Electrochemical methods for the determination of Pedeferri's diagram of stainless steel in chloride containing environment

Abstract

Stainless steels are prone to localized corrosion (pitting and crevice) in the presence of oxygen and chlorides over a critical threshold. Prediction of the corrosion behavior of stainless steels in chloride containing environment is of great importance; for this reason, Pedeferri's diagram is proposed to define corrosion and passivity (perfect and imperfect conditions) as a function of the potential and chloride content. In this paper, the first effort is carried out to define electrochemical conditions to obtain the diagram. Potentiodynamic and potentiostatic polarization tests are performed on AISI 304L type stainless steel. Localized corrosion potential and critical chloride content found with two different methods are in fair agreement. Pedeferri's diagram is built for type 304L stainless steel at room temperature and pH 6. Corrosion, perfect and imperfect passivity regions are separated by scattering band for localized corrosion and repassivation potentials.

Experimental design to study the influence of temperature, pH, and chloride concentration on the pitting and crevice corrosion of UNS S30403 stainless steel

“…This is related to the stochastic nature of pitting corrosion that must be analyzed from a statistical point of view to determine potential distribution [43]. As can be seen, the solid line is passing through the results and scattering band, moreover, this is in accordance with the previous work in which Pedeferri's diagram is determined without using DOE [41]. The scattering of E bd in each chloride concentrations is about 100 mV (±50 mV from the solid line) which is lower than the scattering reported in ASTM G61 [51].…”

“…The solid line is generated by the semi-logarithmic equation (4) and points are the experimental points. Since in Pedeferri's diagram, localized corrosion potential is a scattering band [38,41], it is necessary to define maximum and minimum passivity breakdown potentials for different chloride concentrations (pH = 6 and T = 20°C) by which ranges are defined (dotted lines). This is related to the stochastic nature of pitting corrosion that must be analyzed from a statistical point of view to determine potential distribution [43].…”

“…Corrosion potential for freshwater, brackish water, and the acidic condition are considered in the range of -0.14 ≤ E corr ≤ 0.14 V vs. Ag/AgCl/KCl sat . This range is selected on the basis of the E corr range obtained in the present research and in the previous paper [41], while the passivity breakdown potentials are derived from Equation 4. In the case of seawater, two conditions were considered, with or without bacterial activity.…”

“…During the period of pre-conditioning prior to the potentiodynamic test, free corrosion potential was monitored with a frequency of 1 point every minute. For less aggressive conditions (pH > 7 and Cl -< 10000 mg⋅L -1 ), the pre-conditioning time was 24 hours according to the previous experience [41,50]; for more aggressive conditions, it decreased to one hour as reported in ASTM standard [51]. The sample holder configuration was based on ASTM G61 [51] in which the exposed area of the specimen to the test environment was 1 cm 2 .…”

“…potentiodynamic and potentiostatic polarization tests) to determine localized corrosion characteristics (passivity breakdown potential, E bd , re-passivation potential, E rep , and critical chloride concentration) along with the comparison of the two methods were done. Furthermore, the definition of Pedeferri's diagram of UNS S30403 (AISI 304L) stainless steel for chloride containing environments, neutral pH, and the room temperature was discussed [41].…”