CUI – Corrosion Under Insulation
By Chris Savage – January 2025
“Solid as steel”, “strong as steel”, “ steel-like resolve”, “forged in steel”, “tough as nails”, or even the “Man of steel”, are all terms that invoke thoughts of strength, invulnerability, durability, etc. Our world relies heavily on steel for almost anything you can think of; buildings, cars, toys, electronics, ships/boats, armor, air planes, the space station, submarines, robots…..you get the picture! In the past, present, and future, we rely on steel to support our lives.
Carlson & Stewart Refrigeration lives in the world of refrigeration, a big portion of which is ammonia refrigeration, and we too rely very heavily on steel. Almost every component of an ammonia refrigeration system is made of steel. Why is this? Well, there are a lot of reasons but the ones that I want to focus on for a minute are the first ones that come to mind, strength and durability. There is no doubt that the steel components in an ammonia refrigeration system are strong and durable. But we have to be careful not to think of strength and durability as invulnerability.
Although strong and durable, steel does have a few weaknesses. And the one we are talking about today is corrosion, rust! I think we all know what corrosion/rust is so I’m not going to spend any time talking about what it is. Instead, I’d like to talk a little bit about how it can affect the integrity of our ammonia refrigeration systems, primarily focusing on piping.
I don’t know the exact numbers, but nearly all of the refrigerant containing piping in an ammonia refrigeration system is made of steel. There is a small percentage out there, which is steadily increasing, with portions of ammonia systems being piped with stainless steel piping. But stainless steel remains a very small minority when looking at all ammonia refrigeration systems out there. Where stainless steel piping is used, most of the corrosion challenges are significantly diminished if not eliminated. But stainless steel piping continues to be cost prohibitive when considering a full piping system. So it is limited to small portions. So, we must continue to be aware of our steel pipe systems and the effects of corrosion.
I recently read an IIAR Technical Paper written by Suzy Vohsen titled “Cues and Confirmation Your Piping is in Jeopardy: A comparison of 20+ Years of NDT Data With Field Observations”. Suzy did a nice job of describing how corrosion occurs under insulation and what signs to look for to indicate you may have CUI (Corrosion Under Insulation). Corrosion on any steel ammonia pipe is concerning, but CUI can be a bigger threat because it is somewhat out of sight and therefore is easy to be out of mind. Her paper can be found on the IIAR website or in the Fall of 2024 IIAR Condenser magazine. I won’t try to regurgitate what she said so I’d recommend you give it a read.
As an ammonia refrigeration contractor, Carlson & Stewart Refrigeration has seen a lot of CUI. As a result, we have recognized patterns regarding where to expect to find CUI. We have harnessed these patterns over the years to try to find the “needle in the haystack” per say. There can be hundreds of miles of insulated piping in a large refrigeration system and none of it will corrode in the same way or at the same rate. So sometimes it can be tricky to find.
There are a few ways to find CUI. Some are destructive, meaning the insulation has to be removed and others are non-destructive, meaning the insulation does not need to be removed. As you can imagine, the non-destructive methods require fairly sophisticated equipment to be able to see what is going on below the insulation. This equipment is expensive. The destructive methods are, well, destructive. Meaning the insulation system has to be disturbed, removed, in order to see below the insulation. This is not ideal, as moisture is the enemy and disturbing the insulation lets moisture in.
All in all, it is important to be aware that corrosion under insulation, CUI, is a real thing and it can affect the integrity of your piping system. It is important to be proactive about finding and retarding CUI. As an industry, we are getting better at slowing CUI, but I don’t think we can say that we have a way to stop it. So, be on the lookout for signs of corrosion and proactively inspect your system so that you can stay on top of CUI in your facility.
And if you would like some help, give Carlson & Stewart Refrigeration a call.