Corrosion Prevention: What You Need to Know
There have been many studies done and academic papers written about the subject of corrosion under insulation (CUI) on metal piping, ducts, tanks, etc. CUI is a concern for insulation used on pipes operating in the temperature range of 32°F to 250°F for carbon steel and 140°F to 250°F for austenitic stainless steel. It is also of particular concern in cycling temperature systems because the cold portion of this cycle draws moisture into the insulation system and then the hot portion of this cycle speeds up the corrosion chemical reactions. In addition, cycling temperature systems undergo repeated expansion/contraction, which can be damaging to insulation and vapor retarder systems. This article will cover general facts regarding corrosion, 2 examples of where CUI occurred on applications, and a summary of recommendations that should minimize the risk of CUI. While the recommendations may not apply to all insulation materials, for the purposes of this article, when I mention insulation systems, I am referring to systems used for the applications described above.
Corrosion can occur on any kind of metal piping (iron, copper, aluminum, or stainless steel). Some austenitic stainless steels are particularly susceptible to stress corrosion cracking (SCC) from corrosive ions at temperatures between 140°F and 250°F. When it occurs, corrosion can create safety issues and is a costly problem to correct, so it should be taken seriously. Corrosion can occur on indoor or outdoor systems, and on hot or cold systems. It is much more likely to occur on industrial outdoor systems that operate between the 140°F to 250°F range for stainless steel, or 32°F to 250°F for carbon steel. Pipes or tanks that repeatedly cycle from hot to cold are particularly prone to corrosion. Indoor commercial applications are less likely to encounter issues with corrosion, mainly due to there being less water and water vapor present in the ambient environment in most indoor applications. However, as we will see in the examples, it is certainly possible. This is especially true if the insulation becomes wet.
There are 3 necessary ingredients for corrosion to occur: metal, air (oxygen), and an electrolyte (usually water). Since the metal and air are always present, the controlling factor contributing to CUI is the electrolyte, that is, moisture that penetrates through the insulation system and comes in contact with the metal pipe. Without the moisture, corrosion does not occur. The insulation and/or vapor retarder system must be selected and designed to minimize moisture penetration into the insulation system. Insulation materials with low water vapor transmission rates may be good options for inhibiting moisture vapor ingress.
Most insulation types, by themselves, neither cause nor prevent CUI. Instead, insulation systems prevent corrosion by preventing moisture ingress from reaching the metal pipe. Most insulation materials do not contain corrosion inhibitors in sufficient amounts to be a significant factor in preventing corrosion.
When corrosion occurs, there is often a corrodent/accelerant, (i.e., corrosive ion) involved. An example would be halogens—typically chlorides—that exacerbate corrosion. Although the first reaction is often to blame the insulation, the fact that moisture has made its way into the system means that the system is not completely sealed. Therefore, the actual source of the corrodent is often difficult to detect and can come from many sources, often at a great distance from where the actual corrosion occurred. Water intrusion into the insulation system provides a near inexhaustible source for these corrosive ions, which greatly exceed the level of ions present in the insulation. Other potentially corrosive halogens include fluorine, bromine, and iodine. Sulfides, while not halogen based, are also potentially corrosive.
The first step to preventing corrosion is proper insulation system installation. Ensuring that an appropriate insulation is used and that the system is properly installed and sealed is of vital importance. Additionally, there should be a continual maintenance program that includes visual inspection of the insulation system to identify damage, since damage elevates the corrosion potential. Such a maintenance program can help to reduce costly repairs.
Clearly, there are a number of factors that contribute to corrosion that are not directly related to the insulation itself. Additionally, there are certain applications that have a higher risk of corrosion: outdoor applications, and indoor applications where the operating temperature cycles from hot to cold.
In terms of preventing corrosion, there are several steps to take. First and foremost, preventing moisture ingress from penetrating the insulation system can prevent corrosion from occurring. Moisture must be present for corrosion to occur. Notice I said penetrating the “insulation system,” as it is not just the insulation that is at play here, but also the jacket, moisture vapor retarder, etc. Insulation materials with low water absorption and water vapor transmission properties are generally a good choice for minimizing moisture ingress without the use of an additional concentrated moisture vapor barrier for many applications. The system also includes the fittings, insulation-covering hangers, valves, vapor stops, and any other potential weak spot in the piping layout. Sealing the entire system from moisture can’t be stressed enough in the fight against corrosion. Ideally, if an operating cold line is wet, it should not be insulated, since that would trap moisture under the insulation.
It is not acceptable to succumb to the belief that moisture will get into the system sometime and, as a result, neglect this aspect of the project. Installation of the system is equally as important as the material selection. A poor installation will doom the best material selection to failure. Proper use of vapor stops/dams limits the damage that can be done if there is a break in the insulation system that would allow moisture to contact the piping. Chapter 23 of the ASHRAE Handbook—Fundamentals (specifically Section 23.8 “Corrosion Under Insulation”) provides a basic understanding of the issue of CUI, its causes, effects, and remedies.
It is erroneous to believe that insulation by itself will prevent CUI. The best that can be said is that the insulation itself will not be or provide the corrodent accelerant to the corrosion process in the presence of moisture. Insulation is one part of the insulation system, which, in some cases, can minimize/prevent moisture ingress to the piping. If installed properly, it can help prevent corrosion. There are some insulations that do contain sufficient corrosion inhibitors. However, they may not meet the facility owner’s/engineering designer’s primary requirements for the insulation. For example, they may be open cell, fibrous, or granular, which—without an appropriate vapor barrier and accessories—offer no resistance to water vapor penetration, making them less desirable in cold applications or cold/hot cycling applications. There are also coatings, gels, films, etc. that can be applied to the piping that are corrosion inhibitors and/or moisture barriers.
In below-ambient operating systems, or applications where moisture is a constant issue (e.g., offshore oil rigs or very high-humidity areas such as the U.S. Gulf Coast), the insulation system should be designed to minimize or prevent moisture penetration. If the vapor retarding aspects of the insulation system are damaged and moisture does penetrate the insulation system, the system will also trap this moisture and reduce its evaporation rate, thus allowing the metal pipe to remain wet much longer than if there was no insulation present. This only highlights the need for a strong design and good installation of the insulation system with no open seams and a continuous high-quality vapor retarding system.
As my expertise is in elastomeric insulation, I’d like to share 2 examples where corrosion became an issue involving elastomeric insulation. One stress crack corrosion example involving elastomeric insulation occurred around 25 years ago. It involved elastomeric insulation on copper refrigeration piping lines in supermarkets. The application exhibited ideal conditions for stress crack corrosion (which is usually seen in austenitic stainless steel but can also occur on copper): cyclic (cold/hot) operating conditions that went well above 200°F (hot gas defrost system), corrodents were present in the form of floor-cleaning products, there was high hoop stress in some of the copper piping manufacturer’s products, and high pH in some of the elastomeric insulation manufacturer’s product. Investigation into the cause of the problem resulted in many potential causes. The industry learned from this incident and improvements were made not only to the insulation products, but also to the copper pipe manufacturing (drawing) process to reduce hoop stress as well as improvements to the design and installation of the insulation system in supermarkets.
A more recent corrosion problem regarding an elastomeric insulation system involved an aluminum piping HVAC system (which is not common) that included both indoor and outdoor sections. Once again, the cause of the problem was not focused on one issue, but several factors, starting with poor installation (open seams), environmental conditions (coastal area), and the type of solder used in the fittings. Moisture and corrosive ions (accelerators) were introduced into the insulation system through the open seams (rain water and condensation) and accumulated in areas most susceptible to corrosion (fittings, elbows, etc.). These 2 indoor applications were both “perfect storms” for corrosion to occur, but could have been prevented with better design, material selection, and installation techniques.
The most common test methods referenced for evaluating the influence of thermal insulations on corrosion are:
- ASTM C692: Standard Test Method for Evaluating the Influence of Thermal Insulations on External Stress Corrosion Cracking Tendency of Austenitic Stainless Steel
- ASTM C1617: Standard Practice for Quantitative Accelerated Laboratory Evaluation of Extraction Solutions Containing Ions Leached from Thermal Insulation on Aqueous Corrosion of Metals
- ASTM C665: Standard Specification for Mineral-Fiber Blanket Thermal Insulation for Light Frame Construction and Manufactured Housing (imbedded test method)
These test methods are designed to test systems operating at or above 120°F. There are no test methods designed for systems operating at low temperatures or cyclic hot to cold temperatures. ASTM C1617 is currently being reviewed to expand its scope to include metals other than just carbon steel, such as copper and aluminum. Some of the tests are comparative-type tests where a pass means that the test run with the material in question did not exhibit more corrosion than the control test run without insulation. ASTM C1617 is a quantitative test (mass loss).
ASTM C871 Test Method for Chemical Analysis of Thermal Insulation Materials for Leachable Chloride, Fluoride, Silicate, and Sodium Ions is a measure of the concentrations of corrosive ions and inhibitor ions found in the insulation. ASTM C795 (Standard Specification for Thermal Insulation for Use in Contact with Austenitic Stainless Steel) establishes requirements for insulation materials when tested according to ASTM C871 as well as ASTM C692. Insulation manufacturers should be contacted regarding the acceptability of their products for use on austenitic stainless-steel applications—especially applications for operating temperatures above 140°F. Some insulation materials may contain corrosive ions from the fillers, plasticizers, and flame retardants used in their manufacturing process and may not contain inhibitors in sufficient amounts to buffer them and meet the requirements of ASTM C795. The presence or absence of corrosive or inhibiting ions alone is not the determining factor in whether an insulation is likely to contribute to CUI. Instead, the entire scenario must be considered— including water resistance of the insulation system, amount of water and corrosive ions in the environment, and the operating temperature(s) of the metal pipe, tank, or duct. ASTM C534 Grade 3 materials contain no PVC and less halogens than Grades 1 or 2, but still may contain corrosive halogens. Test results for specific products should be obtained from the insulation manufacturer, as well as specific installation instructions. This is advisable for all Grades of materials listed in ASTM C534. If a test method or standard—such as ASTM C795—is not noted on an insulation product’s data sheet, it most likely means it does not conform to the standard or it not applicable to the typical applications for the product.
A quick review of the ASTM Standard Specifications for 15 mechanical insulation types indicates that 9 (typically fibrous, granular, and cellular glass types) reference corrosion test methods, and 7 include a requirement that an insulation material pass a specific corrosion test.
If we look beyond the insulation material, where else do corrosion accelerators come from? If corrosive ions are present in the ambient environment, in even small amounts, in a cyclic (wet/dry or hot/cold) application, after several moisture exposure/evaporation cycles, the corrodents will accumulate. Corrosion can occur where dirt, oils, grease, fluxes, and other impurities are introduced during the installation process. Indoor applications are often exposed to cleaning products. Rooftop applications are potentially exposed to rain water passing through industrial air pollution. Outdoor applications near the coast are subject to ocean mist. Some outdoor applications are subjected to a cooling tower spray/mist, which can contain various water treatment chemicals that can exacerbate CUI. Buried applications are exposed to ground water contaminants. As mentioned above, key entry points for moisture (with contaminants) would be any point that is hard to seal (e.g., fittings, flanges, hangers, valves, etc.) The use of factory-fabricated fittings and insulated hangers helps to ensure sealed joints. Use of high quality, low permeance, and continuous vapor retarders to prevent moisture from penetrating the insulation system limits moisture intrusion even in extreme conditions.
There are numerous recommended methods to prevent, or at least minimize, moisture ingress of the insulation system:
- Start with a good insulation system design and select the right materials for the application.
- Use factory-made insulation and jacketing fittings, pre-insulated hangers, tight seams and butt joints, and a vapor retarder that is fully adhered to the insulation. This prevents moisture from flowing and accumulating in low areas of the system. Also, be sure all vapor retarders are fully sealed and resist mechanical abuse, or are covered by a protective jacket, assuring the protective jacket is installed with joints sealed or oriented to naturally shed water.
- Use water/moisture vapor stops/dams to isolate moisture issues if they arise. A good installation with no breaks in the insulation system will allow the materials to do the job for which they were selected.
- In potentially severe corrosive situations, the National Association of Corrosion Engineers (NACE) recommends the use of corrosion inhibitors: coatings, gels, etc. on the piping under the insulation to add another level of protection (according to the Standard Recommended Practice: The Control of Corrosion Under Thermal Insulation and Fireproofing Materials—A Systems Approach). NACE is concerned that an insulation system, if poorly designed, installed, or damaged, will trap moisture and the piping system will be in contact with moisture longer than if there were no insulation at all. Because of this, for applications where personnel protection is the only concern, NACE often recommends, where it is possible, the use of a wire cage as a barrier to physical contact rather than insulating the system. This reinforces the point that it is the insulation system and not just the insulation that can prevent CUI. A proper design that is installed and maintained correctly is mandatory.
- Periodic inspection and maintenance of the insulation system is always necessary and should be planned for before the system is installed. Timely repair of any suspected breakdowns in the insulation system before they result in a complete failure completes the steps required to prevent CUI.
The ASHRAE Handbook notes, “Corrosion of metal pipes, vessels, and equipment under insulation, though not typically caused by the insulation, is still a significant issue that must be considered during the design of any mechanical insulation system.” Understanding the issue of corrosion, its potential causes, and designing to prevent them from occurring will go a long way to eliminating CUI.
This article was published in the November 2018 issue of Insulation Outlook magazine. Copyright © 2018 National Insulation Association. All rights reserved. The contents of this website and Insulation Outlook magazine may not be reproduced in any means, in whole or in part, without the prior written permission of the publisher and NIA. Any unauthorized duplication is strictly prohibited and would violate NIA’s copyright and may violate other copyright agreements that NIA has with authors and partners. Contact firstname.lastname@example.org to reprint or reproduce this content.