ABSTRACT
Critical Pitting Temperature (CPT) is used to measure the breakdown of passive layers on corrosion resistant alloys. A large oxidative potential, typically +720 mV (SCE) is imposed on a metal surface. Then, the reaction temperature is increased from 0 °C until a breakdown of the metallic passive layer occurs. This breakdown is observed as an abrupt increase in the current. For example, in the case of UNS S31600, the failure of its metallic passive layer occurs around 12 °C. The high current observed is due to localized corrosion occurring where the layer broke. An oxidative potential of -625 mV (SCE) was selected to perform this same methodology on carbon steel. It was found that both un-inhibited and inhibited carbon steel corrode following a Butler-Volmer behavior. This means that: 1) in the case of the uninhibited metal, the passive layer does not protect. 2) in the case of inhibited metal, the increased temperature just dilute the adsorbed inhibitor from the interphase. This makes the inhibitor film more permeable to diffusion of species.
Focused on the influence of the reaction temperature on localized corrosion, CO2 corrosion of carbon steel was followed with electrochemical impedance spectroscopy at open circuit potential and using reaction temperature ramps starting at 0 °C. By this way corrosion was prevented by temperature control. An equivalent circuit that explain the experimental corrosion data was found. Efforts were put into finding the significance of each component in the circuit. It was possible to identify: 1) general corrosion, 2) the formation of an iron carbonate colloid, 3) a 2-dimensional nucleation and crystal grow process, 4) which covered the surface following a Frumkin isotherm and 5) localized corrosion rates occurring after the metal surface was covered by the iron carbonate crystal. All these processes were interdependent.
INTRODUCTION
Critical Pitting Temperature (CPT) methodology has been used to determine how easily a material develops localized corrosion. Generally, a high oxidative potential, + 720 mV (SCE), is imposed on the material. Which is in 1 M NaCl solution and at 0 °C. Then, the temperature is increased until a burst of current developed. The temperature at which such burst occurred is called the critical pitting temperature.1 This method is normally used to rank materials for localized corrosion susceptibility. This was the starting point to explore if inhibited carbon steel behaves in the same manner. The first part of this publication reports the development of such a method for carbon steel. It was found that inhibitor films do not behave like a solid passive layers and they are influenced by temperature changes in a different way.
