Electrochemical Noise Measurements of Steel Corrosion in the Molten NaCl-K[sub 2]SO[sub 4] System

Electrochemical noise measurements have been carried out on AISI347, 10CrMo910, 15Mo3, and X20CrMoV121 steels in molten NaCl-K2SO4 at 630°C. Different types of current noise have been identified for pitting, intergranular and peeling corrosion. The corrosion mechanism was the so-called active corrosion ~i.e., the corrosion proceeds with no passivation due to the influence of chlorine!, characterized by the formation of volatile metal chlorides as a primary corrosion product. It was found possible to obtain an empirical separation of general and intergranular corrosion using kurtosis ~a statistical parameter calculated from the electrochemical noise data!. It was found that average kurtosis values above 6 indicated intergranular corrosion and average values below 6 indicated general corrosion. The response time for localized corrosion detection in in-plant monitoring was approximately 90 min on this basis. Approximate values of polarization resistances of AISI347 and 15Mo3 steels were determined to be 250 and 100 V cm 2 , respectively. Electrochemical noise measurements ~ENMS! is a term used for the registration of small fluctuations in current and potential around a mean value ~the steady state!, typically at the corrosion potential. The relationship between these current and potential variations is determined by the processes occurring on the electrode. An electrode undergoing localized corrosion displays low-frequency, stochastic transients originating from occurrence of events on the electrode surface. While being random in nature, it still makes intuitive sense to expect a jump in current to be accompanied by a change in potential and visa versa. One application of the ENM technique is to describe this relation between current and potential in terms of a polarization resistance which can be related to the corrosion rate using the procedures developed for potential sweep techniques like steady-state voltammetry. The first papers concerning electrochemical noise in corrosion were published as early as the 1960s. 1,2 However, breakthrough improvements of the data analysis occurred mostly in the 1990s 3-21 and have established their use and reliability in the present day for many low-temperature applications ~construction, nuclear waste barrels, industrial process monitoring, etc.!. Some research on hightemperature applications of an ENM technique has been carried out,