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December 15 - December 19, 2014

What factors would justify coating stainless steel in water or wastewater service? What standards and methods apply?

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From Vaughn O'Dea of Tnemec Company, Inc. on January 1, 2015:
The grades of stainless that I see most commonly used in water and wastewater exposures are typically 316, and to a lesser extent, 304. Although stainless steel is much more resistant to corrosion than ordinary carbon steel, in some circumstances I have observed corrosion in these environments, especially when the stainless is exposed to solutions or corrosive atmospheres containing elevated levels of fluorides, chlorides, or sulfates. Most commonly, however, stainless is coated in water and wastewater services because it is galvanically coupled with carbon steel. In this case, the application of a protective coating to the stainless steel is necessary in order to reduce the galvanic effects of the area ratio. This is best accomplished by abrasive blasting the stainless steel components in accordance with SSPC-SP16 and producing 3.0 mil anchor profile using aluminum oxide, then applying a minimum of 15 mils (381 microns) DFT of a high-quality, epoxy protective coating. This will greatly reduce, but not eliminate, the effects of bi-metallic coupling. Under no circumstances should the anodic carbon steel alone be topcoated—this will result in accelerated galvanic corrosion of the carbon steel. For more information on galvanic coupling of stainless and carbon steel, refer to SSPC 2014 paper by Vaughn O’Dea, Travis C. Tatum, Pat Barry, and Paul Higgins titled, “Galvanic Corrosion in Water & Wastewater Structures: Coupling Stainless and Carbon Metals Leads to Accelerated Corrosion."

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Tagged categories: Asia Pacific; EMEA (Europe, Middle East and Africa); Immersion service; Latin America; North America; Program/Project Management; Stainless steel; Wastewater Plants; Water Tanks

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