There are a number of effective ways to prevent corrosion under insulation (CUI) on pipes and equipment that operate at above ambient temperatures. Some people think that the best approach is to coat the pipes prior to installing the thermal insulation and not worry about what type of insulation to use. Others believe the best approach is to find the perfect type of insulation—the “silver bullet”—that will never allow CUI. Still others think that if facility owners would simply spend the appropriate money, time, and effort to maintain their insulation systems so as to keep out water, CUI would not be a problem. This article explores these different approaches in more detail.

Operating Temperature As a Variable

It is impractical to discuss the prevention of CUI without first considering the effect of pipe operating temperature. For corrosion of steel to occur, four factors need to be present: (1) water; (2) oxygen; (3) corrosive chemical(s); and (4) a suitable temperature. Since only a small percentage of pipes actually operate in this range, CUI is only likely to occur when a pipe operating at an above-ambient service temperature is shut down for service. That happens to all pipes sooner or later. As water vapor does not lead to corrosion, liquid water must be present for CUI to occur. That will be the case at 100° to 300°F (50° to 150°C), according to an article by Dr. Hira S. Ahluwalia (see “CUI: An In-Depth Analysis” in the November 2006 Insulation Outlook). Corrosion will not occur on a 600°F pipe, at least not while it is operating at that temperature. It will occur when the pipe is shut down, is in the process of being shut down, or is being brought back to temperature. If a pipe operates continuously at a temperature where water is likely to be present much of the time (such as on a 150°F pipe), the probability of CUI occurring is much greater.

For a carbon steel pipe with a process temperature between 100°F and 300°F then, water that leaks into the insulation system will sooner or later find its way to the interface between the pipe and the insulation. Invariably, there will be some chlorides or other corrosion-causing chemicals in the environment that will dissolve in the water that gets to the pipe. Consequently, for a low operating temperature, with an insulation system that leaks and an uncoated pipe, the conditions for CUI are close to ideal.

Available Tools To Prevent CUI

In terms of preventing CUI, it is worth examining what tools are available to prevent it. There are a number of different tools that can be used, some more effective than others and all with limitations.

These can be classified in the categories of protective jacketing materials, insulation system maintenance, protective coatings, and insulation materials.

Protective jacketing. The first rule to understanding prevention is to keep water out of the insulation. Regardless of the type of thermal insulation, keeping water out starts with the protective jacketing. The quality of the design, specification, procurement, installation, and maintenance of the protective jacketing system is always critical to preventing CUI. Standard 0.016-inch-thick aluminum jacketing, as well as steel sheet jacketing, installed with caulks and mastics, can effectively keep water out of the insulation system. To be effective, it is critical for everyone involved (the general contractor, insulation contractor, design engineer, and facility owner) to make certain that no shortcuts are taken in design, material specification, and installation. Both conventional aluminum jacketing and steel jacketing can be effective at keeping out the intrusion of water and preventing CUI. Hence, protective jacketing is the most important tool in the CUI prevention toolbox.

A new type of protective jacketing material that is seeing increasing interest is multilaminate, pressure-sensitive jacketing that can be purchased for either field installation or can be purchased factory applied to certain types of insulation. This family of materials essentially consists of industrial-grade tapes, available in 3 foot widths, that are weather resistant; impermeable to water or water vapor; resistant to many chemicals; and able to seal tightly with their pressure-sensitive, “peel-and-stick” surfaces. Some of these are available in industrial-grade weights with a thickness of almost 0.016 inches. An important accessory to making the system effective in keeping out water is a 2- to 4-inch-wide roll of tape to seal the joints and the penetrations. This type of material offers a jacketing that can be adhered to the insulation, thereby preventing moisture from accumulating between the jacket and the insulation. Its flexibility allows for easier installation and sealing at joints and penetrations as well as at termination points, making it very effective at keeping out water. Therefore, wide multi-laminate tapes should be included in the CUI prevention toolbox.

A third type of jacketing, that is durable and effective at keeping insulation beneath it dry, is a glass fiber lagging cloth with an acrylic weather coating mastic. This type of jacketing has the advantage of being able to seal penetrations effectively, to prevent water leakage. Another advantage is that is can be extremely durable, as well as being water proof. Therefore, lagging cloth with an acrylic weather coating mastic should be included as a tool in the CUI prevention toolbox.

Insulation system maintenance. If the pipe insulation is covered with well-designed, well-installed, and well-sealed jacketing, as discussed in the above section, then it is well protected against CUI. However, once this has been accomplished, the system must be maintained. Therefore, the next tool in the CUI prevention toolbox is insulation system maintenance.

For example, consider an insulated pipe that:

• is several decades old;
• has an original insulation system of the same age;
• is located in a relatively rainy climate (something other than the U.S. desert southwest and, at the extreme, along the coast of the Gulf of Mexico);
• has been shut down for an extended period of time during the system’s life;
• is uncoated; and
• has been poorly maintained.

In this case, it is highly likely that the pipe has suffered from CUI. It would be completely unrealistic for the owner or anyone else to expect to not find CUI. In such a case, the CUI is not necessarily the fault of the insulation system design. It further is probably not the fault of the protective jacketing or the insulation material(s). The problem is that when a system has not been well maintained, water—perhaps with dissolved salts from the Gulf of Mexico spray—eventually will get beneath the jacketing between the pipe and the insulation materials, and it will lead to corrosion of the pipe.

In his article, “Is There a Cure for Corrosion Under Insulation?” (Insulation Outlook, November 2005), Mike Lettich emphasizes the necessity for effective maintenance in the battle to prevent CUI. For example, if the insulated pipes have been walked on, resulting in denting of the metal jacketing, there will be water intrusion. If the caulk along the overlapped butt joints has not been periodically replaced, it will become brittle and lose its sealing capability, and rainwater eventually will leak into the system. If caulk around the penetrations—particularly along the top sides of the pipes—has not been examined and replaced as necessary, water will intrude.

For better or worse, the world experienced a prolonged period of very low energy prices from about 1985 to about 2002, with a few, occasional short-term spikes in natural gas prices. With low energy prices, many process pipelines simply were not well maintained due to inadequate maintenance budgets. (This was not the case with all facilities, but it occurred all too often.) With a poorly installed jacketing system—and one inadequately maintained—the insulation system will simply leak rainwater, and CUI will eventually occur. All types of insulation material can be used effectively, up to their design temperatures, if water is kept out of the system. If water gets into the insulation and CUI results, to then simply put all the blame on the materials that make up the insulation system is to put the blame where it should not be placed.

Protective coatings. These coatings can protect carbon steel pipe from water, air, and corrosive chemicals. With those three elements, as well as time and a certain temperature range, corrosion will occur. The first line of defense is the protective jacketing. The second line of defense is insulation system maintenance. Protective coatings on the pipe provide a third line of defense in the prevention of CUI.

Immersion-grade coatings, which are organic, are becoming widely used to coat and protect pipes that operate at or below 300°F. The reason for this temperature limitation is that above that temperature, most organic coatings thermally decompose. Therefore, immersion-grade coatings are an effective tool up to a 300°F operating temperature. They can be considered a valuable tool on carbon steel pipes. The drawback to facility owners, of course, is that they are a financial investment—one that not every facility has been willing to make when operating on inadequate budgets.

In the article, “Corrosion Under Insulation: Prevention Measures” (Insulation Outlook, October 2007), Dr. Hira S. Ahluwalia describes thermal spray aluminum (TSA) coating in great detail. He points out that TSA coating is effective up to a maximum temperature of 1,000°F, much greater than the 300°F limitation of organic, immersion-grade coatings. This type of coating is reportedly more expensive than the immersion-grade coatings, and some facility owners may not think that CUI prevention is worth the investment. However, the expense needs to be evaluated financially through a life-cycle cost analysis, considering not just the initial cost of the TSA coating, but also the value of the pipe, its life expectancy, and the financial risks associated with repairing corroded pipes, fittings, and other components. If the facility is to be brought out of service for any extended period of time, offering an opportunity for water to intrude and CUI to occur, then TSA coatings should be considered. Therefore, TSA coatings are a valuable tool to prevent CUI for piping systems that operate at temperatures between 300°F and 1,000°F.

Insulation materials. Many different types of insulation materials are used on above-ambient pipes and equipment in industrial facilities. It is important to understand that if the facility owner keeps water from intruding into the insulation system in the first place, the facility is not likely to suffer from CUI. If water does occasionally intrude, coatings on the carbon steel pipes and equipment are a backup defense in the CUI prevention battle, as they protect the pipe itself. If this has been done, then any type of thermal insulation, well maintained and operating within its normal temperature limits, can be used without CUI occurring. Nevertheless, under certain circumstances, water, with dissolved corrosive chemicals, sometimes does intrude. It sits on an uncoated carbon steel pipe for extended periods of time. In those cases, certain types of insulation have features that make them tools in the CUI prevention toolbox.

The first of these is hydrophobic (or waterproof) insulation. Several types of commercially available insulation materials have a chemical hydrophobe added in sufficient quantity to make them truly water repellent. These include perlite block and pipe, aerogel blankets, and certain designated hydrophobic microporous insulations (those specifically coated with a hydrophobe). Mineral fiber insulation materials use the same type of hydrophobe, but in lesser percentages than the other materials, and they can still absorb water. Hence, while mineral fiber insulation is somewhat water repellent, it is a wicking material and cannot really be considered water repellent in the way that the other three materials can.

The hydrophobe typically used for some of these hydrophobic insulations is an organic silicone emulsion. It can be added during material manufacturing process. In the case of aerogel insulation, the material is made hydrophobic by virtue of the manufacturing process, by which organic methyl groups are added to the inorganic silica aerogel material. In all cases, this hydrophobic treatment will remain functional apparently up to a temperature range of 400° to 600°F. In that temperature range, the organics, that make the insulation hydrophobic, start to decompose and the insulation becomes less hydrophobic. Therefore, service temperature is the major limitation of the hydrophobe within hydrophobic insulation, regardless of insulation type.

There is an ASTM test for insulation hydrophobicity after heat aging. ASTM C610, the standard for expanded perlite block and pipe material, includes a water-absorption test for material first heat aged in a 600°F oven. The maximum allowable water absorption, after subsequent immersion in water for 48 hours, is 50 percent by weight. Nevertheless, when this material does absorb water and remains wet for a prolonged period of time, the chemical bonding agent is susceptible to failure, possibly leading to physical degradation of the material. However, a major benefit of this behavior is that the bonding agent is an excellent chemical inhibitor against CUI. Expanded perlite has this advantage and hence can be considered a tool in the CUI prevention toolbox for two reasons: One is its hydrophobicity and the other is that it contains a chemical inhibitor against corrosion.

As discussed above, aerogel blanket insulation and hydrophobic microporous insulations are hydrophobic in the service temperature range where the organic material, that make the insulation hydro-phobic, does not thermally decompose. Once the organics decompose, if these insulations are later exposed to water, they will then absorb the water. Later, if dried out, thermal conductivity will have permanently increased due to damage to the tiny pores that make these types of insulation so thermally effective when new. At that point, the insulation will no longer be as effective a thermal insulation as when new. Nevertheless, these two types of insulation can be valuable tools in the CUI prevention toolbox for service temperatures from ambient to the range of 400° to 600°F. For use above that temperature range where CUI prevention is a goal, the insulation manufacturers of these materials should be consulted to ensure that conditions are avoided where excessive loss of hydrophobic treatment and subsequent absorption of water might occur.

How about the effectiveness of closed cell inorganic insulation? There is only one: cellular glass insulation. Indeed, it holds very little water due to its closed cell structure and the fact that water does not pass between the cell walls. While not exactly hydrophobic, it will not absorb water. This behavior can be an important potential contributor to preventing CUI.

In “Corrosion Under Insulation: Prevention Measures” (Insulation Outlook, October 2007), Dr. Hira S. Ahluwalia recommends the use of cellular glass. However, since cellular glass is fragile, it is susceptible to vibration-induced damage and can suffer from boiling water trapped between the pipe and the insulation. Therefore, its effectiveness can be limited. Further, as with many types of insulation, the boiling of water is damaging to the cellular glass structure. One point worth noting, since stress relief cracking of cellular glass typically begins to occur at service temperatures above 450° to 500°F, the manufacturer should be consulted for the best method for insulating these systems. While cellular glass insulation has some limitations in above-ambient applications, it can be considered an effective tool against CUI for applications up to 450° to 500°F.

What about corrosion inhibitors? It was already mentioned that expanded perlite contains an excellent corrosion inhibitor. Some types of calcium silicate also contains a considerable quantity of chemical inhibitor—not as a bonding agent but as an additive specifically intended to be a chemical inhibitor and thereby to prevent CUI. If calcium silicate insulation with a chemical inhibitor absorbs water, the chemical inhibitor dissolves and inhibits against corrosion.

In general, high-compressive-strength insulation provides better resistance to external loads than low-compressive-strength insulation does. These materials provide better support for the metal jacketing, limiting its compression, denting, and opening of gaps.

Expanded perlite and calcium silicate insulations both have high compressive strengths. The compressive strength for expanded perlite, per ASTM C610, is a minimum of 60 pounds per square inch (psi). For calcium silicate, the compressive strength is a minimum of 100 psi, per ASTM C533. That is the highest value of any commercially available block and pipe insulation.

On an insulated pipe or surface that is subjected to external loads, such as foot traffic, the high compressive strengths of perlite and calcium silicate will provide extra support to the metal jacketing system, helping prevent the jacketing from “fish-mouthing” at the overlaps. Fish-mouthing of the metal jacketing will allow for rainwater intrusion. By virtue of providing better support of the metal jacketing, calcium silicate and perlite insulations are considered valuable tools for CUI prevention, with calcium silicate being the strongest material available. As mentioned above, both of these materials contain a corrosion chemical inhibitor. Furthermore, expanded perlite has been included above due to its hydrophobicity.

Summary

There really is no single “silver bullet” that will prevent CUI in all circumstances and all applications. However, there are a number of different tools that can be used, each of which brings numerous features and benefits. By combining several of these tools, the facility owner can reduce the instances of CUI to the point of prevention. This may require spending more money up front on the new facility, specifying and selecting the protective jacketing system and insulation materials more carefully and thoroughly, and spending time and money maintaining the insulation system. If it is done, however, it will reduce the overall operating cost for the facility. As always, “an ounce of prevention is worth a pound of cure.”

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