Electrochemical tests cyclic polarization

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. 

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. 

Factors affecting the electrochemical behavior and See of Alloy 690 in a chloride environment were investigated by Chen et al. [164] using a cyclic polarization method and a SSRT test. 

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. 

Electrochemical tests provide a means to understand the corrosion process, simulate service conditions, or accelerate evaluation of a material [27]. ASTM G 3, Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing ASTM G 5, Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Polarization Measurements and ASTM G 61, Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys provide background in some of these techniques. 

Once power production has been maximized and the system is at steady state, we generate a polarization curve with sequential load changes as described [17,18]. Additional electrochemical tests such as linear scan voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy can be performed to further characterize each electrode. Samples can also be collected for analysis of the microbial community and microscopy. 

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-... 

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. 

Chandrasekhar et al. [934] used several independent methods to monitor corrosion, including Tafel Scan, Cyclic Polarization and Polarization Resistance among electrochemical methods Immersion methods Salt Spray Fog techniques vide ASTM B-117-94 and MIL-C-5541E scratch tests and adhesion tests. They found that their coatings had excellent adhesion, passing the ASTM tests mentioned above, and excellent durability in salty atmospheres, as evidenced by the samples differing very little in appearance before and after the Salt Spray Fog tests. 

Apparatus for electrochemical measurements during corrosion fatigue. CF tests can be done using an apparatus designed by the Continental Oil Company, as shown in Figure 6.52.110,111 The polarization potential and current can be controlled for the four samples tests at the same time. The apparatus consists of a Monel tank in which four specimens are subjected to cyclic bending. The preliminary step in the experiment is to determine the displacement caused by the desired applied load. The exact stresses are then determined with the use of strain gages. 

Testing procedures for cracked concrete are essentially the same as used for other laboratory specimens exposed to chloride solutions continuously or cyclically. Methods that can be used include polarization resistance, electrochemical impedance, and macrocell corrosion which were discussed above. Procedures for conducting these tests are described in Refs 27 and 28. 

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. 

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