Corrosion Measurements by Titration, (CMT). Alone or Combined With Electrochemical Measurements(EC). Examples: Corrosion of Zinc, Nickel, Aluminium and Iron

At the corrosion potential the anodic dissolution of metal, Me = Me(z+) + z (e-), is accompanied by a cathodic reaction, z H(+) + z e(-) = z/2 H(2), for example. In order to maintain a constant pH-value addition of acid is needed, at a rate, which is equal to the rate of metal dissolution. Measurement of the rate of addition of acid is then equivalent to a measurement of the rate of metal dissolution. By using a pH-stat and a simple PC-program the rate of corrosion can be measured continuously and very precisely; this is the basis for CMT measurements. Electrochemical measurements of corrosion rate (EC) can be made, at intervals, during a recording of CMT measurements, and by comparison of the two kinds of measurements possible differences can be interpreted in terms of such phenomena as metal disintegration during corrosion (chunk effects), effects of dissolution of low valence metal species or non-electrochemical dissolution reactions.A great number of examinations of corrosion has been made with the following metals: Zinc, nickel, aluminium and iron, as pure metals or alloys and also, with zinc, as post-treated electrodeposits.Limitations and restrictions in the use of CMT measurements should be recognized: Non-buffer solutions at pH values between 3 and 9 are preferable, an atmosphere free of acid or alkaline components is required, and deposition of corrosion products as hydroxides or carbonates should be excluded.