BACKGROUND
Some recent papers contain inaccurate information regarding the validity of the -850 mVCSE current applied criterion, the so-called on-potential criterion. To qualify as a corrosion control criterion the -850 mVCSE value must imply a sound engineering judgment of the corrosion rate. Only the -850 mVCSE polarized potential criterion does this, as supported theoretically and empirically through its development, where a corrosion rate of less than 1mpy (<0.025mm/a) is generally accepted. There is no similar scientific backing for the -850 mVCSE on-potential criterion. Numerous comparative studies of these two criteria illustrate that the corrosion rate cannot be judged on the basis of an on-potential criterion when the IR drop is ignored. Failure to correct the on-potential measurement, by removing the IR drop, leaves the public exposed to an unnecessary safety risk.
Ever since the first publication of the NACE SP0169 standard in 1969, a clause has been included in this standard to caution users that voltage drops (IR drop), other than across the structure/electrolyte boundary, need to be considered for valid interpretation of the -850 mVCSE current applied criterion, often referred to as the on-potential criterion. Consideration of these inherent voltage drops in the potential measurement has often meant ignoring their effect. A survey, conducted by the NACE T-10-1 criteria committee, during the revision of the RP0169-1983 standard, found that of 1166 responses, 813 (70%) did not consider IR drop. As a result, a -850 mVCSE polarized potential criterion was inserted in the RP0169-1992 standard, which reflected the fundamental science and the empirical evidence from both laboratory and field trials. Unfortunately, this created ambiguity in determining which criterion was correct and did little to dissuade operators from continuing to neglect the IR drop in the - 850 mVCSE on-potential criterion.
