Abstract
This work treats the effects of welding, alloy composition, mechanical surface treatment and heat treatment on the susceptibility of several different stainless steels to Microbiologically Influenced Corrosion (MIC). Optical and scanning electron microscopy were used to document surface structure and microstructure. Quantitative microscopy was used to characterize microstructural constituent size, shape and distribution. Samples were autogenously welded, heat treated and surface conditioned. Samples were subsequently exposed to tap water in a recirculating system. At different intervals samples were retrieved and inspected. The biofilms that had formed were evaluated by electron microscopy after biologically fixing and sputter-coating the samples.
Microbial attachment, film development and the evolution of microbial consortia were documented. The extent of corrosion and the corrosion morphology was documented. Microbes were found to be crucial to the development of corrosion in the tap water environment; no corrosion was observed on control samples exposed to sterile tap water. Furthermore, welded samples were much more susceptible to corrosion than unwelded samples. Though initial attachment was random, and occurred on all samples, microbial colonies developed more frequently and evolved more rapidly in heat affected zone (HAZ) regions. The fusion line and the partially melted zones (PMZ) were most heavily colonized. Surface grinding with 240 and 600 grit papers greatly reduced susceptibility, whereas wire brushing increased it vis-a-vis the as-welded condition.
Several different biofilm structures were observed. One evolution led to a general thickening of the film with a structural differentiation by depth. A second evolution led to the formation of tubercules and the localization of corrosive attack. Prior thermomechanical history, sample chemistry, sample microstructure and the microbial population produced this differentiation.
