Sulphuric acid anodising

In general, the suitability for anodising depends on the composition and the microstructure of the aluminium, and may vary firom one alloy series to another. The resulting anodic coatings will not necessarily have the same properties (Table B.5.1). 

This is the most common anodising process. It is used for decorative purposes, for hard coatings and for protection against weathering. The classic treatment parameters (architectural quality) are 

This treatment is performed discontinuously on profiles, castings or sheet, or continuously on coiled strip. The oxide thickness depends on the intended use  

The structure of anodic coatings depends on the composition of the bath and the process parameters. It consists of hexagonal cells pierced by micropores, whose diameter, say for a coating thickness of 15 [xm, is about 1000 times less than their film thickness. 

These porous coatings lend themselves to adsorption coloration, either by immersion in a bath of special dye or by electrolytic coloration treatments. The latter are performed with alternating current, directly after anodising. 

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Chromic acid anodise O Chromic-sulphuric acid etch A Sulphuric acid anodise A Vapour degrease... 

Electrochemical chromic acid anodising Electrochemical phosphoric acid anodising Electrochemical sulphuric acid anodising Electrochemical hard anodising Electrochemical boric acid/sulphuric acid anodising Chemical sol-gel procedures Activated plasma... 

Sulphuric acid anodising techniques are more generally used for the production of decorative aluminium sheets to improve the durability of the aluminium (cf. chromic acid anodising when 20 ixm, or greater, oxide films are grown and then sealed in hot water/steam) and for the preparation of surfaces prior to priming... 

This is a variant of the standard sulphuric acid anodising procedure where the bath temperature, anodising time and current density parameters are somewhat modified to give the densest oxide film growth of all. The hardness of the oxide film increases and the pore size is significantly reduced. 

The cleaning, alkaline etch and desmutting stages are carried out as for conventional sulphuric acid anodising. 

As already indicated, it is generally accepted that the oxide film produced either by conventional sulphuric acid anodising or by hard anodising in sulphuric acid, does not give the optimum surface structure for structural adhesive bonding, which is why these techniques seem to be rarely used in the aerospace industry. 

Boric acid-sulphuric acid anodising (BSAA)... 

Research by Critchlow and co-workers [35] has shown that these pretreated substrates can be made suitable for structural adhesive bonding if either the anodising temperature is raised to 35 C or the anodised adherends are dipped in dilute phosphoric acid prior to bonding - i.e. similar to the techniques employed by Arrowsmith et al. [34] when using sulphuric acid anodising. 

Unlike aluminium, DC sulphuric acid anodising is an acceptable pretreatment prior to bonding titanium components. 

Sulphuric acid anodising (continuous or static) Transparency, reflectivity, brilliance. TTie lower the Fe and Si content, the more transparent the layer >ec< ation, optical reflectors... 

Sulphuric acid anodising Alloys easy to anodise. Protection, decoration, good corrosion resistance, resistance to scoring Architecture, building, kitchen utensils... 

Chromic acid anodising, sealed with dichromate. Sulphuric acid anodising, sealed with dichromate Surface preparation for adhesive bonding. Limited film thickness, more or less porous layer due to copper dissolutioiL Provides limited protection against corrosion Aeronautical and mechanical applications... 

Sulphuric acid anodising, colourless or electrolytically coloured Most alloys easy to anodise. Decoration, good durability of surface aspect, good corrosion protection MetaUic fittings, buildings, boat masts... 

Sulphuric acid anodising Good durability of surface aspect, corrosion protection. Grey colour due to presence of silicon Mechanical and electrical applications 42100 (A-S7G03), 44200 (A-S13)... 

Velterop L. (2003), Phosphoric sulphuric acid anodising An alternative for chromic acid anodising in aerospace applications , ri7BA7era//Mrgie, 43, 284-9. 

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