Hard Anodising

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The anodising process provides a method of electrolytically increasing the thickness of the naturally occurring oxide film on aluminium and its alloys, to give a coating which may be considered in many ways analogous to the case hardening of steel.
It is distinguished from ‘decorative’ or ‘architectural’ anodising in that the process is operated in a manner, which gives optimum technical properties without regard to aesthetic effect; thus, the coatings may be grey or tinged with yellow or brown, according to the alloy and coating thickness. Colouring may be carried out by dyeing after anodising, but the colours so obtained will be diluted by the ‘natural’ colour of the coating. Dichromate sealing also imparts a colour to the film. Anodising lowers the fatigue strength of alloys but this reduction is said to be restored to some extent by hot sealing.

It is difficult to present a simple picture in writing of the possibilities and limitations of the process because there are many factors that affect the performance of the coatings, but the following will give a general picture that can be expanded in more detail when we know the nature of your interest.

Hard anodising produces darker coatings than the normal ‘decorative’ process, but the exact shade depends upon coating thickness and anodising conditions (which we can control), as well as alloy, thermal history, the degree of hot or cold work, etc, over which we have no control. We cannot, therefore, colour match except between components from the same batch of metal. Some specifications allow the anodiser wide tolerances in his process, which must be closed down if matching is to be achieved. Matching must, therefore, be covered at the time of order.

Total growth

The total major and minor growth on internal diameter is determined by the flank angle of the thread and the requested coating thickness.

This is the allowance that must be used to calculate allowances for growth due to anodising.

Please remember that on any blind deep holes the anodic film thickness will drop off considerably with depth.

Sulphuric Acid Anodising

Often known as natural, clear, or silver anodising, this is the most common type of anodising. The description covers a wide range of processes at different levels and differs from hard anodising in that the electrolyte temperature is higher, and the current density employed is lower. The types of sulphuric acid are sub-divided into classes mainly determined by the field of application. All anodising processes are sealed unless the film is used as a primer for paint or adhesives.

Anodising For Protection

This is the most common form of anodic coating. Film thickness usually between three and thirty microns. The thinnest films stopping aluminium blackening the hands when handled, the thickest offering substantial corrosion resistance and electrical insulation without the loss of fatigue strength that can accompany hard anodising. The oxide layer is less dense than that of hard anodising and readily accepts a dye. Thicker, high-quality films are often referred to as semi-hard anodising.

Decorative Anodising

As the name suggests the main purpose of this process is to provide a pleasing aesthetic appearance of components and covers a wide range of products of differing levels of integrity from publicity gimmicks to high-quality instrument panels. As the natural colour is silver or light grey, dyestuffs may be used as colouring dyes. Prior to anodising the aluminium may be brightened or dulled chemically to achieve a required texture.

Architectural Anodising

This is a specialised area aimed toward components used for external architectural applications where designs dictate a combination of aesthetic appearance, corrosion resistance and mechanical performance. Very attractive effects can be obtained by choosing the correct alloys and mechanical finishes prior to anodising. These films are often coloured using integral colour anodising or dyes that are plated in after anodising. Sealing is therefore very critical to ensure good corrosion resistance. Architectural Anodising is, seemingly, much more common in the US and mainland Europe than it is in the UK.

We are a dedicated provider of specialist metal finishing services, concentrating primarily on the anodising of aluminium.

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