Long-term coating solutions to stop CUI

technology belzona 3. Surface PreparationSurface preparation must be carried out during shut down periods preparation, says García.Corrosion Under Insulation (CUI) is not a new problem, but it can be a serious one if it is not tackled on time, writes Belzona Polymerics' Adriana García

Commonly shared by the oil and gas industry, CUI has caused major leaks which led to loss in production and health and safety incidents. Thus, the consequences of CUI can be expensive, accounting for as much as 60 per cent of a company’s static equipment maintenance costs.

Taking place underneath the thermal insulation due to water ingress, CUI is a real threat for asset integrity. Therefore, asset owners are looking for cost-effective maintenance and repair options that can solve this unresolved problem.

A CUI profile

CUI refers to the external corrosion of piping and vessels fabricated from low-alloy steels, carbon-manganese or austenitic stainless steels that occurs underneath thermal insulations due to water ingress. On stainless steel, CUI can cause induced stress corrosion cracking while on carbon steel corrosion manifests itself as generalised or localised wall loss.

Common causes of water ingress are cladding or jacketing poorly installed, deterioration of the insulation over time and substrates lacking in protection, ie, barrier coatings. Along with the presence of water or moisture on the substrate, there are other conditions that accelerate the corrosion rate, these being the presence of contaminants either in the water, the insulation and/or the substrate and the operating temperature of the piping and vessels.

The source of the contaminants can be external (ie, marine environments) or be produced by the leaching from the insulation material itself. However, high chloride contents of water can contribute to the appearance of one of the most dangerous types of corrosion in the industrial sector, which is chloride external stress corrosion cracking (Cl-ESCC) if operating temperatures exceed 60˚C (140˚F).

With regards to the operating temperature of piping and vessels, there is different evidence to suggest that the temperature range should be between -4˚C (39˚F) and 175˚C (347˚F) for CUI to occur. However, the most critical temperature range identified has been between 30˚C (86˚F) and 120˚C (248˚F), as it is cited on the UK Health and Safety Executive (HSE) website.

Along with the HSE, US data has indicated typical corrosion rates of 0.5 mm/year at temperatures of 80˚C (176˚F) under insulation for carbon manganese steels.

Within the oil and gas industry the most critical range has been identified by asset owners to be between 30˚C (86˚F) and 80˚C (176˚F). Though failures can occur over a wide range of temperatures, CUI will rarely occur when equipment is operating continuously at temperatures above 150˚C (302˚F); however there is increased risk during periods of shutdown.

The thermal insulation itself can contribute to the acceleration of corrosion rate by holding the moisture and any dissolved chemicals (i.e. chlorides) on the surface even as the temperature increases. This situation creates conditions that are more aggressive than those associated with a typical marine atmosphere where typical corrosion rates are 0.1 mm/year.

Some types of insulation are more susceptible to trap moisture without letting it dry out properly and others (i.e. those manufactured with calcium and silicate) may contain leachable chloride compounds.

As the effects of CUI are not visible and can be highly localised, they can potentially cause disastrous loss of containment and equipment failure if they are not detected by inspection. Therefore, the prevention of CUI is the best way to reduce those risks.

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