Influence of metallurgy on the protective mechanism of chromium‐based conversion coatings on aluminum–copper alloys

Abstract

Several important aspects relating to the mechanism of formation of protective Cr-based oxide films on aluminum alloy 2024T3 generated from CrO, + NaF-containing solutions were observed with electrochemical, AES and XPS measurements. Although the film deposition rate and surface composition were very much influenced by the presence of both Fe-and Cu-containing intermetallic phases, a uniform composition and thickness was eventually reached with increased coating time. This behavior is believed to be responsible for obtaining corrosion-resistant films on such heterogeneous surfaces. Surface Cr was consistently found to be in both the 3+ and 6+ oxidation states in an approximately 40 : 60 ratio, respectively, if x-ray beam reduction of Cr(V1) was accounted for. The enriched Cu layer found on the surface of polished 2024-T3 was found to remain intact during Cr-0 film formation. The presence of Cr(V1) provides a 'self-healing' aspect to the film's protective mechanism by remaining available for reduction to Cr(1II) during oxidative attack. Addition of potassium ferre or ferricyanide to the bath resulted in a film which obtained Fe, C and N enriched at the surface on both the matrix and intermetallic regions. These constituents were found, however, to be enriched and distributed throughout the entire depth of the film on the high Cu-bearing intermetallic regions, suggesting that the formation of a Cwferrocyanide complex is responsible for the benefit derived from these compounds.

Formation of Chromate Conversion Coatings on Al-Cu-Mg Intermetallic Compounds and Alloys

“…After 5 min of immersion time, coating thickness appeared to be similar on the different phases. These workers proposed that ferrocyanide interacted with Cu-rich IMCs to form Cu 4 Fe(CN) 6 or Cu 2 Fe(CN) 6 and that these compounds promoted corrosion resistance by eliminating the galvanic couples that would otherwise form between the noble particles and the matrix phase.Lytle et al used a variety of surface analytical techniques, including X-ray absorption spectroscopy (XAS), X-ray absorption near edge spectroscopy (XANES), extended X-ray absorption fine structure (EXAFS), and Fourier transform infrared spectroscopy (FTIR) to study the chemistry of accelerated CCCs on 2024-T3 and 7075-T6. 7 Their results, made with low spatial resolution probes, showed Fe(CN) 6 3Ϫ present in the outermost 20 Å of the CCC.…”

“…2 Ostensibly, accelerated CCCs are used on corrosion-prone Al-Cu-Mg and AlZn-Mg-Cu alloys to ensure maximum protection. 3 Ferricyanide [Fe(CN) 6 …”

Corrosion behaviour of a 2219 aluminium alloy treated with a chromate conversion coating exposed to a 3.5% NaCl solution

“…With regard to the formation of chromate conversion coatings on intermetallic sites in Al alloys, Hagans et al [12] have studied coating formation on a 2024Al alloy. They observed a different deposition rate of the coating on the intermetallics and on the matrix.…”

“…A common additive to the coating solution is potassium ferricyanide, K 3 Fe(CN) 6 6 ] 3 complex is proposed [13]. The couple reacts on the intermetallics as well, suggesting the formation of CuFe(CN) 6 [12][13].…”

Critical Factors for the Transition from Chromate to Chromate-Free Corrosion Protection

“…Nevertheless, specific interaction between CCC and intermetallic particles in Al 2024 has been observed. CCC is not uniform all over the surface but thinner on the Cu-rich particles (26,27). Hagans and Haas observed a strong presence of Fe on those particles suggesting the formation of a Cu-ferrocyanide complex (26).…”