Chromate (Cr6+)-free surface treatments for active corrosion protection of aluminum alloys: a review

For many decades, chromium and its derivatives have been the integral part in the corrosion protection of structural parts and have a wide application in the aircraft industry. It is used in the pre-treatment of aluminum substrate in the form of anodization or conversion coating and in the primer layer as a chromate additive. However, harmful impact of hexavalent chromium (Cr6+) compounds on the human health and the environment has triggered the need to find alternative corrosion protection measures. Finding an alternative to chromate is an arduous effort as chromates are known to provide a corrosion protection under extreme environmental conditions over a very wide range of pH. Active corrosion protection ability, commonly known as self-healing ability, is a hallmark of chromates. In a pursuit to realize an eco-friendly coating system, there are efforts worldwide to find alternatives both for primer component and for the pre-treatment process. This review focuses on work that has been carried out to replace Cr6+ in the pre-treatment process. Importance is given to those alternative pre-treatment processes that have the potential to provide active corrosion protection and possibly replace chromic acid-based anodization and conversion coatings.