Corrosion issue in process industry
Why corrosion happen?
Corrosion occurs in the presence of water and oxygen. So, obviously, if the steelwork under insulation remains dry there is no corrosion problem. Keeping insulation dry can be difficult. It is essential that every effort is made to keep the insulation dry during storage and installation. Failure to do this correctly leads to water ingress into the installation which can lead to steelwork corrosion, commonly referred to as corrosion under insulation (CUI). In practice CUI especially appears in the temperature range between -50 Celsius and 200 Celsius or in case of cyclic operation of the equipment. CUI is found underneath all types of insulation when you have installation issues or damage as detail above.
A common but incorrect assumption is that insulation also protects against corrosion. but if steelwork is not protected with a suitable coating and the insulation is not installed in a dry state under dry conditions and protected by adequate weather-resistant cladding, CUI will be a strong possibility
Types of corrosion
When applying insulation and taking measure to prevent corrosion, the corrosion resistance of the metal surface to be insulated is an important factor. The most frequently occurring types of CUI are:
- General and pitting corrosion of carbon steel, which may occur if wet insulation comes into contact with carbon steel, particularly if acidic product can be extracted from the insulation material itself
- External stress corrosion cracking (ESCC) of austenitic stainless steel, which is a specific type of corrosion mainly cause by the action of water-soluble chlorides from rainwater or, say, the insulation material not meeting the appropriate requirement. Austenitic stainless steel is generally susceptible to this of attack in the temperature range of 50 Celsius to 200 Celsius.
The corroded surface is mostly hidden by the insulation system and will not be observed until the insulation is removed for inspection or in the event of metal failure leading to health and safety incidents. The necessity of protection against corrosion must be determined for each individual plant.