Influence of pre- and post-treatments on formation of a trivalent chromium conversion coating on AA2024 alloy

Abstract

A comparison has been made of the influence of two alloy pre-treatments and two coating post-treatments on the formation, composition and corrosion protection of a trivalent chromium conversion coating on AA 2024-T351 alloy. The investigation employed analytical electron microscopies, ion beam analysis, X-ray photoelectron spectroscopy (XPS) and electrochemical tests. The pre-treatments used alkaline etching followed by de-oxidizing in either nitric acid or a commercial de-oxidizer. The conversion coatings were formed in SurTec 650 chromitAL and revealed two-layers, comprising an inner aluminium-rich layer and an outer chromium-and zirconium-rich layer, with a Cr:Zr atomic ratio in the range ~0.73 -0.93. XPS indicated a chromium-enriched near-surface region that contained ~2 at.% of Cr (VI) species. Potentiodynamic polarization and electrochemical impedance spectroscopy revealed an improved corrosion protection for a pre-treatment that left copperrich sponges, probably de-alloyed S phase, and fewer residues of other intermetallic particles on the alloy surface. Post-coating immersion treatments in deionized water at 20 °C or 40 °C resulted in a significant difference in the zirconium species in the region adjacent to the coating surface that is accessible to XPS, with oxide and hydroxide dominating at the respective temperatures.

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

“…This suggested an insufficient barrier to penetration of the coating by chloride ions and that low concentrations of chromate species formed in the coatings offered negligible anodic inhibition. 10 The findings from the PDP measurements are consistent with results from previous polarization tests in 0.05 wt% NaCl + 0.35 wt% (NH 4 ) 2 SO 4 solutions 4 and 0.5 M Na 2 SO 4 and 0.5 M Na 2 SO 4 + 0.05 M NaCl solutions. Effect of the time of conversion treatment on electrochemical behavior.-In order to determine the preferred time of conversion treatment of the alloy prior to the immersion step in DI water, EIS measurements were carried out in 0.05 M NaCl solution on specimens treated for 600, 900, or 1800 s followed by the standard post-treatment.…”

“…The plots of the coatings with a standard post-treatment were presented in a previous paper in Ref. 10. The steady OCPs of all specimens prior to the EIS measurements were around −0.52 V SCE .…”

Influence of post-treatment time of trivalent chromium protection coating on aluminium alloy 2024-T3 on improved corrosion resistance

“…Two main types of localized corrosion occur on AA 2024 in the neutral chloride solutions, namely, pitting corrosion and corrosion along the grain boundaries. Thus, effective inhibitors and coatings are necessary to suppress the dealloying of Al 2 CuMg and also protect the Al matrix from localized corrosion [7,8].…”

Influence of the alloy microstructure and surface state on the protective properties of trivalent chromium coatings grown on a 2024 aluminium alloy

“…temperature [16,24]. The solution was deaerated for 15 min with ni trogen bubbling before CV experiments.…”

“…the nature of the IMCs present on the surface before the pre-treatrnents and the evolution of their distribution due to their reactivity in the pre-treatrnent baths. Considering that (i) the IMC dissolution can lead to copper redistribu tion (ii) incomplete dissolution of the IMC can leave significant loca lized quantities of copper [16,18,41] and (iii) copper can have a sig nificant impact on the conversion process [42--44], the arnount of electroactive copper remaining on the surface was a major pararneter considering the surface properties; it was determined by CV experi ments for al! the samples.…”