Electrochemical tests potentiodynamic methods

Pitting and crevice corrosion are associated with the breakdown of passivity [SO]. Electrochemical tests for evaluating the susceptibility of a material to pitting and to crevice corrosion include potentiodynamic, potentiostatic, scratch potentiostatic, potentiostaircase, tribo-eUipsometric methods, pit-propagation rate curves, galvanostatic, and electrochemical noise measurements [80-S2]. 

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. 

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 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. 

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. 

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... 

A detailed and well-referenced account of electrochemical methods of testing has been written by Dean, France and Ketcham in a section of the book by Ailor. ASTM G5 1987 outlines standard methods for making potentiostatic and potentiodynamic anodic polarisation measurements and ASTM G3 1974 (R1981) gives conventions applicable to electrochemical measurements in corrosion testing. 

The potential-independent CPT can be determined by two different electrochemical methods a potentiostatic test method using a sufficiently high potential, and a potentiodynamic test method. The potential-independent CPTs are well-defined, experimental results having a reproducibility of approximately l°Cby potentiodynamic testing and approximately 2°C by potentiostatic testing. 

It is preferable to carry out laboratory corrosion tests and to validate the data with service tests for the selection of materials. It is needless to note that the chosen test method be reliable and cost effective. Some of the test methods in use in industry are service tests, field tests, laboratory tests, and rapid electrochemical methods such as potentiodynamic polarization, linear polarization, electrochemical impedance and electrochemical noise. 

The methods of measuring corrosion rates in the course of testing corrosion inhibitors are conventional weight loss, electrochemical techniques such as linear polarization resistance, potentiodynamic polarization, AC impedance, and electrochemical potential or current noise. 

The veirious types of measurement technologies for assessment of corrosion may be summarized as shown in Tables 2 to 5. These techniques cover both laboratory Jind field use. However, many of the direct methods, particularly the electrochemical methods of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) are generally more suited to laboratoiy evaluation. In the laboratory, test conditions are clean and more controlled. Consequently, more sophisticated measurement electrode systems can be used that take advantage of their more sophisticated measurements technologies. In the field, practicalities of changing process conditions, high flow rates, debris, electrical noise, and electrical safety limit their use. 

These standard methods and practices provide the necessary information for electrochemical potentiostatic and potentiodynamic anodic measurements, calculation of corrosion rate from electrochemical measurements, and conducting potentiodynamic polarization resistance measurements. Recently, Electrochemical Impedance Spectroscopy (EIS) htts been introduced for corrosion measurements of steel structures corroding in soils. These tests can be... 

Corrosion test methods can be divided into electrochemical and non-electrochemical methods. Among the electrochemical techniques that have been used successfully for corrosion prediction are potentiodynamic polarization scans, electrochemical impedance, corrosion current monitoring, controlled potential tests for cathodic and anodic protection, and the rotating cylinder electrode for studies of velocity effects [3i,32]. Though not literally a test, potential-pH (Pourbaix) diagrams have been used as road maps to help understand the results of other tests. 

Immersion testing wiU generate weight loss data, or corrosion current measurements can be obtained from stan-deird electrochemical polarization tests (see ASTM G 5, Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements see also Ref 27). Corrosion rates in millimeters per year (mpy) for titanium alloys can be calculated from sample weight loss data as follows ... 

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 corrosion characteristics of iron were determined by potentiodynamic and impedance spectroscopy techniques. Tests were applied to chemically pure iron Fe made by electrocrystalization method and to carbon steel S235JR with the chemical composition shown in Table 3. 

Electrochemical corrosion characteristics of nickel were carried out by potentiodynamic polarization and impedance spectroscopy methods. Corrosion tests of nickel produced by electrocrystallization were ap>plied to its micrometric (Nim) and nanometric (Ni ) crystalline structures and for NiP amorphous alloy of nickel with phosphorus at content of 10.7% by weight (Eftekhari, 2008), (Kowalewska Trzaska, 2006). 

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