The evaluation of corrosion rates with the generalised inflection point method

Abstract The mathematical theory of the generalised inflection point method is expounded and the proof that the resolving equation has only one root is given. The validity of this method has been assessed by examining some theoretical curves having arbitrary values of B a and B c . The results were very promising and demonstrated that the numerical sequence generated by the Newton method is convergent also when − w Δ E i takes values as small as 1×10 −4 . It was proved that for a correct application of this numerical technique it is not mandatory to know a priori if B a > B c or B a B c , and Δ E i takes values quite different from zero also for the most critical cases so that F (1) is always >0. Experimental results refer to the behaviour of iron in 1 N H 2 SO 4 solutions containing KCl at various concentrations and 25°C, the polarization curves being performed under current control. Experimental polarization curves were best-fitted with a polynomial of the fourth degree to determine the actual position of Δ E i and the values of i ′(0) and i ″(0) necessary to compute the quantity w . Usually the best-fitting of the experimental polarization curves refers to the [−50,50] mV Δ E interval. Comparison of the values of B a , B c and I c with those obtained using the NOLI method shows that the inflection point method works properly and is a valid tool to determine corrosion rates.