Due to water penetration, piping and vessels can be damaged by corrosion under insulation. Rain water, wash water, deluge system water, leakage or temperature cycling sweat or operations that involve low temperature like refrigeration are the most common cause of CUI.
This corrosion is hidden underneath the insulation and may remain undetected until removal and inspection is done or when actual occurrence of leak is noticed. CUI had been and still a growing concern in many industries such as refineries, power plants, petrochemical or industries based onshore or offshore.
There are two main ingredients for CUI to develop: moisture and warm temperatures. For carbon steel piping oxygen is needed. For stainless steel under 300 series, chloride ions are needed to have SCC or stress corrosion cracking.
Oxygen, obviously, is abundant and available but surprisingly, so is chloride ions, too. Rich sources of them are sea water, process and drinking water and chemical compounds like chloride which bring out de-icing salts. Add the presence of strong bases like acid gases and salts and the development of corrosion are accelerated.
There are various forms of moisture and they can get into the insulation system in many ways. Rainwater, for obvious reasons is the common method but the insulation system can also be flooded in the low-lying areas by pressurized water or steam leaks. Sprinkler systems from fire protection can do damage that is equal to what a rainstorm can do.
Water vapor can penetrate and soak the insulation systems by operating at ambient temperatures. Ice, as a final source, normally operates on or below freezing point. Insulated piping or equipment under layers of ice escapes corrosion due to limited supply of oxygen and heat. But as the ice freeze and thaw continually, a near perfect corrosion area is provided.
Now comes operating temperature as the next important element. Over 300 F, moisture that finds the way in the insulation systems evaporates prior to its hitting the surface which causes corrosion. Below 32 F, because the energy level is relatively low, corrosion rate is drastically reduced as a result of limited oxygen supply which is also triggered by the forming of ice. However, at the range of 32 F up to 300 F, CUI happen to carbon steel and from 140 F to 300 F to 300 series kind of stainless steel. The optimum range of temperature to achieve an aggressive corrosion on carbon and stainless steel is at about 200 F to 240 F. at this range, there is heat energy but insufficient to completely evaporate the moisture before it hits contact with the surface of the equipment.
The common and direct way to inspect corrosion under insulation is cutting the insulation plugs that are removable so that it can be tested ultrasonically. But occasionally, is likely to create moisture leakage. The disadvantage of this approach is that corrosion might be missed if the inspection plug is not positioned accurately.
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