Corrosion of Stainless Steels...

Corrosion of Stainless Steels Stainless steels (SS) are alloys of iron (Fe) which containing a minimum of 10.5 % chromium (Cr). With increasing content of Cr and with the presence or absence of many of other elements, SS can provide an extraordinary range of corrosion resistance. Different grades of SS are being used since several years in atmospheres which are mild (open air, in architectural applications) as well as extremely severe (chemical-processing industries). Stainless steels are classified in five families as per the crystal structures and the strengthening precipitates. Each family of SS shows its own general features in terms of mechanical properties and corrosion resistance. Within each family, there is a range of specifications which varies in composition, corrosion resistance, and cost. Stainless steels are vulnerable to several types of localized corrosive attack. The avoidance of such localized corrosion is the focus of most of the efforts made in the selection of SS. Also, the corrosion performance of SS is strongly influenced by practices of design, fabrication, surface conditioning, and maintenance. The selection of a grade of SS for a specific application involves the consideration of several factors, but the main factor remains corrosion resistance. It is the first necessity to specify the likely service environment. Besides considering the design conditions, it is also necessary to consider the reasonably anticipated exposures or upsets in service conditions. The suitability of a specific specification can be assessed from laboratory tests or from the documented field experience in similar atmospheres. Once the specification with satisfactory corrosion resistance has been identified, it is then appropriate to consider other factors such as mechanical properties, ease of fabrication, the types and degree of risk present in the application, the availability of the necessary product forms, and cost. Families of...

Corrosion of Cast Irons...

Corrosion of Cast Irons Cast iron is a standard term which is used for a large family of alloys of ferrous materials. Cast iron is mainly alloy of iron (Fe) which contains higher than 2 % of carbon (C) and more than 1 % of silicon (Si). Low cost of raw materials and relative ease of production make cast iron the last cost engineering material. Cast iron can be cast into intricate shapes since it has excellent fluidity and comparatively low melting point. It can also be alloyed for improvement of corrosion resistance and strength. With suitable alloying, the corrosion resistance of cast iron can equal to or exceed that of stainless steel and nickel (Ni) based alloy. Since outstanding properties are obtained with this low cost engineering material, cast iron finds extensive use in atmospheres which need good corrosion resistance. Services in which cast iron can be used for its good corrosion resistance include water, soils, acids, alkalis, saline solutions, organic compounds, sulphur compounds, and liquid metals. In some cases, alloyed cast iron is the only economical choice for the equipment manufacture. Cast iron and the basic metallurgy The metallurgy of cast iron is similar to that of steel except that Si in sufficient quantities is present to necessitate use of the Fe-Si-C ternary phase diagram rather than the simple Fe-C binary diagram. A section of the Fe- Fe3C (iron carbide)-Si ternary diagram at 2 % Si is shown in Fig 1. Iron carbide is also known as cementite. The eutectic and eutectoid points in the Fe-Si-C diagram are both affected with the introduction of Si into the system. With normal Si in the range of 1 % to 3 % in cast irons, eutectic C percentage is related to Si percentage as...

Corrosion of steel reinforcement bars in concrete Apr07

Corrosion of steel reinforcement bars in concrete...

Corrosion of steel reinforcement bars in concrete Reinforced concrete is a composite material comprising steel reinforcement bars embedded in a concrete mass.  Reinforcement bars carries the bulk of the tensile load and imparts a degree of cracking resistance to the concrete which itself is compressively loaded. The corrosion of steel reinforcement bars in concrete is a big universal problem. The damage which happens from corrosion may due to a large extent reduce the serviceability and structural integrity of reinforced concrete. Concrete provides stable long term corrosion protection to steel reinforcement bars because of passivation of the surface of steel by the highly alkaline property of the concrete. As long as this passivity is maintained, the corrosion of the reinforcement bars will not take place. For this the concrete should be sufficiently dense and impermeable so as not to allow transport of chlorides, carbon di oxide and oxygen. The passivation of steel reinforcement bars is deteriorates greatly due to chlorides even at high pH values. The carbon oxide in solution neutralizes the calcium hydro oxide saturated pore water (carbonation process) and lowers the pH value below the pH value at which the steel can remain passivated. Oxygen also plays a vital role in the passivation process. Whenever the reinforcement bar embedded in steel corrodes, the corrosion products increase its volume. All forms of iron oxide and hydro oxide have specific volumes greater than that of steel. The volume of the corrosion products vary by a factor which can be even more than 5 as indicated in Fig. 1. The expansive forces generated by the steel corrosion leads to tensile cracking and rust staining of the concrete. This in turn causes reduction in the serviceability and structural integrity of concrete besides affecting its aesthetics. Once the...