ALWC is characterised by soft orange deposits overlying thin black biofilms under which the steel surface is bright, shiny and pitted. Normally it is found in a narrow band around lowest astronomical tide level. It has baffled corrosion scientists by the seemingly random nature of its attack, with only certain areas of certain piles being affected. These can be holed within 20 years, seriously reducing the life of sheet piled structures. The precise cause of ALWC attack is still unknown.