Chromate conversion coatings (CCCs) have been employed in the surface finishing process for AA2024-T3 for their excellent ability to resist localized corrosion and to promote paint adhesion. However, due to the toxic effects of chromium compounds, a significant amount of effort has been extended to develop alternative corrosion inhibitor systems. Trivalent Chrome Process (TCP) coatings recently have gained wide acceptance and are considered an environmentally friendly replacement for chromate conversion coating, because the TCP bath and the resulting film contain no Cr (VI) species.
In this study, the application of TCP coatings as an alternative to CCCs has been investigated. During TCP coating formation, activation of the Al surface leads to the reactions of oxygen reduction and hydrogen evolution, which result in the local pH increase and the deposition of the TCP coating. TCP coating is characterized as a dense layer consisting of rounded particles hundreds of nm in size, similar to the CCCs. The thickness of the TCP was in the range of 40-120 nm depending on the conversion time, considerably thicker than the zirconium based coating without chromium species. No Cr (VI) was found on the TCP surface assures its application as an environmentally friendly replacement for CCC. A two layered structure is suggested, with zirconium-chromium mixed oxide in the outer layer and aluminum oxide or oxyfluoride at the metal/coating interface. The high vacuum condition in the traditional SEM dehydrates the coating quickly; consequently, mud-crack artifacts were always observed. The TCP coating provides corrosion protection to the AA2024-T3 through suppressing the oxygen reduction reaction on aluminum alloy surfaces by acting as a protective barrier layer.
The effects of two pretreatments on the TCP formed on AA2024-T3 surface were investigated. The growth of TCP following Process I (Henkel Chemicals) started faster compared to Process II (silicate treatement). The size of the particles was around tens of nm which was smaller compared to Process II. The abnormal round clusters were not easily observed after Process I. Nucleation uniformly occurred on the sample surface after Process I, while non-uniform nucleation occurred after Process II. Longer immersion in the TCP bath resulted in increased thickness of TCP film for both pretreatments. Considering all that was found, there was not much difference of TCP formation on the aluminum surface after these two pretreatments.
The self healing properties of TCP on AA2024-T3 have been assessed and quantified using the artificial scratch technique. TCP treatment can greatly improve the corrosion resistance of AA2024-T3 to sustain the long time exposure to simulated corrosive environments. The chromium is able to be released from the TCP coating according to ICP-OES and the released chromium can be transported to uncoated area nearby. EIS data showed the polarization resistance of uncoated surface was twice as much as uncoated controls, which suggested that TCP can provide mild active corrosion inhibition for Al alloys.