Corrosion is the biggest enemy of metals. It comes about when metal reacts with oxygen and moisture under the influence of the temperature. If it is under an insulation layer, it is difficult to detect and counteract. This “corrosion under insulation” (cui) poses a serious threat to the stability of the metal. It is not until a critical stage has been reached that it becomes visible on the surface.
The insulation layer is not the cause of the corrosion. At most, it creates a space where oxygen and moisture can accumulate. In some cases the insulation layer intensifies the corrosion, for example when it is made of material that absorbs a lot of moisture or when chlorides and acids leach out of the insulation layer.
That gives rise to the question of how moisture gets under the insulation layer. Two causes can be identified:
- The first is when moisture seeps through small leakages in the watertight covering of the insulation layer. This gives rain, production water, steam or ground water the chance to reach the metal surface. These leaks could in most cases have been prevented. They are the results of poor design, poor fitting of the insulation, poor use of material or poor maintenance. The forming of condensation can also cause moisture under the insulation layer. If the temperature of the metal surface is lower than the atmospheric dew point, condensation forms on the surface. This possibility must be taken into account as early as during the design of an item so that the appropriate type of insulation can be chosen.
- The corrosion process can be intensified by dirt. Here too, cracks in the insulation layer can result in the penetration of moisture. Chloride, acid and salt pose a special threat to the metal. These substances are sometime even present in the insulation layer itself. If the insulation layer comes into contact with the moisture, these subjects can leach out and corrode the underlying metal. The dirt concentration increases sharply if the moisture evaporates when the temperature rises.
Temperature is a factor that contributes to corrosion. Although evaporation reduces the duration of contact between the moisture and the metal, the higher temperature intensifies the corrosion. That in turn reduces the metal’s lifetime. There is a good chance of corrosion occurring at the temperatures between -4°C (25°F) and 174°c (350°F). The a lower temperature the metal is protected against corrosion by the cold, at higher temperature the heat keeps the metal dry, these are few objects with a temperature that remains constantly below or above the critical point. This is because of variable operation, temperature variations in parts of the object or different temperatures of parts connected to the object.
Corrosion under insulation (cui) can occur under all types of insulation, but especially insulation where:
- The insulation contains salt that leaches out;
- The insulation easily absorbs moisture or moisture penetrates through the top layer;
- The insulation contains foam containing remnant chloride and acid, which react with moisture.
Insulation, which absorbs the least moisture and dries the quickest, offers the lowest chance of corrosion occurring. Bear in mind that cheaper insulation is not necessarily the most economical choice viewed over the entire life cycle.
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