Electrochemical polarization corrosion 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. 

Figure 5.25 Comparison of the results from stress corrosion tests with those from polarization curves at fast and slow potential sweep rates for different carbonate-bicarbonate solutions, indicating the extent to which the experimentally observed cracking range can be predicted from electrochemical measurements.8...

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. 

The simplest type of HT/HP corrosion test is that which is conducted in a sealed, static, pressurized test vessel as shown schematically in Fig. 1. The test vessel contains a solution uld a vapor space above the solution. The liquid and gas ph2ises will be determined by the amount and vajKrr pressmres of the constituents in the test vessel as well as by the test temperature. Corrosion coupons can be placed in the aqueous phase, vapor space, or at phase interfaces depending on the specific interest involved. Additionally, it is also possible to conduct electrochemical tests (i.e., linear polarization resistance (LPR), polarization curve generation... 

ASTM G 3 Standard Practice for Conventions AppUcable to Electrochemical Measurements in Corrosion Testing Information on reporting and displaying polarization measuring techniques, including both DC and AC methods. 

Corrosion tests that are performed on lollipops are corrosion potential readings as described in ASTM C 876 (Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete), polarization resistance as described in ASTM G 59, using IR correction [3,9], and EIS [9]. Specimens are broken open to visually examine the bars for confirmation of the electrochemical results, and chloride analyses are performed. The chloride analysis correlates the chloride content to the corrosion activity. 

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 tests are rapid techniques to determine mechanisms, determine the effect of various parameters on corrosion rate, and screen out a large number of materials [43]. They usually involve measurement of corrosion potentials, corrosion currents, polarization curves, and electrochemical impedance. They are used to evaluate metals and alloys and the behavior of metallic, inorganic, and oiganic coatings. The simplest test involves the measurement of the corrosion potential and its use in conjunction with other measurements. A zero resistance ammeter (ZRA) is commonly used to measure corrosion currents between dissimilar metals and alloys. Controlled potentitd tests and anodic and cathodic polarization curves using potentiostats are the most commonly used electrochemical tests. These are powerful tools for investigating the effect of various parameters on corrosion behavior. These incorporate the use of cycUc polarization and polarization resistance for localized corrosion and corrosion rate measurements. Table 4 lists electrochemical tests that can be used for corrosion tests in the automobile industry. 

Corrosion tests for hydraulic brake line fluid, as well as for motor oil, grease, and lubricants, are listed in Table 12. In addition, the effects of various fuels on materials have been of importance due to the effect of small amounts of water, which can become trapped in eureas such as the fuel tank [57]. Various additives to fuels can prevent corrosion effects and include inhibitor packages and small amounts of alcohol and aromatics. Recently, corrosion testing of ethanol fuels has been of interest because small amounts of water as well as other contaminants (chlorides) cause severe corrosion to occur [55]. Tests for fuels include electrochemical polarization and immersion tests (Table 12). 

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. 

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