Phosphoric acid anodization

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. 

In primer formulations for adhesive bonding of metals, the coupling agents that are most frequently used are those based on epoxy and amine functionalities. Aqueous solutions of aminosilanes have been successfully used for obtaining stable adhesive bonds between epoxy and steel [10] and epoxy and titanium [11,12], while epoxy functional silanes are preferable for applications involving aluminum substrates [13,14], A simple solution of % epoxy functional silane in water is currently used for field repairs of military aircraft [15] where phosphoric acid anodization would be extremely difficult to carry out, and performance is deemed quite acceptable. 

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

PAA. Phosphoric acid anodization (PAA) was developed by the Boeing Company in the late 1960s and early 1970s to improve the performance of bonded... 

ASTM D3933, Standard Guide for Preparation of Aluminum Surfaces for Structural Adhesives Bonding, Phosphoric Acid Anodization, A.STM, West Conshohocken, PA. El-Mashri, S.M., Jones, R.G. and Forty, A.J., Philo.s. Mag. A, 48, 665 (1983). 

We have also looked at the lap shear strength of selected EB-ciirable epoxy adhesives. Because the adhesives being developed were being used for both aluminum-to-aluminum and composite-to-composite applications the lap shear strengths for both adherends was measured. Aluminum adherends were T2024 phosphoric acid anodized according to ASTM 3933. The composite adherends... 

Phosphoric acid anodizing AMPS-sulfuric acid etchant 11.8 23.5... 

In-service issues. As mentioned previously, many early service failures of bonded structure were due to adherend surface treatments that were unstable in long-term exposure to water. A majority of these problems were resolved by the adoption of surface treatments such as chromic and phosphoric acid anodize for aluminum details. The remaining few were alleviated by the adoption of phosphoric acid anodized honeycomb core and foaming adhesives resistant to water passage. Other service durability issues such as the cracking of brittle potting compound used to seal honeycomb sandwich assemblies, and subsequent delamination, have been minor in scope. 

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). 

Progress has recently been made at successfully retarding this adhesion strength loss mechanism for the case of aluminum oxide through the use of a phosphoric acid anodizing pretreatment or special inhibitors86). These pretreatments are analyzed in Section 6.2. 

One very beneficial chemical pretreatment treatment for aluminum substrates is phosphoric acid anodization (PAA), which provides an oxide coating that is inherently hydration-resistant. Its stability is due to a layer of phosphate incorporated into the outer A1203 surface during anodization. 

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

PAA phosphoric acid anodized PBMG phenolic bulk-molding compound... 

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