Aluminum chromic-sulfuric acid etching

Fig. 5. Effect of outdoor weathering on the strength of aluminum alloy/epoxy-polyamide joints (chromic-sulfuric acid-etch metal surface pretreat-ment)61 (Reprinted from Ref. 61, p. 194, by courtesy of Gordon and Breach...

The outdoor durability of epoxy bonded joints is very dependent on the type of epoxy adhesive, specific formulation, nature of the surface preparation, and specific environmental conditions encountered in service. The data shown in Fig. 15.19, for a two-part room temperature cured polyamide epoxy adhesive with a variety of fillers, illustrates the differences in performance that can occur due to formulation changes. Excellent outdoor durability is provided on aluminum adherends when chromic-sulfuric acid etch or other chemical pretreatments are used. 

The Forest Products Laboratory (FPL) and other chromic-sulfuric acid etching procedures are the oldest surface pretreatments for aluminum adherends,(3) other than simple degreasing or mechanical abrasion. In addition to being used as a complete adherend pretreatment, it is also frequently used as the first step in other pretreatments, such as PA A and CAA. 

Bonding of aluminum comprises greater than 90% of the end uses for epoxy film adhesives. At least three major cleaning methods have been used chromic acid etch, phosphoric acid anodize and sulfuric acid anodize. Each process produces a different microstructure and chemical surface to which to bond. A corrosion-resistant epoxy primer is often used to protect the delicately etched surface during assembly operations. The adhesive must then bond to the primer and provide the myriad of intricate test strengths required by each aircraft producer, each aircraft, and each design. The tedious nature of optimizing each adhesive to meet each detailed specification becomes apparent and will not be further pursued in this discussion. 

Aluminum Sulfuric-chromic acid etched 6 days... 

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