Pharmaceutical Engineering. Corrosion.pptx

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1. C orrosion Dr.P . Ashok kumar , Associate Professor, Department of ph.ceutics Sscp .

2. Corrosion definition It is defined as the reaction of metallic material with its environment which causes a change to the material and results in a functional failure of the metallic component or of a complete system.(Act of eating away or Washing away) FACTORS AFFECTING CORROSION Solution p H Oxidizing agents Temperature

3. Continue….. Solution pH:- As metals dissolve in acidic pH (4-10) hence discharge of hydrogen ions takes place. C ertain amphoteric metals( can react both as an acid as well as a base ) dissolve rapidly in either acidic or basic solutions . Corrosion proceeds e.g : Aluminum(Al) and Zinc(Zn). Nobel metals are not affected by pH e.g : Gold and platinum The H+ ions have tendency to take up electrons from the electrolytic solutions (this leads to anodic corrosion) to form hydrogen gas (H 2 ↑). 2H + + 2e - → H 2 ↑(indicated by formation of bubbles at the surface)

4. Oxidizing agents C opper in sodium chloride follows the below mechanism: Oxygen combines with the hydrogen to form water, oxidizing agents that enhance the corrosion of some materials may also retard/ reduce corrosion others through the formation on their surface of oxide or layers of absorbed oxygen which make them more resistant to chemical attack. This film is destroyed only in the presence of suphuric acid(H 2 SO 4 ) and hydrochloric acid.

5. T emperature When temp is increased, the solution may be of homogeneous nature or heterogeneous nature. T his leads to increase the rate of corrosion. As temp rises solubility of oxygen is decreased hence oxygen is released into the solution. This released oxygen increases the rate of corrosion. Oxygen is responsible for formation of protective oxide film which prevents corrosion. Corrosion of stainless steel will increase rapidly through the loss of the oxidizing substance dissolved oxygen needed to maintain protective film (absence of oxygen).

6. Continue……. Copper based alloys do not depend on the oxide film for corrosion. VELOCITY V elocity of the moving corrosion solution on the metallic surface tends to accelerate corrosion. Corrosion products are formed rapidly because chemicals are brought to the corroding surface at higher rate. P revention of insoluble film on the metal surface decreases the rate resistance of corrosion. S tifling( heat or air ) of corrosion products exposes a new surface area to corrode. This leads to corrosion of condensers, evaporator, propellers, agitators and centrifugal pumps.

7. SURFACE FILMS Once corrosion has started its further progress is often controlled by the nature of films that may accumulate on the metallic surface. P assive oxide films form on stainless steel. O il and grease films may occur on the surface. I nsoluble salts such as carbonates and sulphates may be precipitated from hot solution on the metal surface. These protect the metal surface. I f the film is porous ( e.g : Zn oxide), corrosion continues, non porous films ( e.g : Chromium oxide film on iron) prevent further corrosion.

8. Other factors Corrosion rate is influenced by the concentration of corrosive chemicals in the environment. I ncreased corrosion is seen in types 304SS at 40% acid concentration whereas it is decreased at 90% acid concentration. C orrosion will depend on the nature of impurity and their concentration. In some cases impurities may act as an inhibitor and retard corrosion. Other impurities can have deleterious effect on materials e.g : small amount of chloride can break the oxide film on stainless steel.

9. Types of corrosion Pure metals and their alloys tend to chemically react with the elements of a corrosive medium forming corroded metal. The corrosion media are generally liquids(mostly aqueous solutions) but solids and gases can also corrode metallic parts. Generally corrosion is usually confined to metal surfaces or lines of weakness of the metal. It is further divided as Physicochemical corrosion: The effects of this type are swelling, cracking, softening etc E.g : Plastics and non metallic material.

10. Cont …… b) Electro chemical corrosion: This type of corrosion occurs as discrete points of metallic surfaces when electricity flows from cathode to anode. LOCALISED CORROSION: I t is most frequently observed on different locations in a material, it is further classified as Specific corrosion: Mechanically weak spots or dead spots in a reaction vessel causes specific site corrosion. Inter granular corrosion: It is seen in the grain boundaries( are defects in the structure ) of a metal or alloy. It leads to loss of ductility ( ability to reform into original shape when changed) of metal and its strength.

11. Austenitic stainless steel and Aluminium, Copper(SS 18/8:cr/Ni) ↓Improper heating Precipitation of intergranular compounds are seen, hence corrosion occurs. SS is stabilized by incorporation of niobium/titanium or carbon.

12. PITTING CORROSION Pitting is a severe form of corrosion that develops in highly localized areas on a metal surface. This results in the development of cavities or pits which may range from deep cavities of small diameter to relatively shallow depressions. If severe pitting can sometimes leads to actual perforation of the metal. Aluminum and stainless steel alloys in aqueous solutions containing chloride results in pitting. Can be prevented by adding inhibitors.

13. CREVICE CORROSION It occurs in cracks or cervices formed between mating surface of metal and takes the form of pitting. Crevice corrosion is believed to initiate as a result of differential aeration cells.

14. Cont ….. Crevice corrosion occurs only on the film protected metal such as aluminum, magnesium and titanium. Crevice corrosion can result due to deficiency of oxygen in crevice, acidity and build-up of ions in the crevice or depletion of an inhibitor.

15. STRESS INDUCED CORROSION Residual internal stress in the metal or external applied stress accelerate the corrosion. Residual internal stress is produced by Deformation during fabrication. Unequal rate of cooling from higher temperature. Internal stress rearrangements involving volume changes. Stress induced by rivets, bolts and shrink fits. Eliminating high stress areas prevent this type of corrosion.

16. STRESS CORROSION CRACKING At the surface ,if the tensile stress is equal or more than yield stress, the surface develops cracks is known as stress corrosion cracking. Cold formed brass develop cracks in the environment of ammonia. Embrittlement cracking of steel is observed in caustic solution.

17. CORROSION FATIQUE This is a form of stress corrosion in which the stress is applied in a cyclic manner and with in the elastic range. The cyclic stress breaks the productive film which normally retards corrosion. O xygen content, temperature, pH and solution composition affect corrosion fatigue.

18. Highly reactive gases such as hydrogen sulphide cause loss of resistance of metal to corrosion technique. Chromium steels have higher corrosion fatigue resistance than carbon steels.

19. FRETTING CORROSION This occurs when metal move/ slide over one another there by generating heat, it oxidizes the metal surfaces. This oxide wears off the protective oxide layer occurs results in exposure of fresh surfaces for corrosive attack. This type of corrosion can be minimized by using harder materials, minimizing friction by lubrication.

20. FLOW RELEASED CORROSION Liquid metals can cause corrosion, driving force is the tendency of the liquid to dissolve solids or penetrating the metal along the grain boundaries at places of wetting. Corrosion rate is enhanced at high temperatures. Eg : Mercury attacks on aluminium alloys Molten zinc on stainless steel

21. IMPINGMENT CORROSION It occurs when protective film is lost i.e , when the liquid flows turbulently in the pipe bends cause cavitation damage.

22. EROSION It is a type of chemical reaction which usually occurs on the metal surface of steel alloys like chromium, copper, manganese there by destroying it, pH, oxygen content of water and steam also plays an important role in erosion . It can be prevented by using hard metals, decreasing the velocity and adding if inhibitor.

23. CHEMICAL REACTION RELATED CORROSION Corrosion involves chemical reactions such as oxidation and reduction at anode and cathode. Oxygen concentration cells This type of corrosion occurs when oxygen concentration decreases as formation of protective oxide film does not occur.

24. Hydrogen embrittlement In carbon steel, hydrogen penetrates it and reacts with carbon of the steel to form methane(CH4). Depletion( reduced ) of carbon decreases the strength of the metal. Hence corrosion occurs. This can be avoided by alloying the material with chromium and molybdenum.

25. Structural corrosion Structural strength is reduced. This occurs when one component of the alloy is removed or released into the solution.

26. Graphite corrosion It is seen in gray cast iron. Selective dissolution of iron from cast iron is known as graphitization i.e metallic iron is converted into corrosive products leaving a residue of intact graphite mixed with iron corrosive products and other insoluble constituents of cast iron, when the layer becomes porous in nature.

27. Dezincification: (Removal of zinc) This is seen in brass metals whose composition contains greater amounts of zinc i.e , more than 15%.

28. BIOLOGICAL COROSION The metabolic action of micro organisms can either directly or indirectly cause deterioration of a metal. such process is called biological corrosion. Microorganism associated with corrosion are either aerobic or anaerobic. The causes for biological corrosion are Producing corrosive environment or altering environment composition. Altering resistance to surface films. Influencing the rate of anodic/cathodic reaction.

29. Cont ….. The aerobic or anaerobic nature of micro organisms affects and alter the oxygen content on the surface of the metal, also the pH Anaerobic sulphate reducing bacteria promotes corrosion of metals. sulphates When these compounds come into contact with underground pipes convert iron to iron sulphide , this reaction was continue until failure of the pipe occurs. Reducing(reduced in to ) Anaerobic bacteria Hydrogen sulphite (H 2 S) + Calcium sulphite (CAS)

30. PREVENTION AND CONTROL OF ERROSION Selection of proper material : Corrosion should not be permitted in fine mega wire screens, non-metal should be selected, right material is to be selected by performing corrosion test on it. It is desirable to obtain advice and assistance of persons experienced in corrosion engineering work, literature.

31. Proper design of equipment: During installation of pipes, the fittings (like baffles, valves, pumps etc ) should be proper i.e , easily maintained, liquids can be drained completely , ease of cleaning, ease of inspection and maintenance.

32. Coating and linings Nonmetallic coating and linings can be applied by steel and other materials of construction. Organic coatings are used as lining on materials to prevent corrosion 100 mills thickness. Linings of plastic, elastic, ceramic, organic are effective. Zinc coating - on steel, fuel storage tank. Aluminum coated steel- high temperature conditions. lead – roofings . Tin coated steel- food containers.

33. Altering environment Keep (eliminate) the moisture content, low and reducing the temperature, liquids should be deaerated, pumping inert gas( To discourage the growth of aerobic microbes ) etc. By use of inhibitors Inhibitors materials protected in media Chromates, phosphates- iron and steel in aqueous solution. S ilicates - iron and steel in acidic media. O rganic sulphides , - stainless steel in hot diluted solutions of Amines, copper sulphate . Sulphuric acid.

34. CATHODIC PROTECTION Sacrificial anode method: In this method, anodes are kept in electrical contact with the metal to be protected (cathode). The anodes are sacrificed, since it goes into the solution (erodes). For the protection of iron and steel tanks. The metals such zinc, aluminum, magnesium and alloys of these. C hoice of anode metal should be non poisonous.

35. 2. Impressed emf method: This is also known as applied current system i.e , an external voltage is impressed between tank and electrodes. The negative terminal of power supply is connected to the material to be protected. Natural galvanic effect(   effect  of an electric shock ) is avoided, anode is maintained positive. In case of sulphuric acid and deionized water, compressed graphite and high silicon steels used as anodes . Anodes are non sacrificial anode eg . Carbon , Platinum.

36. ANODIC PROTECTION This method involves the protection of anode with the aid of current supply. The anode metal to be protected is set at a potential and current is applied initially, rise in current is seen as the metal gets dissolved. At a certain stage critical point of current reaches known as passivating potential. Above this potential(Passivating current) the flow of current gets reduced. Hence the potential of the metal is increased it does not corrode.

37. Thank you