Materials Performance

DEC 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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19 NACE INTERNATIONAL: VOL. 53, NO. 12 MATERIALS PERFORMANCE DECEMBER 2014 Information on corrosion control and prevention or exposed to the vapors in the headspace above the medium—conditions that mimicked those around the sump pumps. The test medium was a mixture of bacte- ria grow th media and ethanol inoculated with Acetobacter sp. At regular intervals, the coupons were pulled from the test set- up and evaluated for corrosion. Both vapor phase and immersion corrosion rates were measured, and x-ray diffrac- tion (X R D) was used to identif y corrosion products. According to the researchers, labora- tory exposure to Acetobacter sp. produced corrosion damage on the samples that was similar to the damage seen on USTs inspected in the f ield. Results of the vapor phase experiments indicated corrosion rates of ~0.06 mm/y for copper coupons and >1 mm/y for steel coupons in the headspace. The copper coupons experi- enced general and pitting corrosion, with corrosion products that included copper acetate dehydrate, copper hydroxide ace- tate, and cuprite, and small pits that coalesced into larger pits. The researchers noted that it would take 15 years in this environment for external corrosion to break through cop- per tubing with a wall thickness of 1.2 mm. Corrosion products on the steel cou- pons included iron oxides and iron hydroxides that grew thicker with longer exposure times and f laked easily from the coupons. General pitting was found underneath the corrosion products and pit initiation appeared to be connected to non-metallic inclusions. The corrosion rate measured for the immersed copper coupons was 0.051 mm/y ± 0.010 mm/y after 36 days of expo- sure and 0.061 mm/y ± 0.003 mm/y after 39 days of exposure for the steel coupons. Compared to the headspace coupons, cor- rosion was not as extensive on the immersed coupons. The researchers remarked that such a low corrosion rate for steel may be related to bacterial attachment, which could protect the steel by a passivation mechanism, and that an Acetobacter biof ilm could actually decrease corrosion. Deep intergranular attack was noted on the copper coupons, an indication that stress corrosion crack- ing (SCC) could lead to accelerated perfo- ration of bent copper tubing. The surface of the steel coupons contained pits within ferrite grains and attack at interphase boundaries was noted. Based on these f indings, NIST re- searchers suggest that gas stations may need to replace submersible pump cas- ings made of steel or cast iron sooner than expected, with retrof its costing an estimated $1,500 to $2,500 each. There are more than 500,000 underground gas storage tanks around the country. Future investigations of special coat- ings, biocides, and other ways to prevent sump pump failures and leaks could ben- ef it from the NIST test equipment devel- oped for this study. In July 2013, NIST held a workshop on biocorrosion associated with alternative fuels. Presentations and information from this workshop can be found at Source: NIST Tech Beat, July 29, 2014, References 1 J.W. Sowards and E. Mansfield, "Corrosion of Copper and Steel Alloys in a Simulated Un- derground Storage-Tank Sump Environment C o n t a i n i n g A c i d - P r o d u c i n g B a c t e r i a ," Corros. Sci. 87, 10 (2014): pp. 460-471. 2 "NIST Finds That Ethanol-Loving Bacteria Accelerate Cracking of Pipeline Steels," NIST Tech Beat, August 2, 2011, mml/acmd/201108_ethanol_pipelines.cfm (October 29, 2014).

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