Anodising process

COATINGS PRODUCED BY ANODIC OXIDATION Tabic 15.1 Traditional anodising processes... 

Figure 2. The dependences of porous anodising process parameters (a) and porous anodic film parameters (b) on the forming voltage.

In the aerospace industry, aluminium and alloys are used widely and there are essentially two steps used to prepare these surfaces for bonding, i.e., an etching or anodisation process (to maximise adhesive strength) and a priming process (to prevent corrosion of assembled joints). 

The Phosphoric Acid Anodisation process utilising phosphoric acid solutions. This was originally developed by the Boeing Company and is the treatment of choice for critical applications in the USA. 

For metallic adherends most of these prelreatments either involve acid etching or an acid etch followed by an acidic anodising process. However, individual alloys and the particular surface structures caused by different heat treatments may respond differently to a given pretreatment for example, aluminium clad aluminium alloys (Alclad ) as opposed to the bare , unclad alloys. 

Anodising in chromic acid is the electrochemical pretreatment procedure favoured in Europe. European aerospace companies view this as giving a less friable oxide film than the phosphoric acid anodising process favoured in the United States and believe that it imparts much better durability to the bonded joint. This latter statement is somewhat debatable in the light of both experience in the laboratory and the actual chemical stmcture of the oxide films themselves. 

BSAA, now being called a chromate-free anodising process [35], theoretically uses no chromate salts in the cleaning stages or as the electrolyte. A minor exception to this chromate-free concept does, however, occur as some operators use a tri-acid based deoxidiser which does contain some chromic acid together with sulphuric and hydrofluoric acids. 

Yendall, K. A., Critchlow, G. W., Andrews, F. R., Bahrani, D. (2002). An evaluation of chromate-free anodising processes for aerospace applications. Extended abstracts for Euradh 2002. lOM Communications, London. 

This idea was taken up at the beginning of the 20th century, and a first patent was filed in 1911 by the Frenchman de Saint-Martin, who proposed an anodising process in a sulphuric medium containing iron sulphate. 

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... 

Three processes are believed to compete during the anodisation process field-assisted oxidation of Ti to TiOi, field assisted dissolution of Ti metal ions in the electrolyte and the chemical dissolution of Ti and TiOi due to etching by fluoride ions, enhanced by the presence of (Equations 3.27-3.29). ° The chemical steps are not thought to... 

An innovative technology has been developed by UK anodisers (HMF Ltd. and ASE Ltd.) that allow for the production of anodised surfaces that prevent micro-crack formation and can make possible the production of aluminium tools that have improved wear resistance with cyclic heat resistance up to 500°C and, in addition, very high thermal emissivity. This anodising treatment is referred to as the ALAMO anodising process. 

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