Types of corrosion

There are many different types of corrosion, which can be classified by the cause of the metal's chemical deterioration. There are such as:

• General 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. When metal, specifically steel, is exposed to water, the surface is oxidized and a thin layer of rust appears. Like galvanic corrosion, general corrosion is also electrochemical.

• Localized Corrosion

Localized corrosion specifically targets one area of the metal structure. Localized corrosion is classified as one of three types:

• Pitting

Pitting is a form of extremely localized attack that results in holes in the tube walls. It occurs when the corrosive environment penetrates the passivated film in only a few areas as opposed to the overall surface. This area becomes anodic, while part of the remaining metal becomes cathodic, producing a localized reaction galvanic. Pitting corrosion is therefore simple galvanic corrosion, occurring as the small active area is being attacked by the large passivated area. It is one of the most destructive forms of corrosion and also one of the most difficult to predict in laboratory tests. It is generally promoted by low-velocity or stagnant conditions and by the presence of chloride ions. Once a pit is formed, the solution inside it is isolated from the bulk environment and becomes increasingly corrosive with time. The high corrosion rate in the pit produces an excess of positively charged metal cations, which attract chloride anions. In addition, hydrolysis produces H+ ions. The increase in acidity and concentration within the pit promotes even higher corrosion rates, and the process becomes self-sustaining.

• Crevice corrosion

Similar to pitting is crevice corrosion; this corrosion occurs any time liquid flow is kept away from the attacked surface. It is common between single or twin ferrule fittings and tube clams surfaces, it is found in many split seal applications. Salt water applications are the most severe problem because of the salt water low pH and its high chloride content. Due to the tight connections no oxygen is available to passivate the stainless steel, chloride pit the passivated stainless steel surface, the low pH salt water attacks the active layer that is exposed. The corrosion unhampered under the tight fitting clamp.

• 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.

• Galvanic Corrosion

Galvanic corrosion is extraordinarily common, and occurs when two metals with different electrochemical charges are linked via a conductive path. Corrosion occurs when metal ions move from the anodized metal to the cathodic metal.

• Environmental Cracking

Environmental cracking is a corrosion process that can result from a combination of environmental conditions affecting the metal. Chemical, temperature and stress-related conditions can result in the following types of environmental corrosion:

• Stress Corrosion Cracking (SCC)
• Corrosion fatigue
• Hydrogen-induced cracking
• Liquid metal embrittlement
• Flow-Assisted Corrosion (FAC)

Flow-assisted corrosion, or flow-accelerated corrosion, results when a protective layer of oxide on a metal surface is dissolved or removed by

• Erosion-assisted corrosion wind or water, exposing the underlying metal to further corrode and deteriorate.
• Impingement
• Cavitation
• corrosion
• Intergranular corrosion

Intergranular corrosion is a chemical or electrochemical attack on the grain boundaries of a metal. This often occurs due to impurities in the metal, which tend to be present in higher contents near grain boundaries. These boundaries can be more vulnerable to corrosion than the bulk of the metal.

• De-Alloying

De-alloying, or selective leaching, is the selective corrosion of a specific element in an alloy. The most common type of de-alloying is de-zincification of unstabilized brass. The result of corrosion in such cases is a deteriorated and porous copper.

• Fretting corrosion

Fretting corrosion occurs as a result of repeated wearing, weight and/or vibration on an uneven, rough surface. Corrosion, resulting in pits and grooves, occurs on the surface. Fretting corrosion is often found in rotation and impact machinery, bolted assemblies and bearings, as well as to surfaces exposed to vibration during transportation.

• High-Temperature Corrosion

Fuels used in gas turbines, diesel engines and other machinery, which contain vanadium or sulphates can, during combustion, form compounds with a low melting point. These compounds are very corrosive towards metal alloys normally resistant to high temperatures and corrosion, including stainless steel. High temperature corrosion can also be caused by high temperature oxidization, sulfation and carbonization.