Here at Hempel we are often asked our views on the new ISO 19277 standard for testing coatings to Corrosion Under Insulation. To answer this we took the opportunity to talk to our Businesss Development Manager for CUI and High Heat coatings, Simon Daly to explain a little bit more.
Can you explain what Corrosion Under Insulation (CUI) is?
It is well known that CUI is a relentless form of corrosion, localised within insulated, low alloy and carbon steel equipment. It occurs wherever water makes its way under layers of insulation and is unable to evaporate and, over time, the trapped water vapour causes the equipment to corrode and lose structural integrity. Normally found in the insulation layers of operating parts, process equipment, valves and refining assets CUI has unfortunately become well-known amongst operators in the process industries causing loss of revenue due to maintenance, downtime and the replacement of corroded components.
Why can CUI be more problematic that other forms of corrosion?
Within the interior layers of insulation, corrosion can go unseen and is difficult to test for, in a way that won’t delay the daily operations of oil and gas assets. Over time, this can lead to significant corrosion of crucial infrastructure, which can be costly to repair or replace, and sometimes hazardous to leave unattended. Additionally, the removal and disposal of insulation materials needed to inspect, and repair has a significant environmental impact. Here is where the right protective coating can play a significant part. But choosing a protective coating to fight corrosion under insulation is not straightforward, given the sheer number of products on the market today.
What should be considered when choosing the correct coating to protect against CUI?
Is there any guidance available to help customers to make the correct choice of coating?
Can you explain ISO 19277: 2018 in more detail?
ISO 19277: 2018 introduces the concept of CUI categories based upon temperature.
These categories are:
Table 1 - CUI classification environments
| Classification | Minimum temperature | Peak temperature |
| CUI-1 | -45°C | to 60°C |
| CUI-2 | -45°C | 60°C to 150°C |
| CUI-3 | -45°C | 150°C to 204°C |
allowing coatings to be classified to one or more categories, dependent upon the temperature in which they are intended to operate. Additional categories are used for coatings intended for cryogenic service.
The standard describes procedures for sampling, application and heat conditioning of coatings prior to testing. Heat conditioning is extremely important, as CUI does not generally occur the instant that a plant becomes operational. Therefore, the effects of heat/temperature on a coating must be considered, before the coating’s exposure to a CUI environment. Dry film thickness measurement and adhesion testing are carried out before and after each kind of exposure.
The standard proceeds to examine the following areas in detail:
Artificial ageing – whilst not an immediately obvious choice for a CUI testing programme, it is very important to consider 1) the exposure of the coating during transport and installation before plant start-up and 2) situations such as extended shutdowns where the insulation may be removed. Additionally, it is useful when considering bulk pipework which may subsequently be insulated or left exposed.
The artificial ageing process uses a combination of salt spray and water immersion. Afterwards, checks for blistering, cracking, rusting and flaking are carried out to ISO 4628 specifications. This is a common feature throughout the standard.
Cyclic Testing - Cyclic testing with immersion functions involves the repeated heating and cooling over multiple cycles from the maximum temperature of the relevant CUI category then quenching into cold water at 5°C.
CUI cyclic corrosion test (mandatory) – perhaps the most controversial part of this standard is the selection of the condensation chamber as an indication of CUI performance. This test is not widely used in the industry and numerous other, larger-scale testing methods are currently being evaluated. The absence of any insulation in this test makes it an arduous task to form conclusions that would closely reflect real-world performance under insulation.
Besides the test programme, there are several optional tests.
Cryogenic exposure – optional cryogenic cyclic testing is described for those cryogenic categories described in the standards.
Vertical pipe/Houston Pipe test – As an optional CUI test method that is included within this standard, the vertical pipe test faces criticism because of the temperature gradient that exists throughout the pipe samples. This test is also highly dependent upon the testing conditions, notably the insulation type.
What do you believe the future holds for testing of coatings to protect against CUI?
Although this standard is a very positive start to prescribe a documented and reproducible CUI coating test methodology, the CUI testing element faces significant push back from the industry.
With many test programmes active in this area, it seems probable that this standard will be refined as time passes by. Additionally, with a handful of key oil and gas standards such as NORSOK M-501, currently due for revision, it seems likely that there will be continued activity in this important area. The performance of protective coatings under a wide range of CUI conditions is important, which is why such a plethora of testing methods exists, to help avoid choosing the wrong coating specification.
The process of testing the effectiveness of coatings on their ability to prevent or delay CUI is complex, although necessary. Combining these test methods into a coherent standard with clear acceptance criteria is what ISO 19277 seeks to do. Further refinements to this document as improved test methods for CUI become available will allow even better coating selection to be taken in the future.
