Q&A: David Shong on the New CUI Test Method ASTM C1617
In an in-depth interview, David Shong explains corrosion under insulation (CUI) test method ASTM C1617, and discusses how the company’s XOX corrosion inhibitor works, plus the relative costs of the various insulation materials.
Back in 2004–2005, just after aerogel blanket hit the market, were you member of the standards committee that developed the new CUI test method ASTM C1617? And do you know if facility owners, engineers and maintenance personnel were well-represented in the ASTM C16.31 task group?
I personally was not, but there were several members of our company involved in that committee.
It seems that this test method in no way simulates how insulation is actually used in the field or how CUI occurs. Why is the insulation ground up in a blender?
In order to understand the ASTM C1617 test method, which is the latest version of CUI range testing, it’s important to know it’s the only standard that is actually quantitative. The other insulation and corrosion test methods are qualitative, meaning that basically you have to look to determine whether corrosion is occurring. ASTM C1617 provides actual mathematical data that says there is metal loss occurring due to the chemical composition of the insulation.
The first corrosion and insulation test methods developed in 1971 and 1977 were ASTM C692, 871 and 795. They are based on coupons of stainless steel, which has a different corrosion mechanism referred to as external stress-corrosion cracking or ESCC. (Learn about corrosion coupons in the article Corrosion Coupons: Why Relying on One Test Method Isn't Enough.)
So up to that time, insulation testing focused on stainless steel. It involved placing insulation samples on top of a coupon that had a heated pipe under it, then deionized water was dripped onto the insulation for about 28 days. Insulation that absorbed water would pass the water through the material onto the steel where it would become vaporized. The pipe was heated to the boiling point, ± 10° to around 212°F (100°C), which is the CUI range.
The ASTM C1617 test method was developed in order to compare the chemical compositions of various insulations to control solutions.
The purpose of grinding the different insulations into powder is to extract a solution using boiling water and narrow down the actual chemical compositions of the materials. It also allows people to control for that hydrophobic nature.
If you drip water for 28 days onto insulations that have a hydrophobic treatment on them, like perlite and aerogel, they will just shed all the water. That doesn’t allow people to compare chemical compositions and also doesn’t account for what happens when the hydrophobe is burned off in higher temperatures and brought back down within the CUI range.
So the purpose of grinding it into powder and then extracting it into solution is so we can get down to the chemistry of the materials and be able to compare them.
Once you extract the solutions, there’s a metal plate heated to 212° ± 10°, and the ferrous metal (not stainless) coupons are weighed prior to the test. The solutions are then repeatedly pumped in small doses onto the coupons. Each coupon has a piece of plastic pipe adhered to it to isolate the solution in a small area and so it doesn’t spill out onto the other coupons. Because the plate is heated to boiling temperature, the liquid immediately boils off.
This test creates a perfect storm for corrosion to occur in a very short amount of time. The test is only four days. What you're doing is inducing every one of the five conditions that propagate corrosion. (The five conditions are discussed in Q&A with David Shong: Views on CUI & Hydrophobic Insulation.) This test creates a very visual and measurable amount of metal loss in a very short amount of time.
You compare each insulation solution sample to three different controls:
- Deionized water
- 1 ppm chloride solution
- 5 ppm chloride solution
A higher chloride content results in a higher corrosion rate. After testing each one of those materials against the controls, you can then compare the performance of each type of insulation to each other. This is something that has never been done prior to the ASTM C1617 test method.
How does XOX work to inhibit corrosion?
Using all of the ASTM test methods, we have been able to determine that the corrosion rates of our XOX-treated expanded perlite and calcium silicate insulations are already lower than deionized water and are therefore, by definition, a corrosion inhibitor.
The way XOX works is really quite simple, and it’s a proven chemistry that works on two levels.
First, it works to create a passivation layer in the presence of liquid water. The insulation absorbs water, whether it be expanded perlite in humid conditions or in hot service where the hydrophobe is being burned off, or calcium silicate, which is water-absorbent at all temperatures. When the water is present in the insulation, it activates silicate anions contained in the insulation itself. One of the most commonly known and most soluble versions of the silicate anion is sodium silicate. Water will cause some of the sodium silicate to dissolve out of the insulation, where it migrates via osmosis onto the surface of the metal. Through heat and time, XOX forms an iron silicate gel on the surface of the steel. It creates a coating on the pipe where acidic electrolytes contained in the water cannot contact the surface of the pipe and create what’s called cathodic flow. So this is the first level of protection—a passivation layer.
The second method in which XOX provides continuous corrosion inhibiting is through what are called silicate cations. Those are sodium silicate, calcium silicate, magnesium silicate, potassium silicate and aluminum silicate. These five silicate cations work to continuously buffer the pH of any water that may absorb into the insulation. Also, because each silicate compound has different solubility rates and constitutes over 95% of the raw material inputs, the corrosion inhibiting capabilities will continue well beyond the typical aggregate insulation service life of 25+ years.
Acidic compounds can get into or under the insulation in harsh environments, such as a chemical processing unit or an oil refinery. In those plants, you can have little drips of carbonic acid, hydrochloric acid or sulfuric acid fall and settle on the insulation jacketing. So when people wash down the pipes or there’s a rainstorm, the water will dissolve those acids. Eventually, they can find their way through gaps in the jacketing and onto the insulation, and become absorbed.
These acids can make their way to the metal surface and really exacerbate corrosion unless there is an inhibitor to buffer the pH of the acidic components. In that case, what those various silicate cations do is strip off the hydrogen atoms from those acids, effectively neutralizing them. This is much like what occurs when you mix highly basic baking soda and acidic vinegar—they form a neutral solution.
That’s how XOX works.
Just one final question: in terms of cost, can you rank the types of insulation you sell from least to most expensive?
It is unknown to some in the market that every type of insulation will do the exact same thing at a different thickness, based on k-value and cost to produce each formulation. With a hot pipe, for example, what you’re trying to do with insulation is protect workers and prevent heat loss, which if not designed correctly, can negatively impact safety and process control.
It’s just a matter of how thick the insulation is going to be and how much it is going to cost per linear foot. Here’s a direct comparison: On a 6-inch pipe at 600°F, the least expensive is mineral wool, followed by calcium silicate, expanded perlite and aerogel. So aerogel is the most expensive and mineral wool is the least expensive. The installation labor also tracks the same way, and this pattern is consistent across all pipe sizes and all above ambient operating temperatures.
We also manufacture a microporous thin blanket that has the exact same properties as aerogel blanket, for the exact same cost per square foot. However, new data has revealed that in-situ thermal performance could be achieved with fewer layers using our thin blanket vs. aerogel. This would provide a lower installed cost and thinner profile than using aerogel.
The idea is that we know all insulation does the same thing; we just need to match up the thickness requirements with material budgets for a project application with the right material. Mineral wool has been used extensively because it is the lowest cost material. But in some cases, it doesn’t make sense because it has low compressive strength and has been associated with CUI.
In the past, calcium silicate was definitely associated with CUI, so the observations by another industry technical expert finding more corrosion under calcium silicate is accurate—in the past. Just like CUI coatings have improved since the 70s, so have some insulations. We have more than a decade of solid data that shows our XOX-treated calcium silicate causes significantly less corrosion than the old formula discontinued in 2002.