EFFECT OF TEMPERATURE AND CONCENTRATION RATIO ON PITTING RESISTANCE OF 316L STAINLESS STEEL IN SEAWATER

Abstract

Due to a serious shortage of natural fresh water in many areas all over the world, the seawater desalination has emerged as an effective compensation way to meet the consumption requirements. Due to the good corrosion resistance and low cost, stainless steels have been used extensively to construct the multi effect distillation(MED) plants, especially type 316L stainless steel for the evaporation chambers. However, with the application and development of low temperature MED, there is increasingly need of higher temperature distillation and higher brine concentration in the desalinators to reduce the drainage of hot brine and increase the water production ratio, which may cause more serious corrosion on the stainless steel components in the plants. Pitting corrosion of 316L stainless steel was studied in the concentrated environments of seawater with different temperatures(25, 50, 63, 72, 85 and 95 ℃) and concentration ratios(1, 1.5, 2, 2.5 and 3 times) by using cyclic anodic polarization measurement and SEM surface observation. The results show that both pitting potential and repassivation potential of 316L stainless steel decrease linearly with temperature in the concentration ratio range of 1 to 3 times for seawater, but the change of pitting potential is very slight when the solution temperature is higher than 85 ℃ in the case of concentration ratio larger than 2 times. Both pitting potential and repassivation potential reduce linearly with the logarithm of the concentration ratio of seawater in the range of 25 to 95 ℃. It is apparent that increasing temperature and concentration ratio of seawater will deteriorate the pitting resistance of 316L stainless steel noticeably. The influence of temperature and concentration ratio is analyzed on the basis of the point defect model. Nevertheless, the concentration ratio of seawater has a weaker influence on pitting resistance of 316L stainless steel in comparison with temperature as revealed by the pitting potential changes resulted from the concentration ratio around 1.5 times and solution temperature around 72 ℃. Therefore, compared with temperature, the corrosion resistance of 316L stainless steel for low temperature MED plants may be relatively tolerant of the adjustment or fluctuation of seawater concentration.