The anodic coating is essentially aluminium oxide, which is a hard substance. Attack by hot acid at the interface will soften the coating. lf there is a high degree of agitation and the electrolyte temperature is lowered to around 0°C (32° F) the softening effect is suppressed and a hard, wear resistant film is produced.

A further advantage of such conditions is that by increasing the current density thicker films can be produced. Most Defense Specifications require a minimum of 50 microns (2 mil), but coatings up to 125 microns (5.0 mil) can be produced by use of special techniques far build-up applications. Coating thicknesses in the range of 15-40 microns may be specified far special applications.

Production economics lead to the use of higher current densities than the 1.0-1.5 A/dm2 usually employed far decorative and protective anodizing. Current densities of 2.5 A/dm2 (24 A/ft2) or higher are normally employed. However, it should be noted that both British and US specifications allow electrolytes other than sulphuric acid to be employed. They do not restrict operating conditions, provided the film passes the tests required in the specification.

Such coatings have become increasingly important where a combination of weight saving with high performance and wear resistance are required.

When used in conjunction with solid film lubricants important improvements in per­formance are obtained, such as allowing hard anodizing to be used in “sealed far life” applications.