In situ spectroscopic ellipsometry studies of trivalent chromium coating on aluminum

Abstract

In situ spectroscopic ellipsometry in the spectral region of 1.3–4.3 eV has been employed to monitor, in real-time, the formation of trivalent Cr process (TCP) conversion coatings on polished Al substrates. The measured ellipsometry parameters (Ψ and Δ), as a function of immersion time, reveal that the initial stages of film formation include the chemical thinning of the native oxide layer, formation of a very thin initiation layer and the subsequent rapid formation of the TCP film. The film optical constant is modeled using Cauchy dispersion relation and its thickness was determined as a function of immersion time during growth.

Formation of a Trivalent Chromium Conversion Coating on AA2024-T351 Alloy

“…3 for aluminum was obtained. 8 They also reported a lengthy induction time of ∼100 s before growth of the coating commenced, which was not observed with the current treatment. 13 However, the temperature of the coating bath, which was not specified in Ref.…”

“…13 However, the temperature of the coating bath, which was not specified in Ref. 8, may have differed between the studies.…”

“…4,5 One promising candidate treatment uses a trivalent chromium bath, generally consisting of zirconium hexafluoride and trivalent chromium salts. 6,7 The coating formation involves the dissolution of the native oxide on the aluminum surface due to the acidic, fluoride-containing solution, 8 and the subsequent pH-driven deposition of zirconium and chromium species. The increase in interfacial pH is promoted by the cathodic reactions, i.e.…”

Role of Post-Treatment in Improved Corrosion Behavior of Trivalent Chromium Protection (TCP) Coating Deposited on Aluminum Alloy 2024-T3

“…As it has already been widely discussed, the coating formation mechanism occurs by multiple chemical steps. 10,11,13,[15][16][17] The initial step involves dissolution of the aluminum oxide layer by fluorides. The removal of the oxide layer exposes the metal surface where cathodic reactions take place.…”

“…8 Therefore, new conversion coatings have been developed, and one of the most promising solutions is the Trivalent Chromium Protection coating called also as the Trivalent Chrome Process (TCP). [10][11][12][13][14] TCP conversion coatings are formed by immersion in a solution containing Zr 4+ , Cr 3+ and F − ions, at a pH between 3.8 and 4.0. As it has already been widely discussed, the coating formation mechanism occurs by multiple chemical steps.…”