In this study, we investigate the impact of ageing and high vacuum on existing environmentally friendly Zr4/Cr3-based conversion coatings. The freshly formed coating undergoes several changes during ageing and exposure to high vacuum. Based on the present data, we propose that the coating formed over AA6082 and AA7075 alloys is sol-gel in nature, confirmed by secondary neutral mass spectroscopy (SNMS) using the depth profiling technique. Our findings reveal that there are elemental level changes that result in shrinkage of the coating. Most Zr ions in the coating are in the solute form, with lesser number of Cr and Al ions that disappear under high vacuum over a certain period of time. The remaining Cr, Zr and O atoms exist in a gelatinous state. During ageing, there is a continuous transition of ions from solute to gelatinous state. In addition, the deposition of coating ions is directly influenced by the substrates and their constituents. The extent of dissolution of aluminium in the conversion bath determines both Zr and Cr ion deposition. For a highly alloyed metal like AA7075, the dissolution rate is disturbed by copper and zinc.

Considering aviation and space sectors, aluminium alloys are commonly used due to its excellent mechanical and physical properties. Though satellite hardware is confined to controlled environment, it requires anticorrosive treatment over metal substrate followed by a systematic coating scheme. The trivalent chromium coating was deposited over three aluminium alloys namely AA6063, AA7075 and AA6082. The variation in corrosion resistance property of trivalent chromium over each aluminium alloy has been studied in detail. The Neutral Salt Spray (NSS) test result shows that trivalent chromium coating over AA7075 alloy is affected by pitting corrosion compared to other two alloys. In addition to that, NSS test also proves that thickness of the layer does not have any influence corrosion resistance property of trivalent chromium coating. Furthermore, ions in trivalent chromium coating was identified using Secondary Neutral Mass Spectroscopy (SNMS) and degradation of coating in a corrosive liquid studied using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) instrument to understand mechanism of corrosion. The results indicated that both coating and substrate is involved in corrosion process. The number of elements dissolved in to salt solution reveals the weak ionic bonding of coating towards substrates. In comparison, AA7075 alloy has weaker bonding than AA6082 and AA6063 series. The alloying elements such as zinc and copper are removed from substrate by corrosive solution.

In this study, the structure of Zr4/Cr3 based conversion coating on aluminium alloys was studied by using the Glow Discharge-Optical Emission Spectroscopy (GD-OES) method. Samples were examined before and after exposure to high vacuum. The results revealed that the structure of the coating has two different phases, solute and gelatinous state. The growth kinetics of conversion coating is investigated by considering the thickness of the coating under various immersion times inside the coating bath. The present data suggest that coating growth kinetics has five different states, namely activation, linear growth, limited growth, saturation limit and detachment state. The conversion coating structure over aluminium alloys such as AA2024 and AA6082 was investigated in detail to understand the influence of the substrate in coating formation. The results imply that the formation mechanism strongly depends on alloying elements of the substrate. Conversion layer growth involves dissolution and deposition of ions. It was found that the alloying elements have a slowdown influence on aluminium dissolution. The results imply that high alloyed aluminium tends to form a coating with less thickness and lower Zr-Cr concentration.

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