Corrosion coupon testing in plant

Guide for conducting corrosion coupon tests in plant equipment... 

ASTM G4-84 (1984) Standard Method for Conducting Corrosion Coupon Tests in Plant Equipment. 

G 1 Preparing, Cleaning, and Evaluating Corrosion Test Specimens G 3 Conventions Applicable to Electrochemical Measurements in Corrosion Testing G 4 Conducting Corrosion Coupon Tests in Plant Equipment... 

Because of the extensive use of copper-nickel in shipboard piping, the potential corrosivity of RO ultrapure water on these systems is considered important, ASTM G 4 (Standard Guide for Conducting Corrosion Coupon Tests in Field Application) covers in-plant testing of metals and alloys without regard to alloy t q)e, family, or class and generates test results by exposure of the materials to filtered seawater (RO feed stock), an intermediate-pass brine byproduct, and ultrapure product water. 

Bleach plant corrosion testing has included corrosion coupon testing, electrochemical testing, electrochemical protection testing, and nonmetallic materials evaluation. Coupon exposure tests have been used to rank candidate materials for these applications. Nonmetallic materials including thermoplastics, thermosets, brick, and tile are widely used in the bleach plant. 

Pilot plant tests and laboratory corrosion tests under simulated plant conditions will help in the selection of suitable materials if actual plant experience is not available. Preliminary tests can be carried out by inserting coupons of different materials into an apparatus that is known to resist corrosion before testing plant components. This reduces the likelihood of component failure and possible release of chemicals during testing. Care is needed in the interpretation of laboratory tests. 

A typical rack employed for installation of specimens in pilot plants is shown in Fig. 6. Both corrosion coupons, 2 X 1 X 0.35 in. thick, and bend specimens intended to determine stress-corrosion cracking susceptibility, are included in the installation for aqueous corrosion testing. Specimens are separated by high density alumina spacers to eliminate electrochemical effects. During exposure, the racks are welded to existing components in the pilot plant equipment. 

Corrosion resistance of paints for structural steel in paper machine environments has been studied by Bermett and Magar [44,45]. A short-term laboratory testing method was developed to evaluate maintenance coatings. The test method used coupons with one half initially coated and the other half left bare to corrode in the environment, and then later the bare half was coated to simulate a repair coating in the field. Salt fog and Atlas cell immersion tests were also conducted on the paint panels. Rendahl et al. [46] used in-plant exposures of pamt test panels. 

Factors may throw off these rates—these are outlined in ASTM G3I, Standard Practice for Laboratory Immersion Corrosion Testing of Metals. Coupon-type tests cannot be correlated with changing plant conditions that may dramaticahy affect process equipment lifetimes. Other methods must be used if more frequent measurements are desired or correlation with plant conditions are necessary. 

ASTM G4 (latest revision) gives guidance for conducting plant corrosion tests, and in particular, for various methods for mounting specimens (coupons) in process plant. This standard evolved from ASTM and NACE Technical Committees. 

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