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Title: A study of the contaminants on the surface of sheet steel important to its corrosion performance
Author: Richards, D. C.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1991
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The aim of this work has been to study the contamination on the surface of sheet steel and its influence on autobody paint systems. The findings showed that the contamination did have an effect when considering some paint systems. However, when using other systems (e.g. a cathodic electroprimer), or including a top coat, the results showed little association with steel surface contamination, especially in outdoor corrosion tests. The mechanism for paint system failure was investigated and found to be closely associated with the porosity of the zinc phosphate coating normally applied before priming. This was influenced by the presence of carbon in either the amorphous or graphitic form. Using Glow Discharge-Optical Emission Spectroscopy and a combination of plant and laboratory based experiments, it was confirmed that graphitisation was the result of the decomposition of a metastable iron carbide (cementite) at temperatures in excess of 680oC during annealing. In agreement with other workers, strong carbide formers such as Cr and Mn, which form stable mixed carbides with Fe, were found to control the extent of graphite formation. Sulphur bearing species, such as those present in rolling lubricants, also reduced graphitisation. The mechanism was consistent with the formation of an unstable intermediate compound from FeO and CO gas at active surface sites. Other elements were also found to segregate during annealing (such as Mn,Cr,Si,S and P), the driving force probably being oxidation at the surface. Cr appears to be the principal graphite moderator. This, and the presence of an oil film on the pre-anneal surface, were the only two controllable factors which influenced graphitisation. The formation of amorphous carbon from rolling oils was also studied. The breakdown of the synthetic ester molecules was consistent with a free radical oxidation, the rate being strongly influenced by the extent of unsturation present. Radical scavengers were therefore effective antioxidants. The formation of surface carbon was probably the result of polymerisation via the free radical intermediates, which carbonised on the steel surface during the heating phase of annealing.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available