Corrosion is the deterioration or decay of a material (usually a metal), or the deterioration of its properties because of a reaction with its environment. Corrosion not only eats away at capital equipment, but also profits. With this in mind, it’s not hard to see why protection against corrosion is a vital issue in extending the working life of plant and equipment, reducing total life cycle costs.
We are chiefly concerned with the corrosion of metals and to a certain extent the corrosion or deterioration of concrete. Along with our partners at Corrocoat, we mitigate corrosion via the following:
- Mechanical Engineering
Refurbishment and designing mechanical components and equipment. - Chemistry
Developing and manufacturing protective coating systems through our understanding and expert knowledge of service environments.
Our protective coatings are perfect for withstanding harsh operating environments. Our unique high-performance composites and coatings not only fight against numerous types of corrosion in particular service environments, they also prolong the life of metals and machinery. Below are the 5 main types of corrosion we commonly treat:
1. Uniform Corrosion
Uniform corrosion, also referred to as general corrosion, happens when the surface layers of the metal are converted to corrosion products materials. Corrosion products are derived from the destruction of another material by a chemical reaction with the environment in which it resides, in such a way that the thickness of a section is uniformly decreased. Uniform attack over a large area of the metal surface is the most common form of corrosion in material exposed to open air atmospheres, soils and natural waters. It is not the general type found in process environments. Uniform corrosion usually leads to a reddish-brown oxide, giving a rusty appearance. We see and repair uniform corrosion, from clients in all industries.
This type of corrosion occurs on all unprotected steel substrates over a period of time with the rate of corrosion higher in marine environments due to the salinity of the air and the humidity. This is usually a problem associated with the externals of components such as storage tanks.
2. Crevice or Concentration Cell
Intensive localised corrosion occurring in crevices and other shielded areas on metal surfaces exposed to corrosive environment is called crevice concentration. It can also be referred to as concentration cell corrosion.
Crevice corrosion occurs as a result of small amounts of stagnant solution caused by holes, gasket surfaces, lap joints, surface deposits and crevices under bolt and rivet heads.
Additional factors affecting crevice corrosion are different areas of solution, oxygen and metal ion concentrations; with all three being separate situations.
The most common form is oxygen differential cell corrosion. This occurs because moisture has a lower oxygen content when it lies in a crevice than when it lies on a surface. The lower oxygen content in the crevice forms an anode at the metal surface. The metal surface in contact with the portion of the moisture film exposed to air forms a cathode.
Common areas subject to crevice corrosion include gaskets, bolted and riveted connections and clamps.
3. Erosion Corrosion
When movement of a corrosive material over a metal surface increases the rate of attack due to mechanical wear and corrosion, the attack is called “erosion corrosion”. Corrosion products that would normally afford a protective film can be scoured off by moving fluids, particularly when abrasive materials are present. This will expose fresh metal and develop a pattern so obviously a result of erosion, that the corrosive factor may be overlooked entirely. This is a particular issue in substrates which rely on a passive oxide layer for corrosion protection such as stainless steel or aluminium.
We see a lot of erosion corrosion in pumps, valves, clarifiers, hoppers, pipeline elbows and cyclones.
4. Galvanic Corrosion
Galvanic or two-metal corrosion is when a potential difference occurs between two dissimilar metals, when they are immersed in a corrosive or conductive solution.
Corrosion of the less resistant material is usually increased, and attack of the more resistant is decreased. When two metals or alloys are electrically connected in an electrolyte, the less resistant material becomes anodic and the more resistant cathodic. Passivity influences galvanic corrosion behaviour. This type of corrosion is often seen in pumps between the casing and rotating element. Casings are often made of something easy to machine such as cast iron, whilst the rotating element is often made of an alloy such as a bronze, brass or titanium. Galvanic corrosion is also common in tube sheets, where the sheets are often made of steel and the tubes are made from a different metal, with a high heat transfer coefficient but more noble, such as copper or titanium. Bolted connections such as flanges, where bolts are made of a different material to the flange, also commonly suffer from galvanic corrosion.
5. Microbiological Influenced Corrosion
Microbiological Influenced Corrosion (MIC) is corrosion that is affected by the presence of microscopic, one-celled living organisms, including algae, fungi, and bacteria.
These organisms can initiate corrosion, affect the corrosion rate, destroy protective coatings, and affect cathodic protection.
Corrosive bacteria can thrive in all aqueous environments, from fully aerated to low oxygen to oxygen-free conditions, as long as the environment has a nutrient source for the bacteria.
This can occur underneath the coating and large deep pits is usually the outcome. If MIC is suspected, the substrate can be tested and cleaned with a 2% Sodium Hypochlorite solution to remove the bacteria.
For almost 50 years we have been renowned for our products and services in corrosioneering, and providing long-term protection to all. Our expertise has lead us to operate from sites in 30 locations across five continents; it’s made us the go to choice for all kinds of industry sectors – the name you can trust to provide corrosion protection to metals, machinery and more.
Contact a member of the team today to discuss how we can provide long-term corrosion protection for your equipment.
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