Pre-treatment of aluminium

In contrast to the aerospace industry (see Aerospace applications), conditions prevailing in automobile manufacture dictate that pre-treatments (see Pre-treatment of aluminium) should have short process times and be tolerant of production variables. The surfaces produced should retain adequate bonding properties during extended storage under poorly controlled conditions. The surface may need to be sufficiently conductive to permit weld-bonding techniques. Pre-treatments applied at the coil stage must be compatible with mill oils and press lubricants, and withstand mechanical deformation in the press. 

When optimized, these are suitable preparations for both adhesive bonding and painting their effect is to give a reproducible, oxidized surface on the aluminium. Initial adhesion is generally good, and durability fair, but not as good as with Anodizing pre-treatments (see Durability fundamentals. Pre-treatment of aluminium). 

Chromic acid etching and other techniques are compared in Pre-treatment of aluminium. 

Pre-treatment of aluminium). These surfaces all had whiskers of oxide on the outermost parts and in some cases, depending upon the details of the treatment, there were also pores similar to those already described. These whiskers were 100-400 A long and 50-100 A in diameter and the pores were about 400 A in diameter. Their work quite clearly implies that there is a double interlocking, with the whiskers being embedded in adhesive and with the adhesive penetrating the pores. Thus, the region between metal and adhesive has the character of a composite. The mechanism of failure depends upon a viscoelastic deformation of the polymer-adhesive together with a rupture of the other adhesive bonds that have been formed. 

A summary of the progress in evaluating adhesive bond performance over a 25-year period has been pubhshed by Minford. More recent papers relating to Famborough work include a comparison of anodized and chromate etch treatments for clad aluminium (the durability of the former was superior, see also FPL etch. Anodizing, Pre-treatment of aluminium) and a comparison of the natural weathering and accelerated ageing of joints. ... 

Pre-treatment of aluminium G W CRITCHLOW Chronic acid etch, chronic acid anodizing, phosphoric acid anodizing relative bond durability... 

Painting zinc and aluminium makes special demands on the pre-treatment of the substrate. Chromate treatment provides a good basis for paint, and it is still in widespread use in spite of the toxicity of chromate. Phosphate treatment or washpriming can also be used. Alternatives for aluminium are light blast cleaning or anodizing. 

Similarly, with aluminium honeycomb structures, failure may occur at the skin or the surface of the honeycomb itself, especially if the respective surface preparation is poor (see Pre-treatment of aiuminium). Untreated aluminium alloy honeycomb can result in disbonding in service. The use of anodized aluminium alloy honeycomb is strongly recommended. 

The article. Pre-treatment of metals prior to bonding, provides a general survey this article is concerned with the industrially important aluminium and its alloys. 

The importance of a clean surface for painting is paramount (see Engineering surfaces of metals). For metallic substrates, either mechanical or chemical surface treatment is almost always needed to remove surface contamination and promote adhesion. Chemical pre-treatment of metals prior to painting is an effective method of surface conversion that can provide both corrosion resistance and improved paint adhesion. For ferrous metals, the most common chemical pre-treatment is a phosphate for aluminium (and many other non-ferrous metals), the most effective pre-treatment is chromate. Metals must first be cleaned with an alkaline cleaner and then rinsed before they can be converted chemically in an immersion bath or by spray. 

The more common environmental problem with metal joints is attack by water. In addition to alloy composition, pre-treatment and primer, durability is affected by the adhesive used (see Pre-treatment of metals prior to bonding. Pre-treatment of polymers. Primers for adhesive bonding). Sell has ranked adhesives in the order of durability they provide with aluminium substrates ... 

Bmsciotti F., Batan A., De Graeve I., Wenkin M., Biessemans M. et al. (2010), Characterization of thin water-based silane pre-treatments on aluminium with the incorporation of nanodispersed CeOj particles . Surf. Coat Teeh., 205,603-13. 

The painting procedure for other metal surfaces, although similar, the process of pre-treatment for cast iron components or non-ferrous metals, such as aluminium and copper, may need more care. The process of pre-treatment in such cases may vary slightly than for MS, as noted below. Such surfaces may require a change in the type of chemicals, their concentration and duration of treatment. The final surface preparation and painting procedure, however, will remain the same for all. 

In addition to examining pre-exposure effects, the slow strain-rate testing technique has been used increasingly to examine and compare the stress-corrosion susceptibility of aluminium alloys of various compositions, heat treatments and forms. A recent extensive review draws attention to differences in response to the various groups of commonly employed alloys which are summarised in Fig. 8.57. The most effective test environment was found to be 3 Vo NaCl -F 0.3 Vo HjOj. The most useful strain rate depends upon the alloy classification. 

So far, emphasis has been placed on the treatment of polyolefins to improve printability, but the same treatments affect also other processes. As an example, while an untreated polyethylene surface will not retain an adhesive the treated and modified surfaces are receptive to adhesives. A field of increasing importance for packaging is the production of laminates of film with other materials and, as examples, suitably pre-treated polyethylene film can be bonded securely to substrates such as cellulose or polyester films and aluminium foil, using standard adhesives, whereas this cannot be achieved without pre-treatment. 

It may be necessary to exclude certain undesirable contaminants from starting materials, such as pseudomonads from bulk aluminium hydroxide gel, or to include some form of pre-treatment to reduce their bioburdens by irradiation, such as for ispaghula husk and spices. For biotechnology-derived drugs produced in human or animal tissue culture, considerable efforts are made to exclude cell lines contaminated with latent host viruses. Official guidelines to limit the risk of prion contamination in medicines require bovine-derived ingredients to be obtained from sources where bovine spongiform encephalopathy (BSE) is not endemic. 

Leung and Henderson (1982) have not been able to confirm these findings of grossly different curve slopes for different types of serum and standard samples. Bettinelli et al. (1985) made a thorough evaluation of the direct measurement of aluminium in serum using GF-AAS with the stabilized temperature furnace with the L vov platform. Their recommendation was to use pyrolytically coated graphite tubes and to keep the method as simple as possible with minimal sample pre-treatment. Slavin (Personal Communication) also has been unable to observe major slope change problems between aqueous and... 

L. Cecchetto, A. Denoyelle, D. Delahoughse, and J.-P. Petit, A silane pre-treatment for improving corrosion resistance performances of emeraldine hse-coated aluminium samples in neutral environment App/. Surf. Set, 254, 1736-1743 (2008). 

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