Pragmatic Protection Criteria for Nonalloyed Ferrous Materials

1 Pragmatic Protection Criteria for Nonalloyed Ferrous Materials 

In this section the pragmatic protection criteria of NACE [23] given in Table 3-3 are commented on in light of present knowledge. 

This criterion is derived from the fact that the free corrosion potential in soil is generally I/cu Cuso4 -0-55 V. Ohmic voltage drop and protective surface films are not taken into consideration. According to the information in Chapter 4, a maximum corrosion rate for uniform corrosion in soil of 0.1 mm a can be assumed. This corresponds to a current density of 0.1 A m l In Fig. 2-9, the corrosion current density for steel without surface film changes by a factor of 10 with a reduction in potential of about 70 mV. To reduce it to 1 jum a (0.14 V would be necessary. The same level would be available for an ohmic voltage drop. With surfaces covered with films, corrosion at the rest potential and the potential dependence of corrosion in comparison with act contrary to each other so that qualitatively the situation remains the same. More relevant is 

Here U/ is measured after switching off of the protection current and after step polarization. The potential difference corresponds to an IR-fiee potential decay. From the slope in Fig. 2-9, a reduction in the corrosion rate of 100 to 4 /im a results. 

300-mV negative change from rest potential on switching on protection current 

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