An Investigation of Zinc Based Polyphosphates As Corrosion Inhibitors for Use in Organically Coated Steel
Authors: Griffiths Christian , McMurray Hamilton Neil , Williams Geraint 
Published: February 2020

Strontium chromate has long been the industrial standard for corrosion inhibition in organically coated steels (OCS) but legislation will ban its use from 2017due to its carcinogenic properties. Research is focused on sourcing a suitable replacement to strontium chromate. Results indicate that no replacement fully matches the corrosion inhibition performance.. Zinc phosphate is the leading replacement and has shown promising corrosion inhibition performance but cannot match the effectiveness of chromate in organic coatings (1) (2) (3). The poor performance of zinc phosphate has led to research into polyphosphate-based corrosion inhibitors which have shown promising results (4) (5).

Ongoing studies at Swansea University have shown that a range of polyphosphate-based inhibitors demonstrate an effective corrosion-inhibition on galvanised steel. The polyphosphates, supplied by Heubach Ltd, are widely used throughout the coating industry. In-situ scanning Kelvin probe (SKP) technique has proven the ability of polyphosphates to stall and resist cathodic delamination. The technique is further utilised to investigate the effect of various weightings of polyphosphates in model coatings of polyvinyl butryal (PVB). It is thought that the various cations incorporated within the polyphosphates will improve the precipitation layer that is typical in corrosion inhibition for zinc phosphate. The corrosion products were analysed using SEM, EDX and XPS which allows for further evaluation of the corrosion mechanisms that govern the corrosion-inhibition. Current theories attribute the inhibition to electrochemical and mass-transport blocking effect.

Further work has shown how polyphosphates perform within an industrial coating. The salt spray technique (ASTM B117-11) has been used to evaluate the performance of industrial coatings containing a range of polyphosphates perform compared to strontium chromate and zinc phosphate.

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