Corrosion

1. Corrosion

2. Metals spontaneously undergo gradual destruction when exposed to the atmosphere. In chemical terminology, such destruction is known as corrosion. Corrosion is defined as the reaction of a metallic material with its environment, which causes a measurable change to the material and can result in a functional failure of the metallic component or of a complete system

3. Theories of corrosion The metal surface undergoes an electrochemical reaction with the moisture and oxygen in the atmosphere. This theory is known as electrochemical theory of corrosion. The mechanism involves the formation of a galvanic cell, by different metals or different areas on the same piece of metal.

4. Corrosion reaction on single metal

5. Corrosion reaction between metals

6. Types of corrosion • Fluid corrosion, General • Fluid corrosion, Localised • Fluid corrosion, Structural • Fluid corrosion, Biological Fluid corrosion, General: When the corrosion confined to metal surface. General corrosion is of two types. • Physicochemical corrosion: swelling, Crazing, Softening, Cracking • Electrochemical corrosion

8. Intergranular corrosion Pitting corrosion

9. Fluid corrosion, Structural In structural fluid corrosion type, the structural strength is reduced on account of corrosion. Two examples are presented here. • Graphite corrosion: The metallic iron is converted into corrosive product leaving residue of intact graphite mixed with iron corrosive products and other insoluble constituents of iron. If the layer is porous, the corrosion will progress by galvanic behaviour between iron and graphite. • When carbon steel is heated for prolonged periods at temperature higher than 4550C, carbon may get segregated, which is than transformed into graphite. Hence the structural strength of steel is affected.

10. Fluid corrosion, Biological

11. Prevention & Control of Corrosion • Proper design of equipment • Coating and linings • Inhibitors

12. Proper design of equipment: • Elimination of crevices • Complete draining of liquid • Ease of cleaning • Ease of inspection and maintenance Coating and linings • Non-metallic coating and lining can be applied on steel and materials of construction in order to combat corrosion. • Appropriate methods such as electroplating, cladding, organic coating (ceramic, plastic, carbon brick, glass coated))

13. Inhibitors: • Adsorption type • Scavenger type • Cathodic Protection • Impressed emf method • Anodic Protection

14. Materials for construction A number of equipment are used in the manufacture of pharmaceuticals, bulk drugs, antibiotics, biological products etc. The success or failure of a new chemical plant or in the improvement of an existing facility depends on. • Design of equipment • Selection of material • Technique of fabrication This will determines weather a process is expensive or complex.

15. The selection of a material for the construction of equipment depends on the fallowing properties • Chemical resistance • Structural strength • Resistance to physical shock i.e operating pressure • Resistance to thermal shock i.e operating temperature • Ease of fabrication • Cost • Maintenance

16. Classification of materials for plant construction Materials for construction Metals Non-Metals Organic Inorganic glass Non-Ferrous Ferrous Cast Iron Steel carbon Stainless steel Aluminium Rubber Plastic Lead

17. Cast Iron It consist of a iron with a proportion of carbon (beyond 1.5%). The properties of iron depends on the amount of carbon present in it. Cast iron abundantly available, inexpensive and therefore widely used. A number of cast iron are available. • Gray cast iron (carbon & silicon) • Maleable iron (carbon 2.5%) • High silicon cast iron (silicon 13-16%) • Nickel resistant cast iron

18. Advantages: • Cast iron is cheap. Therefore it is used in expensive plant material with enamel or plastic lining. • It is resistant to concentrated sulphuric acid, nitric acid and dilute alkaline. • It has low thermal conductivity, it is used for jackets of steam pan. Disadvantages: • Cast iron is very hard and brittle. Uses: • Supports for plant • Jackets of steam pans • Lining with enamel, plastic or suitable protective material

19. Carbon steel/ Mild steel Carbon steel is an iron alloy, which contains only a small percentage of carbon Advantages: • It is cheapest • It is easily weldable and is frequently used in fabrication Disadvantages: • It reacts with caustic soda and sea water. Uses: It is used for construction of bars, pipes, plates, large storage tanks for water, sulphuric acid, organic solvents etc.

20. Stainless Steel Stainless steel is an alloy of iron. Usually, it contains chromium and nickel, which make the steel corrosion resistant. Stainless steel is stabilised by the addition of titanium, niobium or tantalum. Minor amounts of other elements such as copper, molybdenum and selenium are added. Uses: • Heat resistance • Corrosion resistance • Ease of fabrication • Cleaning and sterilisation • Tensile strength

22. Aluminium • Aluminium is cheap, light in weight and offers adequate mechanical strength. In addition, aluminium equipment can be easily fabricated. Their maintenance and cleaning is easy. Advantages: • High resistance to atmospheric conditions, industrial fumes, vapour and fresh or salt water. It can be used for nitric acid (82%) and acetic acid. • Thermal conductivity of aluminium is 60% Disadvantages: • The mechanical strength decreases greatly above 1500C • it cannot be used with strong caustic solution • Many mineral acid attack aluminium • Oxide and hydroxide films are formed rapidly, when its surface is exosed.

23. Uses: • A super grade is used for food and pharmaceutical use. • These are used as meat storage container.

24. Glass Glass has the advantages of superior protective qualities,attractiveness and low cost. It is chemically inert to a large extent and available in a variety of sizes, shapes and colours. Glass is considered as a super-cooled liquid, though it is seen in solid state. Glass is composed of fallowing constuents. • Sand-silica pure-Base material • Soda ash-sodium carbonate-improves the property • Lime soda-calcium carbonate- improves the property • Cullet- broken glass- fusion agent

25. Types of glass • Soft glass – sodium and calcium silicate • Hard glass – potassium and calcium silicate • Flint glass – potassium and lead silicate • Jena glass – zinc and barium borosilicate • pyrex glass – silicon, boron, sodium oxide • Quartz glass- pure silica

26. Rubber • Natural rubber from latex of tree • Soft rubber it is manomeric isoprene • Hard rubber the soft rubber mixed with sulphur • Synthetic rubber they are neoprene, nitrile, silicon, polyisoprene

27. Plastic Plastics are synthetic resins containing long chains of atoms linked to form giant or macromolecules. They have high molecular weight Advantages: • low thermal and electrical resistance • excellent resistance to weak mineral acids • unaffected by inorganic salts • resistance to slight changes in pH Disadvantages • Low mechanical strength • High expansion rates

28. Based on the utility of plastic in plant construction these can be categorised as. • Rigid material – pipes, bearings, vessels, valve • Flexible material – tanks, pipes, funnels, buckets • Metallic material – tanks, vessels, fans • Plastic cement – acid resistance tiles and bricks • Special case plant – aseptic screening, guards in moving parts