Phosphoric acids anodizing with

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

Although distinct "metal" and "adhesive" sides were apparent upon visual examination of the debonded surfaces treated with 100 ppm NTMP, SEM analysis showed the presence of an adhesive layer on the "metal" side. XPS analysis indicated low A1 and 0 and identical high C levels on both debonded sides, confirming a failure within the adhesive layer (cohesive failure), i.e., the best possible performance in a given adherend-adheslve system. This result is similar to that obtained using a 2024 A1 alloy prepared by the phosphoric acid-anodization (PAA) process (16) and indicates the importance of monolayer NTMP coverage for good bond durability (Fig. 4). 

Phosphoric acid anodizing can be preformed in accordance with ASTM D 3933. 

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. 

Several surface preparations of aluminum are widely used in the aircraft industry. " The important steps in all surface preparations are vapor degrease, alkaline clean, rinse with water, chromic acid etch (FPL Etch), rinse with water, anodize with chromic acid or phosphoric acid, rinse with water, and dry at 140-160°F. Details of the procedures for the surface preparation of aluminum and other metals and composites are available from a number of sources. 

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. 

Phosphoric acid anodization (PAA)( > is a common method of surface preparation of aluminum adherends prior to adhesive bonding for aerospace applications. PA A surfaces are microscopically rough(6) and are more resistant to hydration than aluminum surfaces prepared by other treatments.(2,90) The microroughness provides for mechanical interlocking with the primer or adhesive, which results in a strong adhesive bond, while the environmental stability of the oxide (together with its porosity) results in excellent bond durability in hot, humid environments. 

FIGURE 2. Scanning electron micrographs of phosphoric acid anodized aluminum 7010 at higher magnification (49,000 X) (a) view perpendicular to outer surface (b) view at 45° to outer surface (c) cross section of anodic oxides showing barrier layer at base of pores. Reprinted with permission from Reference 9. Copyright 1983 Butterworth Science Ltd. 

FIGURE 32. Magnesium concentration as a function of argon ion bombardment dose for the various pretreated alloys (a) degreased (note no magnesium detected in the EIC oxide layer) (b) phosphoric acid anodized. Reprinted with permission from Reference 36. Copyright 1982 Gordon and Breach Science Publishers. 

Phosphoric acid anodization was developed by the Boeing Company in the late 1960s and early 1970s to improve the performance of bonded primary structures.(4) Bonds formed with PAA-treated adherends exhibit superior durability during exposure to humid environments compared to those formed with FPL-treated adherends, especially when epoxy adhesives are used. In addition, PA A bonds are less sensitive than FPL bonds to processing variables, such as rinse-water chemistry and time before rinsing. As a result, the PAA procedure has become the treatment of choice in the United States for critical applications. 

Samples of 0.030 in 2024 T3 bare aluminum were pre-cut, to suitable size for mounting in the spectrometer, with a tab to facilitate handling. The metal was subjected to 10 minute etch at 62-63 C in a sodium dichromate-sulfuric acid solution (per Section One, Handbook of Adhesives, Bloomingdale Aerospace Products, Havre de Grace, Maryland 21078), rinsed ten minutes in running, deionized water, and dried one hour at 63 C. One sample was then additionally subjected to the phosphoric acid anodization as outlined in BAG 5555. ... 

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. 

The next major improvement in aerospace adhesives occured in the late 1950s with the introduction of adhesives based on epoxy resins. Since these adhesives crosslink via an addition reaction, no volatiles are released during heat cure. This made low pressure bonding possible and the use of nonperforated honeycomb feasible in sandwich structure. Other improvements followed that resulted in more durable bonded structure. These include the development of corrosion inhibiting adhesive primers in 1968, corrosion resistant aluminum honeycomb in 1969, and the phosphoric acid anodizing process for preparing aluminum for bonding in 1974. 

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. 

Boeing Specifications, BAC 5555 (2001). Phosphoric Acid Anodizing of Aluminum, Issue N (Deoxidising in accordance with paragraphs 8.2.3 and 9.3). 

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