Biofilm formation on 316L stainless steel was investigated in a pilotscale flow-through system fed with brackish surface water using an alternating flow/stagnation/flow regime. Microbial community analysis by denaturing gradient gel electrophoresis and sequencing revealed the presence of complex microbial ecosystems consisting of, amongst others, Leptothrix-related manganese-oxidizing bacteria in the adjacent water, and sulfur-oxidizing, sulfate-reducing and slime-producing bacteria in the biofilm. Selective plating of the biofilm indicated the presence of high levels of manganese-oxidizing microorganisms, while microscopic and chemical analyses of the biofilm confirmed the presence of filamentous manganese-precipitating microorganisms, most probably Leptothrix species. Strong accumulation of iron and manganese occurred in the biofilm relative to the adjacent water. No evidence of selective colonization of the steel surface or biocorrosion was found over the experimental period. The overall results of this study highlight the potential formation of complex microbial biofilm communities in flow-through systems thriving on minor concentrations of manganese.

The high temperature cracking of 17 wt-%Cr ferritic stainless steel during continuous casting was investigated in detail. No relationship between the high temperature internal cracking and the ductile to brittle transition temperature (DBTT) was found. The experimental results imply that the cracking is owing to a combination of factors: a high ferrite potential, a hot ductility gap, and the weaker grain boundary cohesion. The 17 wt-%Cr Ti stabilised grades were found to be the most sensitive ferritic grades for crack formation at high temperatures. The ductility was evaluated by means of hot tension tests. At 1200°C, the fracture surfaces of the Ti free ferritic steel had an intergranular character. At temperatures between 700 and 1100°C, the fracture surfaces were dimpled. Below 600°C cleavage fracture occurs. The 17 wt-%Cr Ti stabilised steel contained solidi® cation related cracks. These cracks widen during the further cooling and could clearly be seen on the surface of a cracked specimen. The Ti stabilised grades had a ductility gap at 1100°C. The DBTT was measured by means of Charpy tests. Most grades had a DBTT higher than 200°C. Type AISI 430Ti and AISI 409 ferritic steel both had a lower DBTT of ~90°C.MST/5106

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