If mild steel is exposed to an aerated neutral aqueous solution, for example a dilute solution of sodium chloride in water, then corrosive attack will begin at defects in the oxide film on the steel. These defects may be present as a result of mechanical damage such as scratches, or may be due to natural discontinuities in the film, i.e. inclusions, grain boundaries or dislocation networks at the surface of the steel.
At each defect the steel is exposed to the solution (electrolyte) and an anodic reaction occurs, resulting in the formation of iron ions and free electrons. These electrons are then conducted through the oxide film to take part in a cathodic reaction at the surface of the film. This reaction requires the presence of dissolved oxygen in the electrolyte and results in the formation of hydroxyl ions.
The hydroxyl ions react with the ferrous ions produced by the anodic reaction to form ferrous hydroxide, which is then converted into a hydrated oxide called, ‘rust'. Gradually a scab of rust may form over the top of the pit, but this is too porous to completely block the anodic area. This allows the corrosion process to continue, resulting in deeper attack and widening of the anodic area as the surface oxide film breaks away.
If the pH of the solution in contact with the steel is low, for example a dilute acid, then the surface oxide film will be removed and the cathodic reaction will be different. Hydrogen gas will be liberated as gradual dissolution of the steel occurs. With oxidising acids, a number of alternate cathodic reactions may take place.