V. Mitchell Liss
Engineering consultant, La Grange, TX
Summary: The following article is a part of the National Board Technical Series. This article was originally published in the January 1988 National Board BULLETIN. (6 printed pages)
Galvanic corrosion, alkaline acidic corrosion, and stress-corrosion cracking have all been known to happen in certain conditions. Here are ways to reduce attack.
Although the extent and resultant cost of corrosion under insulation (CUI) are not known exactly, J. F. Delahunt in "Corrosion Control Under Thermal Insulation and Fireproofing"1 shows the seriousness of the problem by presenting case histories. These cover deep pitting, as well as general corrosion, that have occurred on galvanized steel tanks under 12-year-old polyurethane foam.
Delahunt advises that metal loss can be the least of the problem. For example, an eight-inch carbon steel pipeline carrying heavy fuel oil was insulated with calcium silicate block and protected with a metal weather jacket. The pipe corroded, resulting in a leak. The oil was ignited and a large fire ensued, causing hundreds of thousands of dollars in damage to process equipment.
Types of Corrosion Under Insulation
By understanding the types of corrosion that can occur under insulation, the proper materials and construction can be employed to prevent them. Intruding water is the key problem in CUI. Special care must be taken during design not to promote corrosion by permitting water to enter a system either directly or indirectly by capillary action. Moisture may be external or may be present in insulation.
Corrosion may attack the jacketing, the insulation hardware, or the underlying piping or equipment. Depending on other factors, chloride, and galvanic, acidic or alkaline corrosion may occur.
Galvanic corrosion generally results from wet insulation with an electrolyte or salt present that allows a current flow between dissimilar metals (i.e., the insulated metal surface and the outer jacket or accessories). The extent and severity of the attack on the less noble metal depends not only on the difference in potential of the two metals, but also on their relative areas. The complete galvanic series and the voltage potential for each metal or alloy appear in handbooks and other standard references. The mechanism of galvanic corrosion is detailed by G. Butler and H. C. Ison in "Corrosion and Its Prevention in Waters."2
Alkaline or acidic corrosion results when an alkali or acid and moisture, are present in certain fibrous or granular insulations. For hot service above 250o F, most of the water is driven off. This water vapor may condense at the edge of the insulation, and dissolve the alkaline or acidic chemicals there, resulting in corrosion of the aluminum or steel jacketing.
Some alkaline waters with aluminum produce etching and pitting. Pitting can be severe, especially when chloride ions are present. Insulating cement may also contain alkaline chemicals and water (while the cement is still drying). Below 250° F, an alkaline water may cause corrosion if the substrate or insulated surface is stainless steel, copper, brass or aluminum. Steel would normally not be affected in the time needed for the cement to dry. Fresh, potable water is recommended when mixing insulating cement.
Delahunt reported on leaching tests performed on polyurethane foam insulation containing fire retardant chemicals (i.e., brominated or chlorinated compounds). Distilled water was used, and aggressive acidic solutions were formed. The same was found true for phenolic foams. The pHs of the solutions were often two to three. Laboratory corrosion rates have been shown to be 15-20 mils/yr. Of the two foams, the phenolics are by far the more corrosive.
Chloride corrosion can be caused by the combination of insulation containing leachable chlorides with the 300 series austenitic-stainless-steel surfaces, when moisture is present and temperatures are above 140o F. Concentration of the chloride ion usually results from the evaporation of rain water,or of water used to fight fires, or of process water. Stress-corrosion cracking of insulating jackets often results from airborne salts in coastal regions.
The probability of failure and the speed of crack propagation are governed by the temperature of the stainless steel and the chloride concentration at the metal surface. Solutions containing less than 1 ppm are normally considered safe. Below 176°> F, levels of 100 ppm are not particularly dangerous if continuous surface-wetting occurs; but at higher temperatures, lower levels can result in failure.
In practice, it should be assumed that evaporation of the solution will inevitably occur. Because local concentration of chlorides take place at the metal surface, the bulk concentration may be of little importance. Above 390° F, external stress-corrosion cracking is normally not experienced. The stress required to cause cracking of stainless steel may result either from fabrication or operation (or shutdown).
Water entering the insulation and diffusing inward will eventually reach a region of dryout at the hot pipe or equipment wall. Next to this dryout region is a zone in which the pores of the insulation are filled with a saturated salt solution-this includes any chlorides present. When a shutdown or process change occurs and the metal-wall temperature falls, the zone of saturated salt solution moves into the metal wall. Upon reheating, the wall will temporarily be in contact with the saturated solution (e.g., chlorides), and stress-corrosion cracking may begin.
The major factor in preventing CUI is to keep liquid from intruding into the insulation. Water decreases the effectiveness of the insulation and leads to corrosion of pipe or equipment. Poor conditions caused by wet insulation can be aggravated by weathering, vibration or abuse from people.
There are five factors in preventing CUI: (1) insulation selection; (2) equipment design; (3) protective paints and coatings; (4) weather barriers; and (5) maintenance practices. If an area is subject to spills or high humidity, special consideration must be given to selecting the insulation. Some insulations leave the system less sensitive to defects in weatherproofing or paint films because the insulations are nonabsorbent and chemically nonreactive.
Unfortunately, the insulation picked is normally based on installed costs versus energy saved, and maintenance or corrosion costs are not considered. The following should be considered:
- The cost of repairing the insulation if corrosion is detected. Insulation should be removed in limited sections for inspection. If insulation is subject to damage by abuse, the cost of periodic replacement must be considered.
- The cost of the protective paint.
- For nonabsorbent insulation, a "credit" should be given for the energy saved by eliminating periodic water invasion to absorbent insulation during washdowns and storms.
Insulations such as calcium silicate, glass fiber and, to some extent, cellular plastic foams absorb and retain liquids and vapors. Additional flashing is required where spills, leaks or drippings may occur, or where washing and hosing are carried out. The only fully nonabsorbent insulation is cellular glass. Cellular glass should be used where corrosive or flammable liquids are present.
The proper design of insulation for pressure vessels, tanks and piping includes consideration of the support and connection of the material. Details can be found in a handbook from Midwest Insulation Contractor's Association.3 According to plant operators, weather barriers for insulation are frequently broken either because inappropriate details were originally given for equipment or not enough space was allotted around the insulation. Improvement in design can be accomplished by handling the insulation specifications early during the vessel design and by "simplifying" the surface to be insulated.
The coating system must protect for long periods against water or corrosives. Highly permeable coatings allow corrosion to start behind the coating even in the absence of breaks or pinholes.
An epoxy or epoxy-phenolic should be applied in two coats over an abrasive blast cleaned surface. Inspection of the surface preparation is critical at welds.
In selecting coatings, consider temperature and abrasion resistance, and a service rating for water (or corrosive-chemical) immersion. It is difficult to visually inspect coatings under insulation to find points that need touching up. Unless corrosion or insulation failure causes reinsulating an entire insulation setup, recoating is done every 10-15 years.
The weather/vapor jacket of the insulation provides the primary barrier to water. This covering is the only part of the system that can be inspected quickly and repaired economically. It must not only keep liquids out but also allow for evaporation of any liquid that manages to get into the insulation system. For weatherproofing, a rating of two perms, measured according to ASTM Standard E 398, is acceptable. Also, it should be durable, offer flame-spread resistance, and be economical. The material must be maintained periodically (usually, two to five years) to remain effective.
Further, routine maintenance is needed to catch defects due to deterioration or abuse. If the system is opened in any way for maintenance or inspection, it should be closed promptly after work is completed. One plant reported that openings made in a metal vessel's insulation for acoustic emission tests were never closed. Severe corrosion occurred.
Extensive use of a nonbreathing metallic jacket is believed to contribute greatly to corrosion of warm equipment. Without a permeable jacket, water is trapped. Water in the insulation reaches a point where it is vaporized. Vapor travels to the jacket and condenses; the cycle repeats itself.
Preventing Galvanic Corrosion
Since this form of corrosion results from water invasion in a wet or humid atmosphere, selecting cellular insulation may be the only answer. Also, it might be best to use a plastic or synthetic-rubber jacket. Such jackets are factory applied and are fire and weather resistant. They offer protection from normal abuse and from easy penetration of water. Commercial plastic jacketings normally will not corrode. For example, Hypalon resists sodium chloride solutions to 260 degrees F.
Metal surfaces can, of course, be painted. Paints can inhibit cathodic and anodic reactions, and provide a highly resistive path to current flow. However, some pigments can promote corrosion, especially in the first coat of primers. Common ones are red oxides, gypsum, ochre, graphite and lamp black. Even in the normal corrosion process, alkali is formed at the cathode; this alkaline area grows, even under paint films pigmented with zinc or aluminum.
In selecting coatings to prevent galvanic corrosion, consider the following in regard to sacrificial materials: (a) as temperature increases, there is a chance of reversal in the polarity of galvanic couples; (b) salts carried into insulation and deposited onto surfaces interfere with, or destroy, the effectiveness of corrosion inhibitors; and (c) the insulation system is not freely ventilated and may have inadequate oxygen for sacrificial reactions to occur.
Preventing Alkaline or Acidic Corrosion
Metal jackets should contain moisture barriers on the inside. When corrosion of the jacket is the problem, plastic weather types are a good answer. Precautions with water and the insulation cement are needed if the piping or equipment is stainless steel. If certain combinations of metals cannot be avoided, paint the substrate metal.
For underground pipes, the pipe should be painted before it is insulated, and then a coating should be applied over the insulation. The pipe should be coated with a coal-tar epoxy or an extruded polyethylene jacket. A polyethylene jacket should also be placed over the insulation.
The most critical part of this system is protection of the field-made joints. Rigorous inspection methods are needed to ensure that joints are done correctly. An internally mounted anode beneath the primary weathering barrier and above the secondary coating has been found effective as an additional measure.
Preventing Chloride Corrosion
With stainless steels, problems have resulted because of leachable chlorides contained in some insulation. For the 300 series stainless, use insulation that meets MIL-I-24244 or ASTM C-795 specifications. This provides sufficient control, unless there is an external invasion of chlorides.
A further way to reduce the chance of chloride attack is to use an inhibited insulation. Other points should also be considered:
- With some insulation, such as polyurethane, it is not possible to add inhibitors. Thus, fire-retarded polyurethanes are not recommended for use over austenitic stainless steels.
- For specifications that require less than 10 ppm of chloride in the insulation, attention must be paid to the mortars and cements used, since these may contain chlorides.
- To prevent water and chlorides from reaching the stainless steel, the external barrier must be designed properly and maintained periodically.
When steam tracing is used within insulation, extra precautions are needed to resist stress-cracking corrosion.
Be sure that stainless steel is not coated. In case of a fire, catastrophic embrittlement could result if zinc, titanium or other metallics were used in previously applied paints.
The metal jacketing should be securely fastened and banded to prevent water entry at joints, or where the insulation is supported with attachment angles. Cement-coated insulation can be finished with a suitable waterproof mastic to prevent water ingress. The type of insulation and method of application chosen should assure the absence of shrinkage cracks.
Wrapping equipment with aluminum foil before applying insulation will reduce the risk of corrosion, since the foil provides a physical barrier that prevents the saturated chloride solution from reaching the metal surface. Due to its high thermal conductivity, the aluminum will be at substantially the same temperature as the equipment, and the chloride solution will shift to the foil, rather than the stainless.
When steam tracing is used within insulation, extra precautions are needed to resist stress-cracking corrosion. Extra protection via insulation selection or use of foil wrap is necessary to protect the costly stainless steel surface.
Knowing where corrosion is likely to occur, such as in low spots and at certain temperatures, helps establish an inspection regime. The National Board Inspection Code 4 and the American Petroleum Institute's API 510 require removal of some insulation at least every five years on all vessels where external corrosion is possible. Records of the frequency of trouble spots can assist in setting up an adequate inspection program.
1 Delahunt, J. F. "Corrosion Control Under Thermal Insulation and Fireproofing." Proceedings: Exxon Research & Engineering Co. Internal Conference on Corrosion Under Insulation (1984): p 554.
2 Butler, G., and H. C. Ison. "Corrosion and Its Prevention in Waters." Melbourne, FL: Robert E. Krieger. (1976): Ch. VI, p lO2.
3 Midwest Insulation Contractors Association. "Commercial and Industrial Insulation Standards." Omaha, NE. (1983): Plate No.1-50.
4 National Board Inspection Code, NB-23, Rev.6. Columbus, OH: The National Board of Boiler and Pressure Vessel Inspectors. (1987).
Reprinted by special permission from Chemical Engineering © 1987. New York, NY: McGraw Hill, Inc. (March 1987): pp 97-100. Reprinted text has been edited for clarity and space requirements.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.