Metal samples were removed from the systems and were examined visually and under a stereo- with an The chemical co of the pipe material and the weld metal were analysed optical emission spectrometer(CES Metallurgical e consisted of optical light microscopy of cross sections prepared from the scanning electron microscopy(SEM) studies from failure areas and energy dispersive spectroscopy(EDS) of corrosion products or of the deposits on the inner surfaces of the pipes. In Case 1, the metal samples were completely dry when arriving at the laboratory. Therefore, traditional methods like cultivation could not be applied to detect possible bacteria. However, preliminary trials with a new PCR method were performed with one sample showing corrosion failure. In Case 2, microbiological assays were also performed. The total counts of aerobic and anaerobic bacteria and the amount of sulphate reducing bacteria were analysed by cultivation from the samples taken at site, from the water inside the pipes as well as from the deposits on the surfaces of the pipes. Samples were scraped from several places on the surface of the opened pipe. The system had been opened half an hour before the sampling