Using electrochemical impedance spectroscopy to measure corrosion protection of thin-film magnetic recording disks

Electrochemical Impedance Spectroscopy (EIS) is a powerful method for determining corrosion resistance of coatings. This method uses a spectrum of alternating current frequencies and is employed on magnetic thin-film media used in hard disk drives. Requiring an equivalent circuit model, EIS generates resistance and capacitance data depending upon the model chosen. When applied using a Randles cell, corrosion current and rate are determined. Furthermore, comparisons are drawn regarding density of the protective carbon overcoats and substrate roughness. The results provide a more detailed and accurate view of thermodynamically controlled media corrosion than is available with direct current methods. The results further corroborate data accumulated via kinetic techniques. These methods have been used to compare carbon films prepared by two vacuum depositions: solid target sputtering and ion beam. The carbon films deposited by ion beam on thin-film magnetic recording disks show superior corrosion resistance than the sputtered films, even when thinner.