Real 'in situ' electrochemical potential measurements of welds in deaerators feedwater storage vessels during service

摘要

The present paper describes a series of electrochemical potential, ECP, values which were recorded in two weld run locations of the storage vessels of three small deaerator units, two 20 MW and one 40 MW units, associated with the production of electric power over a period of some 5 years. The deaerators had water temperatures ranged from 110 to 130 deg C while the pressure was only a few bar. It was established that (a) both ECP readings exhibited good agreement, (b) the reference electrodes showed good durability over testing periods of between 1000 and 1700 h service and (c) an initial hysterisis period of around 400 h was observed before ECP levels attained a minimum value. Also, it was shown that when few transients occurred during the start-up period the minimum ECP value was around -350 mV while in the case where significant transients occurred the minimum ECP value was only-100 mV. This effect of transients was important in the prediction of the occurrence of environmentally assisted cracking, EAC, in deaerator welds inasmuch that when few transients occurred EAC was only expected in oxidized weld runs with bulk sulphur levels of around 0.025 percentS and in Pd coated weld runs of some 0.05 percentS. In the case where a significant amount of transients took place EAC was predicted to occur in all oxidized weld runs and in Pd coated weld runs with bulk sulphur levels above only 0.015 percentS. Such effects of start-up conditions will have a significant influence on the working life of deaerator storage vessels since EAC growth rates can be typically 10 to 100 times faster than normal air based fatigue crack growth rates.