Corrosion of Cast Steels...

Corrosion of Cast Steels Cast steels are generally classified into the categories of (i) carbon (C) steels, (ii) low alloy steels, (iii) corrosion resistant steels, and (iv) heat resistant steels, depending on the alloy content and the planned usage. Steel castings are categorized as corrosion resistant if they are capable of sustained operation when exposed to attack by corrosive agents at operating temperatures which are generally below 300 deg C. The high alloy iron base compositions are generally given the name ‘stainless steels’, though this name is not recognized universally. Actually, these steels are widely referred to as cast stainless steels. Some of the high alloy steels (e.g. 12 % chromium steel) show many of the familiar physical characteristics of C steels and low alloy steels, and some of their mechanical properties, such as hardness and tensile strength (TS), can be altered by suitable heat treatment. The alloy steels of higher chromium (Cr) content (20 % to 30 % Cr), Cr-Ni (nickel)  steels and Ni-Cr steels do not show the changes in phase observed in ordinary C steel when heated or cooled in the range from room temperature to the melting point. Consequently, these steels are non hardenable, and their mechanical properties depend on the composition instead of heat treatment. The high alloy steels (stainless steels) differ from C steels and low alloy steels in other respects, such as their production and properties. Special attention is required to be given to each grade with regard to casting design and casting practice in the foundry. For example, such elements as Cr, Ni, C, N2 (nitrogen), Si (silicon), Mo (molybdenum), and Nb (niobium) can exert a deep impact on the ultimate structure of these complex steels. Hence, balancing of the alloy compositions is normally required to...

Corrosion of Carbon Steels...

Corrosion of Carbon Steels Carbon (C) steel is the most widely used engineering material. Despite its relatively limited corrosion resistance, C steels are used in large tonnages in marine applications, power plants (both nuclear based power and fossil fuel based power), metal processing equipment, power transmission, transportation, chemical processing, petroleum production and refining, pipelines, mining, and construction etc. The annual cost of metallic corrosion to the total economy of a country is very high. Because C steels represent the largest single class of alloys in use, both in terms of tonnage and total cost, it is easy to understand that the corrosion of C steels is a problem of enormous practical importance. This, of course, is the reason for the existence of entire industries devoted to providing protective systems for iron and steel. Because of corrosion, the design aspects are also very important. Indeed, design changes are often the most efficient manner of dealing with a particular corrosion problem. C steels also often called mild steels have limited alloy content, usually less than 2 % of the total of all the additions. These levels of additions do not generally produce any remarkable changes in general corrosion behaviour. One possible exception to this statement is the weathering steels, in which small additions of copper (Cu), chromium (Cr), nickel (Ni), and/or phosphorus (P) produce significant reductions in corrosion rate in certain environments. At the levels present in the C steels, the usual impurities have no significant effect on corrosion rate in the atmosphere, neutral waters, or soils. Only in the case of acid attack is an effect observed. In this latter case, the presence of P and sulphur (S) markedly increase the rate of attack. Indeed, in acid systems, the pure irons appear to show the...