A Safer Alternative to Chromate Passivation for Aluminum Pretreatment
Nabil Zaki, SurTec International, GmbH
Track: VALUE-ADDED PROCESSES - Paint
ABSTRACT --- Conversion coating passivation on aluminum plays a crucial role
in corrosion protection and serving as a base-coat for powder coating, painting,
lacquering, and other surface finishes. Traditional passivation for the past 50
years or more is based on hexavalent chromium, Cr(VI), known as chromates.
However, recent environmental concerns have defined Cr(VI) as extremely
hazardous, and new regulations banned its use from protective films on metals.
As a result, various alternatives such as non chrome systems were investigated,
with the aim of matching all the properties of hexavalent chromates with limited
success. Recently, a new Cr (VI) -free conversion coating process for aluminum
was introduced, based on low concentration of trivalent chrome Cr(III). Trivalent
chrome compounds are non hazardous and much safer to use. The process has the
unique advantage of producing coatings with similar or better properties than its
Cr(VI) counterpart. This includes high corrosion resistance on high copper and
silicon alloys, low electrical contact resistance, and excellent paint adhesion. The
technology has been extensively tested by the US Navy and is applicable to the
aerospace, construction, electrical, and electronics industries. It meets MIL Spec
5541 F for classes I and III, is RoHS and ELV compliant, and QPL and Qualicoat
approved. The process solution is very stable and easy to control with indefinite
tank life. This paper describes the process, compares its performance with other
alternatives, and describes field applications in the aluminum extrusion industry.
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