The present paper describes a detailed assessment of the factors which affect the electrochemical potential (ECP) of storage vessel welds in a working deaerator of a small 20MW boiler. Unit load changes, which occasionally occurred during service, significantly affected the ECP level in as much that a one MW change altered the ECP by some 40mV. Such effects of load change on ECP were related to water flow rates through the storage vessel when the Reynolds number was less than 1500. At higher values, the effects of flow rate were much reduced.