The anodic coating is essentially aluminium oxide, which is a hard substance. Attack by hot acid at the interface will soften the coating. If 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 Defence 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 for build-up applications. Coating thicknesses in the range of 15-40 microns may be specified for special applications.