Disregard the insidious effects of pitting corrosion at your peril
Pitting corrosion is an extremely localised form of material attack, during which a material surface is perforated by a series of holes (or pits) in the presence of an aqueous solution. Corrosion of this nature is highly destructive, as these pits can rapidly perforate material causing severe damage, while the surrounding material regions remain unaffected. Pitting generally takes months or years to initiate, but once initiated the pits penetrate the material at exponential rates
The process is thought to initiate as a result of a momentary increase in the rate of metal dissolution at a particular location (possibly due to a scratch, or other surface defects), resulting in ion migration and the subsequent formation of an anodic pit and adjacent cathodic regions.
When metal corrodes in an aqueous solution, the metal anode is oxidised to form metal ions (typically M+), releasing electrons in the process. These electrons are consumed in the cathodic (reduction) reaction, during which the oxygenated water is reduced to hydroxyl ions (4OH-). During the process of pitting corrosion, metal dissolution occurs within the pit, while the reduction of the oxygenated water occurs in regions surrounding the pit. Owing to the rapid localised dissolution of material in the pit, it becomes positively charged, attracting ions (such as hydrogen from the hydrolysis of metal chlorides in water and chlorides that may be present in the electrolyte) into the pit. This increases the concentration of metal chlorides and hydrogen ions in the pit, which subsequently increases the dissolution rates.
Conditions present within a corrosion pit are both self-perpetuating causing penetration of metal at an ever increasing rate (usually in the direction of gravity as ions are retained in the pit). The increase in concentration of ions within the pit reduces the reduction of oxygen, while oxygen reduction continues adjacent to the pit, effectively providing a degree of cathodic protection. Often areas with little or no corrosion damage are evident adjacent to the pit itself.
In some cases, pits can be surrounded by the formation of a tube of corrosion product that is formed by the oxidisation of the product formed by the interaction of the oxidation products within the enclosed pit and those from the surrounding cathodic regions. These tubes, or rust rims as they are commonly known, tend to shield the local environment from the electrolyte, promoting rapid dissolution of the material.
In a recent investigation of a liquid cooled heat exchanger core, the progressive removal of the tin coating on the fins of the tubes, due to the condensation of corrosives inherently present in the operating environment, resulted in the onset and progression of pitting damage of the base copper material. The extent of pitting was severe enough to result in the complete structural degradation of the heat exchanger. Initially, the more active (anodic) tin material would have provided a sufficient degree of cathodic protection by galvanic action between the tin and the more passive (cathodic) exposed copper surface. Continual removal of the tin progressively reduced the degree of protection, resulting in eventual pitting corrosion and subsequent (inevitable) structural degradation.
Pitting corrosion can be considered as a special case of crevice corrosion (discussed in our Nov 2017 tech tip), with most materials susceptible to pitting also being susceptible to crevice corrosion (although the reverse is not always true). As such, mechanisms to combat crevice corrosion are also applicable to pitting corrosion. More specifically, these would include the choice of suitable pitting resistant material (for the particular environment) and the prevention of any local variations in surface energy, or electrolyte concentrations that would promote local material dissolution.
The message is simple; select materials with care and take precautions to prevent the formation of local areas of high material dissolution that would promote pitting corrosion.
Published in Technical Tips by Origen Engineering Solutions on 1 January 2018