Crevice corrosion is a type of localized corrosion that occurs when an electrolyte becomes trapped in the crevices, cracks, or joints of any metal surface.
When this happens, it can lead to pitting and deep crevice corrosion which are both detrimental to the structural integrity of your metal surfaces.
When it comes to preventing structure failure, there are four types of crevice corrosion that you need to look out for:
1. Ohmic drop of potential
Crevice corrosion by ohmic drop of potential affects passive alloys or metals that show an active-to-passive current peak in their anodic polarization curve. This is caused by anodic dissolution of the passive metal at cathodic current peaks.
This type of crevice corrosion occurs in low alloy steels (carbon steel, ferritic, and martensitic stainless steel) due to their high susceptibility for pitting corrosion degradation. Generally speaking, carbon steels are more susceptible than alloys because they have higher iron content.
Passive alloys are less susceptible to this type of corrosion than active alloys, so if you're using a passive alloy in an environment with high electrolyte concentration, you're at a lower risk for ohmic drop of potential.
2. Electrochemical depassivation
Crevice corrosion by electrochemical depassivation is localized corrosion that happens in the presence of specific anions above a critical potential.
This is a well-known type of crevice corrosion that is mostly seen in nickel alloys. If you're using a metal with high susceptibility to electrochemical depassivation, then the only way to prevent it from happening is by increasing your pH and flow rate or choosing another alloy that's less susceptible.
The main reason why nickel alloys are so vulnerable to this type of corrosion is because they have low resistance to anodic dissolution and high cathodic current density.
When looking for signs of electrochemical depassivation crevice corrosion, you should look for pits that are surrounded by a white or colorless film.
3. Atmospheric crevice corrosion
Atmospheric crevice corrosion is caused by water that's gathered or retained in crevices when your structure is exposed to the elements.
Unfortunately, it's very easy for dew or rainwater to enter these crevices. Even with the sun and wind drying these surfaces, corrosion can still continue to wreak havoc within the crevices. This constant wet-dry cycle can expedite corrosion by concentrating salts inside the crevices.
4. Differential aeration
Crevice corrosion by differential aeration cells is caused by varying oxygen concentrations on a metal surface. Once metal has been in contact with high oxygen concentration, it becomes a cathode, slowing down corrosion. Metal in contact with a low oxygen concentration becomes an anode, often greatly increasing its corrosion rate.
This type of corrosion is usually seen in carbon and low alloy steels. The main reason why it's so common in these types of metals is because they have a high porosity which allows oxygen to come into contact with the metal surface.
Corrosion can be tricky, but your offshore structure doesn't need to fall victim to it. Mark Tool's SplashTRON coating can be applied to various areas on an offshore platform, and is most commonly used to protect risers, j-tubes, spools, bends, and more from the corrosive elements of the splash zone. Contact us today to request a quote!