Predicting Localized Corrosion in Seawater, October 2004

• Published in: Corrosion (Houston, Tex.) Vol. 60; no. 10
• Main Authors: Sridhar, N., Brossia, C.S., Dunn, D.S., Anderko, A.
• Format: Journal Article
• Language: English
• Published: NACE 01.10.2004; NACE International
• ISSN: 0010-9312; 1938-159X

ABSTRACTThe geographical variation in the corrosivity of natural seawater results from the variations in the salinity, microbiological activity, dissolved oxygen concentration, and temperature.1 Discounting the inland seas, such as the Dead Sea, the chloride (Cl­) concentration of seawater varies from about 5.8 g/kg to about 2­ 24 g/kg, the sulfate (SO4 ) concentration varies from ­ 0.8 g/kg to 3.4 g/kg, and the bicarbonate (HCO3) con1 centration varies from 0.01 g/kg to 0.2 g/kg. Except in the case of Dead Sea, the sodium to magnesium weight ratio (Na/Mg) remains about 8 in these waters and the sodium to calcium weight ratio (Na/Ca) remains about 26.1 Natural seawater is more aggressive than artificially made seawater (by mixing the appropriate compounds found in seawater) or seawater that has been sterilized. It has been argued that the microbial organisms in natural seawater increase the open-circuit potential, a process called ennoblement.2-3 More recently, Salvago and Magagnin4-5 observed that the corrosion potential of stainless steel (SS) in seawater had a broad distribution, spanning a range of about 500 mV, although the distribution of the corrosion potential narrowed considerably after prolonged exposure with a mean value of about 400 mV vs saturated calomel electrode (SCE).4 The mechanism of ennoblement is still under some debate,4 and recent experiments by Salvago and Magagnin5 argue that the ennoblement may not necessarily be explained by cathodic depolarization arising from microorganisms, but may involve lowering of passive dissolution kinetics. Lacking a sound theoretical basis for the anodic and cathodic kinetics in natural seawater, calculation of the corrosion potential must use of necessity "apparent" anodic and cathodic parameters. Temperature and chlorination level can determine the corrosion behavior of SS. In natural seawater, the 915