Pitting cyclic potentiodynamic polarization

The most common electrochemical test for localized corrosion susceptibility is cyclic potentiodynamic polarization. As was discussed briefly in the section on the electrochemical phenomenology of localized corrosion, this test involves polarizing the material from its open circuit potential (or slightly below) anodically until a predetermined current density (known as the vertex current density) is achieved, at which point the potential is scanned back until the current reverses polarity, as shown in Fig. 42. The curve is generally analyzed in terms of the breakdown (Ebi) and repassivation potentials (Elf). Very often, metastable pits are apparent by transient bursts of anodic current. The peaks in current shown in Fig. 42 for a potentiodynamic scan are due to the same processes as those shown in Fig. 25 for a potentiostatic hold. 

Cyclic potentiodynamic polarization method. Electrochemical studies of pitting corrosion usually indicate that pitting occurs only within or above a critical potential or potential range. Therefore the susceptibility of passive metals to pitting corrosion is often investigated by electrochemical methods such as potentiodynamic or potentiostatic... 

Potentiostatic methods. Once the breakdown potential is determined by cyclic potentiodynamic polarization methods, polarizing individual samples at potentials above and below this value will indicate the validity of the chosen scan rate and give some kinetic data on the initiation and propagation of pits at different levels. Another possibility is to initiate pits above the pitting or breakdown potential and then shift to lower values above or below the protection potential. It is assumed that at imposed values below the protection potential, one should observe current decrease until complete repassivation. 

A correlation has been between pitting potential and pitting in the field the tendency to form pits naturally at open circuit increases as the experimentally determined pitting potential decreases [51]. The difference between E-p and E, which is related to the extent of hysteresis in a cyclic potentiodynamic polarization curve, has also been considered to be a measure of the susceptibility to localized corrosion [3,52]. Issues associated with this interpretation have been reviewed [53]. 

Cyclic potentiodynamic polarization used in determining pitting potential consists of scanning the potential to more anodic and protection potentials during the forward and return scans and compare the behavior at different potentials under identical conditions. The polarization curve of an alloy (with or without coating showing active-passive behavior may be obtained in a chosen medium as a function of chloride concentration). E, or Ep represent pitting potential or breakdown potential,... 

FIG. 8—Schematic representations of experimental data for (a) a cyclic potentiodynamic polarization curve (b) galvanostatic potential-time curve for a material (c) potentiostatic current-time curve for a previously passivated surface which pits ad < Egg < Ej-, and (d) potentiostatic current-time curve for active surface. The protection potential is found as... 

As stated above, Ejj and Eprot often dejiend strongly on the method by which they are determined and, therefore, do not uniquely define intrinsic material properties. The Eprof values determined from the scanning method can be complicated by scan rate, pit size or depth, vertex potential/current, polarization curve shape, and specimen geometry [86,87]. Investigators have found more consistent Eprof values after a critical charge has passed, while others report a single critical potential [85]. Often this potential is difficult to choose from E-I data and has been taken at various points on the reverse scan of a cyclic potentiodynamic polarization curve [89]. 

Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibilily of Iron-, Nickel-, or Cobalt-Based Alloys, is quite effective [8,9,22]. A sample curve showing the effects of a corrosion inhibitor is in Fig. 1. An increase in the pitting (Ep), and breakdown (E),) potentials is indicative of a good anodic inhibitor. 

ASTM G 61, Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, and Cobalt-Based Alloys, outlines an electrochemical procedure to screen and rank the pitting susceptibility of various metals. However, the electrochemical measurement does not necessarily correlate with pitting rates actually encountered in service. 

Pitting/crevice corrosion Cyclic potentiodynamic polarization Electrochemical... 

The basic electrochemical method for evaluating the susceptibihty of metals to pitting and crevice corrosion is covered by ASTM G 61 [30]. An anodic polarization curve is measured by a cyclic potentiodynamic polarization method using a specific scanning rate, from which breakdown potential and protection potential can be determined. See Fig. 11. 

Electrochemical tests such as the cyclic potentiodynamic polarization test (ASTM G 61) or the constant potential test in which the temperature is increased at a constant rate until pitting corrosion occurs (ASTM G 150) are also good tools for evaluating the susceptibility of alloys to localized corro-... 

Potentiodynamic polarization measurements are quite appropriate for determination of the pitting susceptibility of aluminum coatings, and/or the corrosion current density/ corrosion rate of coated steel products in general. ASTM G 102, Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements, describes the calculation of corrosion rates and other information from electrochemical measurements. Another example of the use of DC electrochemical methods to examine the corrosion performance of coated sheet materials is a study by D. A. Jones et al. [48]. The study used polarization resistance measurements to examine the mechanism of steel and coated sheet degradation under conditions of alternate immersion. Jones compared the polarization resistance of samples of low-carbon steel, unpainted galvanized, aluminum-coated, and Zn-Ni alloy coated steel during continuous immersion and alternate immersion. Alternate immersion cyclic exposure produced a thick oxide that led to significant underfilm attack. Jones found that phosphate pretreatment tends to increase the resistance of these materials to underfilm attack. This study is an excellent example of the way electrochemical measurements can be used as a complement to other techniques to elucidate mechanistic information. 

Cyclic anodic polarization procedures based upon ASTM G5 (Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements) have been used to evaluate the localized corrosion resistance of stainless steel alloys in paper machine white waters. The difference between the open circuit or naturally occurring corrosion potential and the pitting breakdown potential of various materials has been reported by many investigators [lO-Id]. Bowers [14] called the difference between the breakdown potential and the potential of the cathodic/anodic current reversal the margin of safety. He also noted the effect of the sulfate to chloride concentration ratio on localized corrosion of Types 304 and 316L stainless steels, These results permitted alternative materials of construction to be ranked and their limits of resistance to be defined. 

An important variant of potentiodynamic polarization is the cyclic polarization test. This test is often used to evaluate pitting susceptibility. The potential is swept in a single cycle (or slightly less than one cycle), and the size of the hysteresis is examined along with the differences between the values of the starting open-circuit corrosion potential and the return passivation potential. The existence of the hysteresis is usually indicative of pitting, while the size of the loop is often related to the amount of pitting. 

ASTM G 3 (Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing) [74] and Refs 49, 55, and 73 show the schematics for the apparatus for corrosion measurements and schematic drawings for cathodic and anodic polarization diagrams and polarization plots. ASTM G 5 (Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements) [74] and ASTM F 4 [55] test methods and practices describe the setup and procedures for making potentiostatic and potentiodynamic anodic polarization measurements. A cyclic polarization curve that contains both the cathodic and anodic portions provides data that can be used to describe corrosion behavior in terms of passivity, breakdown, corrosion rate, and susceptibility to pitting. 

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