Corrosion -testing methods polarization

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. 

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... 

Types of polarization test methods. Polarization methods such as poten-tiodynamic polarization, potentiostaircase, and cyclic voltammetry are often used for laboratory corrosion testing. These techniques can provide significant useful information regarding the corrosion mechanisms, corrosion rate, and susceptibility to corrosion of specific materials in designated environments. Although these methods are well established, the results they provide are not always clear and occasionally can be misleading.2 ... 

Several methods may be used in polarization of specimens for corrosion testing. Potentiodynamic polarization is a technique in which the potential of the electrode is varied at a selected rate by application of a current through the electrolyte. It is probably the most commonly... 

Measurement of mass lost is the conventional method for determining the corrosion rate. The mass loss of an Fe specimen immersed in a corrosion test potential is determined by weighing, (b) Convert the mass loss rate 2.34 x 102 g tf rrT2 into icon using the atomic weight 55.847. (c) What is the difference between the results of the mass loss measurement and the polarization resistance measurement (Numata)... 

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 test method ASTM F7464 covers the determination of the resistance to either pitting or crevice corrosion of passive metals and alloys from which surgical implants are produced. The resistance of surgical implants to localized corrosion is carried out in dilute sodium chloride solution under specific conditions of potentiodynamic test method. Typical transient decay curves under potentiostatic polarization should monitor susceptibility to localized corrosion. Alloys are ranked in terms of the critical potential for pitting, the higher (more noble) this potential, the more resistant is to passive film breakdown and to localized corrosion. (Sprowls)14... 

ASTM G 61-86, Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys, ASTM International, West Conshohocken, PA, 2003. 

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. 

This test was designed for electrodeposits of nickel and chromium on less noble metals, such as zinc or steel [738-740]. Special solutions are used, and the metal is polarized to + 0.3 V versus the standard calomel electrode. The metal is taken through cycles of 1 min anodicaUy polarized and 2 min impolarized. An indicator solution is then used to detect the presence of pits that p>enetrate to the substrate. Each exposure cycle attempts to simulate one year of exposure imder atmospheric-corrosion conditions. The former standard ASTM B 627, Test Method of Electrolytic Corrosion Testing (EC Test), described the method but has been withdrawn. 

ASTM G 96, Practice for On-Line Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods)—This standard outlines procedures for online corrosion monitoring in operating systems. The test methods described in this standard are used to determine the cumulative metal loss (electrical resistance method) or instantaneous corrosion rates (electrochemical method). Reference 15 provides a summary of electrical resistance and polarization resistance theory. 

ASTM G 61, Test Method for Conducting Cyclic Potentio-dynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys— This standard presents a procedure for performing cyclic poten-tiodynamic polarization testing to determine the relative susceptibility of iron-, nickel-, and cobalt-based alloys to localized corrosion (pitting or crevice corrosion). It illustrates and presents test apparatus, reagents, and materials, test procedures, and interpretation of results. 

In general, the most widely used field technologies for inspection are ultrasonic thickness measurement, while for online methods they are corrosion test specimens, electrical resistance, and linear polarization probes. Both of the inspection methods and the first two online methods measure metal loss. The last method measures corrosion rate, but only in a sufficiendy conductive process environment, normally water. 

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. 

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