Corrosion of metals:

Corrosion prevention

Corrosion may be prevented by considering the following points.Material selection Metals and alloys which resist corrosion in a particular environment can be used. Proximity of metals with large potential difference, e.g. a copper pipe on a steel
tank, should be avoided. Galvanic protection can be used, e.g. by use of a 'sacrificial anode' of zinc close to
buried steel pipe or a ship's hull.Appropriate design Crevices which hold water, e.g. bad joints and incomplete
welds, should be avoided as should high tensile stresses in material subject to stress corrosion. Locked-in internal stress due to forming should be avoided. Modijied environment Metals can be enclosured against a corrosive atmosphere,
water, etc. Drying agents, e.g. silica gel, and corrosion inhibitors, e.g. in central-heating radiators can be used.

Protective coating
Metals can be coated to make them impervious to the atmosphere, water, etc., by use of a coating of grease,
plasticizer, bitumen, resins, polymers, rubber latex, corrosion-resistant paints or metal coating.
6.12.2 Corrosion resistance of metals Ferrous metals Stainless steels Generally the best of all metals. All
types have good resistance to atmospheric corrosion except gases such as chlorine and sulphur. Some types
are suitable up to 1100 "C. Some resist sulphuric acid and some nitric acids, but not hydrochloric or hydrofluoric
acids. All resist uncontaminated organic solvents and foods and also alkalis at room temperature,
but not bleaches. They resist neutral water, but stress corrosion cracking may occur above 66 "C.
Alloy steels Chrome steel has good resistance which is improved by the addition of nickel; it can be used in
sea water. Iron-nickel steel has good resistance with over 20% nickel plus 2-3% carbon; it is used in a
marine environment.

Iron and carbon steel These readily corrode in air and especially sea water. They are subject to stress corrosion
cracking and internal stress corrosion, and require protection by painting, plating, tinning, galvanizing, etc.
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Copper and copper based alloys

Copper An oxide coating prevents corrosion from water and atmosphere, e.g. water pipes. Brass ‘Yellow brass’ (> 15%Zn) is subject to ‘dezincification’ in hot water. ‘Red brass’ (85%Cu minimum) is much better. Resistance is improved by the addition of arsenic or antimony.

Bronzes Over 5% tin gives better resistance than brass, especially to sea water and stress corrosion cracking. Aluminium bronze is good at elevated temperatures. Silicon bronze is as good but also has weldability; it is used for tanks. Cupronickel This has the best resistance of all copper alloys and is used for heat-exchanger tubes. Other metals and alloys Nickel alloys These are generally extremely resistant to caustics up to high temperature, and to neutral water and sea water. They resist some acids. Alloys such as Inconel have good resistance up to 1170 “C which increases with chromium content. Nickel alloys
have high resistance to stress corrosion cracking. Different alloys have resistance to different acids. Nickel alloys are used for tanks, heat exchangers, furnace parts, and chemical plant. Magnesium and magnesium alloys These have better resistance than steel in the atmosphere, but are inferior to aluminium. They corrode in salty air. They are fairly resistant to caustics, many solvents and fuels, but not to acids.
Titanium and titanium alloys These have excellent resistance to e.g. seawater and aqueous chloride solutions over a wide temperature range. Most alloys resist nitric acid. When alloyed with noble metals such as palladium they will resist reducing acids. These materials are high in the galvanic series and so should not be used with other metals. Zinc An oxide film gives reasonable resistance to water and normal atmosphere. Aluminium An oxide coating gives good resistance to water and atmosphere, but stress corrosion cracking occurs.

Stress corrosion cracking :Under tensile stress and in a corrosive environment some metals develop surface cracks called ‘stress corrosion cracking’ which is time dependent and may take months to develop. It is avoided by minimizing stress and/or improving the environment.

Galvanic corrosion
For a pair of metals, that highest up the ‘galvanic table’ is the ‘negative electrode’ or ‘cathode’; that lower
down is the ‘positive electrode’ or ‘anode’. The anode loses metal, i.e. corrodes, whilst the cathode remains
unchanged. The greater the potential, the greater the rate of corrosion. Hydrogen is assumed to have zero
potential.