Corrosion is a naturally occurring phenomenon and is defined as the deterioration of a material (usually a metal) as a result of a chemical or electro-chemical reaction between it and its immediate environment. Both the type of material and the nature of the environment, particularly what fluid is in contact with the material, determine the form and rate of deterioration.
It is generally accepted that there are eight different types of corrosion:
- Uniform corrosion;
- Pitting corrosion;
- Crevice corrosion;
- Galvanic corrosion;
- Intergranular corrosion;
- Selective leaching;
- Erosion corrosion;
- Stress corrosion.
The type of corrosion is classified based on the nature and cause of the material's deterioration. Some types of corrosion are unique, but in many cases they are interrelated.
Uniform Corrosion
This is the most common type of corrosion and is characterised by an electrochemical reaction that proceeds more or less uniformly over the entire exposed surface of the material. The thickness of the material is progressively reduced until the component eventually fails.
Uniform corrosion is recognisable from a roughening of the surface accompanied by the presence of corrosion products. As noted above, the corrosion mechanism is electrochemical and takes place at the surface of the material. Differences in composition or orientation between small areas on the surface create anodes and cathodes that facilitate the corrosion reaction.
Uniform corrosion can generally be tolerated because the effect of the material loss is relatively easy to assess and allowances can be made at the design stage. Protective coatings are particularly effective in controlling uniform corrosion. Where it is not possible to apply a protective coating a corrosion allowance can be added to the wall thickness of the component.
Pitting Corrosion
This is an extremely localised form of attack; occurring at discrete sites on the surface of the material. It results in pits and is one of the most destructive and menacing forms of corrosion as it can cause equipment to fail due to perforation. Pits are often difficult to detect because of their small size. In piping applications, pitting is particularly vicious because it is a localised and intense form of corrosion, and failures often occur with extreme suddenness and are nigh on impossible to predict as the pits originate on the inside surface of the pipe.
Pitting is also difficult to predict using laboratory tests. It is not easy to measure quantitatively and compare the extent of pitting because of the varying depths and numbers of pits that may occur under identical conditions. In addition, there are no means available for predicting how long it will take for a pit to develop; it can take several months to a year for pitting to show up in actual service.
Because of the difficulties in predicting the severity of any pitting that may occur, where the risk of component failure cannot be tolerated, the only option is to use a material that is resistant to pitting.
Crevice Corrosion
This is a localised form of corrosion usually associated with the stagnant conditions that occur in crevices and other shielded areas on the surface of a material exposed to a corrosive environment. For crevice corrosion to occur, the ‘crevice’ needs to be wide enough to allow the ingress of the corrosive liquid, but narrow enough to ensure that it remains stagnant.
In general, crevice corrosion only occurs in metals that owe their corrosion resistance to the stability of a passive film, i.e. stainless steels. Stainless steels exposed to an aerated chloride-rich media are particularly susceptible to crevice corrosion. The gradual acidification of the solution within the ‘crevice’ produces a highly aggressive microenvironment that destroys the passive film. Because the liquid within the ‘crevice’ is stagnant, any dissolved oxygen is rapidly exhausted meaning the film cannot reform.
Only a few seconds are required to create a ‘differential aeration cell’ between the ‘crevice’, which acts as the anode, and the remainder of the surface, which acts as the cathode. Once this cell has been created, the cathodic reaction at the surface generates a huge demand for electrons, which is fed by the dissolution of metal within the ‘crevice’. This process is ‘autocatalytic’ so the ‘crevice’ will grow rapidly. If unchecked, the crevice will extend through the complete cross-section of the component.
The risk of crevice corrosion can be eliminated by selecting a suitable material.
Galvanic Corrosion
This refers to the corrosion damage that occurs when two dissimilar materials are coupled in a corrosive electrolyte (e.g. seawater). If a potential difference exists between the two materials, a galvanic couple will be formed. The least noble of the two materials in the couple becomes the anode. The other becomes the cathode. The potential difference that exists between the two materials will produce a flow of electrons between the anode and the cathode. The electrons are produced by the corrosion reaction that takes place at the anode.
The demand for electrons is driven by the area of exposed material at the cathode; the bigger the exposed area, the greater the demand for electrons. In a galvanic couple, the material at the anode corrodes faster than it would do by itself and the material at the cathode corrodes slower than it would otherwise.
Galvanic corrosion can be avoided by ensuring that dissimilar materials are electrically isolated from each other. Where this is not possible, thought should be given to minimising the area of exposed material at the cathode as this is the driving force for the corrosion reaction. This can normally be achieved by applying a non-conductive coating to the surface. Applying a coating to the anode is not recommended, as any failure in the coating would leave the exposed material susceptible to very rapid corrosion.
Intergranular Corrosion
This is a localised electrochemical reaction attack at or adjacent to the grain boundaries of a metal alloy. There is very little corrosion of the actual grains, which can become displaced causing the metal to disintegrate.
Intergranular corrosion can be caused by impurities at the grain boundaries, enrichment of one of the alloying elements, or depletion of one of these elements in the grain-boundary areas. If not properly processed, certain stainless steels are particularly susceptible to intergranular corrosion due to depletion of chromium in the grain-boundary regions.
The risk of intergranular corrosion can be minimised by controlling the chemical composition of an alloy and ensuring that the key manufacturing operations are properly performed. In addition, there are a number of specific corrosion tests available that can be carried out on representative samples of the material to confirm that it has been properly processed.
Selective Leaching
This is the selective removal of a specific element from an alloy by corrosion processes. The most common example is the selective removal of zinc in brass alloys (dezincification), whereby a weakened, porous copper structure is produced. Similar processes occur in other alloy systems in which aluminium; iron, cobalt, chromium, and other elements are removed.
Selective leaching is generally not an issue in the oil and gas/petrochemical industry and can be avoided by appropriate material selection based on the environment in which a material is operating.
Erosion Corrosion
This is the acceleration in the rate of deterioration or attack on a material due to the relative movement between it and a corrosive fluid. The fluid velocity is generally quite high, and mechanical wear effects or abrasion are involved. The combination of erosion and corrosion can result in extremely high pitting rates. The situation is exacerbated by the increased turbulence caused by the pitting, which further increases the erosion rate.
In erosion corrosion, material is removed from the surface in contact with the fluid as dissolved ions. Even if these ions produce solid corrosion products, these are likely to be swept away by the force of the fluid leaving the surface exposed to further corrosion. The damage caused by erosion corrosion usually takes the form of grooves, gullies or rounded holes, which normally exhibit a directional pattern. In many cases, failures due to erosion corrosion occur in a relatively short time and are largely unexpected as the risk of erosion corrosion should have been consider at the design stage.
Whilst materials selection can play an important role in minimising the risk of erosion corrosion, the layout of the piping system is far more significant. Designs that create turbulent flow or involve restrictions to the flow are undesirable; as are abrupt changes in flow direction, which can be avoided by using 3D or 5D bends instead of long radius elbows. As discussed in my blog on pipe sizing, it is generally desirable to reduce the fluid velocity and promote laminar flow. This can be achieved by increasing the pipe diameter, which also reduces problems with abrupt changes in flow direction.
Stress Corrosion
Stress corrosion or as it is more normally knows stress-corrosion cracking (SCC) refers to cracking caused by the combined influence of tensile stress and a corrosive environment. It is common for all cracking failures that occur in corrosive media to be classified as stress-corrosion cracking, including failures due to hydrogen embrittlement. However, this is a mistake as the two types of cracking failure respond differently to environmental variables. When specifying material requirements, it is important to consider stress-corrosion cracking and hydrogen embrittlement as separate phenomena.
Stress corrosion usually occurs where there is a specific combination of susceptible material, environment and stress. It is classified as a catastrophic form of corrosion as it can be very difficult to detect. A disastrous failure may occur unexpectedly and with minimal material loss. A key feature of stress-corrosion cracking is that the material is virtually un-attacked over most of its surface. The damage is confined to ‘microscopic’ cracks, which penetrate into the surface and then advance through the material. These cracks can be either intergranular or transgranular.
The designer has to take the risk of stress-corrosion cracking very seriously; particularly, given that it can occur at stresses well within the allowable design stress of the material. Stresses can be in the form of an applied stress or in the form of a residual stress. The latter is more of a concern because its magnitude and origin may not be obvious. Residual stresses are generated during manufacture; cold forming, welding, heat treatment, machining and even grinding can all be a source of residual stress. If it is impossible to remove the stress or alter the environment, then the only way the designer has of avoiding stress corrosion is to modify or change the material.
One of the most common types of stress corrosion cracking found in the oil and gas/petrochemical industry is sulphide stress corrosion cracking (SSCC). NACE Standard MR0175, more often referred to simply as NACE, specifies material requirements for metals being used in a sour oilfield environment (i.e. the process fluid contains a significant level of hydrogen sulphide). To be on the safe side, a designer will often specify that NACE requirements apply even though the level of hydrogen sulphide in the fluid is well below the threshold limit at which SSCC can occur.