Effects of Aging Temperature and Time on the Corrosion Protection Provided by Trivalent Chromium Process Coatings on AA2024-T3

The effects of aging temperature and time on the physical structure of and corrosion protection provided by trivalent chromium process (TCP) coatings on AA2024-T3 are reported. The TCP coating forms a partially blocking barrier layer on the alloy surface that consists of hydrated channels and or defects. It is through these channels and defects that ions and dissolved O-2 can be transported to small areas of the underlying alloy. Reactions initiate at these sites, which can ultimately lead to undercutting of the coating and localized corrosion. We tested the hypothesis that collapsing the channels and or reducing the number of defects in the coating might be possible through post-deposition heat treatment, and that this would enhance the corrosion protection provided by the coating. This was tested by aging the TCP-coated AA2024 alloys in air overnight at room temperature (RT), 55, 100, or 150 degrees C. The TCP coating became dehydrated and thinner at the high temperatures (55 and 100 degrees C). This improved the corrosion protection as evidenced by a 2X increase in the charge transfer resistance. Aging at 150 degrees C caused excessive coating dehydration and shrinkage. This led to severe cracking and detachment of the coating from the surface. The TCP-coated AA2024 samples were also aged in air at RT from 1 to 7 days. There was no thinning of the coating, but the corrosion protection was enhanced with a longer aging period as evidenced by a 4x increase in the charge transfer resistance. The coating became more hydrophobic after aging at elevated temperature (up to 100 degrees C) and with aging time at RT as evidenced by an increased water contact angle from 7 to 100 degrees C.