Chromic acid anodize pretreatment

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. 

As discussed below, the classic example of the problem of oxide stability upon exposure of bonded joints is with aluminium and its alloys and this aspect has therefore been investigated in detail by the aerospace community. Particularly, the fundamental micro-mechanisms have been studied in order to explain observations such as those shown in Fig. 8.10, which reveals the effect of three common aerospace surface pretreatments upon the subsequent durability of the adhesive joints. The three treatments which have been studied in some detail are chromic acid etch (CAE), chromic acid anodize (CAA) and phosphoric acid anodize (PAA), and details of the processes were given in Section 4.3.4.5. 

Pretreatment is particularly important for structural adhesive joints such as those used in aircraft and motor vehicle construction. Aluminum adhesive surfaces are subjected to chromic acid treatment and an anodizing process, major factors in the overall joining costs. The disposal of used pickling baths is particularly demanding and expensive, especially in view of the CrVl ions resulting from the pickling process. 

J. P. Wightman (VPI, Blacksburg, VA) Why do you think anodization (phosphoric acid or chromic acid) of Ti 6-4 adherends gives better bond properties when compared to other pretreatments ... 

Anodizing pretreatments on aluminum should be ranked as phosphoric acid chromic acid > sulfuric acid with regards to their bond durability under simultaneous stressing and corrosive, saltwater exposure conditions. 

Typical procedures for chromic and phosphoric acid anodizing are given in Pretreatment of aluminium, and the durability (see Durability - fundamentals) of bonds to the surfaces formed is discussed. An example of a profile of elemental composition in depth for a phosphoric acid anodized film is shown in Auger electron spectroscopy. 

Like the aluminum alloys mentioned earlier, the titanium alloy surface can be pretreated by an anodizing process in which there is a controlled rate of surface dissolution accompanied by new oxide formation. Anodizing in bath solutions of either a chromic acid-fluoride or an alkaline-peroxide mixture produces joints with both high initial joint strength and long-term durability. 

Pretreatment of platinum in hot chromic acid was shown [83] to be equivalent to anodic activation. The electrode has a large reactivity after reduction of the oxygen layer that was formed at open circuit. In contrast to the pretreatment with hot chromic acid, the i — U curve of the first sweep after treatment of the platinum electrode in hot nitric acid does not have the shape characteristic for a clean surface. An intermediate product in the reduction of nitric acid seems to be strongly adsorbed on platinum. It takes several hours of continuous cycling between 0.05 V and 1.4 V at 30mV/sec before the i- U curve regains the regular shape. 

Figure 8.16 X-ray photoelectron spectroscopy analysis of aluminium alloy surfaces (A1 2/7 peak) before and after aging [86]. (a) Chromic acid etch (CAE) pretreatment (b) phosphoric acid anodized (PAA) pretreatment.

Conversion coatings are not as protective as anodized coatings. In most cases, conversion coated surfaces are subsequently primed or painted. Xraditionally, pretreatment of Al has relied on chromate-based systems. Xhere are two general classes of chromate conversion processes the activated acidic formulations such as the chromium chromate processes that use a sodium fluoride-chromic acid chemistry, and alkaline oxide processes based on a sodium chromate-sodium hydroxide or carbonate chemistry. 

For each alloy studied, the chromic-sulfuric acid etching, Alodining, and anodizing surface pretreatment produced the most durable joints, although the relative order of effectiveness could vary with different alloys. 

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