Aluminium alloys such as AA6082, AA2024 and AA7075 are widely used in aircraft and space metallic components due to their lightweight and mechanical properties. It is necessary to protect metal with coating that provides good resistance against corrosion and adhesion to top coat. Hexavalent chromium based conversion coating is widely used for aluminium alloys. The toxic, hazardous and carcinogenic nature of hex chrome compounds implies immediate replacement with eco-friendly coating due to restriction from various directives of European union1 .Considering other alternative treatments, Zr4/Cr3 based conversion coating seems to be promising 2. This conversion coating is nowadays widely used for protection of aluminium alloys3. The developments published related to this conversion coating shows successful results in corrosion protection behaviour over aluminium alloys and provides good adhesion over paints.
This conversion bath usually consists of zirconium fluorides and trivalent chromium salts. The formation mechanism of this coating over AA2024 alloy has been studied for several years. Those works reveals conversion coating involves dissolution and redepositing of ions from substrate as well as chromium bath with interfacial pH variation during coating process 45. The previous report by Manchester group about growth of coating over AA2024 alloy signify that coating is comprised of two layers. The outer layer consists of more Zr- and Cr- species, while an inner layer is rich in oxides, fluorides of aluminium 6 7.
In one study, the impacts of aging using AA2024 alloy as a substrate signifies that coating become more hydrophobic nature under elevated temperature 8. Some studies about formation of coating over AA7075 exists but there is not much significant research done previously regarding aging effects related to Zr4/Cr3 based conversion coating over AA6082 and AA7075 alloys.
The structure of coating has neither been studied nor discussed in any of previous paper. The TEM image of coating over AA2024 shows that coating is porous in nature9. However, there is not much significant details interpreted about impact of vacuum or samples preparation influences over image. Most of published papers measured coating thickness under vacuum mentioning coating undergoes severe dehydration. In this sense, we used high vacuum as tool to predict the structure of coating and unrevealed the elemental changes that occur during exposure to high vacuum.
Besides, the previous study about coating growth kinetics over pure aluminium alloy using AFM and TEM indicated that coating growth has three states: an active period up to 30 s, reaches linear state up for 600 s and attains limited growth after 1200 s10. This study was done on pure aluminium alloy thus the influence of alloying elements is not well defined. To discover further changes, we have coated AA6082 alloy sample under various immersion time to predict growth kinetics of conversion coating inside bath.
The formation of coating over AA2024 reveals that coating has several cracks and detachment over substrate. The coating over enrichment copper rich particle shows some change in formation as mentioned in literature11. This indicates that alloying elements have major contribution in the formation of coating12. Despite there are not much studies done previously to understand the influence of substrate in formation of Zr4/Cr3 conversion coating. The effect of alloying elements in substrate was not investigated in any of previous published papers related to this conversion coating.
The aim of these experimental series as reported below using SNMS, GD-OES is to reveal the elemental level aging technique and observe growth kinetics of conversion process. Apart from these, goal of this work is to discover structure this Zr4/Cr3 based conversion coating over aluminium alloys using high vacuum as tool. In this present study, we have also analysed differences in formation mechanism of coating over two different substrate AA6082 and AA2024 using a depth profile technique by GD-OES