Nucleation and growth mechanisms of trivalent chromium conversion coatings on 2024-T3 aluminium alloy

Abstract

This is an author's version published in: http://oatao.univ-toulouse.fr/25262 A Alumlnium alloy A Copper B. EIS B.XPS C. Trivalent chromlum coatlng Nucleation and growth mechanisms of trivalent ch romium conversion coatings on 2024 alumi nium alloy (A A 2024) were studied. Nucleation of 25 nm diameter nodules was observed on the ridges of the scalloped structure of degreased and desmutted AA 2024 after very short tiine of conversion treatment corresponding to the for mation of a 12 nm thick precursor layer. Then, the composition of this layer evolved and concomitantly a chromium and zirconium outer layer deposited on top of it. Rather long-lasting anticorrosive properties were measured even for conversion coatings formed after short exposure to the conversion bath, except for the precursor layer. E-mail address: christine.blanc@ensiacetfr ( C . Blanc).

Formation of a trivalent chromium conversion coating on AZ91D magnesium alloy

“…The OCP evolution during TCC coating formation on magnesium alloys was similar to with that reported for aluminium and aluminium alloy. [17][18][19] TCC coating formation is a pH-driven process where the pH is locally increased by the cathodic reactions leading to enhanced deposition of hydroxides of chromium and zirconium. [20] Fig.…”

Influence of ageing on the corrosion behaviour of 2024 aluminium alloy coated with a trivalent chromium conversion layer

“…As already discussed in the introduction part, a high Cu amount can be associated with a thicker TCP coating due to the catalytic effect of Cu on the protons reduction, leading to a pH increase and promoting the precipitation of both Cr and Zr oxides and hydroxides [28][29][30][31]52,56,88]. It could be assumed that both Cu in the underlying alloy, Cu deposits and Cu-rich intermetallic remnants have this catalytic effect, considering that during immersion in the TCP bath the oxide layer is progressively dissolved by fluoride ions [22]. However, the growth of thicker TCP coating was also associated with a more defective coating due to internal stresses leading to cracking and delamination [28][29][30][31]52,56,88].…”

“…Therefore, a new promising conversion treatment process based on trivalent Cr, known as trivalent Cr process (TCP), has been developed in the aeronautic sector [21]. As described in the literature, this treatment leads to formation of a duplex structure composed of a fluoroaluminate-rich inner part and a Cr and Zr oxide-and hydroxide-rich outer part [22][23][24]. The formation mechanism of the TCP coating is based on the reduction of protons, that leads to a pH increase near the surface and then to the precipitation of Cr and Zr as oxides and hydroxides [24][25][26][27].…”

Formation and corrosion resistance of a novel Co–Ti–Mo composite chromium‐free chemical conversion coating on LY12 aluminum alloy

“…Nevertheless, Cr 6+ in this process has severe toxicity, carcinogenicity, and bioaccumulation, [9,10] leading to great damage and serious threat to the environment and human health, and thus has been restricted in many countries. [11,12] In the effort to eliminate the harm of Cr 6+ to the environment, a series of novel CCPs have been carried out at home and abroad. [13][14][15][16][17] However, according to the results of their research, most of the recent CCPs have their own shortcomings, which limit their further application and promotion.…”