2 - Aluminum and Aluminum Alloys

Abstract

The chapter provides detailed information on Forest Products Laboratory etch (FPL etch). The reaction of FPL etch solution with the aluminum alloy is presented. Various other methods have been developed as improvements over FPL including optimized FPL etch, phosphoric acid anodize (PAA), P and P2 etchants and the chromic acid anodize (CAA) procedures. Optimized FPL etch involves dissolving a known amount of aluminum in the etch solution at the time of preparation. The PAA process produces an oxide structure by a two-stage process. P2 etch provides a chromate-free surface treatment for the preparation of aluminum alloys. CAA process builds a basic oxide layer which supports a tubular cell structure. Durability of the joint is affected by both bath temperature and immersion time. It is also reported that the time between etching and rinsing is critical. Four specific treatments for aluminum and aluminum alloys are presented in a process specification format including optimized FPL etch (sulfo-chrom etch), PAA, P2 etch (sulfo-ferric etch) and CAA. Discussions related to surface chemistry are presented along with available data on the strength and durability for each method.