Chromic acid anodized surface

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... 

For bonding apphcations, the performance of sulphuric acid and chromic acid anodized surfaces may be improved by controlled oxide dissolution in phosphoric acid to yield morphologies more readily penetrated by adhesives and primers. 

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],... 

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. 

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). 

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. 

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. 

More recently the author has published the results of eight-year testing in the seacoast of sulfuric-acid-anodized surface aluminum joints and demonstrated the excellent resistance of anodized surface joints to seacoast and marine conditions.The same excellent performance would also be achieved with aluminum adherends that have been anodized in chromic or phosphoric acids.(23,24,91)... 

Fortunately, a great deal of work has been accomplished in a short time, and notably by aircraft manufacturers as well as adhesives suppliers. There are several important contributions in this area. First, in the area of FPL etch, the important consideration is what kind of bonding surface is provided by the preparation method. The chromic acid/sulfuric acid not only removes air oxide and leaves base metal it also has a chemical potential which produces a very thin anodic type oxide layer of the surface. This oxide layer is porous, due to the dissolving action of the strong acid mixture, and thus the surface produced may be characterized as a thin, porous anodic oxide. (A. W. Smith compared it to a 3V chromic acid anodize based on impedance measurements.) The optimum conditions for this etch as to time, temperature, and composition have been studied at Fokker and by Smith and generally a somewhat higher concentration of sodium dichromate or chromic acid was recommended than was commonly used. 

More recent work with the PPQ in Eq. (3) has involved chromic acid and phosphoric acid anodized Ti surface treatments, which result in better moisture resistance but less thermal resistance than the surface from phosphate fluoride treatment. Chromic acid anodized Ti TSS provided strengths of 5000 psi at 25°C (cohesive failure), 2,000 psi at 232°C after 5,000 hours at 232°C in air (mixed failure) and low strengths at 232°C after 10,000 hours at 232°C in air (100% adhesive failure).An anodized Ti surface degrades when bonding temperatures approaching 370 C are employed. This may have been one of the factors which caused lower strengths at 232 C after aging. 

Ti surface treatment, 10 V chromic acid anodized, 112 glass with A-llOO finish, no filler. Predominantly thermoplastic failure. 

In the BOEING wedge test, the test specimens are immersed in water at 50°C for progressive periods of time, and the length of the crack is measured after each period with a microscope. If the siuface preparation was not good enough, then the crack will progress quickly and far, and the aircraft constructors have their own specifications for this test that they perform with various surface preparations such as PAA (phosphoric acid anodization), CAA (chromic acid anodization), etc. 

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.)...

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