C.U.I.

Corrosion under insulation (CUI) is a major challenge commonly associated with high temperature insulated steel. It is caused by the ingress and trapping of moisture within the insulation system. This results in a warm, damp corrosive environment that can lead to catastrophic and expensive failures.

Installation of protective coatings on insulated steelwork is a highly effective method of combating CUI. Our staff are experienced in CUI surveying and will utilize high temp coating that has been specially designed to accommodate high temperatures and prevent against CUI.

It is important to verify that the high temp coating you choose is suitable for the expected service temperature. To help you better manage the risk that is associated with CUI, we will develop a scheduled coatings maintenance plan to enable periodic non-intrusive inspections to critical pipe work and equipment.

Key Findings: The Costly Impact of Corrosion in Industries

In 2001, a study commissioned by Congress was completed by a research team of corrosion specialists. The study, titled “Corrosion Costs and Preventive Strategies in the United States” reported the direct cost of corrosion to be $276 billion per year. That number can double when indirect costs are also considered.

Another study, commissioned by ExxonMobil Chemical and presented to the European Federation of Corrosion in September 2003 indicated that:

• The highest rate of leaks in the refining and chemical industries are due to CUI and not to process corrosion
• Most piping leaks – 81% – occur in diameters smaller than 4-inch nominal pipe size
• Between 40% and 60% of piping maintenance costs are related to CUI

CAUSES OF CORROSION UNDER INSULATION

Critical Factors for Corrosion Under Insulation (CUI) Formation

In order for CUI to form there must be two basic ingredients: moisture and warm temperatures. For iron products like carbon steel piping and equipment, oxygen is required. For chloride stress corrosion cracking (SCC) to occur in 300 series stainless steel, it requires the presence of chloride ions.

Obviously, oxygen is ample and readily available. However, so too are chloride ions. These are found in a wide variety of places like sea water, drinking and process water, and chloride chemical compounds to roadway deicing salts. The presence of acids, acid gases, strong bases and salts can also create and speed up corrosion.

Moisture

Moisture in its many forms can find numerous ways to get under an insulation system. Rainwater is one of the most obvious examples. However, there is also flooding of insulation systems in low lying areas and water from pressurized water wash down or local steam leaks. Water can also come from fire protection sprinkler systems, which can be as bad as or worse than any rain storm.

The next source of moisture is water vapor penetrating and soaking down the insulation systems operating at or below ambient temperatures. The final source is ice, normally cold service insulation systems operating below the freezing point. Although insulated piping and equipment under a layer of ice do not corrode significantly since the temperature limits the available heat and oxygen (two of the three necessary elements of corrosion), it provides a near ideal corrosion area where the ice is continually freezing and thawing.

Temperature

Operating temperature is the next most important element. Above 300 F, most moisture that finds its way into the insulation system evaporates before it can get to the surface and start corrosion. Below 32 F, because of relatively low energy levels, corrosion rates are significantly reduced and the forming of ice limits the amount of oxygen available. However, between 32 F and 300 F is where CUI can happen for carbon steel, and between 140 F and 300 F for 300 series stainless steel. The “optimum” temperature range for aggressive corrosion on both carbon steel and 300 series stainless steel is between 200 F and 240 F. In this range, there is plenty of heat energy but not enough heat to adeptly evaporate moisture before it contacts the equipment surface.