Types of Corrosion

Types of Corrosion
What are the different types of corrosion?
Corrosion comes in many different forms and can be classified by the cause of the chemical deterioration of a metal. Listed below are 10 common types of corrosion:
1. General Attack Corrosion:
Also known as uniform attack corrosion, general attack corrosion is the most common type of corrosion and is caused by a chemical or electrochemical reaction that results in the deterioration of the entire exposed surface of a metal. Ultimately, the metal deteriorates to the point of failure.
General attack corrosion accounts for the greatest amount of metal destruction by corrosion, but is considered as a safe form of corrosion, due to the fact that it is predictable, manageable and often preventable.
2. Localized Corrosion:
Unlike general attack corrosion, localized corrosion specifically targets one area of the metal structure. Localized corrosion is classified as one of three types:
• Pitting: Pitting results when a small hole, or cavity, forms in the metal, usually as a result of de-passivation of a small area. This area becomes anodic, while part of the remaining metal becomes cathodic, producing a localized galvanic reaction. The deterioration of this small area penetrates the metal and can lead to failure. This form of corrosion is often difficult to detect due to the fact that it is usually relatively small and may be covered and hidden by corrosion-produced compounds
• Crevice corrosion: Similar to pitting, crevice corrosion occurs at a specific location. This type of corrosion is often associated with a stagnant micro-environment, like those found under gaskets and washers and clamps. Acidic conditions, or a depletion of oxygen in a crevice can lead to crevice corrosion.
• Filiform corrosion: Occurring under painted or plated surfaces when water breaches the coating, filiform corrosion begins at small defects in the coating and spreads to cause structural weakness.
3. Galvanic Corrosion:
Galvanic corrosion, or dissimiliar metal corrosion, occurs when two different metals are located together in a corrosive electrolyte. A galvanic couple forms between the two metals, where one metal becomes the anode and the other the cathode. The anode, or sacrificial metal, corrodes and deteriorates faster than it would alone, while the cathode deteriorates more slowly than it would otherwise.
Three conditions must exist for galvanic corrosion to occur:
• Electrochemically dissimilar metals must be present
• The metals must be in electrical contact, and
• The metals must be exposed to an electrolyte