Corrosion rate measurements reliability

Abstract Quality control of corrosion test results implies the validation of the corrosion test method and estimation of the uncertainty of corrosion rate measurement. The corrosion test in an artificial atmosphere of the salt spray mist needs evaluation of corrosivity of the test cabinet by reference specimens. Such calibration of corrosion environment raises very strict requirements for the method description and details of all procedures and used specimens. Reliable corrosion measurements by spray tests require validation of the experimental device together with the experimental procedure and determination of corrosivity uncertainty of the test cabinet environment. 

It turns out that the reliability of the corrosion rate measurements depends on the probe and instrument performance, on a good understanding of the phenomenon (as far as materials and chemistry are concerned) and on a good knowledge of the plant operating conditions. 

Concepts of CP kinetic criteria were postulated from the beginning of the 1970s, but at that time no appropriate measurement method was available to put them into practice. Reliable corrosion rate measurements of metals in CP conditions were ensured by only two physical methods gravimetric and resistometric measurements. Both methods allow the determination of increasing corrosion losses over longer time intervals (due to relatively low sensitivity) however, they do not allow the determination of the instantaneous corrosion rate. Such information would be required by users of CP installations, allowing the regulation of protective parameters in a feedback system. 

Measurement of corrosion rate is essential for the purpose of material selection. The compatibility of a metal to its environment is a prime requirement for its reliable performance. Corrosion rate measurement may become necessary for the evaluation and selection of materials for a specific environment or a given definite application, or for the evaluation of new or old metals or alloys to determine the environments in which they are suitable. Often the corrosive environment is treated to make it less aggressive, and corrosion rate measurement of a specific material in the untreated and treated environments will reflect the efficacy of the treatment. Corrosion rate measurement is also essential in the study of the mechanisms of corrosion. 

The combination of immersion, hydrogen collection and weight loss measurement is an easy corrosion evaluation method, particularly for magnesium alloys. The method, first established and used by Song et al. [5] to estimate and monitor the corrosion rate of magnesium in a NaCl solution, has been widely adopted as a common corrosion rate measurement technique for magnesium alloys in various aqueous solutions. The reliability of the method has been theoretically and experimentally demonstrated [24,25] and the details will not be repeated here. 

Electrochemical tests This group includes the various electrochemical tests that have been proposed and used over the last fifty or so years. These tests include a number of techniques ranging from the measurement of potential-time curves, electrical resistance and capacitance to the more complex a.c. impedance methods. The various methods have been reviewed by Walter . As the complexity of the technique increases, i.e. in the above order, the data that are produced will provide more types of information for the metal-paint system. Thus, the impedance techniques can provide information on the water uptake, barrier action, damaged area and delamination of the coating as well as the corrosion rate and corroded area of the metal. However, it must be emphasised that the more comprehensive the technique the greater the difficulties that will arise in interpretation and in reproducibility. In fact, there is a school of thought that holds that d.c. methods are as reliable as a.c. methods. 

Measurements of corrosion rates and other parameters connected with corrosion processes are important, first as indicators of the corrosion resistance of metallic materials and second because such measurements are based on general and fundamental physical, chemical, and electrochemical relations. Hence improvements and innovations in methods applied in corrosion research are likely to benefit basic disciplines as well. A method for corrosion measurements can only provide reliable data if the background of the method is fully understood. Failure of a method to give correct data indicates a need to revise assumptions regarding the basis of the method, which sometimes leads to the discovery of as-yet unnoticed phenomena. 

It is likely that combined CMT and EC measurements can contribute to further information about the basic details of several other corrosion processes and provide more reliable data for corrosion rates than EC measurements alone. Concerning the possible use of CMT measurements as a routine test of technical products, some of the limitations (it can t be used with iron and steel in aerated solutions or with other metals in bicarbonate solutions) are rather serious. However, there are still many applications where the fast and quantitative results obtained by CMT measurements would seem advantageous. 

This is an electronic method used to measure corrosion rate in mils per year. The corrosion probe can be inserted through a packing gland. It is read periodically with a portable instrument that measures the change in electrical conductivity of the probe. It is simple, but perhaps a little less reliable than the coupon. 

This document summarizes the efforts of the Source Term Working Group to complete the tasked objectives under the lASAP. It presents a detailed discussion of the fission product, actinide, and activation product inventories at each Kara Sea disposal site and a detailed description, with assumptions, of the models used to predict potential release of the radionuclides into the Kara Sea. Results of the release scenario models, reliability of the model input parameters, and an analysis of the sensitivity of the results to changes in the protective barrier lifetimes and SNF corrosion rates are then presented. The potential for recriticality of the SNF bearing cores and considerations for potential remedial measures are next addressed. Finally, conclusions are drawn with respect to the radionuclide releases at each Kara Sea disposal site from the SNF and activated components. 

Structure integrity can be achieved using weU established cathodic protection (CP) criteria, but potential measurements must be carried out at discrete time interval to assure that the stmcture is maintained polarized at the design potential. In general, CP criteria are based on external potential and current, but the former is the parameter that requires close control. Absolute CP is based on the theory of electrochemistry since is predicts zero corrosion rate when the forward and reverse reaction rates are exactly the same. This implies that reliable CP systems are required to be kept polarized at the equilibrium potential so that icorr = 0. 

The gas-phase oxidation of carbon blacks by oxygen and/or water is strongly catalyzed by the presence of catalytically active metals, such as platinum (Rewick et al. 1974, Stevens and Dahn 2005), whereby several weight percent of platinum on carbon can increase the gas-phase oxidation rate by orders of magnitude. This, however, is not the case for the electrochemical oxidation of carbon blacks, where at potentials of 0.8 V and higher (vs. RHE) the carbon corrosion rate is within a factor of 2 between that for noncatalyzed and platinum-catalyzed carbon blacks (Roen et al. 2004, Passalacqua et al. 1992, Kinoshita 1988). Therefore, gas-phase oxidation tests to screen potential carbon-black supports is not a reliable method for predicting their stability in the electrochemical environment, so it is essential to measure the carbon corrosion rates directly in an electrochemical cell. 

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