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Q&A with David Shong: Views on CUI & Hydrophobic Insulation

By Corrosionpedia Staff
Published: September 18, 2015 | Last updated: October 8, 2018 04:09:22
Key Takeaways

There are types of insulation available that have active corrosion inhibitors that activate when water is present—thus water absorption does not contribute to CUI when those insulations are used.

Source: Johns Manville

We recently spoke at length with David Shong about insulation, hydrophobicity and the causes of corrosion under insulation (CUI). The views presented here are those of a manufacturer who makes several types of insulation materials, including mineral wool, calcium silicate, expanded perlite and a thin blanket insulation similar to aerogel blanket.


Please tell us a bit about yourself, and how long you’ve been in the insulation business.

My name is David Shong and I work for Johns Manville Industrial Insulation Group. We focus on mid to high temperature insulation. I’ve been with the company for almost two years now, and in the insulation business for 22 years. I work primarily on the technical side, with engineers, owners, facility maintenance people and mechanical insulation contractors, helping them understand the benefits of various types of insulation materials.

How did Johns Manville get its start, and how long has it been in the insulation business?

The company started in 1858 prior to the Civil War in New York City. It got into the insulation business early on and was very strong throughout the 20th century. In 2001, Johns Manville was purchased by Warren Buffet’s company Berkshire Hathaway. The Industrial Insulation Group (IIG) started as a joint venture between Johns Manville and a company called Calcilite. In 2012, Johns Manville purchased the remaining shares of the joint venture, and IIG became the wholly owned subsidiary of Johns Manville.


So your focus on mid to high temperature service includes the CUI temperature range?

Yes it does. The corrosion under insulation (CUI) range is generally considered to be 25°F to 350°F (-4°C to 180°C) for stainless steel, and up to 300°F (150°C) for mild steel. (Background reading: An Introduction to the Root Causes of CUI with Monica Chauviere.)

Does CUI occur in high temperature service as well?

In continuous operating, the rate of corrosion is not accelerated above 300°F. However, high temperature equipment and pipe can operate cyclically, dropping to 300°F and below. They shut down for maintenance, or cycle down as part of their process, and that’s when high-temp equipment can be subjected to the CUI range.

If water is present on the steel in the high temperature range, does the water evaporate off?

Yes. Generally 300°F is considered the top of the CUI danger zone because that is where H2O ceases to be able to exist in liquid form. In order for CUI to occur there has to be liquid water, not water vapor.

JM sells the full spectrum of insulation materials. Are some types of insulation materials better for high temperature service, and others for the CUI range?

Yes. There are a lot of different types of insulation on the market and Johns Manville Industrial is quite different in terms of our strategy when it comes to the materials. We’re fairly agnostic about which material should be used because we make so many different types.


While most manufacturers focus on making one type of material, we make a lot of different materials because we firmly believe there is no such thing as one material that is good for every application. We want to be able to match up the physical properties of the materials with the operating and the abuse conditions on-site, and make specific recommendations based on the client’s design considerations.

If you look at higher temperature ranges, there really is no such thing as an effective hydrophobic insulation above exposures to temperatures above 450°F over a period of time. There has been a lot of talk recently about aerogel and perlite, that they have excellent corrosion performance because they are hydrophobic. In my opinion, they certainly proved that they have a hydrophobic additive in their insulation. Expanded perlite has a proven history.

But in reality, the silicone-based emulsions that are used in order to create that bulk water shedding ability in aerogel are actually oxidized above 450°F over a period of time.

In those higher temperature ranges, the ability for any insulation to shed bulk water is really permanently reduced. You have to look at the chemical composition of each type of insulation and whether it either promotes corrosion, or has the ability to actually inhibit corrosion.

I have heard from other sources that aerogel blanket is not ideal for high temperature service, but in the CUI temperature range it is an excellent strategy because it is hydrophobic. It sounds like there are other issues in play with CUI in your view. What would you say are the causes of CUI?

One of the main contributors to CUI is the length and duration of contact of wet insulation on the steel. I would expand upon that and say that water in contact with steel doesn’t necessarily induce corrosion.

Let’s look on the inside of a pipe filled with water for example. Just because it is in constant contact with water doesn’t mean the inside of the pipe will corrode. The reason it doesn’t corrode is that for the past 80 years, it has been a common practice for water companies to add a 3% sodium silicate solution to the water. This creates a passivation layer on the inside surface of the pipe. Even though water is in direct contact with the metal, it does not cause corrosion.

I would expand on the statement to say that liquid water containing specific electrolytes, in contact with the metal, will contribute to corrosion. But if water is in contact with the outside of a pipe and there is an inhibitor in play, then corrosion may not physically be mechanized.

For corrosion to occur, there are five conditions that need to be met simultaneously, whether it be in continuous operation or in cyclical operation. You have to have these five things all present simultaneously for corrosion to occur:

  1. Liquid water
  2. Oxygen
  3. Operating temperature between 100°F and 300°F (for mild steel)
  4. Corrosive chemical compounds
  5. Acidic pH (lower than 7)

Corrosive chemicals must be present, whether it is from the insulation itself or from the surrounding environment. And, one of the most important things is that the water that is absorbed into the insulation in that space between the jacketing and the steel has to have an acidic pH of lower than 7.

So, to address CUI, we have to try to address all these factors:

  1. We can’t really do anything about liquid water except to make sure the insulation jacketing is designed to keep most of the water out. We know through maintenance experience that there are many ways that water can get in, despite the best intentions.
  2. Oxygen is in the atmosphere so we can’t do anything about that.
  3. The operating temperature is inherent to the given process.
  4. What we can deal with are the corrosive chemical compounds that can be in the insulation.
  5. We can also alter the pH of the water that will be absorbed into that insulation material.

When we talk about burning off the hydrophobe, it’s important to recognize that CUI doesn’t only occur in process equipment that operates at a maximum of 350°F. CUI can also occur on every type of equipment or piping that is brought down to the CUI temperature range for a given amount of time. And that occurs with almost everything during maintenance shutdowns.

So to create this paradigm that CUI only occurs on these pipes and this equipment operating in this temperature range is really an oversimplification of the issue. The problem expands to include the entire facility, meaning that anything operating above ambient will be in danger of CUI at some point.

Will any insulation that absorbs water contribute to CUI?

That depends entirely on the type of insulation. If it’s an insulation that lacks corrosion inhibitors, like mineral wool, then it could certainly contribute to corrosion. However, there are types of insulation available that have active corrosion inhibitors that activate when water is present—thus water absorption does not contribute to CUI when those insulations are used.

One such insulation is expanded perlite. While there are many experts out there who would attribute the CUI performance of expanded perlite to the hydrophobicity of the insulation, it’s actually a result of its chemical makeup. (For more on this topic, see The Detrimental Effects of Wet Insulation in the CUI Range.)

The fact is, we manufacture both mineral wool and perlite, and we know that both are treated to shed water. However, when these insulations have been used in Gulf Coast refineries, mineral wool has caused notable corrosion while perlite has caused little to no corrosion. Thus, if perlite’s hydrophobicity was the determining factor in its CUI performance, then mineral wool would demonstrate similar effectiveness at preventing CUI. As evidenced by the corrosion in the Gulf Coast refineries, that’s clearly not the case.

Since they both have the same chemical treatment that will make them shed bulk water, stating that one is absorbent and the other isn’t is simply incorrect. Thus, we need to look at it from a different perspective; we need to ask why perlite was able to prevent CUI even after it absorbed water while mineral wool wasn’t.

What a lot of people don’t realize is that expanded perlite will absorb a huge amount of water—up to 30% by volume when just being subjected to heated conditions. So in the Gulf Coast, where you have high humidity, you will see a marked gain in moisture in expanded perlite. Water vapor passes through and condenses in the material. While in other insulations, the water absorbed by the material would be detrimental to its CUI performance, expanded perlite has corrosion inhibiting compounds that are activated in the presence of water. These compounds are ultimately why perlite is so successful at preventing CUI. (To learn more, read Expanded Perlite Insulation: Why Hydrophobicity is Only Part of the Solution to Prevent CUI.)

We call these corrosion inhibiting compounds the XOX package, and it is integral to all domestically produced expanded perlite and, since 2002, calcium silicate. That’s really what the XOX message is. We have perlite that is considered non-absorbent in the lower temperature ranges, and we also have calcium silicate that is absorbent in all temperature ranges, and both have been proven to cause less corrosion than deionized water. This is because they both have the same chemical formula in the XOX package.

The idea that hydrophobic insulation is the ultimate answer is really not the case. When the insulation gets wet, that’s when XOX activates. Water allows the corrosion inhibitors to move from the insulation to the surface of the metal to create a passivation layer, which will prevent the corrosion mechanism from occurring.


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