Corrosion under insulation Frequently Asked Questions
1. What causes CUI?
Corrosion under insulation (CUI) is primarily caused by rainwater entering the insulation. This is particularly the case for new installations, where the insulation’s hydrophobic properties are still intact. In such cases, water doesn’t absorb immediately at the entry point but may accumulate at a lower section of the pipe.
Typically, a coating is applied to protect insulated pipes from corrosion. Until recently, there was no international standard specifically for selecting coatings to prevent CUI. Moreover, standardized tests for CUI coatings were not universally shared across manufacturers. However, in 2018, the International Organization for Standardization (ISO) introduced ISO 19277:2018 “Petroleum, petrochemical, and natural gas industries — Qualification testing and acceptance criteria for protective coating systems under insulation.” This standard establishes guidelines for testing and acceptance criteria of coatings designed to protect pipelines from CUI.
2. What standard for corrosion under insulation coatings was used before 2018?
Before 2018, most major companies relied on the ISO 12944 standard when selecting coatings for high durability (greater than 25 years) and for harsh environments, specifically C5. Over time, many companies developed their own standards by adopting specific coatings designed for corrosion under insulation (CUI).
Despite this, many companies did not use appropriate coatings for CUI prior to 2018. Moreover, even the introduction of ISO 19277:2018 has not fully resolved the issue of corrosion under insulation. This environment remains challenging for protective coatings. According to the RP0198, corrosion under insulation can occur up to temperatures of 175 degrees Celcius.
3. Why does the NACE standard set max 175 degrees Celcius for corrosion under insulation?
There is considerable confusion in the industry, even among corrosion engineers, regarding why the NACE RP0198 standard defines 175°C as the maximum temperature for corrosion under insulation (CUI). It may seem intuitive to assume that at operational temperatures above 100°C, water would evaporate, leaving the insulation dry. However, when rainwater enters the insulation, it is at ambient temperature, and it takes time for the water to heat up sufficiently to evaporate. During this period, before the water fully evaporates, conditions arise that can lead to coating deterioration and, consequently, corrosion.
4. What happens above 175 degree Celcius? Can there be corrosion under insulation above 175 degrees Celcius?
The answer is yes, but at temperatures above 175°C, water would evaporate more quickly, reducing the contact time between the water and the pipe surface. While corrosion can still occur, the shorter contact time means the average corrosion rate would likely be lower and potentially within acceptable limits.
5. Corrosion Rate?
With regard to corrosion rates under insulation, the industry often references a graph published in ASM Handbook Volume 13B, Corrosion, which was later included by NACE in its guideline SP0198-2010, “Control of Corrosion under Thermal Insulation and Fireproofing Materials—A Systems Approach.”
In this study, the authors collected CUI corrosion rate data from various plants and compared the results to theoretical corrosion rate curves for both closed and open systems. The study revealed that the empirical CUI corrosion rate data more closely aligned with the curve for a closed system than with that of an open system.
6. Why the corrosion rate for an open system drops after 80 degrees Celsius?
The explanation lies in the solubility of oxygen in water, which is inversely proportional to temperature. As the temperature increases, the rate of corrosion tends to rise; however, at the same time, the concentration of dissolved oxygen in the water decreases, which in turn reduces the corrosion rate, as depicted in the graph below.
In fact, in the absence of acids, the main cathodic reaction that governs the corrosion rate is:
H20 + 1/2O2 + 2e- = 2OH-
Therefore a decrease in the oxygen concentration will lead to a shift of the equilibrium to the left. Therefore, there is a lower consumption of electrons and a reduced dissolution of iron.
Fe=Fe2+ + 2e-
The study is empirical and does not offer a model explaining why a CUI system behaves similarly to a closed system concerning corrosion rate. It is possible that oxygen does not evaporate easily due to the presence of thermal insulation, leading to a slight increase in local oxygen partial pressure. However, other explanations could also be valid.
7. Why is oxygen not consumed and the rate of corrosion not decreasing?
The cathodic reaction consumes the oxygen dissolved in water. In all cases, new rainwater penetrates the thermal insulation, bringing with it additional dissolved oxygen.
Despite advances in thermal insulation technology, the possibility of water penetrating the insulation remains a concern. For this reason, installing a monitoring system for corrosion under insulation (CUI) is an effective means of enhancing plant safety and reducing costs associated with plant shutdowns.