High Nitrogen Stainless Steel as a Preferred Substratum for Bacteria and Other Microfouling Organisms

Abstract

High Nitrogen steels (HNS) have gained importance in recent years as construction materials. Their resistance to pitting corrosion makes them better candidates for industrial use. In the medical field, these materials are welcomed as their use can minimize the risk of nickel allergy. Nitrifying bacteria are a group of bacteria widely distributed in soils, freshwater, seawater etc. and since they derive their energy by oxidation of nitrogen compounds, they pose a threat to these materials. Experiments on the adhesion of nitrifying bacteria on HNS were carried out following coupon exposure method in the laboratory. Nitrobacter winogradskyi (nitrite oxidizing bacteria) and Nitrosomonas europea (ammonia oxidizing bacteria) were used for the adhesion studies. In addition, adhesion of a heterotrophic strain, Pseudomonas sp. also was studied for comparison. Materials used for the experiment were AISI type 304 L, 304 LN, and high nitrogen stainless steels. The results showed that Nitrobacter winogradskyi, Nitrosomonas europea and Pseudomonas sp., invariably, adhered more on the high nitrogen stainless steel coupons compared to the other two types of coupons tested. During the initial stages, i.e. on the first day, the trend was not so clear, but as the exposure period increased the pattern has clearly evolved. The area of adhesion showed a gradual decrease going from HNS, to 304 L through 304 LN coupons. The acidic metabolic products of nitrifying bacteria might interact with the surface of HNS, disturbing the equilibrium of ammonium ion formation, thereby leading to localized corrosion. Exposure to a natural freshwater pond showed a heavy microfouling load on HNS coupons. The consequences of preferential adhesion of bacteria and other microfouling organisms on the corrosion resistance of HNS are discussed. As a possible means to mitigate bacterial adhesion on HNS, silver contained high nitrogen stainless steel was tested. The results suggested a significant decrease in bacterial adhesion on silver containing HNS compared to 304 L and HNS.