating failure, observed when the pickling step is applied, can be assigned to a faster kinetics of the coating growth leading to formation of thicker conversion coating more susceptible to cracking or to the localized presence of aluminum fluoride species leading to the appearance of coating defects or detachment.