CUI is a common problem shared by the refining, petrochemical, power, industrial, onshore and offshore industries. Corrosion under insulation is difficult to find because of the insulation cover that masks the corrosion problem until it is too late.
The problem occurs on carbon steels and 300 series stainless steels. On carbon steels it manifests as generalized or localized wall loss. With the stainless pipes it is often pitting and corrosion induced stress corrosion cracking.
Though failure can occur in a broad band of temperatures, corrosion becomes a significant concern in steel at temperatures between 0 and 149°C and is most severe at about 93°C. Corrosion and corrosion induced stress corrosion cracking rarely occur when operating temperatures are constant above 149°C.
Corrosion under insulation is caused by the ingress of water into the insulation, which traps the water like a sponge in contact with the metal surface. The water can come from rain water, leakage, deluge system water, wash water, or sweating from temperature cycling or low temperature operation such as refrigeration units.
It is also widely known that the results of CUI are costly. CUI can account for as much as 40 to 60 percent of a company's piping maintenance costs, can result in repairs in the millions, and lead to significant downtime. Most studies on the topic involve all forms of corrosion and their associated costs without providing the individul cost of corrosion related to insulation.
A study completed in 2001 by a research team of corrosion specialists in the USA reported the direct cost of corrosion under insulation to be $276 billion per year, with that number potentially doubling when indirect costs are also considered.
The American Petroleum Institute code, API 570, Inspection, Repair, Alteration and iterating of In-service Piping Systems, the piping code first published in June 1993, identifies CUI as a special concern. Typically, as happened with API 653 and the Clean Water Act, the API codes become an industry standard, and the regulations demand that organizations maintain a program to meet that standard. Occupational Safety and Health Administration (OSHA) 1910 is the rule with the teeth in this case.
API 570 specifies the following areas as susceptibleto CUI..
There are a number of methods used today to inspect for corrosion under insulation. The main ones are profile radiography, ultrasonic spot readings, and insulation removal.
The other method is real-time X-ray. Real-time X-ray has proven to be a safe, fast and effective method of inspecting pipe in plant operations.
In recent years, the CUI prevention philosophy of many large petrochemical companies has been an inspection-free, maintenance-free concept. Insulated systems particularly piping systems are expected to have a service life of 25 to 30 years. Evaluation of life-cycle savings has led to consideration of new, simple approaches to preventing CUI.
Two of the techniques are..
Applying thermal spray aluminum (TSA) on carbon steel to prevent general corrosion, and on austenitic stainless steel to prevent stress corrosion cracking.
All thermal spraying processes rely on the same principle of heating a feedstock, accelerating it to a high velocity, and then allowing the particles to strike the substrate. The particles will deform and freeze onto the substrate. The coating is formed when millions of particles are deposited on top of each other. With TSA, these particles are bonded to the substrate mechanically.
The first step of any coating process is surface preparation. This is done by cleaning and white metal grit blasting the surface to be coated. Masking techniques may be adopted for components that only need specific areas coated. The second step is to atomize the aluminum, which is done by introducing the feedstock material into the heat source. The heat source may be produced by either chemical reaction (combustion) or electrical power (twin wire arc spray). Next, the particles are accelerated to the substrate by the gas stream and deform on impact to make a coating. Finally, the coatings are inspected and assessed for quality by either mechanical or microstructural evaluation.
The two common thermal spray techniques used to apply TSA to components are wire flame spray and twin wire electric arc spray. Adhesion to the substrate is considered largely mechanical and is dependent on the work piece being very clean and suitably rough. Roughening is carried out by grit blasting to a white metal condition with a sharp, angular profile in the 50-to-100 micron (2-to-4 mil) range. Flame and arc spraying require relatively low capital investment and are portable; they are often applied in open workshops and on site. Consumables used for TSA with these processes are more than 99-percent purity aluminum wires.
Using aluminum foil wrapping on austenitic stainless steel pipe to prevent stress corrosion cracking.
Aluminum foil wrapping of austenitic stainless steel has been used successfully for more than 30 years by chemical companies in Europe to prevent ESCC. (This technique has not been widely accepted in the United States.) The aluminum foil provides a moisture barrier and electrochemical protection by preferentially undergoing corrosion and maintaining a safe potential for stainless steel. The system relies on good weatherproofing and the prevention of immersion conditions. It can be applied by the insulation contractor, takes less time to apply than a coating, and requires minimal substrate preparation.
Wrapping pipe with 46 SWG (wire gauge) 0.1-millimeter (mm) aluminum foil can prevent CISCC of stainless steel pipe operating continuously between 60°C and 175°C. The pipe should be wrapped with 50-mm overlap, formed to shed water on the vertical line, and held with aluminum or stainless wire. The foil should be molded around flanges and fittings. Steam-traced lines should be double wrapped, with the first layer applied directly onto the pipe, followed by the steam tracing, and then more foil over the top. On vessels, the aluminum foil is applied in bands held by insulation clips and insulation support rings.