 |
 |
|
|
Corrosion in Insulation Systems
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 corrosion. 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. Alkaline or acidic corrosion results when an alkali or acid and moisture, are present in certain fibrous or granular insulations. For hot service above 120oC, 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 120oC, 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. CUI was reported following 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 pH of the solutions was often measured to be between 2 and 3. Laboratory corrosion rates have been shown to be 4-5 mm/year. Of the two foams, phenolic foams 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 60oC. Concentration of the chloride ion usually results from the evaporation of rain water, or of water used to fight fires, or of process water. Stree 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 80oC, 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 200oC, external SCC 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 dry-out 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.