Sulphuric Acid Anodising

Sulphuric acid anodising is typically 10 – 15µm thick. It is used for its anti-corrosion properties, when additional durability is required or when colouring by dying is required. Sulphuric anodising leaves a clear silver coloured finish but we are able to offer a range of colour dyes giving the part a superior visual finish.

Anodising takes place at the metal interface, so that the oxide grows out of the surface, converted from the metal. Since the oxide has twice the volume of its parent metal, the coating grows 50% out from the original surface and 50% into the original surface. In consequence, the increase in the size of the anodised part is half that of the coating thickness, a factor to be taken into account when considering size tolerance.

As the coating grows, the electrolyte and electrical current must continue to gain access to the oxide metal interface, producing more oxide. Since aluminium oxide is an insulator, this could be a problem, but the coating structure forms a matrix of porous (sub-micron size) columns, from the surface to the interface, and oxide conversion continues at the base of the columns. As the coating thickness builds, and the electrical conductivity to the interface decreases, the rate of coating growth falls exponentially, so that the process cannot continue indefinitely.

Poeton offer sulphuric acid anodising to all major Aerospace and Military specifications.