CORROSION

1. CORROSION

2. INTRODUCTION The word corrosion is as old as the earth, but it has been known by different names. Corrosion is known commonly as rust, an undesirable phenomena which destroys the luster and beauty of objects and shortens their life. The corrosion is the destruction of metal by chemical or electrochemical reaction with its environments The metal is degraded to form ferric rust, a red-brown compound, which is a sure sign of electrochemical oxidation of the underlying metal. • 4Fe + 3O2 + 2H2O → 4FeO.OH

3. Nearly all metals, with the exception of gold and platinum, will corrode in an oxidising environment forming compounds such as oxides, hydroxides and sulphides. • The degradation of metals by corrosion is a universal reaction, caused by the simple fact that the oxide of a metal has a much lower energy than the metal itself. Hence there is a strong driving force for the oxidation of metals.

4. For example the familiar metal aluminium, which is used in aircraft, window frames and cooking utensils, is attacked by oxygen to form the oxide as follows: • 4Al + 3O2 → 2Al2O3

5. There are two general types of corrosion based on the nature of chemical action involved. These are: • Direct chemical corrosion • Electrochemical corrosion

6. Direct chemical corrosion • This is the type corrosion which involves direct combination between the metal and dry gases. There is no flow of electric charge through any perceptible distance. • Oxygen is the most commonly encountered reacting gas, but also SO2 and H2S from fuel and chlorine and other gases in specific situation cause similar reaction. • Oxidation may occur at any temperature, but it is particularly important at high temperature where a more rapid reaction is likely to occur between metal and air • Metal + O2 = Metal Oxide

7. Electro – chemical corrosion • This is the type of corrosion which involves flow of electrical current from one point to another point through some perceptible distance. • Most commonly, the driving forces that causes the corrosion reaction to take place in the metal is electrochemical in nature. • It is difference of potential between the difference points on the metal surface that produces a flow of current from one point to another.

8. At the point from which current flow, called anode, metal dissolver or corrodes. At the point to which current flows, called cathode, no corrosion takes place. The metal is divided into anodic and cathodic areas. • The electrolyte may be moisture or liquid.

9. Potential difference on the metal surface may be set up by any chemical or physical in homogeneity in metal or electrolyte. They are especially very large and capable of causing rapid corrosion when two metals having position far apart in electro – chemical series are connected or are allowed to be in contact in the presence of according environment.

10. The other causes of potential difference in the metal are • Locked – up stresses • Grain size difference • Temperature difference • Non metallic inclusions • Dissolved gases • particles of dirty and dust on the surface

11. The causes of potential difference in the electrolyte are • Differences in aeration or content of dissolved oxygen or other gases • Difference in concentration • Difference in temperature • Suspended solids

12. GALVANIC CORROSION • Galvanic corrosion occurs when two dissimilar metals make contact in the presence of an electrolyte. • It is usually recognizable by the presence of a build-up of corrosion at the joint between the metals.

15. STRESS CORROSION CRACKING. • This form of corrosion involves constant or cyclic stress, acting in conjunction with a damaging chemical environment. The stress may be caused by internal or external loading.

16. Internal stress may be trappedin a part of structure during manufacturing processes such as cold working or by unequal cooling from high temperatures. Most manufacturers follow up these processes with a stress relief operation. Even so, sometimes stress remains trapped. The stress may be externally introduced by riveting, welding, bolting, clamping, press fit, etc.

17. CONCENTRATION CELL CORROSION. • Concentration cell corrosion, (also known as Crevice Corrosion) is corrosion of metals in a metal-to-metal joint, corrosion at the edge of a joint even though the joined metals are identical, or corrosion of a spot on the metal surface covered by a foreign material. • Metal ion concentration cells and oxygen concentration cells are the two general types of concentration cell corrosion.

19. Metal Ion Concentration Cells. • The solution may consist of water and ions of the metal which is in contact with water. A high concentration of the metal ions will normally exist under faying surfaces where the solution is stagnant, and a low concentration of metal ions will exist adjacent to the crevice which is created by the faying surface. • An electrical potential will exist between the two points; the area of the metal in contact with the low concentration of metal ions will be anodic and corrode, and the area in contact with the high metal ion concentration will be cathodic and not show signs of corrosion.

20. Oxygen Concentration Cells • The solution in contact with the metal surface will normally contain dissolved oxygen. An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse into the solution, thereby creating a difference in oxygen concentration between two points. Typical locations of oxygen concentration

21. PITTING CORROSION. • Pitting corrosion is one of the most destructive and intense forms of corrosion. It can occur in any metal but is most common on metals that form protective oxide films, such as aluminum and magnesium alloys. It is first noticeable as a white or gray powdery deposit, similar to dust, which blotches the surface. When the deposit is cleaned away, tiny holes or pits can be seen in the surface. • These small surface openings may penetrate deeply into structural members and cause damage completely out of proportion to its surface appearance.

23. CORROSION PREVENTION AND CONTROL • MATERIAL SELECTION Selection of proper material for a particular corrosive service)

24. Metals and Alloys

26. . INTERNAL MEASURES • Purification of metals. Pure metals have better corrosion resistance than impure metals • Alloying of metals with corrosion resistance elements. • Proper heat treatment of metals. Proper heat – treatment given to a metallic piece also helps to control its corrosion. For example, annealing is often necessary to removal internal stresses of cold worked parts and thus avoid their stress- corrosion.

27. EXTERNAL MEASURES • Design features. Life of equipment can be prolonged by reducing its corrosion by careful designing, such as avoiding of side by side contact of dissimilar metal or to avoid impingement and turbulence in flowing environment or avoid crevices etc. • Application of inhibitors. Inhibitor may be defined as any substance which when added in a small quantity to the environment reduces the corrosion rate appreciably. • Alteration of corrosive environment. Often it is helpful to control corrosion by suitable alteration of the nature, composition and temperature of environment or removal of dissolved gases

28. Cathodic protection. If surface of steel or other metal requiring protection is made cathode, current will flow through an electrolyte only to it and not from it. It will be subjected to a reducing, not an oxidizing effect. Local galvanic cells on its surface will be obliterated by a higher external potential. For these reason it will not corrode. The external potential may be obtained from electric source.

29. Protective coatings. • Metallic coating. These may be provided of zinc, tin. Aluminium. Nickel etc, by either of the method like electroplating, dipping in molten metal, cladding, metal spraying, cementation or vapour deposition. • Non – metallic coatings. The various such protective coatings consists of painting, lacquering, plastic coating, vitreous enamelling, slushing compound coatings, anodising and other oxide films and chemical dipping.

30. Corrosion may severely affect the followingfunctions of metals, plant and equipment: (1) Impermeability: Environmental constituentsmust not be allowed to enter pipes,process equipment, food containers, tanks,etc. to minimize the possibility of corrosion.

31. (2) Mechanical strength: Corrosion should notaffect the capability to withstand specifiedloads, and its strength should not be underminedby corrosion.

32. (3) Dimensional integrity: Maintaining dimensionsis critical to engineering designs andthey should not be affected by corrosion.

33. (4) Physical properties: For efficient operation,the physical properties of plants, equipmentand materials, such as thermal conductivityand electrical properties, should not beallowed to be adversely affected by corrosion

34. (5) Contamination: Corrosion, if allowed tobuild up, can contaminate processing equipment,food products, drugs and pharmaceuticalproducts and endanger health andenvironmental safety.

35. (6) Damage to equipment: Equipment adjacentto one which has suffered corrosion failure,may be damaged.Realizing that corrosion effectively blocksor impairs the . functions of metals, plantsand equipment, appropriate measures mustbe adopted to minimize loss or efficiency offunction.