Basically, the corrosion of pipes and vessels under the insulation due to water penetration is called corrosion under insulation (CUI). This penetration might come from leakage, rain water, and flood, wash water, sweating due to temperature cycling or low temperature from refrigeration units. These include industries such as refineries, power, and petrochemical including those onshore and even offshore ones.
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 is very abundant on Earth; however it is amazing those chloride ions too are present everywhere from seawater, drinking and bottled water, and chloride chemical compounds. In producing corrosion, it is true that it only needs the presence of acids, acid gases, strong bases and salt.
Moisture in its different form can find a lot of ways to get into the insulation system. Obviously, rainwater is the most common method, in low-lying areas there is also a flooding of insulation system but insulation systems which are located in the low-lying regions can be flooded by water that comes from pressurized sources or even steam leaks. Water from fire protection sprinkler systems can be as bad as rainstorm.
Generally, the most vital component is operating temperature. 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, for the reason of relatively low energy levels the corrosion rates are radically reducing as well as the formation and availability of oxygen. On the other hand, corrosion under insulation between 32 F and 300 F may happen in carbon steel, and between 140 F and 300 F for the series of stainless steel.
In fact, the optimum range of temperature to achieve an aggressive corrosion on carbon and stainless steel is at about 200 F to 240 F. In this series, the presence of heat energy is abundant but still its not enough to evaporate the moisture by hitting the surface of the equipment.
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.
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