Replacement for Hexavalent Chromium Conversion Coatings for Magnesium and Zinc-Nickel

The objective of this Air Force SBIR Phase II proposal is to optimize and develop an environmentally friendly replacement for the carcinogenic hexavalent chromium based conversion coatings on AMS 4434 Magnesium and Zinc-Nickel plated AISI 4130 and 4340 steel substrates. The proposed research is a continuation to the successful studies conducted during the Phase I stage of establishing the feasibility of a new trivalent chromium based conversion coating for the aforementioned alloys. The proposed conversion coating is a formulation of trivalent chromium, zirconium and an organic inhibitor in proportions found to offer effective corrosion protection and paint adhesion properties. Design and optimization of an efficient surface activation with the effective pretreatment of the surface for the deposition of reliable conversion coating shall be investigated. Corrosion resistance, coating adherence, multi-layer coating systems, simulated depot-level maintenance procedures including stripping/rework and their effects on the fatigue and corrosion resistance shall be investigated. The end result of the proposed research would be a complete environmentally green Trivalent Chromium based Conversion Coating process and product system as a performance and cost effective replacement for the carcinogenic Hexavalent Chromium Conversion Coatings on AMS 4434 Magnesium and Zinc-Nickel plated AISI 4130 and 4340 steel parts. BENEFIT: Magnesium alloys are used in military applications as helicopter gearbox casings, transmission casings, inline skates and some other high strength-low weight required applications. Kruger flaps, reverse thrust manifolds and control wheels found on many commercial aircrafts such as Boeing 727, 737 are sand-cast out of AZ92A alloy. The longest living aircraft B-52 Stratofortress brake calipers are manufactured out of AZ92A-T6. In spite of its high strength-to-weight ratio, magnesium alloys suffer from high corrosion rate coupled with complicated and expensive pretreatment routines. However, looking at the increasing trend of magnesium alloy usage in Military, Aerospace, Automotive sectors, METALAST believes a breakthrough in developing a successful replacement of hexvalent coatings would find widespread usage and demand. Cadmium plating of steel is another carcinogenic corrosion inhibition system restricted in many military applications. Zn-Ni plating, as a sacrificial layer, was found to be a suitable and economical alternative for various steel applications including fasteners, bolts and rivets. In the wake of recent EPA, OSHA and DoD directives for the replacement or limitation of the typical hexavalent conversion coating, there is a huge demand and need for an environmentally green yet potentially effective conversion coating system. Existing alternatives haven’t been either economical or potentially superior. METALAST International has often understood this need and demand from metal finishing industry. Hence, METALAST envisions the need and plans to utilize this opportunity through SBIR to develop an efficient replacement. Developing an environmentally green conversion coating will not only negate the health hazards but also increase the lifetime of both magnesium and Zn-Ni plated steel parts, the parts adjoined, subsequently reducing the high maintenance costs.