The pitting and localized attack on aluminium is explained by the autocatalytic character of the anodic corrosion reaction ; theoretical considerations suggest that if once, a t a point, the anodic current density becomes high, i t will tend t o rise higher, until checked by the evolution of hydrogen, which tends t o produce a film of oxides or hydroxide over the point and slow down the reaction. Pitting may be expected to occur on aluminium in neutral solutions and on iron in alkaline solutions-as is found to be the case. The theoretical reasoning is supported by observation o f pH changes around corroding specimens and also in divided cells, as well as by experiments on the corrosion cracking of aluminium-magnesium in a susceptible state.Types of Localized Attack.-Aluminium is a metal which, despite a strong affinity for non-metals, is surprisingly resistant to corrosion-a property due largely to the formation of protective films. When, however, corrosion does occur, i t is often localized, causing damage disproportionate to the amount of metal destroyed. It may take the form of pits, involving local penetration without much general corrosion. On alloys containing copper or magnesium which have received a heat-treatment that produces readily attackable matter along the grain-boundaries, intercrystalline attack, which causes more mechanical weakening than isolated pits, is found. If such alloys are subjected to tension during corrosion, i t becomes possible for the corrosion-cracking to be concentrated on a limited number of grain-boundaries running roughly at right-angles

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Corrosion testing frequently requires the use of an inert container material and, for tests in fused chlorides, glass or silica containers have commonly been used. The present work shows that such containers can have a marked effect on corrosion reactions in fused chlorides, and this has two important implications: (1) test results obtained in glass apparatus may not be directly applicable to plant where an all‐metal system is in contact with a fused chloride; (2) test results obtained in all‐metal apparatus may not be directly applicable to plant containing glass or ceramic components as well as metal.