Acid-etched adherend

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. 

Venables and co-workers have reported (2) results of depth profiling of the surface oxide layer on Ti 6-4 adherends by AES. Acidic etches lead to rather thin (20 nm) oxide layers whereas anodized oxide films are considerably thicker (40-80 nm). Venables has further reported (3) that initially an amorphous oxide layer forms on Ti 6-4 adherends which can be converted to a crystalline layer at elevated temperatures. 

In addition to the effect of the chemistry and the physical properties of the adhesives, the surface preparation of the adherend also has a marked effect. Aluminum surface treatments vary from simple solvent wiping to anodization. Steel treatments vary from abrasive grit blasting to acid etching. In general, it is found that the better the initial surface preparation, the more durable the bond. 

Chromic acid-etched A1 adherends. Lap-shear, 90° peel and honeycomb joint geometries. Two years exposure in temperate, hot-dry, and hot-wet regions. 

Although the exact conditions were not specified in their paper, it is reasonable to assume that the results referred to specimens prepared with solvent-degreased adherends, whereas those of Brewis et al were chromic acid etched. The differences in behavior observed in the two studies therefore probably arise because of the different surface treatments. The more durable chromic acid-etch interface was not appreciably attacked during the time of exposure, while the solvent-degreased interface was. 

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. 

The steps used to produce an anodized surface on titanium adherends are similar to those used for aluminum adherends. Initially, the adherend must be degreased to remove organic contaminants. This is followed by an acid etch to remove the oxide scale. The anodization (if used) is done at constant voltage and the adherend is rinsed and dried. The details of the procedure are presented in Tables 4 and 5. Oxide morphologies resulting from the CAA, SHA, and AP processes are presented below. 

The adherends of the specimens were prepared using an acetone wipe, a base/acid etch, or a P2 etch. 

As shown in Table 5, in the mode I test, the thicknesses of the residual adhesive layer on the failure surfaces were about 250 xm for all the specimens with different surface preparations, which indicated that the failures all occurred in the middle of the adhesive layer in the test regardless of the surface preparation method since the total thickness of the adhesive of the specimens was 0.5 mm. When the phase angle increased as in the asymmetric DCB test with h/H = 0.75, which contains 3% of mode II fracture component, a layer of epoxy film with a thickness of around SO xm was detected on the failure surfaces of all the specimens. Although the failure was still cohesive, the decrease in the film thickness on the metal side of the failure surfaces indicated that the locus of failure shifted toward the interface due to the increase in the mode mixity. On the other hand, because the failure was still cohesive, no significant effect of interface properties on the locus of failure was observed. When the mode mixity increased to 14% as in the asymmetric DCB test with h/H = 0.5, where the mode mixity strongly forced the crack toward the interface, the effect of interface properties on the locus of failure became pronounced. In the specimen with adherends prepared with acetone wipe, a 4-nm-thick epoxy film was detected on the failure surfaces in the specimen with adherends treated with base/acid etch, the film thickness was 12 nm and in the P2 etched specimen, a visible layer of film, which was estimated to be about 100 nm, was observed on the failure surfaces. This increasing trend in the measured film thickness from the failure surfaces suggested that the advanced surface preparation methods enhance adhesion and displace failure from the interface, which also confirmed the indications obtained from the XPS analyses. In the ENF test, a similar trend in the variation of film thickness was observed. 

For metallic adherends most of these prelreatments either involve acid etching or an acid etch followed by an acidic anodising process. However, individual alloys and the particular surface structures caused by different heat treatments may respond differently to a given pretreatment for example, aluminium clad aluminium alloys (Alclad ) as opposed to the bare , unclad alloys. 

The adherends are initially degreased using standard procedures. This process then makes use of an acid etch (to replace grit blasting) followed by an alkaline oxidation (to replace a peroxide oxidation). The details are as follows ... 

Although the evolution of surface chemistry depicts the hydration of bare surfaces, the same process occurs for buried interfaces within an adhesive bond. This was first demonstrated by using electrochemical impedance spectroscopy (EIS) on an adhesive-covered Forest Products Laboratory (FPL, a sodium dichromate + sulfuric acid etch) aluminum adherend immersed in hot water for several months (Davis et al. 1995b). EIS, which is commonly used to study paint degradation and substrate corrosion, showed absorption of moisture by the epoxy adhesive and subsequent hydration of the underlying aluminum oxide after 100 days (O Fig. 8.5). At the end of the experiment, aluminum hydroxide had erupted through the adhesive. Later, cross-section micrographs of bonded tapered double-cantilever beam... 

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