Definitions of corrosion
CORROSION is a process where infrastructure, products and parts can degrade through chemical or electrochemical reaction with their environment.
Primary types of corrosion
Six types of corrosion are commonly found in industrial applications today:
• Galvanic corrosion is where two electrochemical different materials (e.g. steel and brass) come into contact in a corrosive environment, causing the least-resistant material to corrode
• Pitting CORROSION occurs when deep, narrow holes penetrate inwards quickly, but the rest of the surface metal remains intact. This usually applies to self-passivating materials like stainless steel or aluminium alloys
• Uniform corrosion happens at a steady rate, leaving an even deposit across the surface of exposed metal
• Crevice corrosion can be found where stagnant liquid builds up locally in small gaps, such as in pockets or corners
• Intergranular corrosion occurs on or near to the grain structure of an alloy, causing localised attacks
• Stress corrosion is where a material is subject to a continuous or changing tension in a corrosive environment, leading to cracks.
What causes corrosion?
The causes of CORROSION are complex and vary across industries. Some examples common to specific industries are listed below:
• Corrosion in construction often occurs due to metals being exposed to outdoor elements and extreme temperatures
• Underground mining is typified by an acidic water environment (often with chlorides and sulfates), combined with high humidity and temperature
• Forestry corrosion often takes place in remote locations, with equipment parked on grass or soil surfaces which draw up large amounts of moisture overnight; this can corrode onboard mechanical systems and components.
Environmental conditions will also affect corrosion rates and spread. Where humidity is present, this generates moisture – causing a reaction where metals corrode much more quickly than they would in dry conditions.
One or more of the following factors will often be present in a corrosive environment:
• Humidity
• Extreme temperatures
• Surface moisture
• Airborne particles
• Salt
• Industrial lubricants.
Preventing and managing corrosion
Engineers can use a range of techniques to reduce or prevent CORROSION. These techniques, explored in more depth in the combating corrosion white paper, include:
• Materials selection – choosing the right materials for the task and environment is key. While all metals can corrode in an aggressive environment, alloy performance can vary dramatically; choosing the right balance of tensile strength with resistance to heat, chemicals and corrosion is critical
• Materials compatibility – engineers need to consider contacts between potentially incompatible materials when designing products; for example, combinations such as copper and stainless steel, or bronze and steel, can lead to galvanic corrosion. Choosing compatible metals and alloys, or using insulation to prevent an electrical path forming, can help combat this challenge
• Protective coatings - some metals, such as steel, iron and aluminium, can have a corrosion-resistant coating applied as protection. Selecting the optimal metal and coating technique relies on careful analysis of strength, durability, friction, torque and corrosion resistance for the task
• Corrosion testing – controlled tests can simulate a range of corrosive atmospheres, including saltwater spray, salt fog, drying and humidity. Such tests are usually performed to very precise specifications, such as recreating seasonal weather cycles to replicate real world environments
• Corrosion management – an effective corrosion management system can help businesses manage threats effectively. Condition monitoring and incident logs can help build a clearer understanding of corrosion practice; and sharing information across departments highlights potential relationships between capital spend, after-care practices and asset life.
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