Electrochemical tests 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. 

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. 

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-... 

Standard corrosion testing such as weight loss. Unear polarization resistance (LPR) and cyclic potentiodynamic polarization (CPP) were used to assess each compound s inhibition efficiency for steel in 0.01 M NaCl solution. Cerium and lanthanum combined with cinnamate and substituted cinnamates produced the most effective corrosion inhibition for steel. As an example, lanthanum 4-nitrocirmamate achieved 92% inhibition for mild steel itmnersed for 7 days in sodium chloride solution. Cerium 4-methoxycinnamate was also effective as a corrosion inhibitor. Electrochemical measurements showed the rare earth cinnamate compounds to be mixed inhibitors, with an initial suppression of anodic processes, followed by a decrease in cathodic processes after longer exposure times. 

Nickel-base alloys respond well to most electrochemical test techniques and show active-passive behavior in many environments. Due to their rapid repassivation, however, the results obtained with potentiod3mamic techniques can sometimes be affected by scan rate and immersion time prior to starting the test [5,6], Electrochemical techniques are useful for investigating localized corrosion resistance, ASTM G 61, Test Method for Conducting Cyclic Potentio-dynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys, and general corrosion resistance, ASTM G 59, Practice for Conducting Potentiodynamic Polarization Resistance Measurements of nickel alloys. Electrochemical impedance measurement techniques have not been extensively applied to nickel alloys. 

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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