9 February 2017
As a specialist repair and refurbishment contractor CSC Services provide range of corrosion protective coatings to protect structures affected by corrosion.
Corrosion is the deterioration of a material (usually a metal) that results from a chemical or electrochemical reaction with its environment. In order to prevent and control corrosion, it is important to understand the different types of corrosion; the mechanisms involved; how to detect them; and how and why they occur.
Corrosion will usually take more than one form and is affected by numerous external factors. In this feature article Mick Flounders, Contracts Director of CSC Services lists the most common types of corrosion.
- Uniform corrosion – According to NACE International (the Worldwide Corrosion Authority), on the basis of material wasted, this is the most common form of corrosion. It is characterised by corrosive attack proceeding evenly over the entire surface area of a metal. General thinning will take place until failure. Uniform corrosion is relatively easy to measure and predict making disastrous failures relatively rare. In many cases the only problem it causes is to the appearance of a material, and in some cases the patina created can add appeal and colour to a surface such as the rust hues produced on weathering steels. The breakdown of protective coating systems on structures often leads to this form of corrosion. Dulling of a bright or polished surface, etching by acid cleaners, or oxidation (discoloration) of steel are examples of surface corrosion. If permitted to continue this corrosion can lead to more serious types of corrosion.
- Pitting – This type of corrosion is characterised by holes or cavities produced in material. It is more difficult to detect, predict and design against than uniform corrosion. The products of corrosion can often cover the pits and the pits can often take different shapes.
- Pack Rust – This is described by ‘Corrosionpedia’ as an electrochemical reaction that results in rust formation within the joints and packs of ferrous metal sheets and components of different structures. Rusting occurs due to electrochemical reactions. Normal rust does not cause bulging and distortion, but a significant pack rust feature is the bulging of parts of parallel plates. If the problem is not resolved, structural strength will reduce continuously, creating potentially serious failures. Pack rust mainly affects ferrous materials. To prevent pack rust, the surfaces must be treated with rust-preventive coatings at the time of component and plate assembly.
- Crevice corrosion – As the name suggests this type of corrosion usually occurs in the gaps and contact areas between parts, under gaskets or seals, inside cracks and seams. It is initiated by changes in local chemistry within the crevice such as the depletion of inhibitor in the crevice or the depletion of oxygen in the crevice.
- Filiform corrosion – this usually occurs under painted or plated surfaces when moisture permeates the coating. It often starts at small, sometimes microscopic, defect in the coating and is common with “quick-dry” paints. Nace recommend that “where a coating is required, it should exhibit low water vapor transmission characteristics and excellent adhesion. Zinc-rich coatings should also be considered for coating carbon steel because of their cathodic protection quality.”
- Erosion corrosion – is described by Nace as acceleration in the rate of corrosion attack in metal due to the relative motion of a corrosive fluid and a metal surface. The increased turbulence caused by pitting on the internal surfaces of a tube or pipe for instance can result in rapidly increasing erosion rates and eventually a leak. Erosion corrosion can also be aggravated by faulty workmanship. For example, burrs left at cut tube ends can upset smooth water flow, cause localised turbulence and high flow velocities, resulting in erosion corrosion. A combination of erosion and corrosion can lead to extremely high pitting rates.
- Galvanic corrosion – this refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. According to Nace it occurs when two (or more) dissimilar metals are brought into electrical contact under water. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone.
- Cavitation – this occurs when a fluid’s operational pressure drops below its vapour pressure causing gas pockets and bubbles to form and collapse. This is most likely to happen at the suction of a pump, at the discharge of a valve or regulator and at other geometry-affected flow areas such as pipe elbows and expansions.
- Fretting – this refers to corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration. Pits or grooves and oxide debris characterize this damage, typically found in machinery, bolted assemblies and ball or roller bearings. Contact surfaces exposed to vibration during transportation are exposed to the risk of fretting corrosion.
- Intergranular corrosion – is localised attack along the grain boundaries (or immediately adjacent to the grain boundaries) which form the micro-structure of a metal or ally. Nace suggest this is usually associated with chemical segregation effects (impurities have a tendency to be enriched at grain boundaries) or specific phases precipitated on the grain boundaries. Such precipitation can produce zones of reduced corrosion resistance in the immediate vicinity.
For more information about the range of specialist protective coatings CSC Services offers, visit the coatings section of this website. Cathodic protection is also available as a way to control natural corrosion of metal surfaces through an electrochemical process.
Mick Flounders is a certified NACE Coating Inspector.