Aluminum phosphoric acid anodized

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. 

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... 

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. 

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. 

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... 

ASTM D3933-98 Standard practice for preparation of aluminum surfaces for structural adhesive bonding (phosphoric acid anodizing). 

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. 

Figure 13. Isometric drawing of the oxide morphology produced on an aluminum surface by phosphoric acid anodization. From ref. 69 by courtesy of SAMPE.

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. 

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