
Harmonic Analysis. As shown in the LPR technique section, the voltagecurrent response of a corroding electrode in the region of Ecorr is basically the difference of two exponential curves. To calculate the corrosion current density from the slope of the voltagecurrent curve one needs to know the values of the Tafel slopes. Or so it would seem. With a bit of simple algebra (and a couple of Taylor polynomial expansions of Fourier  Bessel functions and some approximations and finger crossing) it can be shown that if the perturbing signal is a sinusoidal voltage then the corrosion current density can be calculated from the current response at the fundamental and the second and third harmonics only, without the need for the Tafel constants. The original work was done by Devay and Meszaros (1, 2 and 3). For more details Google or have a look at the relevant pages from John Gill's PhD thesis. There are a few practical problems with this technique. The levels of the harmonics can be quite low compared to the fundamental and can be hard to measure, especially in the presence of any noise. Electrolyte resistance (conveniently assumed to be zero in the derivation) tends to linearise the response. But it can be made to work surprisingly well (more info is also here). 
