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...

Hydrogen in Steels

Hydrogen in Steels  Hydrogen (H) (atomic number 1 and atomic weight 1.008) is a colourless gas. It has a density of 0.0899 gm/litre. Melting point of H is – 259.2 deg C and boiling point is -252.8 deg C. The phase diagram of the Fe-H  is given at Fig 1. Fig 1 Fe-H phase diagram  H in steels is considered as an undesirable impurity which is quite harmful in certain applications. It is always a source of various problems within steel production because of its generally detrimental effects on processing characteristics and service performance of steel products. Just a few parts per million of H dissolved in steel is sufficient to do the harm. Hence where necessary, it should be avoided or removed as required. Source of hydrogen There are multiple sources identified for H to enter into steel by any of several routes. In the primary steel making furnace, source of H is water which enter the furnace through wet scrap, flux materials, ferro alloys and refractory materials which are not fully dried. Water dissociates on contact with liquid steel and produces H which is absorbed by the steel bath. This H generally get removed by the purging action of the carbon (C) boil, but some can remain in the steel. Contact between the liquid steel and moisture in refractory materials  of the steel teeming ladles and/or humid air can cause pick up of H by the liquid steel. The hydrophilic calcium oxide (CaO) in the slag and decomposition of refractory binders which are required for sufficient thermal shock resistance, also account for H entering into the steel. Dissociation of water vapour (equation given below) contained in furnace gases generated during the steel making practices employing the combustion of hydrocarbon fuels produces H...