Electrochemical noise at passive iron

Abstract

With the apparatus described it was possible to measure under potentiostatic conditions at frequencies up to about 1 kHz spectral densities of current noise comparable to the thermal noise at resistances of the order M Ohm. The spectral densities of the current noise at passive iron microelectrodes in neutral borate or acetate buffer solutions were nearly frequency independent at frequencies below 10 Hz and increased with the square of the frequency at higher frequencies. The results are described by a nearly frequency independent voltage noise source of an intensity about one order of magnitude larger than the thermal noise at the electrode impedance. The spectral densities of the current noise and the corresponding rms noise currents were found to decrease with time during the approach to the steady state, with electrode potential due to the changes of the electrode impedance, and with electrode area. The current noise was increased by small heat flows through the electrode. At constant temperature, the noise was not affected by the exchange of electrolytes not containing ions causing pitting corrosion. The replacement of such inert electrolytes by electrolytes containing chloride resulted in an immediate increase of the spectral density of the current noise and a subsequent decrease towards a steady state before the end of the incubation time. The rms noise current in presence of chloride normalized to the noise current in absence of chloride was independent of electrode potential and proportional to the logarithm of the ratio between the actual chloride concentration and the critical chloride concentration below which pitting never occurs. The voltage noise source became frequency dependent with a maximum at about 5 Hz. After onset of pitting the low frequency noise of the 1/ f a -type grew by several orders of magnitude. The results are explained in terms of local fluctuations of the thickness of the passivating film due to local adsorption of chloride followed by currentless dissolution of oxide monolayers and subsequent growth or further dissolution of the oxide film at random sites.