The inhibition of the atmospheric corrosion of iron by vapour-phase-inhibitors

Abstract

Several new techniques have recently been introduced in order to analyse the corrosion of metals covered by very thin or even adsorbed electrolyte layers. In this study the same techniques are used for the first time to investigate the inhibition mechanism of a typical vapour phase inhibitor (ammonium benzoate) on iron under atmospheric corrosion conditions. During wet/dry transitions the corrosion potential and the corrosion rate are measured without touching the surface under investigation, the kinetics of the metal dissolution and the oxygen reduction are analysed in bulk electrolytes as well as below very thin electrolyte layers and the corrosion rate and the chemical composition of the inhibitor covered surface are determined under non-condensing reaction conditions, where the surface is covered by monolayers of water only. It is shown that the rate of the anodic and the cathodic partial reactions of the corrosion process are strongly effected by the thickness of the electrolyte layer. Due to the presence of the inhibitor the surface is passivated, but stable passivity is observed only for very thin electrolyte layers. The inhibitor efficiency, therefore, cannot be deduced from conventional electrochemical experiments in bulk electrolytes, but only from studies on a metal surface which is covered either by very thin layers or even adsorbed electrolyte layers.