Anodization, chromic acid

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],... 

EPA, 1993. U.S. EPA, Office of Air Quality Planning and Standards, "Chromium Emissions from Chromium Electroplating and Chromic Acid Anodizing Operations Background Information for Proposed Standards," EPA-453[R-93-030a, Research Triangle Park, NC, July 1993. 

Chemically produced oxides can be formed by treatment in 2-3% sodium carbonate containing 0.1% sodium dichromale for 10-20 minutes at 66°C. followed by sealing" in 5% sodium dichromale at 82-B8 C for 10 minutes. Such coatings are softer, more porous, and not as effective us those produced by chromic acid anodizing. 

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. 

A review of the XPS results in Tables 2 and 3 suggests that for the lOV chromic acid anodized Ti 6-4 adherend, the high Ti surface concentration on the L13-10-50 adhesive failure surface (APS) thermally aged for 10,000 hr is associated with the lowest strength, it is proposed that long-term thermal aging of bonded lap shear samples weakens the surface oxide. The high Ti concentration observed on the L13-10-50 adhesive failure surface (APS) results from the fracture of the surface oxide layer. 

Fig. 1 illustrates the lap shear strengths obtained with LARC-13 using the various surface preparations. Both chromic-acid anodize (CAA) and phosphoric-acid anodize (PAA) exhibit superior bond properties. 

The PPQ adhesive system made up of PPQ adhesive resin, dilute PPQ resin primer, and chromic-acid anodized surface treatment was selected based upon the data generated from this study, as well as from internal Boeing research programs. Lap shear data in Fig. 2 shows the phosphoric-acid anodized surface treatment superior to the chromic-acid anodized. However, Boeing data on both these surface treatments have shown that phosphoric-acid anodize is not stable in long-term (125 hr) elevated temperature (600 F) environments. Chromic-acid anodize exhibited good thermal... 

The phosphoric acid-anodized process provides markedly improved stressed-bond joint durability and retards bond-line crevice corrosion (started at an edge) in severely corrosive environments when compared to chromic acid-anodized and FPL etched. (See Chapter 9 for description.)... 

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). 

Figure 12. XPS atomic concentrations of a chromic acid anodized titanium alloy (34).

The failure surfaces produced following tests of lap shear or wedge samples of titanium alloy bonded with epoxy depended on the surface pretreatment (34). Simple add etching of the adherend produced primarily interfacial failure between the oxide and epoxy whereas chromic acid anodization of the adherend resulted in failure within the oxide layer as in the case discussed above. 

J. P. Wightman and J. A. Skiles, "Analysis of Chromic Acid Anodized 11-6A1-4V Adherends with High Temperature Structural Adhesives," in Proc. 33rd SAMPE Svmtx)sium. pp. 473-483, SAMPE, Azusa, CA (1988). 

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