Surface preparation aluminum

Mazza, J.J. and Kuhbander, R.J., Grit blast/silane (GBS) aluminum surface preparation for structural adhesive bonding, WL-TR-94-4111. Materials Laboratory, Air Force Materiel Command, September 1999. 

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. 

Combined methods. More than one cleaning method is usually required for optimal adhesive properties. A three-step process that is recommended for most substrates consists of (1) degreasing, (2) mechanical abrasion, and (3) chemical treatment. Table 7.11 shows the effect of various combinations of aluminum-surface preparations on lap-shear strength. 

Representation of pores in anodized aluminum surface prepared for bonding (Thrall Jr and Shannon (1985), Chapter 4 Marceau, Boeing)... 

Results from exposure corrosion testing show that aluminum surfaces prepared with a chromate conversion coating and a chromate-free primer perform much better than a chromate-free sol-gel type of conversion coating with the same chromate-free primer, " leading to the necessity for enriching the sol-gel coating with efficient inhibitors. 

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. 

Quantifying the effect of surface roughness or morphology is difficult, however. Surface preparations that provide different degrees of surface roughness also usually produce surfaces that have different oxide thicknesses and mechanical properties, different compositions, or different contaminant levels. The problem of separation of these variables was circumvented in a recent study [52] by using a modified microtome as a micro milling machine to produce repeatable, well-characterized micron-sized patterns on clad 2024-T3 aluminum adherends. Fig. 2 shows the sawtooth profile created by this process. 

Fig. 9. Wedge test results of aluminum adherends with the following surface preparations FPL, PAA, and FPL followed by an NTMP treatment. Adapted from Ref. [42].

Despite the progress outlined in this chapter, much work remains to be done in the metal surface preparation arena. For example, there is still no ideal surface preparation method that does for steel what anodization processes do for aluminum and titanium. The plasma spray process looks encouraging but because it is slow for large areas and requires rather expensive robot controlled plasma spray equipment, its use will probably be limited to some rather special applications. For more general use, the sol-gel process has potential if future studies confirm recently reported results. 

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). 

Effects on lap shear strength of EB-cured epoxy adhesives from different surface preparations on aluminum and composite adherends... 

Bituminous This term is used for products obtained from both petroleum and coal tar sources but the petroleum products are the more widely used. These materials are very resistant to moisture and tolerant to poor surface preparation. They are only available as black, dark brown or aluminum pigmented. The last has reasonable outdoor durability but, without the aluminum, the film will crack and craze under the influence of sunlight. Normally they cannot be over-coated with any other type of paint, because not only will harder materials used for over-coating tend to crack or craze but there is also a possibility that the bitumen will bleed through subsequent coats. The best use is as a cheap waterproofing for items buried or out of direct sunlight. 

Zinc silicate This material has good corrosion resistance and can withstand temperatures up to 540°C, particularly when over-coated with silicone-based aluminum. The zinc silicate requires a high standard of surface preparation before application. 

Depositing, in a vacuum, a thin layer of vaporized metal (generally aluminum) on a surface prepared by a base coat. 

Nearly all aluminum parts are first treated in an alkaline solution. In some cases, this is only a cleaner for removing grease and soils sometimes it is a mild etchant to remove a layer of metal and its oxides. Frequently, this is all the surface preparation that is necessary. Any further preparation... 

The surface preparation operations for strip are dependent upon whether the basis material is steel or aluminum. The surface preparation steps for steel strip are minimal in comparison to porcelain enameling on steel sheets because precleaned strip steel is used. Steel strip is nickel immersion plated prior to the enameling step. Surface preparation for aluminum involves only cleaning. The enamel for either basis material is applied by means of spray guns that are aimed at the surface of the moving strip. Two coats are normally applied, the strip being fired after each coat. 

The introduction of enamel into the wastestream results in an increase in the concentration of metals, but these metals (antimony, titanium, zirconium, tin, cobalt, and manganese) are in solid form whereas the metals generated by surface preparation are normally in dissolved form. These solid metals increase the suspended solids concentration of the stream. Other metals that may be found in the enamel preparation and application wastestream in significant amounts include aluminum, copper, iron, lead, nickel, and zinc. Table 8.2 presents pollutant sampling data for the processes used in the porcelain enameling on steel industry. 

Wastewaters from this subcategory come from surface preparation, enamel application, ball milling, and related operations. Constituents of this wastewater include aluminum and components of the surface preparation solutions and the enamels being applied.3-6... 

This facility produces 360 m2/h of porcelain enameled aluminum for 4000 h/yr, and uses 0.038 m3 of process water/m2 of product coated. The mixed wastewater stream is treated by equalization (settling), pH adjustment (lime or acid), polyelectrolyte coagulation, clarification, and contractor removal of the resulting sludge prior to discharge to a surface stream. Process water flow for this production consists of 8.12 m3/h and 4.37 m3/h for surface preparation and coating operations, respectively.3 5... 

Substrate Characterization. Test coupons and panels of 7075-T6 aluminum, an alloy used extensively for aircraft structures, were degreased In a commercial alkaline cleaning solution and rinsed In distilled, deionized water. The samples were then subjected to either a standard Forest Products Laboratories (FPL) treatment ( 0 or to a sulfuric acid anodization (SAA) process (10% H2SO4, v/v 15V 20 min), two methods used for surface preparation of aircraft structural components. The metal surfaces were examined by scanning transmission electron microscopy (STEM) In the SEM mode and by X-ray photoelectron spectroscopy (XPS). 

Figure 1. Structure of a Forest Products Laboratories (FPL) prepared aluminum surface (7).

Figure A. NTMP coverage as a function of concentration on SAA-prepared 7075-T6 aluminum surface.

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