Iron-oxidising microbial biofilms as possible causes of increased friction coefficient in intermediate and lower guide vane bearing bushings at a hydroelectric powerplant in Brazil

Increased coefficient of friction led to malfunction of many and destruction of some maintenance-free bushings of intermediate and lower guide vane bearings at a hydroelectric power plant in Brazil. Analysis of surfaces of failed bushings revealed the presence of three types of deposits. The contact zones between bushings and the guide vane axis were covered with a thin black graphite film. The remaining bushing surface was covered by a mix of yellow-red coloured deposits, which contained a large proportion of iron-oxide-rich microbial biofilms, and green-coloured deposits, which consisted largely of copper oxides. Biofilms sampled from both the inside and the outside of the bearings contained 75% iron oxides by weight. The iron oxide deposits produced by these biofilms were identified as the primary cause of increased friction resistance between the bushing surface and the guide vane axis. Iron deposition within biofilms was made possible by the action of iron-reducing bacteria in the anaerobic zone of the reservoir immediately in front of the turbine intakes. These bacteria enriched the anoxic reservoir water with Fe(II) and the relatively small oxygen concentrations in turbine feedwater prevented the complete oxidation of Fe(II) in the penstock. Water-proofing of the bearing seals would prevent water penetration into the bearings and biofilm formation on the bushing surfaces and thus avoid the type of failure observed at this plant.