Error Sources and Calculational Methods

Only the systematic errors of the corrosion-rate measurements, those inherent in the theory of the techniques, are treated in this chapter. These errors are a consequence of the assumptions involved in the derivation of Eq. (1), which are enumerated in Section II. 1. Error of the measurement occurs whenever one of the simplifying assumptions is not justified for the existing conditions. Some other errors, as discussed below, arise from further simplifying assumptions, specific for each measurement technique. Each error possibility will be discussed and evaluated separately. Of course, more than one of them will usually have to be considered in a practical situation. Moreover, the random errors of the measurement itself (e.g., current, potential, specimen area, etc.) are seldom negligible (see, for example. Ref. 21, 23, 77), and they should be added to the systematic errors described here. 

The mass-transport correction factor, A, is plotted in Fig. 1 as a function of /corrA / and 2.3AJE/6. As shown, the correction can be considerable even at small values of icovrlh for large cathodic polarizations and for small cathodic Tafel slopes. The value of A, is also large for large anodic polarizations, but under these conditions the cathodic term in Eq. (12) is usually negligible. 

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

Therefore, the error caused by the neglect of mass transport can be expressed analytically by comparing /corr.true calculated from Eq. (14) with /corr.caic Calculated from Eq. (5)  

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

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