Effect of Anions in Seawater to Corrosion Attack on Passive Alloys

Some metals and alloys have special characteristics that enable them to provide superior corrosion-resistant metal surfaces. These protective passive films are responsible for the phenomenon of passivity. Metals with this characteristic can be categorized as passive materials such as stainless steel 316L and nickel alloy Inconel 625. However, in corrosive environment this protective passive film may breach and exposed bare metal for further corrosion attack. Corrosive environment can be refer as metal is susceptible to corrosion attack such as seawater which contain aggressive ions (salt) with negative charge (anion). An electrochemical test has been carried out to determine the effect of anion on corrosion attack of 316L and Inconel 625. The objective is to identify the breakdown potential (Eb) in different anion ratio (chloride (Cl ) : sulphate(SO4 ). The anions concentration was decided as Cl : SO4 ; 35:0, 25:10, 10:25, 0:35 and was compared to artificial seawater, 3.5% NaCl. The pH values were set in neutral in static condition. The materials were run for cyclic polarization with a scan rate of 2mV/sec. The Eb value for 316L decreased in every solution as the temperature increased from 30°C to 80°C. The results explain that SO4 2has inhibiting effect to passive alloy showing that the highest Eb value detect at 30°C and highest concentration of SO4 . Optical observations elucidate that, the number of pits increased in high concentration of Cl than SO4 . Both materials presents pits reducing as the temperature and concentration of SO4 2ions increased. Nevertheless, the Eb value for 316L is likely to has a competition effect between Cl and SO4 2anion as the graph shows no significant effect at Cl : SO4 2,25:10, 10:25. The same scenario is not shown by Inconel 625 because this metal is highly corrosion resistance compared to 316L.