Recent directives and limits have required the surface finishing industry to seek more environmentally friendly finishing processes while still providing protection and performance. A new Navy-licensed, QPL-approved, non-hexavalent conversion coating for aluminum offers a possible solution.

In the past few years, the European Union (EU) has banned the use of certain heavy metals and hazardous materials with European Directive 2002/95/EC (RoHS-Restriction of Hazardous Substances), European Directive 2002/96/EC (WEEE-Waste Electrical and Electronic Equipment) and European Directive 2000/53/EC (ELV-End of Life Vehicles). Most recently, the U.S. Occupational Safety and Health Administration (OSHA) has set a new limit of 5 μg/m³ on hexavalent chromium exposure. These directives and limits have required the surface finishing industry to seek more environmentally friendly finishing processes while still providing protection and performance.

A Non-Hexavalent Conversion Coating

The U.S. Navy chemical researchers at Pawtuxet River, MD, developed a trivalent conversion coating for aluminum that complies with the OSHA and European directives while offering corrosion protection, adhesion and electrical resistance properties that meet or exceed the coating’s hexavalent counterpart. In 2004, the U.S. government offered licenses for the newly developed chemistry to chemical manufacturers. Of the four manufacturers that received the licensing privileges, each was required to submit its own version of the conversion coating within the parameters of the basic chemical formulation.

After more than a year of rigorous testing per MIL-DTL-81706B, a product from Luster-On Products, Inc., called Aluminescent, was placed on the Qualified Products List (QPL) for MIL-SPEC-5541F Type II, Class 1A and 3, Material Form II, Application Methods A and C. The powdered product is used to form a non-hexavalent conversion coating on aluminum surfaces. The process used to apply the coating provides excellent corrosion protection at low cost and requires no electric current, cooling or exhaust. It is suitable for use on both wrought and cast aluminum alloys, and it is designed for ease of operation with minimum control.

Environmentally Friendly Protection

Table 1 shows the various aluminum alloys that have been tested in conjunction with MIL-DTL-81706B and ASTM B-117. As shown in the table, the product meets or exceeds performance results of the hexavalent chromium conversion coatings while still complying with RoHS, ELV, WEEE and OSHA requirements. In addition, the coating provides excellent adhesion on aluminum prior to e-coat and powder finishing.

The environmentally friendly coating is being used to provide protection in a variety of end uses, including computer chassis and aerospace and automotive components.