Uniform corrosion will tend to occur when some surface regions become anodic for a short period, but their location and that of the cathodic regions constantly change. General rusting of mild steel will take place when there is a uniform supply of oxygen available across the surface of the steel and there is a uniform distribution of defects in the oxide film (this is usually the case in the non-protective films formed on unalloyed steel). In the absence of areas of high internal stress (cold-worked regions) or segregated zones (such as non-uniform distributions of sulphide inclusions), a number of anodic regions will develop across the surface. Some areas will become less active while new anodic regions become available. Therefore, overall attack takes place at a number of anodic sites whose positions may change, leading to general rusting across the surface.
If the supply of oxygen is not uniform across a surface, then any regions that are depleted in oxygen will become anodic, for example within a crevice, at a joint or beneath a surface deposit. The remainder of the surface has oxygen available to it and therefore acts as a large cathodic area. When the cathodic area is so much larger, severe local attack will occur in the small anodic region. This can result in perforation of a container, or damaging local loss of load-bearing capacity in structural members.
Crevice Corrosion in Road Vehicles
In road vehicles, local corrosion damage caused by mud and road dirt (especially from salted roads) that is trapped inside bodywork provides an all too familiar example of the effects of differential aeration. Likewise in the partly filled tanks corrosion becomes concentrated just below the waterline, where there is a pronounced fall in the oxygen availability and a short electron path to the water level where hydroxyl ions are formed.
Design to Prevent Crevice Corrosion
Crevice and deposit forms of corrosion can be minimised by correct design, for example by the sealing of joints, by the prevention of deposits and fouling via efficient maintenance and water treatment. The rate of supply of oxygen will govern the rate and mechanism of rusting. Differences in oxygen concentration can result in highly localised damage. Without oxygen, the cathodic reaction could not take place; hence in closed environments such as boiler and heating systems, the removal of oxygen from feedwaters by suitable treatment reduces the corrosion rate. In other systems where the water is in contact with air, corrosion in the system may be reduced by the addition of anodic or cathodic inhibitors to the water.
Cathodic and Anodic Inhibitors
Cathodic inhibitors promote insoluble deposits that have an insulating effect and block the flow of electrons, limiting the cathodic reaction and hence anodic dissolution. Anodic inhibitors assist film repair over the metal at anodic sites and hence passivate these areas. When using anodic inhibitors it is vital to maintain their correct concentration, since if the anodic sites are not completely blocked, then the remaining exposed anodic area will be much smaller than before. The cathodic reaction is not affected and this will give higher current densities at the anode sites, causing highly localised attack called pitting and leading to rapid perforation of sections.