Phosphoric acid anodizing process

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. 

The phosphoric acid-anodized process provides markedly improved stressed-bond joint durability and retards bond-line crevice corrosion (started at an edge) in severely corrosive environments when compared to chromic acid-anodized and FPL etched. (See Chapter 9 for description.)... 

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. 

Boeing Process Specification, BAC 5555 Issue M. Phosphoric acid anodizing of aluminum for structural bonding. Boeing Airplane Company, 1995. 

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). 

Minford also studied the effects of four different phosphoric acid processing conditions under stress and intermittent salt-water immersion testing of 6061-T6 aluminum alloys. None of the joints pretreated by varying phosphoric acid anodizing conditions failed after 480 days exposure, even... 

Bonding of aluminum comprises greater than 90% of the end uses for epoxy film adhesives. At least three major cleaning methods have been used chromic acid etch, phosphoric acid anodize and sulfuric acid anodize. Each process produces a different microstructure and chemical surface to which to bond. A corrosion-resistant epoxy primer is often used to protect the delicately etched surface during assembly operations. The adhesive must then bond to the primer and provide the myriad of intricate test strengths required by each aircraft producer, each aircraft, and each design. The tedious nature of optimizing each adhesive to meet each detailed specification becomes apparent and will not be further pursued in this discussion. 

A further enhancement in aluminum/epoxy joint durability in a moist environment is obtained by anodizing the aluminum after FPL treatment (80). Typically, samples are anodized for several minutes in an aqueous solution of phosphoric acid (phosphoric acid anodization or PAA) before rinsing and drying in warm air. The process produces a thin, uniform oxide layer near the bulk metal and a... 

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. 

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. 

All of this should have come to an end in the middle 1970s when phosphoric-acid anodizing and optimized etch processes were established, particularly since Bethune at Boeing developed the simple wedge-crack test, ASTM D-3762, Marceau et al. (1977) that made it easy to distinguish between properly and improperly prepared bonding surfaces. The properly prepared surfaces had a stable oxide coating, with many pores that the primer could penetrate, as shown in O Fig. 44.7. 

Potrooms and anode baking Wet process phosphoric acid Coed cleaning and dryer Transfer, loading ... 

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