PITTING CORROSION CAUSED BY MICROBIOLOGIC AL ACTIVITY OF AUSTENITIC STAINLESS-STEEL PIPES USED FOR RIVER WATER

Rhine River water used for cooling purposes increasingly causes pittin g at circumferential welds in type 321 and 316Ti stainless steel pipin g. Type 321 is attacked even at ambient temperature, type 316Ti at 35 to 55-degrees-C. At these temperatures both materials did not show pit ting in the past. Failure analysis and corrosion tests, both in the la boratory and in a pilot plant (river water circuit), showed that the o pen circuit potential of stainless steels is shifted into a region of instable pitting resistance by a microbiological effect. Thus, pit nuc lei are able to grow even at circumferential joints welded without any visible annealing colours. The actual concept to select cooling water pipe materials, therefore, has to be reconsidered.