Image Credit: PHATRAWUTH EAWCHAREON/Shutterstock.com
Article updated on 12/02/2020 by Jo Finchen-Parsons
If mild steel is exposed to an aerated neutral aqueous solution, such as a dilute solution of sodium chloride in water, a corrosive attack will begin at defects in the oxide film on the steel. These defects may be a result of mechanical damage, such as scratches, or natural discontinuities in the film, such as inclusions, grain boundaries, or dislocation networks present on the surface of the steel.
At each defect, the steel is exposed to the electrolyte solution and an anodic reaction occurs, thereby resulting in the formation of iron ions and free electrons. These electrons are then conducted through the oxide film, where they participate in a cathodic reaction at the surface of the film. This reaction requires the presence of dissolved oxygen in the electrolyte solution and ultimately results in the formation of hydroxyl ions.
Figure 1. Rusting of mild steel.
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 commonly referred to as ‘rust'. Gradually, a scab of rust may form over the top of the pit; however, it is too porous to completely block the anodic area. Therefore, the corrosion process can continue, resulting in a 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, as would be the case if a dilute acid is used, 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 oxidizing acids, a number of alternate cathodic reactions may take place.
In all cases of corrosion, the anodic reaction cannot proceed in isolation from the cathodic reaction. Furthermore, if either reaction can be limited or stopped, then less or no corrosion will occur.