An Optimized Trivalent Chromium Conversion Coating Process for AA2024-T351 Alloy

Abstract

In this work, a trivalent chromium conversion coating applied on AA2024-T351 alloy has been optimized for corrosion protection in sodium chloride solutions. Scanning electron microscopy, energy dispersive X-ray spectroscopy and electrochemical measurements were employed to characterize the coating. An immersion post-treatment of the coated alloy in 40 • C deionized water for 120 s considerably enhanced the corrosion protection properties, compared with a post treatment at 20 • C, or the absence of a post-treatment. Electrochemical noise measurements, combined with real time imaging, and potentiodynamic polarization experiments indicated a conversion treatment for 300 or 600 s provides optimal corrosion protection. Relatively long conversion treatments decreased the corrosion protection due to an increase in coating defects and cracks, especially around the second-phase particles.

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

“…For all others samples, only one time constant, with high phase angles on a large frequency domain, was distinguished. As shown in a previous paper [29], high impedance modulus and a large frequency range of high phase angles could be related to improved anticorrosion properties for the TCP coating [11,13,29,36]. After 14 days of immersion in NaCl, an enlargement of the frequency domain were high phase angles were observed and an increase by one or two orders of magnitude of the impedance modulus at 10 −2 Hz were noticeable for all samples except for the degreased desmutted and 3 s TCP coated samples that showed similar EIS spectra.…”

Inspection of Trivalent Chromium Conversion Coatings Using Laser Light: The Unexpected Role of Interference on Cold-Rolled Aluminium

“…While those restrictions have led to further investigations of suitable alternatives, one of the most promising substitutes based on Cr(III) and Zr(IV) (SurTec R 650-chromitAL TCP., SurTec International GmbH, Bensheim, Germany) is used exemplarily in this study. According to the literature, several papers have already been published using SurTec R 650 as passivation agent on different aluminium alloys backing our choice [8][9][10][11][12][13]. Additionally, researchers like Kim et al applied this agent on the commonly used AA3003 (here on aluminium foil) which serves as our substrate of choice [14].…”

Chromate replacement: what does the future hold?

“…[34][35][36][37][38][39] The trivalent conversion coating (TCC) typically contains fluoride salts (NaF), trivalent chromium salts (Cr(OH) 3 or Cr 2 (SO 4 ) 3 ) and hexafluorozirconate (ZrF 6 2− ). [40][41][42][43] Surface film formation takes place in an acidic environment where the co-precipitation of hydrated zirconia and trivalent chromium hydroxides occurs, hence providing high barrier properties. 40 However, due to the absence of hexavalent chromium within the coating, a significant decrease in the socalled self-healing properties associated with trivalent chromium protection strategies was noted.…”