In-service electrochemical potential data in a deaerator feedwater storage vessel; the influence of a noble metal coating

摘要

The present paper describes the two separate test runs where the Electrochemical Potential, ECP, of two weldments, one in the normal oxidised condition and one coated with a layer of palladium, were monitored "in situ" in the storage vessel of a deaerator system in a 20MW boiler. Although both the test runs, called gamma and delta, were dogged with abnormal operational conditions, viz. numerous off-load events occurred in one run and intermittent condenser leaks in the other, it was established that the Pd coated weld exhibited ECP levels which were consistently lower than the normal oxidised weld. Indeed the differences varied from 100 to 180 mV and averaged 125 mV.When the normal oxidised weld ECP data from the earlier test runs where little, or no, upset operational conditions occurred was compared to the present oxidised data sets it was evident that the initial conditions after going on-load were critical in determining the ECP levels attained for each test. If off-load events or oxygen leaks were prevalent during the initial 400 h, where the hysterisis effects were noticed, the subsequent ECP levels were approximately 200 mV more positive than those ECP levels attained from a test run where no upset conditions were present during the initial stages.Through the use of the beneficial effects arising from Pd coated welds, it was shown that the ECP values in real working deaerator storage vessel welds could be "conditioned", i.e. lowered, to levels which were well below the critical ECP levels required to induce environmental assisted cracking, EAC, viz. E_(CRIT), during operation. Other locations in a boiler, viz. those that are water filled, which are prone to EAC can also be treated in a similar manner.