What is corrosion
Corrosion is defined as the gradual destruction or failure of materials due to their interaction with the environment. Based on the nature of the corrosive environment, corrosion can be classified as wet or dry corrosion. Wet corrosion occurs in the presence of a liquid or moisture for example the rusting of steel by water. Dry corrosion results from the attack of gases on metals, typically at high temperatures. For example, hydrogen sulphide and chlorine gases react with steel and can cause severe damages to pipelines and furnaces. Although the scope of corrosion extends beyond metals and alloys to other materials such as ceramics, concretes, polymers and biomaterials, the corrosion of metals and alloys remains the principal area of corrosion science and engineering. A wide variety of different corrosive environments exist and the nature of the corrosive environment can induce different types of metallic degradation which in turn be classified as uniform corrosion, galvanic corrosion, crevice corrosion, pitting corrosion, stress corrosion cracking and fatigue corrosion.
- Uniform corrosion, is the most common type of corrosion. It describes the thinning of the metal’s surface uniformly which does not include deep rusting or cracking. Some examples of uniform corrosion are the surface rusting of iron in the air, automotive bodies, heat exchanger tubes and structural steel.
- Galvanic corrosion happens when two metals with different electrochemical potentials are in contact through an electrolyte (E.g. copper (+0.334 V) and iron (-0.440 V)) forming a galvanic cell or battery. The driving force of the galvanic reaction depends on the position of the two metals in the galvanic series. Some examples of the galvanic corrosion include the body of a ship in contact with brass or aluminium, steel bolts in copper joints, rivets in aircrafts (titanium rivets and aluminium body) and the Statue of the Liberty (steel armature and copper skin).
- Crevice corrosion is a localized metal degradation that occurs under the head of rivet bolts or roller joints (E.g. Holes, cracks, lap joints and deposited dirt on the metal’s surface).
- Pitting Corrosion is a type of extremely localized corrosion that results in the formation of cavities or holes in the metals . It mostly happens on the surface of metals with a passive protective coating such as aluminium or steel. Detection of the pits is challenging since the cavities can penetrate the metal’s matrix, leading ultimately to equipment failure. Exposure to a corrosive environment or lack of homogeneity in a coating film, in the presence of halogen ions, is typically responsible for pitting corrosion in metals.
- Stress corrosion cracking (SCC) occurs when metals are exposed to a corrosive chemical under static tensile or residual stress. For instance, steel exposed to chloride ions, oxygen, hydrogen sulphide or hydrogen gas at high-temperature can suffer from stress corrosion cracking. Stress corrosion cracking can be initiated from different sites on a metal such as surfaces with irregularities, laps, pits and grain boundaries. The susceptibility of a metal to SCC also increases with temperature.
- Fatigue corrosion is the degradation of materials due to exposure to the combined presence of corrosion and cyclic stress. Examples of areas where fatigue corrosion occurs include drilling rigs, tubes of heat exchangers or pumps and steam turbine blades. The presence of several cracks, initiated from the surface, is one of the main features of fatigue corrosion. Concentrated corrosive ions from the environment, temperature and cyclic stress decrease the corrosion fatigue resistance of metals.
The Cost of corrosion
Corrosion is ubiquitous and its impact on the economic activity is very significant. According to National Association of Corrosion Engineer (NACE) based in the US, the cost of corrosion for the world were estimated to be in excess of 2.2 trillion US dollars per annum or over 3% of world GDP (source Corrosion cost and preventative strategies NACE). The direct cost in the US amounts to US$276 billion on an annual basis and corrosion control method costs (i.e. direct costs) were US$121 billion, or 1.38% of the U.S. GDP (excluding labor). According to NACE, the largest portion of this cost (88.3%) was attributed to organic coatings.
Corrosion affects a wide range of industrial sectors. Here are some key facts:
- The largest corrosion cost, a staggering 36 billion dollars (for the US only), has to do with utilities and 75% of that cost has to do with drinking water and sewer systems.
- For the cost related to infrastructure, 31% of it has to do with pipeline and 31% is related to chemical storage (i.e. oil and gas storage), which makes about 20 billion dollars for the oil and gas industry (not including exploration). The rest (37%-8.3 billion dollars) is related to highway bridges.
- For transportation, the lions’ share of the cost (79%) is attributed to motor vehicles, with ships representing 10% of the costs and 7% for aircrafts.
- In the production and manufacturing sector, the bigger costs come from the pulp and paper sector followed by petroleum refining. Interestingly the mining industry only represent 1% of this sector.
From an end-user perspective, the largest market segments in the US, which require corrosion protection are:
- Water and sewage systems USD 36 billion
- Oil and gas/petrochemical industry USD 25 billion
- Motor vehicles USD 23 billion
- Bridges USD 8.3 billion
- Pulp and paper industry USD 6 billion
How do you prevent corrosion
Depending on the type of corrosion, the inhibiting strategy can vary. Typically, the use of polymeric coatings, cathodic and anodic protection and modification of the corrosive environment are used to prevent corrosion. Based on the various types of corrosion, some of the practical mitigation methods are described below.
For uniform corrosion, which occurs as a thin layer on the surfaces of the metals and alloys, an organic coating reinforced with corrosion inhibitors can be applied to protect the metallic surfaces from corrosive media. For galvanic corrosion, since corrosion is caused by the contact between two metals with different reduction potential, selecting metals that are close together in galvanic series decrease probability of corrosion. Insulating the dissimilar metals and preventing direct contact, or applying inhibitor coatings are common methods to suppress galvanic corrosion. Crevice corrosion appears underneath of the stagnant areas such as bolted or riveted joints. To prevent it, welding of joints and sealing of the crevices using a polymeric coating loaded with encapsulated inhibitors can be an efficient strategy. Avoiding sharp edges, applying cathodic protections, removing of the stains and deposits can help minimise the progression of crevice corrosion. Pitting corrosion happens as the tiny cavities and holes from the surface grow downward into the alloy structure. Accordingly, providing a uniform and appropriate coating using inhibitors can minimize the effect of the external corrosive factors. Since the coating is the key method for preventing pitting corrosion, covering the surface using smooth polymeric film will minimise potential points of attacks and decrease the chance pitting corrosion to occur. Stress corrosion cracking (SCC) occurs when a metallic material is exposed to corrosive chemicals, mainly chloride ions, and static tensile stress. Modifying the temperature, pH and reducing the tensile stress can also decreases the probability of SCC into the metals and alloys. Fatigue corrosion, which results in random cracks in the alloy structure, appears when the alloy is exposed to corrosive materials and cyclic stress. To avoid the initiation of the cracks, various mitigation strategies can be used such as cathodic or anodic protection, decreasing the induced stress and applying a protective coating.
Paints for corrosion protection
Paints are one of the most commonly used methods for preventing corrosion of metal surfaces; providing a cost effective and practical alternative to other methods such as plating, coating and enamelling, whilst providing the ability to customise the appearance of the surface. Typically, an anti-corrosion paint will be made up of two layers – the primer contains the corrosion preventing agent and will adhere directly to the substrate. Most anti-corrosion pigments will tend to give the primer a matt appearance due to their shape, size and refractive index. This means that typically a topcoat is required to provide the aesthetic quality of the paint, providing gloss and smoothness. Direct to metal coatings (DTM) can also be used in corrosion protection of metals, with the corrosion protecting pigment being applied using a one coat system. Whilst most anti-corrosion pigments can reduce the glossiness of the coating, leading to a compromise between corrosion protection and appearance, Inhibispheres® are able to be incorporated into DTM without reducing the surface characteristics, and at the same time providing effective and long-lasting corrosion protection.
Depending on the environment to which the metal surface is exposed, the rate of corrosion will differ. Exposure to moisture, chlorides and air pollution can all increase the corrosivity of an environment, classified in ISO 12944-2, as shown below:
| Environmental class | Corrosion level | Internal environment | External environment |
|---|---|---|---|
| C1 | Very low | Heated building with low relative humidity, e.g. shops, offices | Dry or cold zones, very low pollution and very short-term humidity, not typically found outside of central Arctic/ Antarctic areas |
| C2 | Low | Unheated buildings with changeable temperature and relative humidity, e.g. sports halls, storage rooms | Temperate zone with low level pollution, e.g. rural areas |
| C3 | Moderate | Production areas with moderate air pollution and high humidity, e.g. food processing plants, laundries, breweries | Temperate zone, moderate SO2 pollution levels, e.g. urban areas, coastal area with low chloride levels |
| C4 | High | Industrial areas with high condensation and high industrial pollution, e.g. swimming pools | Temperate zone, high levels of SO2 pollution or high salinity, |
| C5-I | Very high – Industry | Buildings or areas with very high condensation frequency and high pollution, e.g. mines, industrial excavations | Temperate and subtropical zones, industrial areas with very high humidity, or coastal areas with high salinity. Very high pollution or chlorides, e.g. coastal areas, offshore areas. |
| C5-M | Very high – Marine |
Paints for corrosion protection can be classed as either active, sacrificial or passive depending on their mode of action. Paints which prevent corrosion purely through barrier protection are known as passive corrosion protection, as they do not change the behaviour of the corrosive agent or change the tendency of the substrate to corrode. These paints can also utilise layers of metal or silicate flakes to provide a tortuous path – making it difficult for water which has breached the surface of the paint to reach the substrate. However, if the paint film is damaged, corrosion will occur very quickly due to lack of protection from the environment. Some of the commonly used passive corrosion paints include alkyds, acrylics, amine epoxies, polyamide epoxies, urethane and polyurethane.
These paints are also used in both sacrificial protection and active corrosion prevention. Sacrificial coatings utilise both barrier protection and cathodic protection to protect a substrate, with cathodic protection being used upon disruption of the paint film (e.g. scratch, chip). These coatings utilise the presence of metals which are more active, and will corrode preferentially to the substrate, helping to prevent corrosion of the underneath layer. Typical sacrificial corrosion pigments include zinc phosphate (ZnPO4), zinc dust or chromate 6+ (CrVI) compounds. There are a number of issues with the use of these compounds in anti-corrosion paints, with of serious health and environmental issues resulting from the use of both zinc and chromate compounds. This has led to the restriction in the use of hexavalent chromium in Europe and other countries, with European companies requiring specific and very limited authorisation from REACH to use these compounds. There is strong worldwide encouragement to shift away from using these types of materials in anti-corrosion coatings. Active corrosion protection paints contain an additive which can influence the corrosion reactions caused by exposure to corrosive elements, by disrupting the corrosion chemical reactions. These are frequently used in primers.
Inhibispheres® offer active corrosion protection when incorporated into a traditional barrier paint system and promote self-healing of the paint film upon damage, preventing corrosion from occurring. With their homogeneous dispersion throughout the coating, Inhibispheres® will provide protection regardless of where the coating is ruptured, diffusing to the corrosion site to quickly prevent further corrosion. Designed with specially selected corrosion inhibitors, Inhibispheres® can be used on either steel or aluminium substrates, in both water-based and solvent based paint systems, and the sustained release action of Inhibispheres® means that the coating will provide long lasting corrosion protection.
