Forest Products Laboratory FPL etching

FIGURE 2. (a) XPS survey spectrum of a Forest Products Laboratory (FPL) etched aluminum adherend surface, (b) High-resolution A1 2p spectrum of a native oxide on aluminum. 

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). 

Low-frequency electrochemical impedance of an epoxy-coated Forest Products Laboratory (FPL)-etched aluminum adherend as a function of immersion time in 50°C water (Adapted from Davis et al. 1995b)... 

Wedge test results for Forest Products Laboratory (FPL)-etched and phosphoric add anodized (PAA) adherends (above) and PAA, chromic acid anodized (CAA), and boric-sulfuric acid anodized (BSAA) adherends below). The upper wedge test involved FM-123 adhesive the lower wedge test involved FM-94 adhesive so the crack lengths cannot be compared from one test to the other. Exposures were 50°C at 95% relative humidity (RH)... 

Forest Products Laboratory (FPL)-etched aluminum morphology. Top-. Isometric drawing adapted from Venables et al. (1979). Bottom High-resolution stereo scanning electron micrograph (SEM)... 

Aluminum can be cleaned in the same fashion as steel. Alternatively, it may be prepared for bonding by chemical etching. For instance, cyanoacrylates cure rapidly to give strong, durable bonds on aluminum etched by Forest Products Laboratory (FPL) chromic acid etching solution. 

Based, in part, on the results of the previous section, we decided to investigate the use of several organic hydration inhibitors on PAA and Forest Products Laboratory (FPL)(92) sodium dichromate-sulfuric acid etched surfaces.(3.5,i3,9i,93) One of the inhibitors that is very successful is nitrilotris methylene phosphonic acid NTMP, N[CH2P(0)(OH)2]3. It provides hydration resistance for FPL surfaces in a manner similar to anodization in phosphoric acid, i.e., it forms a protective molecular layer that does not allow the aluminum oxide substrate to hydrate until the inhibitor complex dissolves. 

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. 

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... 

The effect of environment on pretreated oxides has also been studied by STEM. Venables et al.U ) documented the degradation mechanism causing bond failure of FPL (Forest Products Laboratory) etched aluminum alloys in a humid environment. The surface oxide with a cellular and whisker structure converts to a surface hydroxide with a com flake type stmcture as shown by a schematic diagram of the mechanism in Figure 15. 

Aluminum and aluminum alloys. The effects of various aluminum surface treatments have been studied extensively. The most widely used process for high-strength, environment-resistant adhesive joints is the sodium dichromate-sulfuric acid etch, developed by Forest Product Laboratories and known as the FPL etch process. Abrasion or solvent degreasing treatments result in lower bond strengths, but these simpler processes are more easily placed into production. Table 7.14 quahtatively lists the bond strengths that can be realized with various aluminum treatments. 

Chemical treatments are usually most effective with aluminum alloys, especially when long-term environmental exposure is required. The sulfuric acid—dichromate etch FPL etch, named after Forest Products Laboratory, US Department of Agriculture www.fpl.fs.fed.us) has been used successfully for many decades. Techniques more recently developed are often modifications of the FPL procedure. Other important methods include chromate conversion coating and anodizing. Corrosion-resistant adhesive primer, as well as anodic and chromate conversion coatings, helps to prevent corrosive failure of adhesion. ... 

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