Pitting cyclic polarization

Figure 24 Schematic Evans diagram and polarization curve illustrating the origin of the negative hysteresis observed upon cyclic polarization for materials that do not pit. Line a represents the (unchanging) cathodic Evans line. Line b represents the anodic Evans line during the anodically directed polarization, while line c represents the anodic Evans line for the material after its passive film has thickened because of the anodic polarization. The higher corrosion potential observed for the return scan (E (back)) is due to the slowing of the anodic dissolution kinetics.

From polarization curves of the type shown in case 3, three important parameters can be determined ECOSI, Ebth and In the literature there exists a nearly infinite number of variations of nomenclature, many of which are shown in Table 2. The interpretation of cyclic polarization curves has been and continues to be a subject of great controversy. The classic interpretation of case 3 would be that the potential of a material must exceed EM for new pits (or localized corrosion sites) to nucleate, but that at potentials between EM and En existing pits can propagate. At potentials below En all localized corrosion sites repassivate. Thus, from a design or material selection perspective, a material will perform well if its Econ is kept below This criterion can be met by environment... 

Controversy concerning the interpretation of cyclic polarization curves has raged for many years. Of particular interest is which (if either) of the two potentials can be used for material selection and mitigation strategy decisions. The classic interpretation is that a material s potential must exceed Ehl[ in order to initiate pits, but if flaws were introduced into the surface in any way, they could propagate at all potentials above Ew. Thus Eq, could be used in design as a protection potential. 

Figure 42 Cyclic polarization curve for Type 302 stainless steel in 1,000 ppm NaCl. Note the definition of the breakdown and repassivation potentials, the vertex current density, and the appearance of metastable pits.

Figure 43 Cyclic polarization behavior of 430 stainless steel in 1 M NaCl, demonstrating the striking effect of pit propagation on 7ipr,t. (From Ref. 43.)...

These tests focused on the determination of a materials resistance to localized (pitting) corrosion. To accomplish this goal, three types of electrochemical experiments were conducted (cyclic polarization, electrochemical scratch, and potenti-ostatic holds) to measure several key parameters associated with pitting corrosion. These parameters were the breakdown potential, EM, the repassivation potential, Etp, and the passive current density, tpass. 

Fig. 7.15 Schematic cyclic polarization curve fora metal showing susceptibility to pitting. Pitting is initiated at Eb jtand propagation stops...

For alloys showing high susceptibility to crevice corrosion, measurements of the pitting potentials are of limited value since failure in service by crevice corrosion would predominate. Polarization measurements can be useful in showing relative susceptibility of alloys to crevice corrosion. Figure 7.46 shows results from cyclic polarization measurements on specimens of three alloys containing O-rings to produce crevices (Ref 66). The environment was aerated water with 3.5... 

Fig. 7.1 Typical cyclic polarization plot for stainless steel that shows the corrosion potential, c, critical pitting potential, Epn, protection potential, Eprot, and metastable pitting region.

Another more informative solution test is to perform cyclic polarization in the saturated calcium hydroxide solutions mentioned above. This quickly demonstrates whether the corrosion mechanism is one of pitting and determines the primary mechanisms of corrosion inhibitors [7-9]. A modified version of ASTM G 61, Test Method for Conducting... 

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. 

The principles of cyclic polarization are similar to that of cyclic voltammetry (CV). The cyclic polarization technique is used to qualitatively measure pitting tendencies of a metallic sample in a corrosive solution environment. ASTM Standard G6I (35] describes the experimental procedures typically used in this study. The beginning potential scan is toward the anode from a potential in the vicinity of Ecorr- When the measured cur-... 

The cychc polarization method is a standardized traditional electrochemical method to determine relative loealized eorrosion susceptibility. This method involves anodic polarization of a specimen until localized corrosion initiates as indicated by alaige increase in the apphed current. An indicationofthe susceptibility to initiation of pitting corrosion in this test method is given by the potential at which the anodie current increases rapidly, that is the breakdown potential. The nobler this potential, obtained at a fixed sean rate in this test, the less susceptible is the alloy to the initiation of loealized eorrosioa Conventional understanding is that the breakdown potential is the potential above which pits are initiated, whereas the repassivation potential obtained at reverse sean is the potential below which pits repassivate. In cyeUc polarization measurements, scatters in the breakdown potential and its dependence on scan rate are often experienced. It should also be noted that results from a cyclic polarization test are not intended to correlate in a quantitative manner with the rate of localized corrosion. 

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. 

Comparative electrochemical Cyclic Polarization ("pitting") scans in aerated 3.5% NaCl, for aluminum alloy samples. 

Cyclic polarization method is a highly useful method for determining the susceptibility of a metal or alloy to pitting [17]. From Figures 4(a) (f), it can be seen that at the vertex potential of 2 V when the scan reverses its direction, the reverse scan starts left of the forward scan curve, that is, towards the low current density region. This type of the cyclic polarization curve is known to resist localized corrosion [18]. It is also observed that the reverse scan curves meet the forward scan curve along the passive range. The reverse scan curves show lower current densities as can be seen in Table 2 except for Til and Ti2. The potentials... 

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. 

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