How Hard Anodising Is Applied
The process of Hard Anodising seems so simple, just a component dipped in a tank. But it is a very controlled process and each stage of the process is important to ensure that the end product is satisfactory. Here is a description of the anodising process from new bare aluminium component to a wear resistant end product.
Cleaning And Degreasing First
The first step of the process is to clean the part, in a degrease stage using an alkaline or solvent solution, which is then rinsed off. All residues of cutting oil, finger prints and machining marks must be removed in this stage in order to get a good coating. A hasty cleaning of the part in this stage can later result in areas of the coating not being as good as others.
Loading Onto Jigs Or Racks
The component is then fixed to a jig or rack which is then placed into the tank. Care is taken to ensure that the contacts to the component are on a non-critical area of the component. This area will not anodize and can show up as a bare patch on the finished component. This must be arranged prior to dispatch of the component to the anodisers.
Etching Off The Natural Oxide
An aluminium workpiece has a natural oxide layer forming on the surface as soon as it is exposed to the air. This layer has to be removed before anodising. In the etch stage a mild acid or alkaline solution is used to remove the existing oxide layer from the surface of the part to be anodised. This layer has to be removed so that a new, even oxide layer can form on the surface of the part.
The Cold Acid Bath
The part is now lowered into the cold acid bath (usually sulfuric acid). The bath used for Hard Anodising is similar to that used for normal anodising in that it is an electrolytic process using sulfuric acid as the electrolyte. However, it is held at a much colder temperature, around 0-5 degrees C, compared to the normal anodising bath which is operated at a much higher temperature. For details of the process control and the maintenance of industrial anodising baths, see Surface Engineering Association.
Raising The Current Gradually
As the part is anodised the current is gradually increased and then run at a high voltage and current density. In the case of Hard Anodising this is a lot higher than would be used for a standard anodising process to produce a thin decorative coating. The part is slowly ‘built up’ to the high current and then left to coat at this level for the required time.
Running The Tank For Longer
Standard anodising processes can run in 20 minutes to half an hour to produce a relatively thin layer of a soft, anodic coating. To produce a very hard, thick, wear resistant coating, however, requires a long anodising time – typically of the order of 1 hour or more depending upon thickness to be achieved and alloy type. The component material is critical in determining the anodic coating characteristics and processing time required to achieve a specific coating thickness.
Rinsing Thoroughly After The Tank
The rinsing is done thoroughly to ensure that all of the residual acid and electrolyte from the coating process are removed from the part’s surface. The risk of the part reacting with the surface is high if any of the acid is left on the surface of the part after it has been removed from the coating tank. This can cause problems with the part later on.
Sealing Or Leaving It Open
Sealing or leaving the anodized surface open can be used depending upon the intended use of the part. A sealed surface will tend to close up all of the pores in the anodized film. This is often desirable for corrosion purposes or aesthetic reasons. On the other hand an unsealed surface will have open pores and can be used for later impregnation of the part with lubricants. These parts would normally be used for sliding or wear purposes.
This is the full process and each stage has to be done correctly or the coating on your part can suffer severely.