Mechanism of Crevice Corrosion

Abstract

Abstract Failure of structures and of many types of automatic equipment is often due to crevice corrosion. Differential aeration was advanced some time ago as a major cause of such attack. The present investigation was made to demonstrate other factors in the mechanism of crevice corrosion. Electrochemical methods were used to determine effects and relationships of polarization potentials and corrosion current densities in the main volume of electrolyte (which has free access to the open surface of the metal tested) and in the metal's crevices and clearances. Special laboratory apparatus was used with which crevice corrosion conditions could be simulated and in which “crevice” width was adjustable. Iron, aluminum and stainless steels were tested in 0.5N NaCl. Iron was also exposed to a mixture of Na2SO4, NaCl and NaNO2, to 8.6N HNO3 and to 0.2N H2SO4. Observations were also made of linear selective type crevice corrosion at the metal-dielectric interface perimeter in acid and of comparative corrosion rates above, at, and below the water line of partly-immersed iron. (In the latter case, a “crevice” is considered to exist near the liquid's meniscus.) It was concluded that much destruction of metal in clearances is due to peculiar electrochemical behavior which results in acceleration of the anodic metal ionization reaction and deceleration of the cathodic reaction. Insignificant differences in potential between metal on the open surface and that in the crevices initiate operation of corrosion cells. Linear-selective dissolution of iron in acids, on the phase boundaries of the metal-dielectric-acid system, proceeds according to the crevice corrosion mechanism. In water-line zone corrosion, the crevice corrosion mechanism is also in evidence, rate depending on whether the electrolyte is neutral, acid or alkaline. In inhibited media, metal potential near the water line becomes negative, due to difficulty of access of inhibitor, so that macrocells are activated.