Current light metal finishing procedures for industrial, automotive, aerospace, and Department of Defense (DoD) applications center around the use of hexavalentchromium (chromate)-based chemistries for enhancing corrosion resistance and paint adhesion. Aluminum finishing, in particular, utilizes chromate chemistries for anodizing, anodic sealing, and pretreatment. The most ubiquitous use of chromate coatings is in the conversion coating of aluminum alloys for use as-deposited or prior to organic coating application. These coatings are very thin, inexpensive to produce, extremely process flexible, and can be applied by immersion, spray, and wipe techniques.
While there are many advantages to using hexavalent chromium, it is also known to be carcinogenic. The occupational safety and health issues arise from risk of worker exposure to hexavalent chromium. However, there are additional costs and potential liabilities resulting from an accidental leak to the environment and waste disposal issues from normal finishing operations. The objective of this project was to achieve the goal of reducing or eliminating the use of hexavalent chromium in aluminum finishing by demonstrating and validating the performance of non-chromate aluminum pretreatments.
All the alternatives demonstrated in this project are aqueous solutions designed to deposit a conversion coating on aluminum alloy substrates to enhance paint adhesion and painted corrosion performance. Alternatives face the challenge of the low cost and ease of application of the chromate conversion coatings while providing a coating that provides acceptable technical performance. Along with technical performance, processing and toxicity issues are important to consider in capturing the overall impact of an alternative.
There are currently four non-chromate alternatives in various stages of validation or implementation. Alodine 5200/5700, PreKote, and AC-130/131 each provide paint adhesion and painted corrosion protection and are all non-chromium chemistries. The trivalent chromium process (TCP) provides both painted and unpainted corrosion protection as well as electrical conductivity in corrosive environments. However, TCP does contain trivalent chromium, and users will need to balance total chromium wastewater requirements with technical performance requirements when deciding on implementation of TCP. TCP and Alodine 5200/5700 provide the most process flexibility, as they can be applied like a chromate conversion coating, by immersion, spray, or wipe-on methods. AC-130/131 can be used in spray applications. PreKote must be manually applied for proper coating performance. All of the demonstration coatings have shown good paint adhesion and corrosion performance when used under chromated primers and in moderate environments. The PreKote and the AC-130/131 have not demonstrated acceptable performance when combined with non-chromate primers in high corrosion environments. The TCP and Alodine 5200/5700 have shown good paint adhesion and painted corrosion performance when used under both chromated and non-chromated primers. TCP and Alodine 5200/5700 have performed well in high corrosion environment testing.
It is critical with any new non-chromate material that it be tested to failure against the chromated control. Additionally, any new coating application should be demonstrated and validated by field-testing for each operational environment where implementation is being considered. Only then can the complete technical performance of a coating or coating system be determined. Implementation of any alternative must take into consideration the costs, process, health and safety, laboratory and field testing performance, and the specific coating system application and operational environment.