Development of the cyclic potentiodynamic polarization (CPP) technique and interpretation of the resulting curves was based primarily upon the stainless steel-aqueous chloride system. This paper presents and interprets CPP curves for this and other alloy-environment systems. It was concluded from the analysis of CPP curves for these systems that both a forward and a reverse scan should be performed in order to maximize the information on localized corrosion obtainable from the test technique. The pitting potential, which is one parameter obtained from a CPP curve, is a non-conservative parameter for assessing susceptibility of a metal to pitting corrosion while the protection potential is a much more conservative parameter. While conducting CPP tests, attempts to eliminate preferred initiation sites on a metal surface will generally increase the variability in the CPP test results. For some alloy-environment systems, it may be necessary to produce consistent preferred sites for pit initiation, such as the specimen holder-specimen interface, in order to maximize reproducibility of the CPP test results. Hysteresis in CPP curves is not always associated with pitting or crevice corrosion and thus, all specimens should be examined following testing to establish the morphology of attack. Cathodic loops can complicate interpretation of CPP curves, and in some cases, they can be minimized or eliminated by removing oxidants from the test solution.
Over the past twenty-five years, electrochemical test techniques have been used to evaluate pitting tendencies for a variety of alloy-environment systems. These techniques have been used to rank alloys with respect to their pitting resistance in a given environment and to establish the effects of environmental variables on pitting of a given alloy. One of the most popular electrochemical test techniques is the cyclic potentiodynamic polarization (CPP) test technique described in ASTM Standard Practice G 61.? This technique was developed for stainless steels and nickel-base alloys but has been increasingly used for other alloy systems. Interpretation of CPP curves is not always as simple as conventional wisdom would indicate. Care is required when using CPP curves to make engineering decisions on alloy selection and process evaluation. This paper presents and interprets CPP curves for stainless steels and some of these other alloys.
THE CYCLIC POTENTIODYNAMIC POLARIZATION TECHNIQUE
ASTM Standard Practice G 61 provides details on conducting CPP tests but is very limited with respect to interpretation.? The CPP test is a method for scanning, at a constant rate, the electrode potential of a specimen while monitoring the resulting current flow from the specimen surface. Typically, the potential is scanned in the noble (positive) direction until a specified anodic current or potential is reached. The direction of the scan is then reversed and the potential is scanned in the active (negative) direction until the original starting potential is reached, or the direction of the current changes sign, indicating a cathodic current. Figure 1 is a schematic of a typical curve for a stainless steel that has undergone pitting during the test.
In the CPP test technique, two pitting-related parameters are determined: (1) the pitting potential (E,,), which is sometimes referred to as the breakdown potential, and (2) the protection potential(Er,,,), which is sometimes referred to as the repassivation potential. The value of Epi, denotes the potential at which the current rapidly increases