Metallic and organic coating development for high performance pre-finished steels

• Main Author: Taylor, Christopher James
• Published: Swansea University 2009
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.752173

Organic coated steels are extremely popular in the construction industry due to their flexibility, cost, ease of construction and aesthetic nature. However the fact that they can be produced in many colours and finishes in large volumes slightly belies the complexity of the system itself. Many of the components have a degree of environmental sensitivity and therefore with the introduction and implementation of strict environmental legislation constant improvement is needed to keep in line or ahead of such directives. Improvement of the coating system from the substrate up is presented in this thesis. Initial work was undertaken to understand the species that would be released from an organic coating. To this end PVC based model coatings were produced and subjected to natural weathering for one year, the coatings were designed to degrade by addition of a U grade TiO2 to ensure that results were obtained during the period. The leachate species were monitored on a monthly basis and identified using inductively coupled plasma mass spectrometry (ICPMS). From this work it was found that pigmentation has an effect on the degradation of coatings. It was shown that monitoring of metallic run off is important as it provides details on species type, while some species can be identified as markers to photo-degradation of the coating. The comparison of stabilisers historic, present and novel using the same natural weathering method and testing period. Hydrotalcite was identified as a potential novel stabiliser for PVC based coatings. When compared to organotin based stabilisers as used in the past or Barium/Zinc based stabilisers that are currently used Hydrotalcite performed favourably both from a chemical and physical point of view. With regard to improving the coating system as a whole the next phase of work involved the identification of an optimum galvanised coating weight. Samples of Zn/Al galvanised steels with increasing coating weight were subjected to natural weathering for one year and the amount of zinc monitored monthly. Conventional hot dipped galvanised steel and Zalutite coatings were included in the sample field to provide further comparison points. A coating weight of 255 gm-2 was found to be the optimum with respect to the natural weathering, while Zalutite which contains a high aluminium addition performed best overall. This gave rise to the next section of work that involved used of the Scanning Vibrating Electrode Technique (SVET) to characterise the corrosion performance of galvanised coatings with aluminium additions greater than 4.8%. It was found that increasing the aluminium content improved both surface and cut edge corrosion. Under the parameters under which the samples were produced a wholly eutectic structure was achieved at a target addition of 6.1%. The final phase of work was a study on reducing the coating thickness of the high aluminium coatings to see if the improvement in corrosion performance attributed to increased aluminium addition carried across to thinner coatings. The samples were again tested using the SVET. The results suggest that there is some scope for reducing the coating thickness at certain aluminium levels.