Abstract
The present study aims to further the understanding of the galvanic corrosion behaviour of the two systems: titanium/Al brass and 316L stainless steel (st.st)/Al brass in Arabian Gulf water. The study involves the measuring of the open circuit potentials (OCP) of coupons of three metals in seawater at temperatures between 25°C and 65°C. When steady values are attained, the coupons are short-circuited and the couple potentials are followed as functions of time. Corrosion currents are simultaneously recorded. The OCP behaviour of each element depends on the prevailing experimental conditions. With Ti in stagnant and stirred solutions at all temperatures, the OCP shifts towards positive values denoting film formation, repair and thickening. Stainless steel in stagnant seawater at low temperatures undergoes pitting corrosion. At high temperatures the metal is protected by a film of CaCO 3, whereas in stirred solutions the metal escapes pitting. Peculiar behaviour is recorded, when together with the steel, a coupon of uncoupled Al brass is immersed in the same solution. This arrangement seems to protect against pitting attack. The OCP behaviour of Al brass in low-temperature seawater suggests film destruction and dissolution. Coupling of Ti or stainless steel to Al brass produces initial potentials near those of the base metal. At advanced times partial protection due to the electrochemical formation of CaCO 3 takes place. This affects the magnitude of corrosion currents which decrease also with a rise in temperature. These conditions are supported by the results of measurements carried out in synthetic seawater free from HCO 3 and Ca 2+ ions. Because of the large difference between the free corrosion potentials of titanium and stainless steel, on the one hand, and of Al brass on the other, severe corrosion of the latter can occur. The intensity of attack can be greatly reduced when the oxygen content of the brine is reduced. The same can be achieved through the appropriate manipulation of the naturally forming CaCO 3 film.
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