Hard Anodising is formed by using an electrolytic solution of sulphuric acid approximately between 3 to 5 degrees and current density of 23 to 37 amps per square foot. The process will run for 20 to 120 minutes depending on the alloy used and desired coating thickness. This will produces a generally grey coating 10 microns to 50 microns thick with a 50% build up and 50% penetration.
There are hundreds of governmental industrial and commercial anodising specification in use, each with their own method of calling out coating, seals, dyes etc.
Hard anodising works very well with 6000, 5000 and 7000 series alloy. High copper 2000 series can present some difficulties and should be avoided is possible. As with all forms of anodising high-silicon die-castings are not recommended.
Hard anodising provides for several desirable qualities such as:
Hard anodising coating may be dyed a variety of colours. However, due to its naturally darker appearance, dyeing does not produce the vibrant colours that standard sulphuric anodising will. Additionally, the required sealing process after dyeing does slightly reduce hardness.