Effect of Deaeration and Sodium Sulfite Addition to MSF Make-up Water on Corrosion of Evaporator and Heat Exchanger Materials

A near zero oxygen level is normally maintained in MSF make-up water by mechanical deaeration followed by Na2SO3 addition. Is the complete removal of dissolved oxygen from the make up water beneficial to evaporator and heat exchanger materials which invariably require a stable protective oxide layer for corrosion resistance? To investigate this problem, a detailed study has been carried out to determine the effect of trace levels of dissolved oxygen in seawater (in the absence and presence of Na2SO3) on the corrosion behavior of evaporator and heat exchanger materials like mild steel, AISI316L and 70:30 cupro-nickel alloys. The experiments were conducted at 30°, 50° and 80°C, with or without addition of Na2SO3 and at dissolved oxygen levels varying from zero to several ppb and to a few ppm. The effect of other additives such as chlorine and Belgard EV-2000 antisealant has also been studied. The electrochemical polarization resistance has been the main technique used to measure the corrosion rates. The results of the studies indicate that by and large, addition of Na2SO3 to seawater containing up to 20 ppb dissolved oxygen has very little effect on the corrosion rates of evaporator and heat exchanger alloys. A slight negative effect on the corrosion behavior was noticed when Na2SO3 was added in deaerated seawater, which became more pronounced at elevated temperatures. Addition of antisealant in deaerated and Na2SO3-dosed seawater also resulted in slightly higher corrosion rates probably due to descaling effect