roop lal engineering materials &...

Roop Lal Engineering Materials & Metallurgy

(Asst. Prof.) Delhi Technological University

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Unit – IV Engineering Materials and Metallurgy

Materials: Plain carbon steels, effect of alloying elements, properties and uses, heat treatment of tool steel, stainless, spring and wear resisting steels. Production, composition, properties, and use of non ferrous alloys e. g. brasses, bronzes, duralumin, die – casting and bearing alloys. IS standards codes for steels. Corrosion: Types of corrosion, Galvanic cell, rusting of Iron, Methods of protection from corrosion. Materials: Plain carbon steels: Steel is an alloy of iron and carbon, with carbon content up to a maximum of 1.5%, silica upto 0.5%, and manganese upto 1.5% along with traces of other elements is called plain carbon steel. The carbon occurs in the form of iron carbide, because of its ability to increase the hardness and strength of the steel. Other elements e.g. silicon, sulphur, phosphorus and manganese are also present to greater or lesser amount to impart certain desired properties to it. Most of the steel produced now-a-days is plain carbon steel or simply carbon steel. A carbon steel is defined as a steel which has its properties mainly due to its carbon content and does not contain more than 0.5% of silicon and 1.5% of manganese. The plain carbon steels varying from 0.06% carbon to 1.5% carbon are divided into the following types depending upon the carbon content. Plain Carbon Steels are generally classified into following groups:

1. Dead mild steel — up to 0.15% carbon 2. Low carbon or mild steel — 0.15% to 0.45% carbon 3. Medium carbon steel — 0.45% to 0.8% carbon 4. High carbon steel — 0.8% to 1.5% carbon

(i) Dead mild steel — contains up to 0.15% carbon

(ii) Mild steel or Low Carbon Steel: The percentage of carbon in these steels range from 0.15% to 0.25%. It is moderately strong, tough, soft, ductile, low tensile strength steel and have good weldability. The production cost of these materials is also low. It is a general purpose steel and can be easily worked and welded. It is used where hardness and tensile strength are not the most important requirements. Typical applications of mild steel are body work for cars and ships, screws, wires, nails, structural steels, etc.

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(ii) Medium Carbon Steel: Carbon content in this steel is 0.3% to 0.6%. The strength of these materials is high but their weldability is comparatively less Strength and hardness are improved while ductility is reduced. This type of steel can be forged, rolled and machined. It finds uses for agricultural tools, fasteners, dynamo and motor shafts, crank shaft, connecting rods, gears, etc.

(iii) High Carbon Steel or Tool Steel: It is hardest of plain carbon steels and contains

0.6 to 1.5% carbon. It possess good tensile strength. Tool steel is for withstanding wear, where hardness is more necessary requirement than ductility. Tool steel is generally used for machine tools, saws, hammers, cold chisels, punches, axes, dies, taps, drills, razors. These are always used in hardened and tempered condition. These materials get hard and tough by heat treatment and their weldability is poor.

Effect of alloying elements, properties and uses: Carbon steel has wide range of mechanical properties, which can be further improved by the addition of alloying elements and proper control of carbon content. A homogeneous mixture of two or more metals or a metal and a non-metal when fused together at a certain temperature forms a new metal after solidification, termed as an alloy. Metals in actual commercial use are almost exclusively alloys, and not pure metals, since it is possible for the design engineer to have an infinite variety of physical properties in the product by varying the metallic composition of the alloy. Alloys are normally harder than their components, less ductile and may have a much lower conductivity, whereas the highly purified single crystal of a metal is very soft and malleable, with high electrical conductivity. This is why pure metals are used only for specific applications. The alloy is usually more corrosion resistant and less affected by atmospheric conditions. The conductivity of an alloy varies with the degree of order of the alloy and the hardness varies with the particular heat treatment. The alloying elements are added to improve one or more of the following properties: (a) tensile strength, hardness and toughness (b) elasticity, (c) machinability, (d) corrosive and oxidation resistance, (e) hardenability (f) creep strength and (g) fatigue resistance, etc.

Following are the effects of alloying elements on steel: 1. Nickel. It increases the strength and toughness of the steel. These steels contain 2 to 5% nickel and from 0.1 to 0.5% carbon. In this range, nickel contributes great strength and hardness with high elastic limit, good ductility and good resistance to corrosion. An alloy containing 25% nickel possesses maximum toughness and offers the greatest resistance to rusting, corrosion and burning at high temperature. It has proved to be of advantage in the





manufacture of boiler tubes, valves for use with superheated steam, valves for I.C. engines and spark plugs for petrol engines. A nickel steel alloy containing 36% of nickel is known as invar. It has nearly zero coefficient of expansion. So it is in great demand for measuring instruments and standards of lengths for everyday use. 2. Chromium. It is used in steels as an alloying element to combine hardness with high strength and high elastic limit. It also imparts corrosion-resisting properties to steel. The most common chrome steels contains from 0.5 to 2% chromium and 0.1 to 1.5% carbon. The chrome steel is used for balls, rollers and races for bearings. A nickel chrome steel containing 3.25% nickel, 1.5% chromium and 0.25% carbon is much used for armour plates. Chrome nickel steel is extensively used for motor car crankshafts, axles and gears requiring great strength and hardness. 3. Tungsten. It prohibits grain growth, increases the depth of hardening of quenched steel and confers the property of remaining hard even when heated to red colour. It is usually used in conjuction with other elements. Steel containing 3 to 18% tungsten and 0.2 to 1.5% carbon is used for cutting tools. The principal uses of tungsten steels are for cutting tools, dies, valves, taps and permanent magnets. 4. Vanadium. It aids in obtaining a fine grain structure in tool steel. The addition of a very small amount of vanadium (less than 0.2%) produces a marked increase in tensile strength and elastic limit in low and medium carbon steels without a loss of ductility. The chrome-vanadium steel containing about 0.5 to 1.5% chromium, 0.15 to 0.3% vanadium and 0.13 to 1.1% carbon have extremely good tensile strength, elastic limit, endurance limit and ductility. These steels are frequently used for parts such as springs, shafts, gears, pins and many drop forged parts. 5. Manganese. It improves the strength of the steel in both the hot rolled and heat treated condition. The manganese alloy steels containing over 1.5% manganese with a carbon range of 0.40 to 0.55% are used extensively in gears, axles, shafts and other parts where high strength combined with fair ductility is required. The principal uses of manganese steel is in machinery parts subjected to severe wear. These steels are all cast and ground to finish. 6. Silicon. The silicon steels behave like nickel steels. These steels have a high elastic limit as compared to ordinary carbon steel. Silicon steels containing from 1 to 2% silicon and 0.1 to 0.4% carbon and other alloying elements are used for electrical machinery, valves in I.C. engines, springs and corrosion resisting materials. 7. Cobalt. It gives red hardness by retention of hard carbides at high temperatures. It tends to decarburize steel during heat-treatment. It increases hardness and strength and also residual magnetism and coercive magnetic force in steel for magnets. 8. Molybdenum. A very small quantity (0.15 to 0.30%) of molybdenum is generally used with chromium and manganese (0.5 to 0.8%) to make molybdenum steel. These steels possess extra





tensile strength and are used for air-plane fuselage and automobile parts. It can replace tungsten in high speed steel

Heat Treatment of Steels: The term heat treatment may be defined as an operation or a combination of operations, involving the heating and cooling of a metal or an alloy in the solid state for the purpose of obtaining certain desirable conditions or properties without change in chemical composition. The aim of heat treatment is to achieve one or more of the following objects :

1. To increase the hardness of metals. 2. To relieve the stresses set up in the material after hot or cold working. 3. To improve machinability. 4. To soften the metal. 5. To modify the structure of the material to improve its electrical and magnetic

properties. 6. To change the grain size. 7. To increase the qualities of a metal to provide better resistance to heat,

corrosion and wear.

For high-strength steels, the best combination of mechanical characteristics may be realized if a predominantly martensitic microstructure is developed over the entire cross section; this is converted to tempered martensite during a tempering heat treatment. Hardenability is a parameter used to ascertain the influence of composition on the susceptibility to the formation of a predominantly martensitic structure for some specific heat treatment. Determination of hardenability is accomplished by the standard Jominy end-quench test, from which hardenability curves are generated. Other factors also influence the extent to which martensite will form. Of the common quenching media, water is the most efficient, followed by oil and air, in that order. The relationships between cooling rate and specimen size and geometry for a specific quenching medium frequently are expressed on empirical charts; two were introduced for cylindrical specimens. These may be used in conjunction with hardenability data to generate cross-sectional hardness profiles.

Heat treatment of tool steel, stainless, spring and wear resisting steels: Heat treatment of tool steel: Essential requirements for all types of cutting tool steel are cold hardness, red hardness, abrasion resistance, toughness, thermal conductivity and specific heat, coefficient of friction and machinability. Tool steel should be able to withstanding wear, there hardness is more necessary requirement than ductility. Tool steel is generally used for machine tools, saws, hammers, cold chisels, punches, axes, dies, taps, drills, razors. These are always used in hardened and tempered condition. The operation of hardening is applied to all tools. The main purpose of hardening tool is to develop high hardness. The parts subjected to wear in service, such as shafts, etc. need high hardness.





Hardening operation consists of three steps:

i. 1st the specimen is heated to about 850oC and kept at this temperature for 4 to 5 hours. This is done in order to dissolve all the carbides. After it, specimen is heated to 1200oC for 1 – 2 minutes. The purpose of heating to high temperature is that, more the substance is cooled from high temperature to lower, more will be the hardness.

ii. Cooling the parts in a fused salt bath iii. Holding it in the fused salt bath iv. Subsequently cooling it in the open air

Cooling of articles or tools to be hardened is the most difficult and important part of the hardening operation. In hardening martensite to be formed, so cooling rate must be faster than the critical one. The actual cooling rate for hardening should be very rapid. But rapid cooling is the main cause of the development of substantial internal stresses which leads to distortion of the articles and at the worst to the formation of cracks. These internal stresses are dangerous in cooling within the range of martensitic transformation. One of the simplest and most commonly used methods of cooling in hardening articles and tools of high carbon steels is to quench them successively in two media, first in water and then in oil. This method, which is also called quenching ‘through water to oil’ consists of first plunging into watwer for a few seconds to remove a part of the heat and then into oil till the cooling is complete. The object of quick cooling is hardening steels is to suppress the pearlitic transformation of austenite. It is sufficient to cool the steel rapidly through the temperature range A1 to 400oC. If the decomposition of austenite into ferritocarbide aggregate is suspended through a quick cooling, then in further cooling, even though it be comparatively slow, the supercooled austenite can only transform into martensite

Tempering is performed by reheating a hardened steel article to a temperature below the transformation range ( 220oC to 340oC) in order to

i. Reduce brittleness in hardened steel ii. Reduce the internal stresses caused during quenching iii. Improve toughness by sacrificing some hardness

Heat treatment of (18 : 4 : 1) W : Cr : V) High Speed Steel ( Stainless Steel) 1st the specimen is heated to about 850oC and kept at this temperature for 4 to 5 hours. This is done in order to dissolve all the carbides. After it, specimen is heated to 1200oC for 1 – 2 minutes. The purpose of heating to high temperature is that, more the substance is cooled from high temperature to lower, more will be the hardness. However, the specimen con not be kept at such high temperature for longer time , as high temperature grain growth will be tremendous and will get larger grain size which lowers down the impact value. After it





substance is quenched in salt bath at 650oC and kept at this temperature for 10 – 20 minutes. After it the substance is oil quenched. spring and wear resisting steels. Heat Treatment of spring : Helical twist springs are made of carbon steels, silicon steels, manganese steels, silicon manganese steels, and some other complexly alloyed steels. The most typical is silicon steel which is used for coiling most springs. It is comparatively cheap, since it does not contain expensive alloying elements, it is easier to heat treat than other spring steels and has good elastic properties. The heat treatment of springs usually consists of quenching in oil, followed by tempering. In some cases, when the original steel has coarse grain structure, the springs are preliminary normalized. In heating springs for hardening, special care should be taken to prevent them from changing their dimensions and shape. To achieve this the springs should be first of all heated in a horizontal position, but if a spring is placed vertical for heating, it may deflect under action of its own weight while hot. In heating spring for hardening, it should be born in mind that all steels with silicon are liable to decarburization. Therefore, springs must be held at the hardening temperature for a short period of time as possible, calculated on the basis 1 minute for 1 mm of wire diameter Manganese alloyed steels are very sensitive to overheating. In heating springs of Manganese steels it is necessary to control furnace temperature closely. As a rule spring should be quenched in oil. The springs must be immersed in the quenching tank vertically, with their long axis perpendicular to the surface of the quenching liquid. Springs must be tempered in saltpeter baths. Tempering in bath type furnace results in hardness being non – uniform over the length of the spring. The tempering temperature is calculated on the basis of 1.5 minutes for each millimeter of wire diameter, being not less than 1.5 hour in any case





Non-ferrous Metals: We have already discussed that the non-ferrous metals are those which contain a metal other than iron as their chief constituent. The non-ferrous metals are usually employed in industry due to the following characteristics :

1. Ease of fabrication (casting, rolling, forging, welding and machining), 2. Resistance to corrosion, 3. Electrical and thermal conductivity, and 4. Weight.

Copper Alloys The copper alloys are broadly classified into the following two groups : 1. Copper-zinc alloys (Brass). (70% copper–30% Zinc) is much harder and stronger than pure copper. The most widely used copper-zinc alloy is brass. There are various types of brasses, depending upon the proportions of copper and zinc. This is fundamentally a binary alloy of copper with zinc each 50%. By adding small quantities of other elements, the properties of brass may be greatly changed. For example, the addition of lead (1 to 2%) improves the machining quality of brass. It has a greater strength than that of copper, but have a lower thermal and electrical conductivity. Brasses are very resistant to atmospheric corrosion and can be easily soldered. They can be easily fabricated by processes like spinning and can also be electroplated with metals like nickel and chromium. The following table shows the composition of various types of brasses according to Indian standards.





2. Copper-tin alloys (Bronze). The alloys of copper and tin are usually termed as bronzes. The useful range of composition is 75 to 95% copper and 5 to 25% tin. The metal is comparatively hard, resists surface wear and can be shaped or rolled into wires, rods and sheets very easily. In corrosion resistant properties, bronzes are superior to brasses. Some of the common types of bronzes are as follows:

(a) Phosphor bronze. When bronze contains phosphorus, it is called phosphor bronze.Phosphorus increases the strength, ductility and soundness of castings. The tensile strength of this alloy when cast varies from 215 MPa to 280 MPa but increases upto 2300 MPa when rolled or drawn. This alloy possesses good wearing qualities and high elasticity. The metal is resistant to salt water corrosion. The composition of the metal varies according to whether it is to be forged, wrought or made into castings. A common type of phosphor bronze has the following composition according to Indian standards : Copper = 87–90%, Tin = 9–10%, and Phosphorus = 0.1–3%. It is used for bearings, worm wheels, gears, nuts for machine lead screws, pump parts, linings and for many other purposes. It is also suitable for making springs.

(b) Silicon bronze. It contains 96% copper, 3% silicon and 1% manganese or zinc. It has good general corrosion resistance of copper combined with higher strength. It can be cast, rolled, stamped, forged and pressed either hot or cold and it can be welded by all the usual methods. It is widely used for boilers, tanks, stoves or where high strength and good corrosion resistance is required. (c) Beryllium bronze. It is a copper base alloy containing about 97.75% copper and 2.25% beryllium. It has high yield point, high fatigue limit and excellent cold and hot corrosion resistance. It is particularly suitable material for springs, heavy duty electrical switches, cams and bushings. Since the wear resistance of beryllium copper is five times that of phosphor bronze, therefore, it may be used as a bearing metal in place of phosphor bronze. It has a film forming and a soft lubricating property, which makes it more suitable as a bearing metal.

(d) Manganese bronze. It is an alloy of copper, zinc and little percentage of manganese. The usual composition of this bronze is as follows: Copper = 60%, Zinc = 35%, and Manganese = 5%

This metal is highly resistant to corrosion. It is harder and stronger than phosphor bronze. It is generally used for bushes, plungers, feed pumps, rods etc. Worm gears are frequently made from this bronze.

(e) Aluminium bronze. It is an alloy of copper and aluminium. The aluminium bronze with 6–8% aluminium has valuable cold working properties. The maximum tensile strength of this alloy is 450 MPa with 11% of aluminium. They are most suitable for





making components exposed to severe corrosion conditions. When iron is added to these bronzes, the mechanical properties are improved by refining the grain size and improving the ductility. Aluminium bronzes are widely used for making gears, propellers, condenser bolts, pump components, tubes, air pumps, slide valves and bushings, etc. Cams and rollers are also made from this alloy. The 6% aluminium alloy has a fine gold colour which is used for imitation jewellery and decorative purposes.

Aluminium It is white metal produced by electrical processes from its oxide (alumina), which is prepared from a clayey mineral called bauxite. It is a light metal having specific gravity 2.7 and melting point 658°C. The tensile strength of the metal varies from 90 MPa to 150 MPa. In its pure state, the metal would be weak and soft for most purposes, but when mixed with small amounts of other alloys, it becomes hard and rigid. So, it may be blanked, formed, drawn, turned, cast, forged and die cast. Its good electrical conductivity is an important property and is widely used for overhead cables. The high resistance to corrosion and its non-toxicity makes it a useful metal for cooking utensils under ordinary condition and thin foils are used for wrapping food items. It is extensively used in aircraft and automobile components where saving of weight is an advantage. Aluminium Alloys The aluminium may be alloyed with one or more other elements like copper, magnesium, manganese, silicon and nickel. The addition of small quantities of alloying elements converts the soft and weak metal into hard and strong metal, while still retaining its light weight. The main aluminium alloys are discussed below: 1. Duralumin. It is an important and interesting wrought alloy. Its composition is as follows: Copper = 3.5 – 4.5%; Manganese = 0.4 – 0.7%; Magnesium = 0.4 – 0.7%, and the remainder is aluminium.

This alloy possesses maximum tensile strength (upto 400 MPa) after heat treatment and age hardening. After working, if the metal is allowed to age for 3 or 4 days, it will be hardened. This phenomenon is known as age hardening. It is widely used in wrought conditions for forging, stamping, bars, sheets, tubes and rivets. It can be worked in hot condition at a temperature of 500°C. However, after forging and annealing, it can also be cold worked. Due to its high strength and light weight, this alloy may be used in automobile and aircraft components. It is also used in

manufacturing connecting rods, bars, rivets, pulleys, etc.





Die – casting Alloys: The most of the die castings are produced from Zinc Base Alloys. These alloys can be casted easily with a good finish at fairly low temperatures. They have also considerable strength and are low in cost. The usual alloying elements for Zinc Base Alloys are aluminium, copper and magnesium and they are all held in close limits. The composition of two standard die casting zinc alloys are as follows :

1. Aluminium 4.1%, copper 0.1%, magnesium 0.04% and the remainder is zinc. 2. Aluminium 4.1%, copper 1%, magnesium 0.04% and the remainder is zinc.

Aluminium improves the mechanical properties and also reduces the tendency of zinc to dissolve iron. Copper increases the tensile strength, hardness and ductility. Magnesium has the beneficial effect of making the castings permanently stable. These alloys are widely used in the automotive industry and for other high production markets such as washing machines, oil burners, refrigerators, radios, photographs, television, business machines, etc.

Nickel Base Alloys The nickel base alloys are widely used in engineering industry on account of their high mechanical strength properties, corrosion resistance, etc. The most important nickel base alloys are discussed below: 1. Monel metal. It is an important alloy of nickel and copper. It contains 68% nickel, 29% copper and 3% other constituents like iron, manganese, silicon and carbon. Its specific gravity is 8.87 and melting point 1360°C. It has a tensile strength from 390 MPa to 460 MPa. It resembles nickel in appearance and is strong, ductile and tough. It is superior to brass and bronze in corrosion resisting properties. It is used for making propellers, pump fittings, condenser tubes, steam turbine blades, sea water exposed parts, tanks and chemical and food handling plants. 2. Inconel. It consists of 80% nickel, 14% chromium, and 6% iron. Its specific gravity is 8.55 and melting point 1395°C. This alloy has excellent mechanical properties at ordinary and elevated temperatures. It can be cast, rolled and cold drawn. It is used for making springs which have to withstand high temperatures and are exposed to corrosive action. It is also used for exhaust manifolds of aircraft engines. 3. Nichrome. It consists of 65% nickel, 15% chromium and 20% iron. It has high heat and oxidation resistance. It is used in making electrical resistance wire for electric furnaces and heating elements. 4. Nimonic. It consists of 80% nickel and 20% chromium. It has high strength and ability to operate under intermittent heating and cooling conditions. It is widely used in gas turbine engines.





Bearing Alloys The following are the widely used bearing metals Alloys: 1. The copper base alloys (Phosphor bronze, Beryllium bronze) are the most important

bearing alloys. These alloys are harder and stronger than the white metals (lead base and tin base alloys) and are used for bearings subjected to heavy pressures.

2. Lead-base alloys T are employed where a cheap and corrosion resistant material is required. An alloy containing 83% lead, 15% antimony, 1.5% tin and 0.5% copper is used for large bearings subjected to light service.

3. Tin base alloys is An alloy containing 83% lead, 15% antimony, 1.5% tin and 0.5% copper is used for large bearings subjected to light service. Alloy is used for lining bearings subjected to high speeds like the bearings of aero-engines

A tin base alloy containing 88% tin, 8% antimony and 4% copper is called babbit metal. It is a soft material with a low coefficient of friction and has little strength. It is the most common bearing metal used with cast iron boxes where the bearings are subjected to high pressure and load.

Note : Those alloys in which lead and tin are predominating are designated as white metal bearing alloys. This alloy is used for lining bearings subjected to high speeds like the bearings of aero-engines.

4. The cadmium base alloys contain 95% cadmium and 5% silver. It is used for medium loaded bearings subjected to high temperature. The selection of a particular type of bearing metal depends upon the conditions under which it is to be used. It involves factors relating to bearing pressures, rubbing speeds, temperatures, lubrication, etc.

A bearing material should have the following properties:

1. It should have low coefficient of friction. 2. It should have good wearing qualities. 3. It should have ability to withstand bearing pressures. 4. It should have ability to operate satisfactorily with suitable lubrication means at the maximum rubbing speeds. 5. It should have a sufficient melting point. 6. It should have high thermal conductivity. 7. It should have good casting qualities. 8. It should have minimum shrinkage after casting. 9. It should have non-corrosive properties. 10. It should be economical in cost.





IS standards codes for steels. According to Indian standard [IS : 1762 (Part-I)–1974], a new system of designating the steel is recommended. According to this standard, steels are designated on the following two basis :

(a) On the basis of mechanical properties, and (b) On the basis of chemical composition.

The designation of steel on the above two basis: These steels are carbon and low alloy steels where the main criterion in the selection and inspection of steel is the tensile strength or yield stress. According to Indian standard IS: 1570 (Part–I)- 1978 (Reaffirmed 1993), These steels are designated by a symbol ‘Fe’ or ‘Fe E’ depending on whether the steel has been specified on the basis of minimum tensile strength or yield strength, followed by the figure indicating the minimum tensile strength or yield stress in N/mm2. For example ‘Fe 290’ means a steel having minimum tensile strength of 290 N/mm2 and ‘Fe E 220’ means a steel having yield strength of 220 N/mm2.

Table 1. Indian standard designation of steel according to IS : 1570 (Part I)-1978 (Reaffirmed 1993).





Notes :

1. The steels from grades Fe 290 to Fe 490 are general structural steels and are available in the form of bars, sections, tubes, plates, sheets and strips.

2. The steels of grades Fe 540 and Fe 620 are medium tensile structural steels. 3. The steels of grades Fe 690, Fe 770 and Fe 870 are high tensile steels.

Steels Designated on the Basis of Chemical Composition According to Indian standard, IS : 1570 (Part II/Sec I)-1979 (Reaffirmed 1991), the carbon steels are designated in the following order : (a) Figure indicating 100 times the average percentage of carbon content, (b) Letter ‘C’, and (c) Figure indicating 10 times the average percentage of manganese content. The

figure after multiplying shall be rounded off to the nearest integer.

For example 20C8 means a carbon steel containing 0.15 to 0.25 per cent (0.2 per cent on an average) carbon and 0.60 to 0.90 per cent (0.75 per cent rounded off to 0.8 per cent on an average) manganese.

Table 2 Indian standard designation of carbon steel according to IS : 1570 (Part II/Sec 1) – 1979 (Reaffirmed 1991).





Indian Standard Designation of Low and Medium Alloy Steels According to Indian standard, IS : 1762 (Part I)-1974 (Reaffirmed 1993), low and medium alloy steels shall be designated in the following order :

1. Figure indicating 100 times the average percentage carbon. 2. Chemical symbol for alloying elements each followed by the figure for its

average percentage content multiplied by a factor as given below :

Element Multiplying factor

Cr, Co, Ni, Mn, Si and W 4

Al, Be, V, Pb, Cu, Nb, Ti, Ta, Zr and Mo 10

P, S and N 100

For example 40 Cr 4 Mo 2 means alloy steel having average 0.4% carbon, 1% chromium

and 0.25% molybdenum.

Notes : 1. The figure after multiplying shall be rounded off to the nearest integer. 2. Symbol ‘Mn’ for manganese shall be included in case manganese content is

equal to or greater than 1 per cent. 3. The chemical symbols and their figures shall be listed in the designation in the

order of decreasing content.

Table 2.8 shows the composition and uses of some low and medium alloy steels according to

Indian standard IS : 1570-1961 (Reaffirmed 1993). Indian Standard Designation of High Alloy Steels (Stainless Steel and Heat Resisting Steel) According to Indian standard, IS : 1762 (Part I)-1974 (Reaffirmed 1993), the high alloy steels (i.e. stainless steel and heat resisting steel) are designated in the following order:

1. Letter ‘X’. 2. Figure indicating 100 times the percentage of carbon content. 3. Chemical symbol for alloying elements each followed by a figure for its average

percentage content rounded off to the nearest integer. 4. Chemical symbol to indicate specially added element to allow the desired properties.

For example, X 10 Cr 18 Ni 9 means alloy steel with average carbon 0.10 per cent, chromium 18 per cent and nickel 9 per cent.





Indian Standard Designation of High Speed Tool Steel According to Indian standard, IS : 1762 (Part I)-1974 (Reaffirmed 1993), the high speed tool steels are designated in the following order :

1. Letter ‘XT’. 2. Figure indicating 100 times the percentage of carbon content. 3. Chemical symbol for alloying elements each followed by the figure for its average percentage content rounded off to the nearest integer, and 4. Chemical symbol to indicate specially added element to attain the desired properties.

For example, XT 75 W 18 Cr 4 V 1 means a tool steel with average carbon content 0.75 per cent, tungsten 18 per cent, chromium 4 per cent and vanadium 1 per cent.





Corrosion: Most materials, to one degree or another, experience some type of interaction with a large number of diverse environments. Such interactions with environments impair a material’s usefulness as a result deterioration of its mechanical properties takes place, e.g. ductility and strength, appearance, other physical properties.

Control of corrosion and erosion or chemical attack in service and to prevent of surface decarburization during processing or heat treatment. Corrosion is defined as the destructive and unintentional attack of a metal; it is electrochemical and ordinarily begins at the surface. The problem of metallic corrosion is one of significant proportions; in economic terms, it has been estimated that approximately 5% of an industrialized nation’s income is spent on corrosion prevention and the maintenance or replacement of products lost or contaminated as a result of corrosion reactions. The consequences of corrosion are all too common. Familiar examples include the rusting of automotive body panels and radiator and exhaust components. In metals, there is actual material loss either by dissolution (corrosion) or by the formation of non metallic scale or film (oxidation). The following types of corrosions are significant: (i) Direct corrosion, (ii) Electrochemical corrosion, (iii) Galvanic corrosion, and (iv) High temperature oxidation corrosion. (ii) ELECTROCHEMICAL CORROSION This kind of corrosion develops in liquid electrolytes: moist atmosphere and soil, salt and fresh water, aqueous solutions of salts, alkalies and acids. Electrochemical corrosion is characterized by the appearance of an electric current and dissolution of the metal due to its electrochemical interaction with the electrolyte. There is a transfer of electrons from one chemical species to another. Metal atoms characteristically lose or give up electrons in what is called an oxidation reaction. The surface of a metal is electrochemically inhomogeneous so that when the metal is placed into an electrolyte, this leads to the formation of microgalvanic corrosion cells (Fig. 4.1). In some portions of the surface, which are called anodes, there occurs reactions of the type (1) and other portions, called cathodes, of the type(2) and (3)





Fig. 4.1 Scheme of a corrosion cell

The site at which oxidation takes place is called the anode and oxidation is sometimes called an anodic reaction. The electrons generated from each metal atom that is oxidized must be transferred to and become a part of another chemical species in what is termed a reduction reaction (eq. 3). Other reduction reactions are possible, depending on the nature of the solution to which the metal is exposed. For an acid solution having dissolved oxygen, the reduction reaction

O2 + 4H+ + 4e – → 2H2O (6)

may probably occur. For a neural or basic aqueous solution in which oxygen is also dissolved, reduction reaction given by (2) will take place. Any metal ions present in the solution may also be reduced. For ions that can exist in more than one valence state (multivalent ions), reduction may occur in accordance by

Men+ + e– → M(n + 1)+ (7)

in which the metal ion decreases its valence state by accepting an electron. It is also possible that a metal may be totally reduced from an ionic to a neutral metallic state as

Men+1 + ne– → Me (8)

The location at which reduction occurs is called the cathode. We may note that two or more of the reduction reactions above may occur simultaneously. As long as corrosion cell remains open, the reactions at the anode and the cathode portions proceed at the same rate in both directions, i.e. they are reversible. An overall electrochemical reaction must consist of at least one oxidation and one reduction, and will be the sum of them. We may note that often the individual oxidation and reduction reactions are termed as half-reactions. However, there can be no net electrical charge accumulation from the electrons and ions, i.e., the total rate of oxidation must be equal to the total rate of reduction. Obviously, all electrons generated through oxidation must be consumed by reduction. To make it clear, let us consider an example in which zinc metal is immersed in an acid solution containing H+ ions. At some





Silverware gets tarnished in the presence of the atmosphere by hydrogen sulphide. The reaction is as follows

Ag + H2S → AgS + H2 (15) We may note that these reactions are common to all metals and the scales formed are uniform throughout the surface, i.e., direct attack proceeds uniformly over the entire surface of the metal. As a consequence of oxidation, the metal ions may either go into the corroding solution as ions (reaction 9), or they may form an insoluble compound with non metallic elements (reaction 13). (iii) GALVANIC (TWO-METAL) CORROSION A reaction takes place when a metal comes in contact with an electrolyte, releasing the metal ions into the electrolyte. However, very soon an equilibrium is established and the reaction does not proceed further. Now, if a dissimilar metal forms a second electrode, there will be a similar equilibrium at this electrode also. Now, if the circuit is completed allowing for the flow of current, the equilibrium is disturbed. Let us consider Copper (Cu) as the first electrode, Zinc (Zn) as the second electrode (Fig.3), and water as the electrolyte. This arrangement is called as galvanic cell. If the current continues to flow, a less noble metal Zn (in comparison to Cu) will gradually dissolve in the electrolyte or in other words corrode. Zn is the anode of the galvanic cell. The more noble metal copper will act as cathode. The reactions that occur at the two electrodes are as follows:

Zn → Zn2+ + 2e– (anode reaction) (23) Copper will deposit on the cathode through the following reduction reaction

Cu2+ + 2e– → Cu (24)





Fig. 3 Galvanic corrosion

If cupric ions are not available in the electrolyte, the reduction at the cathode may still proceed through other reactions. Hydrogen ion may get reduced, resulting in the evolution of hydrogen at the cathode

2H+ + 2e– → H2 (cathode reaction) (25) Alternatively, the reduction may take place through oxygen and water combining with the excess electrons to produce hydroxyl ions:

O2 + 2H2O + 4e – → 4OH– (26) A galvanic cell may be set up due to the differences in concentration of the metal ion in the electrolyte (Fig. 3). The potential difference and the rate of the anodic metal will depend upon the relative positions of the two metals in the galvanic series (Following Table ). A galvanic cell set up due to such differences in metal ion concentration is also called as concentration cell. Following Table represents the relative reactivities of number of metals and commercial alloys in seawater (galvanic series). The alloys near the top of the table are cathodic and unreactive, where those at the bottom of the table are most anodic; no voltages are provided. Interestingly, a comparison of the standard emf and the galvanic series reveals a high degree of correspondence between the most relative positions of pure base metals. In a wide variety of environments, most metals and alloys are subject to oxidation or corrosion to one degree or another, i.e., they are more stable in an ionic state than as metals. In going from metallic to oxidized states, thermodynamically there is a net decrease in free energy. This is why, essentially all metals occur in nature as compounds, e.g. oxides, hydroxides, carbonates, silicates, sulfides and sulphates. There are two notable exceptions; gold and platinum. For these two metals, oxidation in most environments is not favourable, and therefore, both of these two





metals may exist in nature in the metallic state. In order to reduce galvanic corrosion effects following measures may be taken:

(i) If coupling of dissimilar metals is necessary, one may choose two that are close together in the galvanic series.

(ii) One should use anode area as large as possible and avoid an unfavourable anode-to-cathode surface area ratio.

(iii) One should insulate electrically metals from each other. (iii) Connect electrically third metal to the other two. This is known as cathodic

protection.

Corrosion of dissimilar metals in contact with each other like the galvanic cell is rapid. If we can avoid direct contact between dissimilar metals, we can avoid the corrosion of anodic metal. That is why, in fabrication work, one prefers the composition of electrodes in case of welding and of rivets in riveting should be of the same chemical composition as the base metal. Galvanic corrosion also found to occur on the unprotected steel of ships due to the selective action of steel on the brass screws. An underground pipe line that passes through impervious clays in some regions and through porous sands in some other regions may corrode in clay region. A galvanic cell may also be formed due to different residual stresses in the same metal. The stressed region is more active and is anodic with respect to a stress free region. Such types of





galvanic cells are called stress cells. Such types of cells can be formed between regions of different dislocation density in a cold worked metal or in a polycrystalline metal, where the grain boundaries are anodic to the interior of the grains. Interestingly, a bent wire is likely to corrode at the bend, where it has been plastically deformed CORROSION RATES The rate of material removal as a consequence of the chemical action, or the corrosion rate, is an important parameter. One can express this as the corrosion penetration rate (CPR), or the thickness loss of material per unit time. It is expressed as

CPR = 𝐾𝑊

𝜌𝐴𝑡 (27)

Where W is the weight loss after exposure time t; ρ and A represent the density and exposed specimen area, respectively, and K is a constant, its magnitude depends on the system of units used. For convenience, CPR is expressed in terms of either mils per year (mpy) or millimeters per year (mm/yr). (iv) HIGH TEMPERATURE OXIDATION OR DRY CORROSION Ferrous alloys get rusted at high temperatures forms scales and oxides. One can prevent this corrosion by adding alloying elements which are resistant to oxidation in some metals and also have good properties of high temperature service. When liquid metals flow past other metals, high temperature corrosion also occurs. Essentially, a corrosion reaction is a process of mass transfer and is not dependent upon local cell potentials for its driving force. Truly speaking, the corrosion is due to the tendency of the solid to dissolve in the liquid metal upto the solubility limit at a given temperature. We may note that the liquid-metal attack may either form a simple solution of the solid metal, a chemical compound, or be the selective extracton of one of the component metals in a solid alloy. This occurs when there is a concentration gradient or temperature gradient within the solid-liquid system. Serious damage by liquid-metal attack has been reported in heat exchangers carrying (Bi and Na) liquid coolants. A portion of the solid goes into solution in the liquid as the solid container usually copper tubing, approaches equilibrium with the liquid-metal coolant in the hot zone of heat exchanger. The solids try to deposit on the walls of the heat exchanger tubes, as the liquid moves to the cooler part of the heat exchanger. Obviously, the hot zone is continually corroded and the cold zone becomes plugged with the deposited corrosion products. One can prolong the life of heat exchangers by the addition of certain inhibitors to the liquid alloy to form protective films to prevent high temperature corrosion. SPECIFIC FORMS OF CORROSION One can conveniently classify corrosion according to the manner in which it is manifest. Usually, metallic corrosion is classified into eight different forms: uniform, galvanic, crevice, pitting, intergranular, selective leaching, erosion-corrosion and stress corrosion. In addition, there is hydrogen embrittlement. Truly speaking, hydrogen embrittlement is a type of failure rather than a form of corrosion. However, hydrogen embrittlement is often produced by hydrogen that is generated from corrosion reactions.





(i) Uniform Corrosion: This is a form of electrochemical corrosion that occurs when the whole surface of the metal is corroded to the same degree and when the metal surface and environment are uniform. This type of corrosion often leaves behind a scale or deposit. Under these conditions one can easily estimate the useful life of a given material and there is no need to fear about unexpected failure. One can easily control the uniform corrosion attack by selecting suitable materials. In practice such a corrosion is rare. Uniform corrosion is generally observed in metals like Zn, Pb and Al. (ii) Crevice Corrosion: Electrochemical corrosion can also occur as a consequence of concentration differences of ions or dissolved gases in the electrolyte solution, and between two regions of the same metal piece. For such a concentration cell, corrosion takes place in the local that has the lower concentration. Corrosion occurring in crevices and recesses or under deposits of dirt or corrosion products where the solid solution becomes stagnant and there is localized depletion of dissolved oxygen are few good examples of this type of corrosion. Corrosion preferentially occurring at these positions is known as crevice corrosion. In order to penetrate the crevice must be wide enough, yet narrow enough for stagnacy. We may note that usually the width is several thousandths of an inch. After oxygen has been depleted within the crevice, oxidation of the metal occurs at this position (Eq. 8). Electrons from this reaction are conducted through the metal to adjacent external regions, where they are consumed by reduction (probably in accordance with reaction Eq. (2)). In several aqueous environments, the solution within the crevice has been found to develop high concentrations of H+ and Cl– ions, which are especially corrosive. Since protective films are often destroyed by the H+ and Cl– ions and therefore many alloys that passivate are susceptible to crevice corrosion One can prevent this type of corrosion by using welded instead of riveted or bolted joints, using non absorbing gaskets when possible, removing accumulated deposits frequently, and designing containment vessels to avoid stagnant areas, and ensure complete drainage. (iii) Pitting Corrosion: This type of very localized corrosion attack occurs when the whole surface of the metal is corroded to the same degree and when the metal surface and environment are uniform. This type of corrosion attack produces fine pits or holes at various locations on the metal surface. They ordinarily penetrate from the top of a horizontal surface downward in a nearly vertical direction. Pitting corrosion results either of the following reasons: (a) localized difference in the composition of the metal surface, or its smoothness, (b) inhomogeneous external environment, (c) insoluble corrosion product (d) protective films on the metal surface having not been uniformly perfect (e) crystallographic directions having not been equal in reactivity, and (f) a small spot on the surface being continuously anodic with current flowing from it at a relatively high density. Pitting corrosion results from an electrochemical reaction between the broken film as anode and the surrounding unbroken film as cathode which form a small galvanic cell and produce pits. The mechanism for pitting is probably the same as for crevice corrosion in that oxidation occurs within the pit itself. This type of corrosion is observed in aluminium, steel, copper and





nickel alloys. Stainless steel alloys are somewhat susceptible to this form of corrosion. However, alloying with about 2% molybdenum enhances their resistance significantly. Figure 4 illustrates the pitting corrosion. A pit may be initiated by a localized surface defect such as a scratch or a slight variation in composition. However, pits are randomly formed on the surface. The shapes of pits vary widely, they are normally hemispherical with electropolished inner surfaces. It has been observed that specimens having polished surfaces display a greater resistance to pitting corrosion. Pits have considerable effect on mechanical properties of metals, for example fatigue.

Fig.4 Pitting Corrosion

(iv) Intergranular Corrosion: This type of corrosion occurs preferetially along grain boundaries for some alloys and in some specific environments. This occurs when a pronounced difference in reactivity exists between grain boundaries and the reminder of the alloy. This type of corrosion is especially prevalent in some stainless steels. When heated to temperatures between 500 and 800°C for sufficiently long time periods, these alloys become sensitized to intergranular attack. Probably, this type of heat treatment causes the formation of small precipitate particles of Chromium Carbide (Cr23C6) by reaction between the chromium and carbon in the stainless steel. To form the precipitates, both the chromium and the carbon must diffuse to the grain boundaries. This leaves a chromium-depleted zone adjacent to the grain boundary. This grain boundary region is highly susceptible to corrosion. Selective corrosion attack between the grains of metals and alloys may occur due to potential differences set up there when:

(a) when there is precipitation in the grain boundaries and/or (b) even because of difference in grain orientations.

Intergranular corrosion is normally observed in defective welding and heat treatment of stainless steels, copper and aluminium alloys. One can examine intergranular corrosion with the help of microscope. Figure 5(a) shows the intergranular corrosion, where the grain boundaries have been etched, i.e., corroded, the atoms at the boundaries have a different electrode potential than the atoms in the grain proper. Obviously, anode and cathode are developed,





resulting in galvanic stress cells causing corrosion of the anode, i.e. the grain boundary. We may note that a fine grained metal will have a higher corrosion rate than a coarse grained metal as there is no anode area.

Fig. 5(a) Intergranular corrosion, the grain Fig.5(b) Intergranular corrosion caused by the boundaries act as the anode because the precipitation of certain compound at boundary atoms have a high potentials grain boundaries (v) Selective Leaching or corrosion and Dezincification: This type of corrosion is usually found in solid solution alloys and occurs when element or constituent is preferentially removed as a consequence of corrosion processes. The most common example of selective corrosion is the dezincification of brass. In this type of corrosion, the zinc atoms are dissolved from the solid solution leaving behind copper as the spongy mass which has practically no strength. The mechanical properties of the copper-brass zinc alloy are significantly impaired, since only a porous mass of copper remains in the region that has been dezincified. Moreover, the material changes from yellow to a red or copper colour. It is found that the selective leaching may also occur with other alloy systems in which Al, Fe, Co, Cr, and other elements are vulnerable to preferential removal. In order to overcome from dezincification, one may add tin, antimony or phosphorus to brasses containing Zn more than 15% Zn. (vi) Atmospheric Corrosion: This type of corrosion is very frequent on ferrous materials. Rain water and humidity, in air are mainly responsible for this type of corrosion and act as electrolytes. This type of corrosion is primarily due to the formation and breakdown of films by the moisture and electrochemical attack on the exposed metal. Obviously, this follows the oxygen-absorption mechanism. It is observed that cracks and discontinuities in the film produced expose fresh areas to the formation of corrosion cells. (vii) Erosion-Corrosion: This type of corrosion arises from the combined effect of chemical attack and mechanical abrasion or wear as a consequence of fluid motion. Mechanical abrasion is produced by the turbulent flow of liquids or the impingement of trapped gases. Normally, all metal alloys, to one degree or another, are susceptible to erosion-corrosion. Erosion-corrosion causes accelerated attack because it mechanically removes the protective layer that normally builds up on the corroding surface. Obviously, this type of corrosion is especially harmful to





alloys that passivate by forming a protective surface film. When the coating is not capable of continuously and rapidly reforming as a protective barrier, erosion-corrosion may be severe. This type of corrosion action contributes to the formation of localised pitting cells. Usually, this type of corrosion is associated with the formation of cavities in the metal by fast-moving liquids. Relatively soft metals such as copper and lead are also sensitive to this form of corrosion attack. This type of corrosion is generally found in piping, especially at bends, elbows, and abrupt changes in pipe diameter positions where the fluid changes direction or flow suddenly becomes turbulent. Propellers, turbine blades, valves, and pumps are also susceptible to erosion-corrosion. Figure 7 illustrates the erosion-corrosion in a pipe-line. In order to reduce erosion-corrosion, one can change the design to eliminate fluid turbulence and impingement effects. One can also utilize other materials that inherently resist erosion. Moreover, removal of particulates and bubbles from the solution may lessen its ability to erode. (viii) Stress Corrosion: This type of corrosion also termed as stress corrosion cracking is produced by the combined effect of mechanical stress and a corrosive environment on a metal. The stress that produces this type of corrosion need not be externally applied; it may be a residual one that results from rapid temperature changes and uneven contraction, or for two phase alloys in which each phase has a different coefficient of expansion. One may note that in the presence of corrosive environments, the strained area acting as anode while unstrained area as cathode give rise to a type of galvanic-cell, called stress-cell. Obviously, corrosion starts at the strained area due to electrochemical action of the cell It is found that the stress corrosion rate is dependent upon the corrosive agent, time and temperature of exposure, behaviour of protective films and stress magnitude. Under high tensile stress, the absorption of hydrogen in metal causes embrittleness. This phenomenon is usually known as hydrogen embrittleness. When mild steel exposed to alkaline solutions at high temperatures and stresses, one can observe hydrogen embrittleness

This type of corrosion occurs predominantly in metal components having internal stresses (which may be due to cold working or otherwise) and being used in corrosive environment. In fact, some materials that are virtually inert in a particular corrosive medium become





susceptible to this form of corrosion when a stress is applied. Pile up of dislocations at grain boundaries in cold-worked metals increases the energy in such regions and makes anodes in the structure in certain environments. Small cracks form and then propagate in a direction perpendicular to the stress, with the result that failure may eventually occur. Season cracking in brass, especially in the presence of moisture and traces of ammonia, and caustic embrittlement of steel exposed to solutions containing sodium hydroxide are few well known examples of stress corrosion. The most effective control of stress corrosion is the elimination of tensile stress from the component part or lower the magnitude of the stress. This may be achieved by reducing the external load or increasing the cross-sectional area perpendicular to the applied stress. Furthermore, an appropriate heat treatment may be used to anneal out any residual thermal stresses. In case of cold worked metal, the corrosion takes place at the strained portion. In the strained area, the atoms of metal are not in their lowest energy position. Consequently, strained area of the metal (having internal stresses) has higher electrode potential than the strain-free area. (ix) Corrosion Fatigue: This is a type of failure that results from the simultaneous action of a cyclic stress and chemical attack. Now, it is well understood that corrosion becomes the cause of cracking while fatigue may cause the crack propagation. A corrosion crack may propagate even under static stresses. We may note that the combined action of corrosion and repeated stresses is far more serious than the sum of these two factors acting independently. Corrosion fatigue is also the reduction of fatigue strength of materials due to the presence of a corrosive medium. The alternates slip taking place at granular legal creates minute regions of high strain energy which tend to become anodic with respect to surrounding regions of low strain energy. The effect of corrosion fatigue is clearly observed in heat exchangers where variable stresses are produced because of thermal expansion and contraction. We may say that under corrosive conditions and fatigue loading the material is in most critical state. One can express the effect of corrosion fatigue strength by the damage ratio as

Damage ratio = 𝑐𝑜𝑟𝑟𝑜𝑠𝑖𝑜𝑛 𝑓𝑎𝑡𝑖𝑔𝑒 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ

𝑁𝑜𝑟𝑚𝑎𝑙 𝑓𝑎𝑡𝑖𝑔𝑢𝑒 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ

The damage ratio for salt as a corroding medium is ~0.2 for carbon steels, ~0.5 for stainless steels and bout 1.0 for copper. The treatment of the corroding medium and surface protection of the metal may be suitable protective measures against corrosion fatigue.





(x) Fretting: Fretting corrosion is a type of corrosion which occurs at interface between material surfaces under high pressure, if slight slip occurs. Usually, this type of corrosion takes place when two closely fitted metal surfaces, e.g. when bolted or rivetted joints, or press fitted hubs on shafts are subjected to vibrations. Molecular plucking of the surface particles takes place in the initial stage, which oxidise to form a debris. In case of steel this debris is like red rust. Fretting corrosion depends on load, temperature, humidity, presence of oxygen, and metallurgical factors. One can minimise the effects of this type of corrosion by avoiding relative motion between the two surfaces by introducing compressive stresses and by heat treatment. Fretting corrosion reduces at high humidities and increases in the presence of oxygen. It decreases with the decrease in temperature. It is found to be more in soft materials than that in hard materials. (xi) Hydrogen Embrittlement: A significant reduction in ductility and tensile strength is bserved when atomic hydrogen (H) penetrates into various metal alloys, and specifically some steels. This phenomenon is usually referred to as hydrogen embrittlement and sometimes also called as hydrogen-induced cracking and hydrogen stress cracking. We can say that hydrogen embrittlement is a type of failure, in response to applied or residual tensile stresses. Brittle fracture occurs catastrophically as cracks grow and rapidly propagate. Interestingly, hydrogen in its atomic form (H as opposed to the molecular form, H2) diffuses interstitially through the crystal lattice, and concentrations as low as several parts per million can lead to cracking. We may note that hydrogen-induced cracks are most often transgranular. Hydrogen embrittlement is quite similar to stress corrosion. In order that hydrogen embrittlement may occur, some source of hydrogen must be present, and, in addition, the possibilities for the formation of its atomic species. Pickling (a procedure used to remove surface oxide scale from steel pieces by dipping them in a vat of hot, dilute sulphuric or hydrochloric acid) of steels in H2SO4, electroplating, and the presence of hydrogen-bearing atmospheres (including water vapour) at elevated temperatures, e.g. during welding and heat treatments are such situations wherein these conditions are met. Moreover, the presence of poisons such as sulphur, i.e. H2S and arsenic compounds accelerates hydrogen embrittlement.

Methods of protection from corrosion. Perhaps the most common and easiest way of preventing corrosion is the proper selection of the materials, once we have characterized the corrosion environment. However, one has also to examine the economic feasibility prior to employing the material that provides the optimum corrosion resistance, for example, one can use noble metals in very limited applications such as ornaments and delicate scientific instruments.





In order to control corrosion, the materials would have to be absolutely uniform without any heterogeneities in either composition or structure and at the same time the environment would also have to be entirely uniform. The following methods are widely used for the control and prevention of corrosion:

(i) Using high-purity metals and alloy additions (ii) Proper design against corrosion (iv) Proper modification of corrosive environment (iv) Making use of protective coatings (v) Making use of inhibitors (vi) Cathodic protection

(i) Use of High-Purity Metal and Alloy Additions: One can improve the corrosion resistance of a given metal by avoiding galvanic couples, i.e. limiting designs to only one metal. However, this is not always feasible. By selecting suitable electrically insulating metals of different omposition or by alloy addition, one may avoid the cells in special circumstances. One can increase the corrosion resistance of most metals by alloying them with suitable alloying elements. Alloying may increase corrosion resistance by increasing the energy level of the solution, reducing the driving emf in the galvanic cell, and protective coatings and decreasing the mobility of the corrosive ions. Alloying may however help in the following ways:

(i) To avoid carbon precipitation in heat treatment. (ii) To prepare steels with high chromium content. We may note that chromium

corrodes less readily. (iii) To make steels containing strong carbide formers. Titanium and tantalum are

such elements. Interestingly, these elements do not permit carbon precipitation at the grain boundary. These methods reduce intergranular corrosion in metals. This technique is mainly used in stainless steel which must be fabricated by welding.

(ii) Design Against Corrosion: One can design to avoid physical contact between dissimilar

metals so that a galvanic couple does not form. In addition, the design should allow for complete drainage in the case of a shutdown, and easy washing. It is found that issolved oxygen may enhance the corrosivity of many solutions, the design should, if possible, include provision for the exclusion of air. In order to prevent, one may ensure the following points:

(i) As far as possible avoid of dissimilar metal contracts. When contact between dissimilar metals is unavoidable, one may take care that the metal forming the anode does not have a small surface area as compared to the cathode.

(ii) There must be cathodic protection. (iii) One must avoid recesses and sharp corners. (iv) Avoidance of excessive concentration with proper fabrication. (v) One must prevent the retention of liquid with air tight joints.





(vi) Surfaces must be clean, i.e., free from foreign matter like dust, dirt and soot.

(vii) Wherever possible, one must use corrosion-resistant materials. (viii) We may take care that the two different metals should be as close as

possible. (iii) Modification of Corrosive Environment: It is found that changing the character of environment, if possible, may also significantly influence corrosion. One can achieve large saving in materials if these are used in less corrosive environments. Lowering the fluid temperature and/or velocity usually helps in the reduction in the rate at which corrosion occurs. One can modify the environment in the following ways:

(i) Lowering the temperature, pressure, concentration, velocity, etc. (ii) By changing humidity, oxygen content, oxidizing agents and solid

impurities, one may alter the chemical composition of environment. It is found that purified and dehumidified atmosphere around the structure helps in reducing corrosion.

(ii) By adding inhibitors, i.e. those substances in low concentrations to the environments, which decreases corrosiveness of the environment. For example, alkaline neutralizers are used to prevent the corrosive effect of the environment by neutralizing the acidic character of corrosive conditions. To prevent high-temperature corrosion of ferrous metals and alloys, vacuum and inert gases are also found useful. Of course, the specific inhibitor depends both on the alloy and on the corrosive environment.

(iii) (iv) Use of Protective Surface Coatings: Physical barriers to corrosion are widely applied on

surfaces in the form of films and coatings. There are good number of diverse metallic and non-metallic coating materials available. It is essential that the coating should have a good corrosion resistance when in use, and maintain a high degree of surface adhesion, which undoubtedly requires some preapplication surface treatment. In several cases, the coating must be virtually non reactive in the corrosive environment and resistant to mechanical damage that exposes the bare metal to the corrosive environment. In general, all three material types—metals, ceramics and polymers are used as coatings for metals.

Metallic Coating

In the protection of the underlying metal by metallic coating, two factors are mainly involved: (i) mechanical isolation of the metal from the corroding environment and (ii) galvanic relation of coating metal and the base metal.

If the metal coating is baser than the substrate (e.g., zinc or aluminium on steel), galvanic protection is offered to the underlying metal. The coating is anodic and corrodes first. However, Zn and Al become passive after the initial attack. Nickel and chromium plating give an





decorative attractive appearance. Hard chromium plating produces a thicker, wear resistant surface. The tin coating on steel article is produced so that tin plate may have excellent corrosion and fabrication qualities. The most common use of tin plate is for making food containers. The following methods are widely used to apply metallic coatings: (i) hot dipping, (ii) electroplating, (iii) metal cladding, (iv) high temperature diffusion, (v) metal spraying etc. Zinc coating on steel (galvanized coating) and the tin coating on a steel article are usually produced by dipping clean sheet steel into molten zinc or a tin bath. Galvanized iron (G.I.) is produced by dipping a low carbon steel sheet in molton zinc bath at about 450°C. The zinc coating crystallizeds forming ‘zinc flowers’ when cooled in air. Articles, e.g. buckets and drums prepared from galvanized iron are very suitable for aqueous environments. However, the lives of galvanized coatings depend on the thickness of the zinc layer and environmental conditions. In the presence of oxygen, zinchydroxide is precipitated as a protective layer. One can obtain thinner, uniform coatings, of Zn and Sn by electroplating. Now a days, this process is popular for these metals. Chromium, cadmium and nickel platings are usually produced by electroplating

In this electroplating process, the coating metal is deposited on the base metal by passing a direct current through the solution of electrolyte containing a salt of the coating metal. The base metal is made the cathode and the coating material is made the anode. We may note that the coating properties mainly depends on the current intensity, agitation, temperature of the solution and the chemical composition of the plating solution. Interestingly these coatings arerelatively thick and are mostly applied to cast or machined parts. A comparatively thick lining is placed upon the metal surface forming a strong alloy bond between plates of two metals in metal cladding. This strong alloy bond is formed either by casting or hot rolling. The principal objective for cladding is the production of a corrosion resistant surface. Use of the powder metallurgy technique is made in the high-temperature diffusion process. The base metal is heated with the powder of the metal which forms the coating. When diffusion coatings are formed on steels with the help of high-temperature treatment, it is found that the





innermost core is rich in the base metal followed by successive richer coating surfaces. In the presence of powered aluminium, this treatment is known as calorizing, with powered zinc as shearadizing, and with chromium as chromising. Interestingly, these coatings are also thick. By metal spraying, metallizing or flame spraying, thick coatings of any metal can be produced. This involves the application of a spray of molten-metal droplets on the surface to be coated. We an use variety of metals. Deposits are porous, but one may offset this by applying a sealing coat of paint. If an overcoat is planned for, then this procedure may be beneficial. If the coating is anodic to the base metal as in the case of zinc or aluminium coated steel, then porosity is of less concern. Metal spraying is particularly very advantageous when a very large surface is to be treated, greater speed of working is desired and the coating is to be applied to hidden areas. Normally, three types of metal-spraying processes are in use: wire, powder and plasma. In the wire and powder metal spraying techniques, the coating metal is fed into the spray gun, where it is melted. Compressed air then atomises and sprays the molten particles onto the prepared surfaces to be coated. The new plasma spray process also uses powdered metal. An inert gas, usually argon or nitrogen is dissociated in an electric area, and the energy produced by this raises the temperature of the gas stream. Powered metal is introduced into the gas stream where it immediately melts as it is driven toward the metal surface. The plasma spray process is also used for applying non-metals, e.g. ceramics, cermets, etc., although the reason for applying them is to improve abrasion resistance rather than for corrosion protection.


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