Abstract
This thesis reports upon an investigation into the involvement of microorganisms in an unusual form of pitting corrosion occurring in copper water pipes, which over the last few years has come to be termed Type 1 1/2 pitting. Type 1 1/2 pitting has led to the failure of copper tube carrying warm or cold water in a very small number of institutional buildings spread throughout the world. This form of pitting is very characteristic and has so far been seen mainly in areas supplied with soft waters. Failures of the pipes can be rapid with perforations being reported only a few months after replacement pipe has been installed. This work has clearly demonstrated, for the first time, the involvement of microorganisms with Type 1 1/2 pitting, through surveys using; the scanning electron microscope (SEM), along with field isolations from service water pipes and subsequent examination for corrosion characteristics. A novel form of static bed reactor was designed in which the pitting of copper rings was clearly associated with the presence of a consortium of bacteria. Having demonstrated this link, an investigation of the physiology of the isolated bacteria, particularly in relation to exopolysaccharide production and biofilm formation, clearly showed a link with the conditions known to prevail in water pipes exhibiting Type 1 1/2 pitting. From these studies it has been possible to propose a mechanism for the formation of biofilms within such pipes. Particular attention has been paid to temperature, nutrient levels and nutrient limitation. Flow conditions, including flow rates and stagnation periods were also found to be important. It has been possible to demonstrate that the conditions prevailing in pitted water pipes will favour biofilm formation. Partial characterisation of the isolated exopolysaccharide material produced by the bacteria has been carried out and shown to be similar to those produced by other bacteria known to be involved in corrosion. This has led to the speculation that the exopolysaccharide is directly involved in the pit initiation. The potential for the bacteria and their associated exopolysaccharides to cause the dissolution of copper has also been demonstrated. The possible involvement of low molecular weight metabolic products has been considered with some preliminary studies using hydrogen peroxide yielding a form of pitting reminiscent of Type 1 1/2 pitting.