Polarization-Resistance Technique

Polarization resistance (Rp) techniques can be used to continuously monitor the instantaneous corrosion rate of a metal. Mansfeld provided a thorough review of the use of the polarization resistance technique for the measurement of corrosion currents. R is defined as ... 

A simplification of the polarization resistance technique is the linear polarization technique in which it is assumed that the relationship between E and i is linear in a narrow range around E . Usually only two points ( , 0 are measured and B is assumed to have a constant value of about 20 mV. This approach is used in field tests and forms the basis of commercial corrosion rate monitors. Rp can also be determined as the dc limit of the electrochemical impedance. Mansfeld et al. used the linear polarization technique to determine Rp for mild steel sensors embedded in concrete exposed to a sewer environment for about 9 months. One sensor was periodically flushed with sewage in an attempt to remove the sulfuric acid produced by sulfur-oxidizing bacteria within a biofilm another sensor was used as a control. A data logging system collected Rp at 10-min intervals simultaneously for the two corrosion sensors and two pH electrodes placed at the concrete surface. Figure 2 shows the cumulative corrosion loss (Z INT) obtained by integration of the MRp time curves as ... 

Fig. 10. Time dependence of corrosion rate of Co8P and Co films, electrolessly plated and sputter-deposited onto NiP substrates, in a droplet of DI water. The results were obtained by a repeated application of the polarization resistance technique with the potential scanned at 1 mV/sec in a potential range 15 mV above and below the corrosion potential [125]. (Reprinted by permission of The Electrochemical Society). 

F. Mansfeld, The Polarization Resistance Technique for Measuring Corrosion Currents, Corros. Sci. Technol., Vol 6, Plenum Press, 1976,p 163... 

The most serious problem related to the practical application of the polarization resistance technique concerns the correct use of commercial equipments, which can be conventional-type or computerized. 

F. Mansfeld, The polarization resistance technique for measuring corrosion currents in Advances in Corrosion Science and Technology (Eds. M. G. Fontana, R. W. Staehle), Plenum Press, New York, 1976, p. 163. Vol. 6. 

The constant, fC, in Eq. (5.26) varies from 32 to 52 for a majority of the metal-solution interface systems. The polarization resistance can be determined with minimum system perturbation using the linear polarization resistance technique. 

With the advent of advanced electronics and computerization, electrochemical techniques have evolved rapidly. The most common technologies today are the polarization resistance technique, electrochemical impedance, and Tafel extrapolation. Regardless of the technique used, each relies on the same basic principles in each test, a metallic coupon in an electrolyte is subject to an electrical perturbation. This perturbation is the appUcation of a current from an external source (power supply). This current stimulates the surface corrosion reactions. The voltage (potential) response of the coupon is measured and correlated with the current appUed—a galvanodynamic test. Conversely, the coupon potential is controlled and correlated with the requisite current—a potentiodynamic test. In either case, the resultant current is representative of the rate determining mass transfer or charge transfer rate. This may be related to the corrosion rate. 

There are many other techniques for measuring corrosion. Some of these may rely on chemical solution analysis or physical measurement of metal loss. One widely used additional technique is provided by ASTM G 96, Practice for On-Line Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods). This guide covers two basic procedures. In one, a sample of the material, usually in the form of a continuous wire, is immersed into the environment of interest (liquid, air, solid, or multiphase). The electrical resistance through the wire is determined. As corrosion consumes the cross section of the wire, the resistance increases proportionately. The second procedure uses the polarization resistance technique described in ASTM G 59 to determine the corrosion rate in the environment. 

Nondestructive Electrochemical Methods—Many of the disadvantages discussed above for the Tafel extrapolation method can be eliminated by using the polarization resistance technique. This nondestructive method can be used on systems that are under either activation or diffusion... 

Tafel extrapolation. Polarization can be applied to field testing and monitoring. Computerized equipment is available for both types of tests. Instruments based on the polarization resistance technique are widely used for process monitoring. 

Berke, N. S., Shen, D. F., and Sundberg, K. M., Comparison of the Polarization Resistance Technique to Macrocell Corrosion Technique, Corrosion Rates of Steel in Concrete, ASTM STP 1065, Berke, Chaker, and Whiting Eds., ASTM International, West Conshohocken, PA, 1990. 

Magar [32] evaluated inhibitors in white water by means of the linear polarization resistance technique in a search for effective inhibitors to prevent corrosion of bronze suction rolls. A galvanodynamic testing method was used by DeBerry and Ellis [33] in order to study inhibition of corrosion of a duplex stainless steel alloy. 

The corrosion current can be related to the Rd in the case of mixed control (Epelboin et al., 1972), where the polarization resistance technique fails, according to the following expression ... 

The potential-current density plot is approximately linear in a region of within 10 mV of the corrosion potential. As the slope of the plot has the units of resistance, this technique is called Polarization Resistance technique. The current measured in the external circuit equals the change in corrosion current, A/, caused by a small perturbation. As both anodic and cathodic reactions proceed in the vicinity of corrosion potential ( corr)> they are exponentially dependent upon the applied potential. Over a small potential range (20 mV), however, the exponentials are linearized (because mathematically Logx->x as x->-0) and an approximate linear... 

To the uninitiated engineer, the plethora of available corrosion monitoring techniques can be overwhelming in the absence of a categorization scheme. The first classification can be to separate direct from indirect techniques. Direct techniques measure parameters that are directly associated with corrosion processes. Indirect techniques measure parameters that are only indirectly related to corrosion damage. For example, measurements of potentials and current flow directly associated with corrosion reactions in the linear polarization resistance technique represent a direct corrosion rate measurement. The measurement of the corrosion potential only is an indirect method, as there is at best an indirect relationship between this potential and the severity of corrosion damage. 

With this practical polarization-resistance technique, the current-density-polarization curve is assumed to be linear at small polarizations. 

The disadvantage of the polarization-resistance technique is that, while the values of and cannot be determined, these values must be known for the calculation of i orr- Therefore, the Tafel slopes must be determined with some other technique, or their values must be estimated. 

The mass-transport corrected equation for the idealized polarization-resistance technique can be obtained by differentiating Eq. (12) ... 

For the practical polarization-resistance technique, /corr,true can be calculated from Eq. (12) and /corr,caic Eq. (7), resulting... 

The error of the idealized polarization-resistance technique was also treated by Gerasimenko, in terms of diffuse-layer potential, for a number of special cases (activation control, diffusion control, and passivation control, with and without specific adsorption on the surface). For other corrosion-rate determination techniques, the error can be obtained from numerical simulations using Eq. (17). ... 

The values of the Tafel slopes must be known to calculate the corrosion current density from Eqs. (5) and (7). If the values used in the calculation differ from the true values, an error results in the corrosion current density, and, for the idealized polarization-resistance technique, this error can be expressed as ... 

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