Conversion Coatings

Passivation

Aluminium can also be protected by the use of conversion coatings, sometimes known as passivation, these coatings are usually done by immersion and do not involve electrolysis. These coatings not only protect the aluminium from corrosion but are electrically conductive, they are frequently used on components where electrical continuity is required and also as a pretreatment for subsequent paint application. The most common processes are the “Alocrom” products which are chrome based, both yellow and clear versions are available, but in view of the current requirement to reduce the use of chrome salts we also offer a clear non chrome containing version, Iridite NCP.

Where electrical conductivity is required on assemblies of anodised components it is usual to mask the conducting areas prior to anodising then alocroming the unanodised areas as a final operation.

Phosphating

Phosphating is the application of a porous crystalline coating usually applied to ferrous based materials. The process is non electrolytic with the phosphate layer being produced by catalytic action between the component material and the Phosphating solution. This catalytic action producing an exceptionally strong bond of the phosphate layer to the component surface.

The coating produced is crystalline and porous and forms a very good base for subsequent applications. As an undercoat for paints it dramatically improves adhesion of the painted layer, and the application of specialised oils and greases will enhance the corrosion performance of the component, or provide lubricity for subsequent manufacturing operations.

Although most commonly applied to ferrous materials, it can also be applied to zinc or zinc plated bases where, prior to subsequent painting operations, both corrosion performance and adhesion are considerably improved.

Ashton & Moore Ltd. offers two types of Phosphating, Zinc Phosphate which gives a finer grain with a wider range of thicknesses being achievable, or Manganese Phosphate which is limited to heavier films, The Manganese phosphate coating also has different crystalline structure to zinc phosphate, this structure is laminar rather than columnar, compressive, and imparts greater corrosion resistance and lubricity without the need for additional coating, although these can be specified if required. Manganese phosphate is particularly good for components where “wearing/running in” is required, and because it is a compressive, coating continues to offer lubricative properties for extended life.

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