Corrosion resistance of stainless steel in soil

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The aim of this project is to clarify resistance against external corrosion of stainless steel pipes in soil. This is being done by laboratory screening tests, by exposure in various soil types in the field and exposure to AC current, as well as by collecting operational experiences of existing stainless steel pipelines. Alloys included are austenitic grades: EN1.4307, 1.4404, 1.4436, 1.4429, 1.4547; and duplex grades: EN 1.4162, 1.4362, 1.4462, 1.4410. Two laboratory screening tests are performed: ranking the steels based on critical pitting corrosion temperatures, and testing their ability... to re-passivate after a local mechanical breakdown of the passive film. Two to seven years’ field exposures are performed in clay, peat and sand soils at seven test sites in Sweden and France, evaluating corrosion resistance. Corrosion was found at the three marine test sites, the most extensive being the Mediterranean sea shore site. Attempts are made to correlate corrosion with soil parameters and corrosion potentials. In the field exposure, large specimens are used in order to allow the formation of large cathodic areas promoting possible pitting corrosion, thus simulating the relationship between large cathodic and small anodic areas occurring on buried stainless steel suffering localised corrosion. AC corrosion of stainless steel is investigated, both in the laboratory and in the field, resulting in better understanding of AC induced corrosion of stainless steel. Exposure to AC voltage will cause the steel potential to follow the sinus slope and may cause corrosion if the potential oscillates into the trans-passive and into the immunity regions.