What is Corrosion under Insulation?
Corrosion under insulation (CUI) is, as the name indicates, the corrosion or corrosion-related cracking that occurs under insulation and/or fireproofing. It is a major problem in all parts of the chemical processing industry, and constitutes a major portion of many plants’ maintenance budgets. The corrosion can take the form of general corrosion, crevice corrosion, pitting or stress corrosion cracking. In carbon and low alloy steels, CUI is almost always general or localized corrosion in which there is a significant amount of metal loss. In stainless steels, general corrosion is unusual. Crevice corrosion or pitting can occur, but the most common problem is stress corrosion cracking, especially at welds and cold worked areas, without significant metal loss.
Why does it occur?
CUI occurs because of the combined action of three factors:
- Water and moisture penetrate and accumulate between the insulation and the equipment
- Other corrosive components dissolve in the water/moisture that collects
- The mixture of aqueous corrosives are held in place by the insulation allowing the corrosion to continuously progress.
A fourth factor that occurs in most cases is the elevated temperature of the insulated equipment that increases the rate of corrosion. However, elevated temperature is not always a factor: Carbon and alloy steel equipment with an operating temperature below the dew point or which cycles above and below 32°F (0°C) often suffers CUI even though the metal temperature may never rise above ambient. In all cases, the corrosion is hidden under the insulation and progresses unnoticed for long periods of time.
