The Role of Tin in Suppressing Filiform Corrosion on Tin Coated and Iron-Tin Intermetallic Coated Steels
Authors: Wint Natalie , McMurray Hamilton Neil , Williams Geraint , Geary Steven , de Vooys Arnoud C.A. 
Published: February 2020

This paper describes a high resolution optical and electrochemical study into how metallic tin acts to reduce the rate of filiform corrosion (FFC) initiation and propagation in tin and iron-tin intermetallic (Fe-Sn IM) coatings for packaging steels.
A greater understanding of the role of tin in suppressing atmospheric corrosion such as FFC is of significant interest due to environmental regulation. FFC is aesthetically undesirable, therefore being of concern to the food packaging industry. (1, 2)
Tin and Fe-Sn IM coated steel samples were obtained from Tata Steel. In all cases tin had been electrodeposited onto the steel substrate. In the case of the IM coatings the tin layer had been diffusion annealed in an inert atmosphere.
Two sample types were prepared. In the first type the tin or Fe-Sn IM coated surface was continuous. In the second type the tin or Fe-Sn IM coating was removed from half the sample surface as shown in Figure 1. All samples were solvent coated with a 30μm thick film of polyvinyl butyral (PVB) lacquer. A 1cm line penetrative PVB coating defect was created by scribing with a scalpel blade. In all cases FFC was initiated by introduction of 2μL of 2.5 x 10-3 M aqueous FeCl2 to the scribe.
Two experiment types were carried out. In the first type FFC was initiated on the tin or Fe-Sn IM. In the second type FFC was initiated on the exposed steel substrate and allowed to propagate over the coated portion of the sample.
The time-dependent extent of corrosion was determined optically using time lapse photography and by repeated in-situ scanning using a scanning Kelvin probe (SKP) apparatus.
The capability of the SKP to visualise the spatial distribution of localized free corrosion potential variation with time has been demonstrated previously. (3)
Initiation and propagation of FFC was observed on the Fe-Sn IM but not the tin coating.
The rate of FFC propagation on the Fe-Sn IM was lower than that on the steel substrate. Using the SKP it was found that the FFC cell potential on Fe-Sn IM was approximately a quarter of that on iron as seen in Figure 2. This suggests the driving force and therefore propagation rate of FFC on Fe-Sn IM to be a quarter of that on the bare substrate.
It has been stated that tin is associated with a high oxygen overpotential elsewhere. (4) A rotating disk electrode (RDE) was therefore used as a supportive technique. (5) It was verified that free tin acts as a poor electrocatalyst for oxygen reduction when compared to iron present in the Fe-Sn IM.
The initiation and propagation of FFC is thus prohibited on tin coated samples.
It is consequently suggested that Fe-Sn IM coatings are used in conjunction with a passivating pretreatment when being used industrially.
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