The document discusses various materials used in engineering, particularly for vessels and equipment that are exposed to corrosive environments. It highlights the importance of corrosion allowances, material properties, and specific types of metals like stainless steel and nickel alloys, which are preferred for their corrosion resistance. Additionally, the document covers the uses and characteristics of materials such as aluminum, copper, lead, titanium, and tantalum in chemical and industrial applications.

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Stubborn Engineer
CORROSION ALLOWANCE
The corrosion all is the additional thickness of metal provided to allow the material
loss by corrosion and erosion during the desired life of the vessels. A minimum
corrosion allowance of 1.5 mm should be provided unless a protective lining is
employed.
Vessels may be fully or partially lines with corrosion-resistant materials. Such linings
may be loose, intermittently attached to the vessels base materials or integrally
bonded to the vessel base materials (for example, as clad steel.). These linings are
provided to exclude contact between the corrosive agent and the vessel base materials,
and there is no need to provide corrosion allowance against internal wastage of the
base materials.
Important Materials of construction
Iron and Steel: Low carbon steel (mild steel) is the most commonly used engineering
material: It can be easily worked and welded. It has good tensile and ductility.
The carbon steels and iron are not resistant to corrosion, except in certain specific
environments, such as concentrated sulphuric acid and the caustic alkalis. They are
suitable for use with most organic solvents, except chlorinated solvents. Mid steel is
susceptible to stress-corrosion cracking in certain environments. The high silicon
irons (14 to 15 % Si) have a high resistance to mineral acids, except hydrofluoric acid.
They are particularly suitable for use with sulphuric acid at all concentration and
temperatures.
Stainless steel: The stainless steels are the most frequently used corrosion resistant
materials in the chemical industry. Chromium is added to impart corrosion resistant in
oxidizing conditions and its content must be above 12%. Nickle is added to improve
the corrosion resistance in non-oxidizing environments.
Stainless steels are the most frequently used corrosion resistant materials in the
chemical industry. Chromium is added to impart corrosion resistant in oxidizing
conditions and its content must be above 12%. Nickle is added to improve the
corrosion resistance in non-oxidizing environments.
SS can be divided into three types according to their microstructure.
1. Ferritic 13-20 % Cr. <0.1 % C, with no nickel.
2. Austenitic 18-20 % Cr. > 7 % Ni.
3. Martensitic 12-10 % Cr. 0.2 and 0.4 % C, up to 2% Ni.
The uniform structure of austenite (fcc, with the carbides in solution) is the structure
desired for corrosion resistance. The main grades of austenitic steels are given below:
Type 304 (also called 18/8 stainless steels): It is the most widely used SS. It contains
the minimum Cr and Ni that gives a stable austenitic structure.

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Stubborn Engineer
Type 304L: It is the low carbon version of type 304 (<0.03 %C). It is used for
thicker welded sections, where carbide precipitation would occur with type 304.
Type 321; It is stabilized with titanium to prevent carbide precipitation during
welding.
Type 347; It is stabilized with niobium.
Type 316: It contains molybdenum which improves the corrosion resistance in
reducing conditions, such as it dilute sulphuric acid in particular, to solutions
containing chlorides.
Type 316L; It is a low carbon version of type 316.
The austenitic stainless steels have greater strength than the plain carbon steels,
particularly at high temperatures. Unlike the plain carbon steels, the austenitic
stainless steels do not brittle become at low temperature. The thermal conductivity of
stainless steels is lower than that of mild steel. The higher the alloying elements, the
better the corrosion resistance over a wide range of conditions, strongly oxidizing to
reducing. Intergranular corrosion (weld decay) and stress corrosion cracking are
problems associated with the use of stainless steels.
Nickle and its Alloys
The main use of nickel alloys is for equipment handling caustic alkalies at
temperature above 70°C.
Monel is the classic nickel-copper alloys with the metals in the ratio 2:1. It is easily
worked and has good mechanical properties up to 500ºC. It is not susceptible to stress
corrosion cracking in chloride solutions. Monel has good resistance to dilute mineral
acids and can be used in reducing conditions, where the SS would be unsuitable. It
may be used for equipment handling alkalies, organic acids and salts and sea water.
Inconel contains 76 % Ni, 7% Fe, 15%Cr. It is used primarily for acid resistance at
high temperature. It is resistant to furnace gases, if sulphur free.
Hastelloys were developed for corrosion resistance to strong mineral acids,
particularly HCl. The major constituents of Hastelloy B and Hastelloy C are given
below:
Component % Hastelloy B Hastelloy C
Ni % 65 54
Mo % 28 17
Fe % 6 5
Cr % 15
Copper and its alloys
Copper has been used traditionally in brewing. It is used extensively for small-bore
pipes and tubes. The main alloys of copper are the brasses, alloyed with zinc and the
bronzes, alloyed with tin. The main use of these alloys in the chemical industry is for

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valves and other small fittings and for heat-exchangers tubes and tube sheet. The
copper-nickel alloys (70 % Cu) have a resistance to corrosion-erosion and are used for
heat-exchanger tubes, particularly where sea water is used as a coolant.
Aluminum and its alloys
Pure Al has higher resistance to corrosion than its alloys but it lacks in mechanical
strength. The main structural alloys of Al are the range of Al-Cu alloys (typical
compositions 4 % Cu, with 0.5 % Mg.) These alloys have a tensile strength equivalent
to that of mil steel. The corrosion resistance of Al is due to the formulation of a thin
oxide film (as with the SS). The pure Al can be used as cladding on plates of Al
alloys, to combine the corrosion resistance of pure metal with the strength of the alloy.
It is suitable for concentrated nitric acid, greater than 80 %. It is widely used in the
textile and food industries, where the use of mild steel would cause contamination. It
is also used for storage and distribution of demineralized water.
Lead
Lead is a soft, ductile material and is mainly used in the form of sheets (as linings) or
pipe. It has a good resistance to acids, particularly sulphuric.
Titanium
Titanium has a good resistance to chloride solutions, including sea water and wet
chlorine. It depends for its resistance on the formation of an oxide film. Its alloys with
palladium (0.15%) significantly improves the corrosion resistance, particular to HCl.
Titanium is replacing Cu-Ni for use with sea water.
Tantalum
Tantalum, also called a metallic glass, is used for special applications, where glass or
a glass lining would not be suitable. Tantalum plugs are used to repair glass-lined
equipment.