Micro Laser Raman analysis of failed components in nuclear industry

Micro laser Raman technique was used to analyse the corrosion products on the surface of the failed components received from different plants of DAE. The corrosion of active-passive materials generally occurs due to the failure of the passive film which protects the surface. The paper currently deals with two case studies. Case study I: Al-brass condenser tubes which failed due to erosion corrosion and also due to pitting as these condenser tubes had sea water as the coolant. The failure of the tube was a result of a poor passive film formation on the surface. The remedial measure was to subject these tubes to ferrous sulphate dosing. The Raman analysis was carried out after these tubes were subjected to ferrous sulphate dosing which is essentially done to get a γ-FeOOH layer on the surface. The results showed that a 7 cycle treatment gave an adherent and protective layer but in case of field treatment the concentration of γ-FeOOH was very less as established from the Raman spectrum thereby indicating that the treatment was improper. Case II: The carbon steel column in a heavy water plant showed excessive thinning. The cause for this thinning was attributed to reactive species in the column which reacts with Fe to form a non-adherent layer on the surface. The H2S passing in the column generally results in the formation of an adherent FeS2 coating which is desirable. The Raman analysis carried out on these samples showed the presence of FeCO3 which is an undesirable product along with Fe-oxides. Formation of these products resulted in the consumption of Fe from the surface resulting in thinning of the column. These analyses on the failed samples have shown the importance of surface film on the corrosion resistance of these alloys. The micro laser Raman is a powerful tool in analysing the corrosion surface compounds without disturbing the surface since it doesn't involve any sample preparation. The paper illustrates the importance and application of Raman spectroscopy in establishing root cause of corrosion failures in plants. (author)