Corrosion is caused by specific combinations of metals and chemicals coming into contact with each other. It’s essential that we identify these different combinations in our plant and how they could impact operation of assets. Some common themes observed in mining are uniform corrosion, pitting corrosion, under-deposit corrosion and corrosion-enhanced wear.
Uniform or general corrosion gets its name from the way that rust spreads out more or less evenly over a metallic surface. This type of corrosion is fortunately usually easy to spot, unless surfaces are in hard-to-inspect locations. How fast this mechanism goes depends on the exposure and in many mining environments, it can be rapid and severe. If corrosion rates are known it can be dealt with through adding more thickness to steel to ensure it lasts and by applying a protective surface coating.
Pitting corrosion is a more insidious type of corrosion. Corrosion in very localised areas can form deep pits in the metal, undermining its integrity and leading to leaks or even catastrophic collapse. Pits are difficult to predict and detect. Pitting often occurs when metal sees elevated chlorides, or other aggressive exposures. It can even be caused by certain bacteria! It is important to identify if this type of corrosion might be a concern and then optimise material / coating selection. It is also important to design inspection / monitoring procedures to deal with it.
Under-deposit corrosion occurs when material builds up on a surface due to leakage events or a lack of housekeeping. The environment underneath the build-up can become aggressive and cause metal loss. It can be difficult to see this mechanism as by its nature, it’s hidden. Keeping assets and components clean is important to avoid this attack and also allow for proper inspection.
Mining operations experience many issues with material wear. However, when in combination with corrosion, the impact can be accelerated. For example, abrasion combined with acidic ore can cause rapid metal loss. It is important to identify this risk so that the selection of material / coating / liners is optimal. Understanding this mechanism also helps to ensure inspection/replacement frequency is accurate.
There are many different corrosion mechanisms that can impact our operating assets and it is important to know what the problem is before we can fix it. This is the core of corrosion risk management.