Coatings are the first barriers to protect tanks against corrosion. Due to microscopic pinhole defects in the coating and damage to it whilst in service localised pitting will occur. Tank anodes are used to complement the coating at areas of exposed steel prolonging the life of the coating and structure. Zinc Tank Anodes, Aluminium Tank Anodes and Magnesium Tank anodes are available upon request with various methods of fixing arrangements. It must be noted that hydrogen is evolved from aluminium anodes and thermite sparking may occur when light alloys impact on rusty steel. Therefore there is a height restriction in cargo/ballast tanks in tankers that anodes are not located at a height (in m) greater than 27/W where W is the total mass (in kg) of the anode and insert. Exceptions to this rule are made when the anodes are on wide horizonal surfaces from which they cannot fall, such as webs and girders. For the same reason International Association of Classification Societies forbids the use of magnesium anodes in the cargo tanks of tankers.