What is Corrosion under isolation (CUI)?
Corrosion under isolation (CUI) takes place underneath the insulation which makes it invisible from the outside. It is therefore a hidden threat.
Pipes and equipment are often insulated to reduce heat or cold losses. Since the efficiency of insulation can be greatly reduced by moisture, a metal "weather jacket" or protective sheathing is typically placed around the insulation.
Despite this protective layer, there are locations that are difficult to seal off watertight such as bends, starting and end points of insulation, pipe supports, etc. Here moisture can still intrude, which can lead to corrosion problems.
There are several reasons why corrosion under insulation can proceed at higher rates than atmospheric corrosion: the moisture is difficult to remove, the insulation material can also retain the moisture and possibly leach out certain ions that can reinforce the corrosion.
Temperature also plays an important role. Practically speaking, CUI is a real risk where the surface of the material under the insulation has a temperature between -4°C and 175°C. At temperatures < 4°C the corrosion rates are low enough, at temperatures > 175°C the water evaporates fast enough so that wet corrosion does not have the time to occur. The most critical temperature range is 60-120°C where the corrosion rates for (unprotected) carbon steel can reach 1-2 mm/year.
Which materials are sensitive to corrosion under isolation (CUI)?
CUI affects both carbon steel and stainless steel.
With carbon steel, it is more likely to manifest itself in the form of local or more general wall thickness reduction. In stainless steel, it is more likely to appear in the form of stress-corrosion cracking (practically at temperatures > 50°C), which is a crack-forming mechanism.
Why didn't you hear about CUI very often before and now it seems to be a hot topic?
First of all, the corrosion is hidden under the isolation, making it not easy to detect. In addition, many cases of CUI have become known in recent decades, with enormous consequences. Although nowadays stricter requirements are set for leachable chlorides of insulation materials and new coatings are being developed, there is a growing awareness that a total approach is needed to combat the phenomenon.
What can I do to avoid CUI?
"Prevention is better than cure", which is why it is important first and foremost to prevent moisture from penetrating.
Optimally designed and insulated pipes or appliances are also a good step in the right direction. There are a lot of guidelines for this. The CINI handbook is a good reference in this respect and contains many practical examples.
In addition, it is also important to choose a suitable coating system as a final defense against corrosion. It is common practice to coat carbon steel structures that are in the CUI temperature zone. For stainless steel structures this is only true for temperatures > 50°C, where stress-corrosion cracking is a real risk. Which coating systems are suitable can be found in standards such as NACE SP0198.
How to inspect for corrosion under isolation (CUI)?
There are a lot of techniques to inspect for moisture intrusion/material loss underneath insulation, each with their advantages and disadvantages. We will elaborate on this in a next blog article, so stay tuned!
Now that you already have a first idea about the danger of CUI and possible solutions, you can already better prepare yourself against unpleasant surprises. However, this article is just a tip of the iceberg. Keep an eye on our LinkedIn page for our next articles on, among others, the inspection of CUI. Do you already have a question? The METALogic team is ready to take a closer look at your question, so don't hesitate to contact us!
