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.
Polymer technologies for CUI
In the market there are many options available for the prevention of CUI, the most common being the use of cladding to protect the insulation from the external environment.
However, main disadvantages of cladding are that these are prone to leakage at the joints when poorly installed increasing the risk of water ingress.
There are other options such as surface treatment technologies like petrolatum tapes, solvent-based asphaltic mineral fibre tapes and conventional epoxy coatings, but none of these have proved to be infallible solutions.
Another solution is the use of Thermal Spray Aluminium (TSA) in which a metal or organic powder is melted and spray deposited on to a surface in order to provide protection. Some drawbacks of TSA are that surfaces must be grit blasted and prepared until obtaining a Sa 2½ profile.
The surface preparation must be carried out during shut down periods which are costly for asset owners as they must stop production. In addition, there is evidence that suggests that TSA is failing in service when exposed to prolonged periods of immersion.
Another option to overcome CUI is the use of organic protective barrier coatings developed with the latest polymer technology to protect the substrate. These types of organic coatings offer both a proactive and a reactive solution for CUI. They are reactive as once the problem appears it can be solved easily and proactive as they prevent the onset of CUI if substrates of pipelines and vessels are lined.
However, asset owners have requested a certain performance criteria to manufacturers of organic linings for the development of protective barrier coatings to prevent CUI as they are essential to ensure plant production in the long-term.
These requirements include the application of the coating directly onto hot substrates with a minimal preparation of the surface. In addition, the linings must resist aggressive partial immersion environments, offer long-term corrosion resistance and be safe and easy to apply. Moreover, applications must be multi-coat with fast overcoat times and solutions must be cost-effective.
With these criterias in mind, Belzona Polymerics Ltd, a manufacturer of high performance solutions based on polymer technologies, designed a range of suitable products for CUI. Main features of the Belzona Solutions for CUI are that these can be applied in-situ onto minimally prepared metal substrates. Surface preparation can be done using a simple application technique, ie, with a wire brush.
This way of preparing the surface eliminates the need to grit blast and the additional costs and safety complications of carrying out this procedure such as, environmental control, creation of a habitat (confined space), contamination of the surrounding area, spark risks, among others.
Another important characteristic of the Belzona Solutions for CUI is that they have heat activated curing properties which generate suitable adhesion on the substrate.
Adhesion on the substrate is generated thanks to the opportunities that a hot corroded steel substrate presents as porosity of the steel increases with the heat. Therefore, in this situation the coating is capable of penetrating in to the substrates surface granular structure after minimal surface preparation and creates a similar mechanical lock to the one acquired by grit blasting.
In addition, once the coating gets in contact with surfaces between 30˚C (86˚F) and 150˚C (302˚F), the 100 per cent solids coating viscosity reduces and it penetrates the substrate even more before curing and creating the mechanical bond. Figures 1 and 2 (above) illustrate this idea.
Case study: UK refinery condensate vessel protected against CUI
In June 2001, a severe CUI problem was detected in a condensate vessel which operates at 85˚C (185˚F) during a routine inspection. The corrosion, if left untreated, could have caused considerable damage. Therefore, a suitable solution which did not interrupt operations was required by the asset owner.
The refinery opted for an on-line solution using a mono component; heat activated organic barrier coating Belzona 5851-HA Barrier due to operation and surface preparation requirements. Therefore, the surface was prepared to a St3 (SSPC SP3) standard using power tools and loose surface rust was removed by power wire brushing. The surface was then cleaned and de-greased.
The surface temperatures of the vessel ranged between 50˚C (122˚F) and 110˚C (230˚F) at the top and due to the requirement of a minimum temperature for the application of the heat activated product, the coating was applied by brush down two thirds of the vessel.
The second coat was then applied after the first coat cured. The application was completed in four hours and the product fully cured within three hours.
In February 2002, an initial inspection was carried out after several months of continual operations of the vessel. When sections of the insulation were cut away to inspect the condition of the coating, it was found in good condition.
Later on in 2008 and 2011 other inspections were done, and they confirmed that the original application still remained in good condition after ten years in service.
Adriana García, marketing executive - oil and gas, petrochemical and bulk chemicals, Belzona Polymerics Ltd
