Aluminum chromic acid anodized

CAA. Chromic acid anodization [74-76]. was developed initially as a treatment to improve the corrosion resistance of aluminum surfaces, but it is also used as a surface treatment for adhesive bonding especially in Europe where it is used extensively in aerospace applieations [29,77],... 

Zeolite coatings have been investigated as corrosion-resistant coatings for aluminum alloys in aerospace applications. The currently used chromic acid anodization and chromate conversion coatings are effective but release hexavalent chromium, which is... 

Strong. However, some of the aerospace aluminum alloys (e.g., 2024-T3) are very prone to corrosion. In order to combat corrosion of aluminum alloys, chromic acid anodization and chromate conversion have thus far proved most effective. Both processes, however, use and release hexavalent chromium, a proven human carcinogen, causing serious environmental and worker health and safety concerns. Thus, a chromium-free alternative with equivalent or superior corrosion performance has long been desired. 

Stressed-bond joint durability is markedly affected by the adherend prebond surface treatment and the adhesive/primer system in contact with it. This is evidenced by the poor performance of FM 123-L/BR 123 (non-CIAP) adhesive/primer system on FPL-etched and chromic acid-anodized 2024-T3 aluminum alloy, clad and bare, and the superior performance of the same systems when BR 127 (corrosion-inhibiting adhesive primer (CIAP)) is substituted for BR 123 (non-CIAP). 

Cotter and Kohler likewise concluded that anodized surfaces produce the most durable adhesive bonds to aluminum. Their study included a comparison of the effects of using bare and clad alloys. In humid conditions, no significant difference was observed in the durability of chromic acid-anodized specimens prepared from the different alloys. In a corrosive environment, however, specimens prepared from the bare alloy proved to... 

In contrast to the relatively open cellular structures of etched and phosphoric acid-anodized aluminum, the chromic acid anodize treatment produces a very thick, dense oxide layer, consisting of solid columns, represented by the drawing in Figure 14. This presents a relatively smooth surface with no protrusions. 

Figure 14. Isometric drawing of the oxide morphology produced on an aluminum surface by chromic acid anodization. From ref. 69 by courtesy of SAMPE.

Aluminum surfaces are prepared for adhesive bonding in aerospace applications by either etching or anodization in acid solutions. (For less stringent strength and durability requirements, mechanical abrasion can be adequate.(2)) Common preparations result in microrough adherend morphologies, which studies have shown yield the best overall bond durability. Three of these surface preparations are described in this section Forest Products Laboratory (FPL) etching procedure, phosphoric acid anodization (PAA), and chromic acid anodization (CAA). 

Chromic acid anodization< ) is widely used to improve the corrosion resistance of aluminum surfaces, e.g., on window frames and in other architectural applications. Similarly, it was thought that the use of a good protective coating on the aluminum would protect the metal interface and thereby increase the bond durability of the joint. Although CAA is not as popular as FPL and PAA treatments in the United States, it is widely used for aerospace applications in Europe. 

The most successful application example of this technology is the Fokker F 27/F50 Friendship , in which about 70% of the structure (ca. 550 parts) are bonded. The proven durability (> 1000 aircraft in service for up to 30 years) of this design is also due to the superior fatigue properties compared to a riveted construction. Additionally the chromic acid anodizing (CAA) of the aluminum surftices before bonding resulted in corrosion-resistant joints. 

Figure 2. XPS survey spectra of aluminum alloys following chromic acid anodizing. (a) Commercial-purity aluminum which showed extremely poor durability, (b) Alloy BS LI57 (Al-5Cu-lMg-lSi-lMn) which exhibited extremely good durability despite the high concentration of Mg in the surface analysis. (From Ref. 10.)...

To show the idealized cell structure of a porous aluminum oxide grown post chromic acid anodization... 

It is well known that aluminum alloys require special treatments in order to develop good durability, but the toxicity of the favored chromic acid anodization has led to development of alternative treatments (Armstrong 1997 Critchlow et aL 2006). O Figure 48.14 shows a schematic set of results that illustrate the influence of this type of preparation. Short-term strength... 

Dichromates and chromic acid are used as sealers or after-dips to improve the corrosion resistance of various coatings on metals. Eor example, phosphate coatings on galvani2ed iron or steel as well as sulfuric acid anodic coatings on aluminum can be sealed by hexavalent chromium baths. 

In-service issues. As mentioned previously, many early service failures of bonded structure were due to adherend surface treatments that were unstable in long-term exposure to water. A majority of these problems were resolved by the adoption of surface treatments such as chromic and phosphoric acid anodize for aluminum details. The remaining few were alleviated by the adoption of phosphoric acid anodized honeycomb core and foaming adhesives resistant to water passage. Other service durability issues such as the cracking of brittle potting compound used to seal honeycomb sandwich assemblies, and subsequent delamination, have been minor in scope. 

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