The Common Approach to Evaluation and Repair of Corrosion Under Insulation
The vast majority of insulated equipment and piping in the chemical process industry (CPI) is made of either carbon steel or 300 series stainless steel. This article will review the dynamics of corrosion under insulation (CUI) and explain what typically happens in a chemical plant when CUI is discovered.
Corrosion of insulated carbon steel usually is the result of long-term exposure to wet insulation. Corrosion damage presents itself in the form of either general metal loss, where the entire affected surface loses thickness, or pitting, where the damage is very localized. Normally, failures from pitting corrosion only damage a small portion of the insulated surface. Either form of corrosion can lead to small leaks that, left undetected, may further lead to pipe, equipment and/or insulation damage. These all can be prevented if the corrosion is detected early.
When 300 series stainless steel fails by corrosion it is typically through stress corrosion cracking (SCC). SCC occurs when a susceptible material is exposed to a specific cracking agent in combination with tensile stress. By far the most common cracking agent for 300 series stainless steel is chloride ion combined with water and temperatures above 60 C (140 F). The tensile stress may be the result of applied process loads or residual stress from fabrication welding. The most common method for preventing SCC is to apply a coating to the surface of the steel. Normally, this coating is an immersion-grade epoxy phenolic.
Sample Case: How Plants Approach CUI
A hypothetical case will illustrate how plants approach the process of CUI remediation. When an operating system is damaged for any reason, it would be ideal simply to shut it down and make immediate repairs. However, in reality, if safety is not a concern, that does not always happen. In the CPI, safety and the environment always receive priority, and any operating system damaged by CUI that poses a safety or environmental hazard is immediately shut down and repaired. I have personally never encountered anyone in the CPI who was willing to put either safety or the environment at risk by deliberately ignoring hazardous corrosion damage. But when there is no safety risk, and most often there is not, the approach is often different, influenced by the specific circumstances of the problem. Shutdown can be very costly, both in lost production and direct labor, and will be avoided as much as possible if safety is not an issue.
As an example, the Liquid Widget International Company (LWIC) is the world’s leading manufacturer of liquid widgets, a key ingredient in refrigerant chemicals used in virtually every air conditioner in the world. The plant is highly profitable and is designed to operate continuously with maintenance shutdowns occurring once every 2 years for a maximum of 7 days each time. A portion of the process uses 304 stainless steel piping and equipment that operates at temperatures between 40 and 90 C (104 and 194 F). For process stability, energy conservation and personnel protection, the equipment is insulated with mineral fiber, and calcium silicate is added to the heads of vessels for improved damage resistance. Calcium silicate also is used at pipe supports for added strength. Everything is jacketed with stucco-embossed corrugated aluminum.
When LWIC built this plant in the early ’80s, it was not common practice to use coatings on stainless steel to prevent CUI. Common practice in LWIC’s plant is routine wash downs of plant equipment using water pumped from a local lake. The LWIC plant is located near a saltwater coast where the natural chloride content of the ground water is high—at least 50 ppm. While the specifications used when the plant was built were good and the insulation was properly installed, time has taken its toll: After more than 20 years of service, in many places the insulation will no longer keep out water. Consequently, each time the plant is washed, both the mineral fiber and the calcium silicate absorb water. After wash down, process heat dries the system slightly but never completely. As a result, the chloride level in the insulation rises, and now slow leaks typically associated with SCC of stainless steel in low-pressure equipment have been discovered, both in piping and jacketed equipment. What should be done?
The Process of Elimination
The first question the plant manager will probably ask the materials engineer is, “Why did this happen?” In this case, it is obvious: Hot stainless steel has been exposed to wet insulation and an external source of chloride, a perfect recipe for SCC. However, the root cause is poor maintenance. Had LWIC followed a regular maintenance program on their insulation, as they do on other “critical” operating equipment, this problem would have been avoided. Fortunately, processing liquid widgets requires low pressure and does not involve any chemicals that pose either a safety or an environmental risk. In the absence of pressure, it is highly unlikely for a catastrophic rupture to occur when SCC is the mechanism. This gives LWIC some time to consider the options.
What must first be determined is the extent of the damage, how it will be repaired and how future corrosion will be prevented. This time, a few leaks became obvious enough to alert operations staff to a problem, but how many more leaks is the insulation hiding, and how many more areas of corrosion damage are on the verge of leaking? In this situation, it is highly likely that more damage exists; however, damage to wet insulation can be very subtle and it is not obvious where to look without first removing the insulation. It is expensive to remove and re-install insulation in a large plant, so simply removing the insulation from the entire plant all at once is not an option. Further complicating matters is the fact that the plant operates continuously and insulation is required for process stability. Disrupting the production of a process that makes $1 million per day cannot be tolerated.
Is there a method that can reliably find SCC while the plant is operating without requiring that the insulation be removed? Much research has been done to answer this question, but there is not yet a widely accepted method for finding SCC under insulation. In this case, the recommendation is first to identify all of the piping and equipment operating in the temperature range that promotes SCC. After this list is compiled, the next step is to divide the system into manageable blocks and then prioritize those blocks on the basis of their effect on operations. Items that are critical to operational reliability receive the highest priority rating and are inspected first. The process of inspecting and correcting the entire plant is done online. In a large plant like LWIC, it will take several years to finish.
Because the plant cannot be shut down, a special process is required for conducting the inspection. Starting with the highest-priority blocks, the insulation is removed and the surface prepared for inspection. SCC is usually accompanied by pitting corrosion and produces a network of very fine cracks that typically cannot be seen by the naked eye, unless the cracks are very advanced. Leaks or areas that are close to leaking are easily hidden by surface debris left behind by wet insulation.
Blast or Grind?
Deciding which method is needed to clean the surface depends on how the system will be returned to service after inspection. The proper approach depends on many factors, including how long the plant is expected to remain in operation. In this example, liquid widgets are expected to be in demand for the foreseeable future; therefore, the plant is expected to operate for at least another 30 years. Preventing future corrosion is important, so it is necessary to apply a coating to the stainless steel before it is reinsulated. Because the plant cannot be shut down, this coating must be applied to a hot operating surface. Typical epoxy phenolic cannot be applied to hot surfaces, but there are CUI-resistant coatings made specifically for hot applications and one must be specified in this case. These coatings require blasting of the stainless surface, a process also used to prepare the surface for inspection. If for some reason LWIC decided not to coat the steel before re-insulating, the blast step would be eliminated. In that case, the surface could be cleaned with a very light surface grind, which is designed just to roughen the surface and not to remove significant amounts of metal.
After the surface is blasted clean, an initial visual inspection is done. Many areas will be free of corrosion, but some areas will have a pitted appearance. This pitting is a good indication that SCC is present. The next step is to conduct a dye-penetrant inspection of the suspicious areas. Normal dye-penetrant materials will not work on hot surfaces, so special high-temperature materials must be used. It is important that qualified inspectors conduct this inspection. Specifically, the inspector should have the proper training needed to recognize SCC and on the use of high-temperature inspection materials. A light surface grind may be necessary to reveal the full extent of the SCC since the cracks are often too small and tight for the penetrant to be absorbed.
Crack Depth Helps Determine Replacement
Criteria should be established at the beginning of the inspection process to determine the degree of damage that will be replaced and the degree that will be tolerated. The factors that go into this analysis can be quite complex and depend on the specific situation, but the result of the analysis is deciding what crack depth will require that a part be replaced. If the item is already leaking, a temporary repair is made using a commercially available service until replacement can be completed during a normally scheduled plant shutdown. Crack depth is determined using the ultrasonic shear wave method, often referred to as angle beam inspection. Again, it is critical that the inspector be properly trained to work with SCC of stainless steel.
After inspections are complete, the item is either marked for future replacement or is coated and reinsulated. LWIC elected to take a belt-and-suspenders approach to preventing future problems because the liquid widget process is so crucial to their business. The insulation material specified is changed from fibrous to perlite because perlite is more resistant to mechanical damage and moisture absorption. A non-corrugated jacket material of greater thickness than the original is also specified, again to improve damage resistance. Since wash down of the building cannot be eliminated, and it is not likely that maintenance practices will change, a more robust insulation system makes sense in this case.
The process described is for a large plant, but the principles can be applied to a small plant or even a single pipe loop or vessel: Assess the damage, determine the nature and extent of the repairs required to the pipe and equipment, carry out those repairs as production constraints allow, and then reinsulate. When the replacement insulation is designed, the nature of the damage to the original insulation must be considered and changes must be made to reduce the likelihood of future damage. Experience is the best teacher; learning from past failures provides a better system for the future.