Structure and Corrosion Performance of a Non-Chromium Process (NCP) Zr/Zn Pretreatment Conversion Coating on Aluminum Alloys

Abstract

We report on the physicochemical properties and anti-corrosion performance of a non-chromated Zr/Zn conversion coating (NCP) on AA2024-T3. The immersion coating was formed on polished, degreased and deoxidized specimens. Electrochemical methods were used to assess the corrosion inhibition provided by the coating in laboratory tests. The results were compared with environmental exposure tests to assess the stand-alone corrosion protection. Coated AA6061-T6 and 7075-T6 specimens were also used in the environmental tests. Electrochemical testing in naturally-aerated 0.5 M Na 2 SO 4 + 0.1% NaCl revealed that the NCP coating shifted E corr positive by about 250 mV, suppressed anodic more than cathodic current around E corr by at least a factor of 10x and shifted E pit more noble. The coating functions more as an anodic inhibitor through barrier layer protection. The coating provided excellent corrosion protection to all three alloys during a 14-day full immersion test in 0.5 M Na 2 SO 4 + 0.1% NaCl. However during 14-day neutral salt spray and thin-layer mist tests, NCP failed to provide much stand-alone corrosion protection to the aluminum alloys and the anti-corrosion properties were found to be inferior to TCP conversion coatings of comparable thickness. A 7-day beach exposure revealed the NCP coating also provides little resistance to galvanic corrosion on the aluminum alloys as compared to TCP coatings. The results demonstrate that laboratory evaluation of the anti-corrosion properties of non-chromated conversion coatings does not always reflect coating performance during accelerated degradation or environmental exposure. The inferior anti-corrosion behavior of NCP, as compared to TCP, is due to (i) inherent defect density of the former (i.e., reduced throwing power) and ( AA2024 and AA7075 are high-strength aluminum alloys that derive their properties from their alloying components. They are used on civilian and military aircraft because of their light weight and mechanical strength. 1,2 However, many of the constituent particles, such as the Al 2 CuMg phase (so-called S-phase) in AA2024 3,4 and Mg(ZnCuAl) 2 in AA7075, 5-8 lead to corrosion challenges. More noble intermetallic particles play a critical role in the corrosion susceptibility of aluminum alloys as they can give rise to localized corrosion, such as pitting and exfoliation, because of the formation of galvanic cells with the surrounding aluminum. [9][10][11] The intermetallic phases tend to function as cathodic sites supporting oxygen reduction, which can drive the localized dissolution of nearby aluminum. The shape, size and chemical composition of the intermetallic particles are determined by the processing route (heat-treatment and forming) carried out on the aluminum alloy. [9][10][11] Multilayer coating systems (conversion coating + primer + topcoat) are required to protect aerospace aluminum alloys from corrosion in service. Traditional coating systems contain hexavalent chromium (Cr(VI)) in both the conversion coating and primer, volatile orga...

Review—Conversion Coatings Based on Zirconium and/or Titanium

“…Mechanical pre-treatment in the form of grinding and/or polishing may precede chemical pre-treatment but chemical pre-treatment is often applied without prior mechanical treatment. In some cases, the surface was not mechanically pre-treated but cleaned with ethanol 102 or acetone, 55 For rinsing, deionized water is usually used, although also tap water 92,94,106,112 was applied to make the procedure simpler for industrial processing. Due to environmental and health concerns related to toxicity and carcinogenicity of chromates, procedures that reduce the worker exposure and discharge into the environment were introduced.…”

“…81 The majority of Zr/Ti based conversion coatings are in the category of thin coatings, between 10 nm and 50-80 nm. 6,[32][33][34]44,49,52,76,77,88,90,91,97,102,105,106,108,112 Coatings with thickness between 100 nm and 600 nm were reported as well, 48,81,94,127 and even in the micrometer thickness range from 1.0 μm up to 6 μm. 30,37,43,59,61,80,119,127 Thickness is related to other parameters such as substrate, pH, time of immersion and stirring, as discussed in Conversion bath parameters and Effect of substrate on coating formation sections.…”

“…33,34,36,37,47,51,52,55,59,61,62,[64][65][66]69,70,72,78,82,84,87,90,92,94,95,98,99,101,102,104,[106][107][108]110,[112][113][114] Elevated temperatures were also studied, i.e. between 30 and 55…”

“…112 Both Zr-and Zn-deposits are formed around Cu-rich IMPs and Zn is present throughout the coating at about 10 at.% but is elevated at the outer interface. In aggressive corrosion testing, NCP coating performed worse than a commercial trivalent chromium process (TCP) chromium(III)-based coating, which was ascribed to the higher solubility of Zr(OH) 4 and Zn(OH) 2 and reduced adhesion strength compared to Cr(OH) 3 in the TCP coating.…”

“…In aggressive corrosion testing, NCP coating performed worse than a commercial trivalent chromium process (TCP) chromium(III)-based coating, which was ascribed to the higher solubility of Zr(OH) 4 and Zn(OH) 2 and reduced adhesion strength compared to Cr(OH) 3 in the TCP coating. 112 Reducible hypervalent transition metals (Mo, Mn, V, Tc) were considered as possible chromate replacements, because they, like Cr, form oxoanions in aqueous solution. Anions of Mo and Mn are very soluble and undergo reduction to insoluble products, which could provide extra protection during formation of conversion coatings.…”