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Organic Finishes on Coil Coated Steels
[Thesis]. Manchester, UK: The University of Manchester; 2018.
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Abstract
The replacement of hexavalent chromium corrosion inhibitors from paints is an important venture due to the effects of those compounds on the environment and detrimental health effects. The procedure of replacement is made more urgent by imminent changes on the legality of using these compounds within the European Union. Whilst replacements have been devised, in many cases these replacements are less versatile, less effective and their mechanisms less understood. In aid of this, a methodology was produced that allows some understanding of the potential of an inhibitor on complex metallic systems, without the time restraints involved with more traditional methods, such as atmospheric testing. This methodology, simultaneous electrochemical impedance, utilises multiple zero resistance ammeters attached to a single overriding potentiostat to gain electrochemical information on multiple metals whilst maintaining the mixed potential of the coupled system, and was verified using previously defined inhibitor systems. Three commercial polyphosphate inhibitors were analysed using this technique and were shown to have distinct behaviours on a simple steel/zinc couple. Strontium aluminium polyphosphate exhibited a delayed inhibition response which was absent when the distance between working electrodes was decreased, which was theorized to be due to the mechanism of action requiring the presence of anodically produced zinc ions. This was tested and confirmed through an x-ray photoelectron spectroscopy study which externally introduced zinc ions and showed an increase in the phosphorus content seen on the steel surface. The concentration of externally added zinc required for this effect was tested and determined to be approximately between 1 and 2 mM. Subsequently, alternative cationic species were added to determine whether the efficacy of the phosphorus surface deposition could be potentially improved through other in-paint additives. Calcium was shown to be effective, with magnesium showing some effect on strontium aluminium polyphosphate. Aluminium cations appeared to have little effect. Additionally, the use of the simultaneous electrochemical impedance methodology was shown to have some use in analysing even more complex materials with a zinc/steel/aluminium model. The use of pure aluminium may cause some issues in analysis due to the protective oxide layer. Magnesium aluminium polyphosphate results suggest that it may initially interfere with this oxide layer production.