Corrosion resistance and self-repairing behaviour of Cr(III) contained conversion coating on AA2024-T3

Abstract

A Cr(III) contained conversion coating on AA2024-T3 was prepared in solution containing Cr 2 (SO 4 ) 3 and K 2 ZrF 6 . Its composition was analysed using X-ray photoelectron spectroscopy, and its corrosion resistance was characterised by electrochemical measurements and neutral salt spray exposure. To investigate its self-repairing effect, the coupling current between a small uncoated sample and a large coated sample was monitored through an artificial scratch cell. The coating mainly consists of zirconium oxide and chromium oxide and hydroxide; no Cr(VI) species is detected. It improves the corrosion resistance by hindering the anodic process as well as the cathodic process. Meanwhile, it possesses self-repairing effect, which is generated from the release of Cr(III) species from the coating, migration and then deposition on the nearby scratch area.

Communication—Role of Trivalent Chromium on the Anti-Corrosion Properties of a TCP Conversion Coating on Aluminum Alloy 2024-T3

“…23 Recent published work has produced insight on the TCP coating structure and the underlying mechanisms by which the coating inhibits corrosion on various aluminum alloys. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]23 For example, we have reported on the properties of one commercial TCP coating on AA2024-T3, AA6061-T6 and AA7075-T6. 4,[7][8][9][10] There have also been other reports on another commercial version of TCP on AA2024-T3 and AA7075-T6.…”

“…The coating provides cathodic protection, in part, by serving as a diffusional barrier for O 2 , even though very thin (∼100 nm). 4,[7][8][9][10][11][16][17][18][19] The coating consists of a bi-phasic structure with an outer hydrated zirconia layer rich in Cr(OH) 3 and an interfacial layer consisting of hydrated zirconia, some Cr(OH) 3 and KxAlF 3+x . [4][5][6][7][8][9][10] In this communication, we report on potentiodynamic polarization and rotating disk voltammetric data that show both conversion coatings suppress anodic currents equivalently but the coating with Cr(III) inhibits cathodic currents to a greater extent.…”

Ti/Cr(III) conversion coating on aluminium foil for lithium-ion battery package

“…However, the hexavalent chromium (Cr(VI)) in wastewater produced during the manufacture of Al-polymer composite film is highly toxic and carcinogenic [7]. For this reason, a number of environment-friendly surface treatments have been developed in response to environmental protection requirements, such as titanium (Ti) conversion coatings [8], zirconium (Zr) conversion coatings [9], phosphate conversion coatings [10], vanadium conversion coatings [11], trivalent chromium (Cr(III)) conversion coatings [12] and rare earth conversion coatings [13]. However, as it is difficult for a single-element conversion coating to achieve excellent overall performance, research interest has been directed towards multiple-elements conversion coatings.…”