Electrochemical tests polarization methods

Evidence of localized corrosion can be obtained from polarization methods such as potentiodynamic polarization, EIS, and electrochemical noise measurements, which are particularly well suited to providing data on localized corrosion. When evidence of localized attack is obtained, the engineer needs to perform a careful analysis of the conditions that may lead to such attack. Correlation with process conditions can provide additional data about the susceptibility of the equipment to locaHzed attack and can potentially help prevent failures due to pitting or crevice corrosion. Since pitting may have a delayed initiation phase, careful consideration of the cause of the localized attack is critical. Laboratory testing and involvement of an... 

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. 

Immersion tests provide no information about reaction mechanisms and often they require relatively long exposure times. Electrochemical tests do not have these drawbacks and they are therefore widely used in practice. In the following electrochemical polarization methods are presented that provide information on the rate of uniform corrosion under conditions where the rate is controlled by charge-transfer. Other electrochemical test methods will be presented in subsequent chapters. 

Standard test procedures are defined within ASTM standards ASTM G 59, Practice for Conducting Potentiodynamic Polarization Resistance Measurements G 5, "Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements G 106, Practice for Verification of Algorithm and Equipment for Electrochemical Impedance Measurements and G 102, Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements. Each of these methods describes a standard procedure or practice for the test method. A complete discussion of the technologies is beyond the scope of the current text. For the current text, the focus is on the application of the most simple and most widely used of these techniques, the polarization resistance measurement, ASTM G 59. The parameters discussed are, however, applicable concerns for all electrochemical tests. 

The most common test method is the coupon immersion test, usually conducted under static or near-static conditions. Other methods, used to a lesser degree, include rotating electrodes, of which the best arrangement for water is probably the rotating cylinder, and small recirculating systems. Electrochemical tests and polarization studies are used primarily to elucidate the corrosion mechanism. Surface analysis is also used, primarily for mechanistic studies. 

Most electrochemical testing conducted to date has used various DC approaches. The most common methods involve linear polarization (to determine the polarization resistance for calculation of corrosion current via the Stem-Geary equation) [44] and potentiodynamic polarization (to determine breakdown and repassivation potentials). Other tests are also conducted, however. For example, long-term open circuit potential versus time measurements, potentiostatic chronoamperometry, and galvanostatic measurements are occasionally conducted for specialized purposes. 

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 tmodic polarization tests (ASTM G 5 and G 61 ) are useful corrosion test methods for alloy and process development work on P/M materials. Reference 23 describes the application of potentiostatic anodic polarization to steam-treated P/M carbon steel in neutral salt and acidic environments. References 13 and 19 describe the application of potentiodynamic polarization to sintered austenitic stainless steels. These test methods are very effective in revealing metallurgical weaknesses of sintered stainless steels. Sintered stainless steels, due to their large surface areas, exhibit large corrosion currents, compared to the wrought stainless steels, and frequently the current rises with increcising potential. Furthermore, sintered stainless steels do not always exhibit a pronoimced transition ftom... 

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. 

Aqueous corrosion is electrochemical in nature. It is therefore possible to measure corrosion rate by employing electrochemical techniques. Two methods based on electrochemical polarization are available The Tafel extrapolation and linear polarization. Electrochemical methods permit rapid and precise corrosion-rate measurement and may be used to measure corrosion rate in systems that cannot be visually inspected or subject to weight-loss tests. Measurement of the corrosion current while the corrosion potential is varied is possible with the apparatus shown in Figure 1.4. 

The corrosion resistance of various Al- and Al/Zn-coated AZ91D Mg alloys has been evaluated by salt spray and electrochemical methods. In the following, the results from salt spray test, polarization curve and electrochemical impedance measurements manifesting the benehcial effects of Al and/or Al-Zn coating on AZ91D Mg alloy are demonstrated. 

Potentiostatic and Potentiodynamic Anodic Polarization Measurements and ASTM G106 - 89 Standard Practice for Verification of Algorithm and Equipment for Electrochemical Impedance Measurements. Although most ASTM electrochemical testing techniques are developed for stainless steels, the test methods and procedures can be adapted for noble metals used in implantable medieal devices. 

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. 

In ASTM G 3, Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing, there are several examples of polarization curves. Figure 7.15 illustrates the ideal polarization behavior one could obtain, for example, using the linear polarization method briefly described below. Figures 7.16 and 7.17 show hypothetical curves for, respectively, active and active-passive behavior, while Fig. 7.18 was plotted from actual polarization data obtained with a S43000 steel specimen immersed in a 0.05 M H2SO4 solution. 

A wide variety of steady and transient, in situ and ex situ, AC and DC methods have been developed to smdy electrochemical system behavior. To fully understand and apply these techniques requires a greater depth than the brief summary in this chapter can provide. For a more detailed explanation, the reader is referred to various texts devoted to electrochemical testing techniques, such as by Bard and Faulkner [1]. This chapter is designed to serve as an introduction to the potential techniques available to obtain a greater understanding and parameterization of the various polarization losses. 

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