Why is pig iron liable to corrode in humid air than pure iron?
Pig iron is an alloy of iron and carbon carbon content more than 2%, pig iron carbon content is generally in 2.11%--4.3%, including C, Si, and Mn, S, P and other elements, is the use of iron ores in blast furnace smelting products. According to the different form of carbon in pig iron, it can be divided into several kinds, such as steelmaking pig iron, casting pig iron and nodular cast iron. Pig iron performance is hard, wear-resistant and casting good, but pig iron is crisp, can not forge. The impurity elements of pig iron are mainly silicon, sulfur, manganese, phosphorus and so on. Raw iron is hard and brittle, lacks toughness, has almost no plastic deformation ability, so it can not be machined by forging, rolling and drawing. But with high silicon cast iron (gray iron casting) and good cutting performance. Pig iron is a blast furnace product, and can be divided into two categories: steelmaking pig iron and casting pig iron according to its use. Traditionally pig iron is called pig iron and cast pig iron is referred to as cast iron. Cast iron by forging, metamorphism, spheroidizing method can change the internal structure and improve its mechanical properties, therefore, cast iron can be divided into white cast iron, gray iron, malleable iron, ductile iron and special cast iron etc..
Because the carbon content of pig iron is higher, in the humid air, the iron surface will form a water film, carbon and iron between the formation of a number of primary batteries, will produce electrical corrosion, accelerate the corrosion of iron corrosion. Iron and carbon form primary batteries, iron is negative, and carbon is positive. In the primary battery, the negative electrode is corroded and the positive electrode is protected, thus the pig iron is easy to corrode. The iron content of pure iron is very high, it is not easy to form a galvanic cell, pure iron is naturally not easy to corrode. In addition, corrosion and metal composition, metal environment and oxygen concentration and many other factors are related.
Pure iron is iron alloy carbon content is less than 0.02%, also known as iron (carbon content in 0.02 - 1.7% for steel, the carbon content in 1.7 - 4.3% is called iron). Wrought iron by iron ore by direct reduction of carbon, or by iron after melting and impurity oxidation products obtained; the former smelting temperature is low, the latter is earlier, although the temperature is higher, but the iron to carbon due to melting point increased thickening; both are not easy to complete separation of slag and iron, so iron in often contain a small amount of slag, showed fibrous tissue after processing. General industrial pure iron, especially soft, toughness is particularly large, electromagnetic performance is very good. There are two kinds of common specifications, one is as deep drawing materials, can be punched into very complex shapes; the other is as magnetic materials, magnetic resistance high sensitivity magnetic low are widely used in electronics, electrical components, magnetic material, non crystal products, relay, sensor, automobile brake, textile machinery, electric meter solenoid valve products etc.. Melting point is higher than iron, it is difficult to rust in humid air than iron, and can be passivated in cold concentrated sulfuric acid.