12 C O N T E X T 1 3 9 : M A Y 2 0 1 5 ‘Corten steel’ in the USA, for which they do make such claims. One of the chief uses for pure iron in today’s industry is as sacrificial anodes to protect steelwork tanks and ships, as it has been found to corrode preferentially to mild steel. Verity’s examination of puddled wrought iron and mild steel was a chemical analysis, which appears to have overlooked the presence of the slags. During the manufacturing process, wrought iron does not become molten, as do more highly refined metals, so impurities are included in the matrix of the iron rather than being separated and disposed of. At these high temperatures the impurities are turned to glass, and are commonly known as ‘slag’, consisting of carbides and silicates which give wrought iron its fibrous structure. Approximately 250,000 siliceous fibres appear in each cross-sectional square inch of high-quality wrought iron. The slag present in the structure of wrought iron inhibits corrosion in a number of ways: 1. Slags themselves are non-corrodible and serve as an effective mechanical barrier against the progress of corrosion. 2.The structure of the iron gives rise to a very rough (microscopically speaking) surface texture, which interlocks with the oxide layer, be it rust or mill scale, preventing it from flaking off the surface.The oxides act as a protective coating, preventing further corrosion. 3. Electrically speaking, where the slag appears on the surface it acts an insulator between the areas of reactive iron, retarding electrolytic action. When wrought iron is heated in the fire to a high temperature, the slag melts and covers the surface of the iron in a rather similar manner to flux. This glassy layer retards oxidation to the extent that the iron can be Polish blacksmith Arek Kropidlowski forging at Chris Topp’s Setting the shape of a hot scroll
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