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Revision Date: August 15, 2013
Updated On: September 17, 2013
5.1 The corrosion observed on steel and other materials under thermal insulation is of great concern for many industries including chemical processing, petroleum refining and electric power generation. In most cases, insulation is utilized on piping and vessels to maintain the temperatures of the operating systems for process stabilization and energy conservation. However, these situations can also provide the prerequisites for the occurrence of general or localized corrosion, or both, and in stainless steels, stress corrosion cracking. For example, combined with elevated temperatures, CUI can sometimes result in aqueous corrosion rates for steel that are greater than those found in conventional immersion tests conducted in either open or closed systems (see Fig. 1).4 This figure shows actual CUI data determined in the field compared with the corrosion data from fully immersed corrosion coupons tests.
5.2 This guide provides a technical basis for laboratory simulation of many of the manifestations of CUI. This is an area where there has been a need for better simulation techniques, but until recently, has eluded many investigators. Much of the available experimental data is based on field and in-plant measurements of remaining wall thickness. Laboratory studies have generally been limited to simple immersion tests for the corrosivity of leachants from thermal insulation on corrosion coupons using techniques similar to those given in Practice G31. The field and inplant tests give an indication of corrosion after the fact and can not be easily utilized for experimental purposes. The use of coupons in laboratory immersion tests can give a general indication of corrosion tendencies. However, in some cases, these procedures are useful in ranking insulative materials in terms of their tendencies to leach corrosive species. However, this immersion technique does not always present an accurate representation of the actual CUI tendencies experienced in the service due to differences in exposure geometry, temperature, cyclic temperatures, or wet/dry conditions in the plant and field environments.
5.3 One of the special aspects of the apparatus and methodologies contained herein are their capabilities to accommodate several aspects critical to successful simulation of the CUI exposure condition. These are: (1) an idealized annular geometry between piping and surrounding thermal insulation, (2) internal heating to produce a hot-wall surface on which CUI can be quantified, (3) introduction of ionic solutions into the annular cavity between the piping and thermal insulation, (4) control of the temperature to produce either isothermal or cyclic temperature conditions, and (5) control of the delivery of the control or solution to produce wet or wet-dry conditions. Other simpler methods can be used to run corrosion evaluations on specimens immersed in various solutions and leachants from thermal insulation. In some cases, these procedures may be acceptable for evaluation of the contribution of various factors on corrosion. However, they do not provide accommodation of the above mentioned factors that may be needed for CUI simulation.
5.4 With the CUI-Cell, the pipe material, insulation and environment can be selected for the desired simulation needed. Therefore, no single standard exposure condition can be defined. The guide is designed to assist in the laboratory simulation of (1) the influence of different insulation materials on CUI that, in some cases, may contain materials or additives, or both, that can accelerate corrosion, (2) the effect of applied or otherwise incorporated inhibitors or protective coatings on reducing the extent and severity of CUI. This guide provides information on CUI in a relatively short time (approximately 72 h) as well as providing a means of assessing variation of corrosion rate with time and environmental conditions.
Note 1—The actual CUI corrosion rates can be in excess of the those obtain in conventional laboratory immersion exposures.