VCI containing package material - mode of functioning

I. L. Rozenfeld was one of the pioneers in the field of corrosion research and the application of vapor phase corrosion inhibitors (VCIs, volatile corrosion inhibitors). He found that no substance can sufficiently inhibit the corrosion by its own, what is reflected on conventional corrosion protection schemes. Nowadays, three strategies are established in the field of iron protection to reach a passive state: 1. the installation of buffer systems (combination of a weak acid with its corresponding base) to keep the pH value of the surface electrolyte layer constant in the alkaline range; 2. the combination of anodic and cathodic inhibitors to achieve a synergistic effect; 3. the supply of (at least one) oxidizing agent in the present of oxygen in the atmosphere or already dissolved in the surface electrolyte layer. In general, polymer based packaging materials are used in order to protect metallic equipment (mostly made of iron materials) during transport and storage against atmospheric corrosion. This temporary corrosion protection is achieved by incorporation of VCI in the polymer film, whereby the packaging material functions as a VCI source. Protection is necessary especially during the time of wetness, where the metal surface can corrode due to the formation of a thin electrolyte layer on the surface. The efficiency of a selected combination of VCI compounds mainly depends on three parameters: 1. their vapor pressure (more exactly: their tendency to sublime) under atmospheric conditions are high enough allowing significant vapor phase transport of the compounds within an enclosed space to the metal surface; 2. their adsorption strength on the oxide covered metal surface (directly or after dissolving in the condensed water film) inhibiting the metal corrosion during storage and transport by interaction with the surface; 3. the achieved mixture of different VCIs on the metal surface to cover different strategies of corrosion protection Within this paper calculations should be presented to estimate the minimum need of VCI in dependence of the package volume, the surface roughness of the goods and the exposed temperature. Further, the transport of the VCI in the atmosphere of a densely closed package by diffusion and natural convection should be discussed. These findings can be used to approximate the conditioning time. In the second part results of electrochemical measurements like the measuring of the free corrosion potential and recording of electrochemical impedance spectra should be presented characterizing the interaction of