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Corrosion in Metals

Corrosion in Metals and Protections

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Corrosion in Metals

  1. 1. Corrosion in Metals Mohd. Hanif Dewan, IEng, IMarEng, MIMarEST (UK), MRINA(UK), Maritime Lecturer and Consultant
  2. 2. Corrosion WHAT IS CORROSION • Corrosion is the deterioration of materials by chemical interaction with their environment. • The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals. The most widely used metal is iron (usually as steel) and the following discussion is mainly related to its corrosion.
  3. 3. RESULTS OF CORROSION The consequences of corrosion are many and varied and the effects of these on the safe, reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal. Some of the major harmful effects of corrosion can be summarised as follows: 1. Reduction of metal thickness leading to loss of mechanical strength and structural failure or breakdown. When the metal is lost in localised zones so as to give a crack like structure, very considerable weakening may result from quite a small amount of metal loss. 2. Hazards or injuries to people arising from structural failure or breakdown 3. Loss of time in availability of profile-making industrial equipment.
  4. 4. 4. Reduced value of goods due to deterioration of appearance. 5. Contamination of fluids in vessels and pipes. 6. Perforation of tanks and pipes allowing escape of their contents and possible harm to the surroundings. For example corrosive sea water may enter the boilers of a power station if the condenser tubes perforate. 7. Loss of technically important surface properties of a metallic component. These could include frictional and bearing properties, ease of fluid flow over a pipe surface, electrical conductivity of contacts, surface reflectivity or heat transfer across a surface. 8. Mechanical damage to valves, pumps, etc, or blockage of pipes by solid corrosion products. 9. Added complexity and expense of equipment which needs to be designed to withstand a certain amount of corrosion, and to allow corroded components to be conveniently replaced. RESULTS OF CORROSION
  5. 5. Galvanic Corrosion • Noble or Cathodic – Platinum – Gold, Titanium – Silver – Stainless steel – Bronze/Copper/Brass – Cast Iron – Steel – Aluminium – Zinc – Magnesium • Active or Anodic Electrolyte Copper Zinc
  6. 6. Isolation of dissimilar metals by electrical insulation.
  7. 7. Galvanic Corrosion: • Possibility when two dissimilar metals are electrically connected in an electrolyte* • Results from a difference in oxidation potentials of metallic ions between two or more metals. The greater the difference in oxidation potential, the greater the galvanic corrosion. • Refer to Galvanic Series (Figure 13-1) • The less noble metal will corrode (i.e. will act as the anode) and the more noble metal will not corrode (acts as cathode). • Perhaps the best known of all corrosion types is galvanic corrosion, which occurs at the contact point of two metals or alloys with different electrode potentials.
  8. 8. GALVANIC SERIES Galvanic Series in Seawater ( supplements Faraq Table 3.1 , page 65), EIT Review Manual, page 38-2 Tendency to be protected from corrosion, cathodic, more noble end Mercury Platinum Gold Zirconium Graphite Titanium Hastelloy C Monel Stainless Steel (316-passive) Stainless Steel (304-passive) Stainless Steel (400-passive) Nickel (passive oxide) Silver Hastelloy 62Ni, 17Cr Silver solder Inconel 61Ni, 17Cr Aluminum (passive AI203) 70/30 copper-nickel 90/10 copper-nickel Bronze (copper/tin) Copper Brass (copper/zinc) Alum Bronze Admiralty Brass Nickel Naval Brass Tin Lead-tin Lead Hastelloy A Stainless Steel (active) 316 404 430 410 Lead Tin Solder Cast iron Low-carbon steel (mild steel) Manganese Uranium Aluminum Alloys Cadmium Aluminum Zinc Beryllium Magnesium Note, positions of ss and al**
  9. 9. Big Cathode, Small Anode = Big Trouble
  10. 10. Figure 1 illustrates the idea of an electro-chemical reaction. If a metal is placed in a conducting solution like salt water, it dissociates into ions, releasing electrons, as the iron is shown doing in the figure, via the ionization reaction Fe  Fe++ + 2e- The electrons accumulate on the iron giving it a negative charge that grows until the electrostatic attraction starts to pull the Fe++ ions back onto the metal surface, stifling further dissociation. At this point the iron has a potential (relative to a standard, the hydrogen standard) of –0.44 volts. Each metal has its own characteristic corrosion potential (called the standard reduction potential), as plotted in Figure 2. If two metals are connected together in a cell, like the iron and copper samples in Figure 1, a potential difference equal to their separation on Figure 2 appears between them. The corrosion potential of iron, -0.44, differs from that of copper, +0.34 , by 0.78 volts, so if no current flows in the connection the voltmeter will register this Figure 1. A bi-metal corrosion cell. The corrosion potential is the potential to which the metal falls relative to a hydrogen standard. Figure 2. Standard reduction potentials of metals. Galvanic Corrosion Potentials:
  11. 11. Liquid Cell Battery: dry cell is a galvanic electrochemical cell with a pasty low- moisture electrolyte. A wet cell, on the other hand, is a cell with a liquid electrolyte, such as the lead-acid batteries in most cars
  12. 12. Zn(s) → Zn2+(aq) + 2 e- - oxidation reaction that happens at zinc = anode Dry Cell - Zinc-carbon battery 2MnO2(s) + 2 H+(aq) + 2 e- → Mn2O3(s) + H2O(l) - reduction reaction at carbon rod = cathode
  13. 13. How to avoid Galvanic Corrosion • Material Selection: Do not connect dissimilar metals! Or if you can’t avoid it: – Try to electrically isolate one from the other (rubber gasket). – Make the anode large and the cathode small • Bad situation: Steel siding with aluminum fasteners • Better: Aluminum siding with steel fasteners • Eliminate electrolyte • Galvanic of anodic protection
  14. 14. • Galvanic severity depends on: – NOT • Not amount of contact • Not volume • Not mass – Amount of separation in the galvanic series – Relative surface areas of the two. Severe corrosion if anode area (area eaten away) is smaller than the cathode area. Example: dry cell battery
  15. 15. Steel bolt (less noble) is isolated from copper plates. See handout! – Read Payer video HO
  16. 16. Prevention of Galvanic Corrosion  Use a single material or a combination of materials that are close in the galvanic series.  Avoid the use of a small ratio of anode area to cathode area. Use equal areas or a large ratio of anode to cathode area.  Electrically insulate dissimilar metals where possible. This recommendation is illustrated in figure. A flanged joint is equipped with bolts contained in insulating sleeves with insulating washers under the head and nut. Paint, tape, or asbestos gasket material are alternative insulations.  Local failure of the protective coating, particularly at the anode, can result in the small anode-to-cathode area syndrome marked by accelerated galvanic corrosion. Maintain all coatings in good condition, especially at the anode.
  17. 17.  Decrease the corrosion characteristics of the fluid where possible by removing the corrosive agents or adding inhibitors.  Avoid the use of threaded or riveted joints in favor of welded or brazed joints. Liquids or spilled moisture can accumulate in thread grooves or lap interstices and form a galvanic cell.  Design for readily replaceable anodic parts or, for long life, make the anodic parts more substantial than necessary for the given stress conditions.  Install a sacrificial anode lower in the galvanic series than both the metals involved in the process equipment.
  18. 18. Pitting Corrosion • Pitting corrosion is the phenomenon whereby an extremely localized attack results in the formation of holes in the metal surface that eventually perforate the wall. The holes or pits are of various sizes and may be isolated or grouped very closely together.
  19. 19. Preventive measures There are several preventive approah to avoid pitting. There are : 1. Proper material selection e.g. SS316 with molydenum having higher pitting resistance compare to SS304 2. Use higher alloys for increased resistance to pitting corrosion 3. Control O2 level by injecting O2 scavenger in boiler water system 4. Control pH, chloride concentration and temperature 5. Cathodic protection and/or Anodic Protection 6. Proper monitoring of O2 & chloride contents by routine sampling 7. Agitation of stagnant fluid
  20. 20. Selective Leeching Corrosion • Selective leaching is the term used to describe a corrosion process wherein one element is removed from a solid alloy. The phenomenon occurs principally in brasses with a high zinc content (dezincification) and in other alloys from which aluminum, iron, cobalt, chromium, and other elements are removed. • Grey cast iron is subject to leeching known as graphitization, whereby the iron is dissolved leaving behind a weak porous graphite network.
  21. 21. Dealloying: • When one element in an alloy is anodic to the other element. • Example: Removal of zinc from brass (called dezincification) leaves spongy, weak brass. • Brass alloy of zinc and copper and zinc is anodic to copper (see galvanic series).
  22. 22. Dealloying:  Two common types: – Dezincification – preferential removal of zinc in brass • Try to limit Zinc to 15% or less and add 1% tin. • Cathodic protection – Graphitization – preferential removal of Fe in Cast Iron leaving graphite (C).
  23. 23. Prevention of Selective Leeching/dealloying  The only effective method of preventing corrosion by selective leaching is to avoid the use of materials known to be subject to it in association with the fluids concerned. Brasses with high zinc content (> 35 percent) in acid environments are particularly susceptible.
  24. 24. Erosion Corrosion • Erosion corrosion is the term used to describe corrosion that is accelerated as a result of an increase in the relative motion between the corrosive fluid and a metal wall. The process is usually a combination of chemical or electrochemical decomposition or dissolution and mechanical wear action.
  25. 25. Erosion Corrosion of Condenser Tube Wall
  26. 26. Prevention of Erosion Corrosion  Use materials with superior resistance to erosion corrosion.  Design for minimal erosion corrosion.  Change the environment.  Use protective coatings.  Provide cathodic protection
  27. 27. Cavitation Erosion Cavitation erosion is a special class of erosion corrosion that is associated with the periodic growth and collapse of vapour bubbles in liquids. Normally it occurs to the diesel engine (wet cylinder liner), cooling systems and pumps. • Prevention of Cavitation Erosion: - Using of cavitation inhibitor chemicals or supplemental coolant additives, that form a film on surfaces, in cooling system can reduce the cavitation erosion. - Using of pressurized cooling system can reduce the cavitation erosion. - pH control to avoid corrosion.
  28. 28. Fretting Corrosion • Fretting corrosion occurs at the contact points of stressed metallic joints that are subject to vibration and slight movement. It is also called friction oxidation, wear oxidation, chafing, and false brinelling. Fretting corrosion to be a special case of erosion corrosion occurring in air rather than aqueous conditions.
  29. 29. Tube-tube-sheet & Tube-baffle Fretting
  30. 30. Theories of Fretting Corrosion
  31. 31. Essential Elements for Fretting Corrosion  A loaded interface. Tube-tube-sheet joints are heavily loaded by the strains induced in rolling the tubes in the tube-sheet.  Vibration or repeated relative motion between the two surfaces.  The load and relative motion of the interface must be sufficient to produce slip or deformation on the surfaces.
  32. 32. Prevention of Fretting Corrosion Eliminate vibration Eliminate high-stress interface Lubricate the joint Use hard surface Increase friction at the interface Use soft metallic or non-metallic interface gaskets
  33. 33. Stress Corrosion Cracking • Stress corrosion is the name given to the process whereby cracks appear in metals subject simultaneously to a tensile stress and specific corrosive media. The metal is generally not subject to appreciable uniform corrosion attack but is penetrated by fine cracks that progress by expanding over more of the surface and proceeding further into the wall.
  34. 34. Factors: • Must consider metals and environment. What to observe for: – Stainless steels at elevated temperature in chloride solutions. – Steels in caustic solutions – Aluminum in chloride solutions • 3 Requirements for SCC: 1. Susceptible alloy 2. Corrosive environment 3. High tensile stress or residual stress
  35. 35. Stress Corrosion Cracking: See handout, review HO hydron!
  36. 36. Prevention of Stress Corrosion Cracking  Reducing the fluid pressure or increasing the wall thickness.  Relieve residual stress by annealing.  Change the metal alloy to one that is less subject to stress- corrosion cracking. E.g. carbon steel is more resistant than stainless steel to corrosion cracking in a chloride-containing environment, but less resistant to uniform corrosion. Replacing stainless steel with an alloy of higher nickel content is often effective.
  37. 37. Prevention of Stress Corrosion cracking  Modify the corrosive fluid by process treatment or the addition of corrosion inhibitors such as phosphates.  Apply cathodic protection with sacrificial anodes or external power supply.  Use shot peening method to induce surface stress.  Use venting air pockets to avoid concentration of chloride in the cooling water
  38. 38. Intergranular Attack: • Corrosion which occurs preferentially at grain boundries. • Why at grain boundries? – Higher energy areas which may be more anodic than the grains. – The alloy chemistry might make the grain boundries dissimilar to the grains. The grain can act as the cathode and material surrounding it the anode.
  39. 39. Intergranular Attack: • How to recognize it? Near surface Corrosion only at grain boundries (note if only a few gb are attacked probably pitting) Corrosion normally at uniform depth for all grains.
  40. 40. Example 1: Intergranular Attack Sensitization of stainless steels: – Heating up of austenitic stainless steel (750 to 1600 F) causes chromuim carbide to form in the grains. Chromuim is therefore depleted near the grain boundries causing the material in this area to essentially act like a low-alloy steel which is anodic to the chromium rich grains.
  41. 41. Example 2: Intergranular Attack Sensitization of stainless steels: – Heating up of austenitic stainless steel (750 to 1600 F) causes chromuim carbide to form in the grains. Chromuim is therefore depleted near the grain boundries causing the material in this area to essentially act like a low-alloy steel which is anodic to the chromium rich grains. – Preferential Intergranular Corrosion will occur parallel to the grain boundary – eventually grain boundary will simply fall out!!
  42. 42. How to avoid Intergranular Attack: • Watch welding of stainless steels (causes sensitization). Always anneal at 1900 – 2000 F after welding to redistribute Cr. • Use low carbon grade stainless to eliminate sensitization (304L or 316L). • Add alloy stabilizers like titanium which ties up the carbon atoms and prevents chromium depletion.
  43. 43. Intergranular Attack:
  44. 44. Factors affecting corrosion rates Temperature As a rule of thumb for each 10'C rise in temperature doubles the rate of corrosion.
  45. 45. Corrosion in Close system and Open system • The rate of oxygen diffusion increases in an open system with temperature up to around 80'C. A rapid tailing off (to reduce in amount) then occurs due to the solubility of oxygen. For this reason open system feed tanks seen on many vessels have heating coils which maintain the temperature at 85'C or higher.. In a closed system there is no such tail off as the oxygen cannot escape
  46. 46. pH/Alkalinity The electrochemical nature of the metal will determine its corrosion rate with respect to pH. The corrosion rate of iron reduces as the pH increases to about 13 due to the reduced solubility of the Fe ions. Aluminium and zinc, being ampoteric, have rates of corrosion that increases with pH higher or lower than neutral
  47. 47. Methods to Control Corrosion There are five methods to control corrosion:  material selection  coatings  changing the environment  changing the potential  design
  48. 48. How to avoid Corrosion? 1. Material Selection. 2. Eliminate any one of the 4 requirements for corrosion. 3. Galvanic - Avoid using dissimilar metals. – Or close together as possible – Or electrically isolate one from the other – Or MAKE ANODE BIG!!!
  49. 49. How to avoid Corrosion? 4. Pitting/Crevice: Watch for stagnate water/ electrolyte. – Use gaskets – Use good welding practices 5. Intergranular – watch grain size, environment, temperature, etc.. Careful with Stainless Steels and AL.
  50. 50. How to avoid Corrosion? 6. Consider organic coating (paint, ceramic, chrome, etc.) – But DANGER IF IT GETS SCRACTHED!! 7. BETTER to consider cathodic protection (CP): – such as zinc (or galvanized) plating on steel – Mg sacrificial anode on steel boat hull – Impressed current (ICCP) etc..
  51. 51. Corrosion Control:
  52. 52. Surface Treatment (Coatings)  Organic paints  Chromating and phosphating:  The Process - chromating and phosphating are surface-coating processes that enhance the corrosion resistance of metals. Both involve soaking the component in a heated bath based on chromic or phosphoric acids. The acid reacts with the surface, dissolving some of the surface metal and depositing a thin protective layer of complex chromium or phosphorous compounds  Anodizing (aluminum, titanium) – The Process - Aluminum is a reactive metal, yet in everyday objects it does not corrode or discolor. That is because of a thin oxide film - Al2O3 - that forms spontaneously on its surface. This oxide formed by anodizing is hard, abrasion resistant and resists corrosion well. The film-surface is micro-porous, allowing it to absorb dyes, giving metallic reflectivity with an attractive gold, viridian, azure or rose-colored sheen; and it can be patterned. The process is cheap, an imparts both corrosion and wear resistance to the surface.
  53. 53. Surface Treatment (Coatings) • Electro-plating – The Process -Metal coating process wherein a thin metallic coat is deposited on the workpiece by means of an ionized electrolytic solution. The workpiece (cathode) and the metallizing source material (anode) are submerged in the solution where a direct electrical current causes the metallic ions to migrate from the source material to the workpiece. The workpiece and source metal are suspended in the ionized electrolytic solution by insulated rods. Thorough surface cleaning precedes the plating operation. Plating is carried out for many reasons: corrosion resistance, improved appearance, wear resistance, higher electrical conductivity, better electrical contact, greater surface smoothness and better light reflectance. • Bluing – Bluing is a passivation process in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting protective finish. True gun bluing is an electrochemical conversion coating resulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite (Fe3O4), the black oxide of iron, which occupies the same volume as normal iron. Done for bolts called “blackening”.
  54. 54. • Hot-dip Coating (i.e. galvanizing) – Hot dipping is a process for coating a metal, mainly ferrous metals, with low melting point metals usually zinc and its alloys. The component is first degreased in a caustic bath, then pickled (to remove rust and scale) in a sulfuric acid bath, immersed (dipped) in the liquid metal and, after lifting out, it is cooled in a cold air stream. The molten metal alloys with the surface of the component, forming a continuous thin coating. When the coating is zinc and the component is steel, the process is known as galvanizing. – The process is very versatile and can be applied to components of any shape, and sizes up to 30 m x 2 m x 4 m. The cost is comparable with that of painting, but the protection offered by galvanizing is much greater, because if the coating is scratched it is the zinc not the underlying steel that corrodes ("galvanic protection"). Properly galvanized steel will survive outdoors for 30-40 years without further treatment. Surface Treatment (Coatings)
  55. 55. Material Selection:  Importance of Oxide films • The fundamental resistance of stainless steel to corrosion occurs because of its ability to form an oxide protective coating on its surface. This thin coating is invisible, but generally protects the steel in oxidizing environments (air and nitric acid). However, this film loses its protectiveness in environments such as hydrochloric acid and chlorides. In stainless steels, lack of oxygen also ruins the corrosion protective oxide film, therefore these debris ridden or stagnant regions are susceptible to corrosion.
  56. 56. The “Right” material depends on the environment. Polarization can have a major effect on metal stability. Recall CES Rankings: strong acid, weak acid, water, weak alkali, strong alkali
  57. 57. Corrosion Control for Iron -2 2 0
  58. 58. Often several approaches to control corrosion Often several “system” constraints pertain
  59. 59. Cathodic Protection (CP) • Cathodic protection (CP) - It is a technique to control the corrosion of a metal surface by making it work as a cathode of an electrochemical cell. - This is achieved by placing in contact with the metal to be protected another more easily corroded metal to act as the anode of the electrochemical cell. Uses: Cathodic protection systems are most commonly used to protect steel, water or fuel pipelines and storage tanks, steel pier piles, ships, offshore oil platforms and onshore oil well casings.
  60. 60. Cathodic Protections: 1. sacrificial anodes – zinc, magnesium or aluminum. The sacrificial anodes are more active (more negative potential) than the metal of the structure they’re designed to protect. The anode pushes the potential of the steel structure more negative and therefore the driving force for corrosion halts. The anode continues to corrode until it requires replacement, 2. Galvanized steel (see above slide) – again, steel is coated with zinc and if the zinc coating is scratched and steel exposed, the surrounding areas of zinc coating form a galvanic cell with the exposed steel and protects in from corroding. The zinc coating acts as a sacrificial anode.
  61. 61. 3. Impressed current Cathodic Protection (ICCP)  ICCP is Using an arrangement of hull mounted anodes and reference cells connected to a control panel(s), the system produces a more powerful external current to suppress the natural electro-chemical activity on the wetted surface of the hull.  This eliminates the formation of aggressive corrosion cells on the surface of plates and avoids the problems which can exist where dissimilar metals are introduced through welding or brought into proximity by other components such as propellers.  An essential feature of ICCP system is that they constantly monitor the electrical potential at the seawater/hull interface and carefully adjust the output to the anodes in relation to this. Therefore, the system is much more effective and reliable.
  62. 62. ICCP System advantages: 1. Increased life of rudders, shafts, struts and propellers as well as any other underwater parts affected by electrolysis 2. Anodes are light, sturdy and compact for easy shipping, storage and installation 3. Anodes, reference cells and automatic control systems maintain just the right amount of protection for underwater hulls and fittings, unlike standard zinc anodes, which can't adjust to changes in salinity or compensate for extreme paint loss 4. Automatic control equipment ensures reliable, simple operation 5. Optimum documented corrosion protection at minimum overall cost 6. Only one installation required for the life of the vessel or structure 7. Increased dry-dock interval 8. Approved by all classification societies for all types of vessels 9. Designed to provide a 20 plus year service life
  63. 63. MGPS Working Principle: • Basic principle on which MGPS runs is electrolysis. The process involves usage of copper, aluminum and ferrous anodes. The anodes are normally fixed in pairs in the main sea chest or in such place where they are in the direction of the flow of water. • The system consists of a control unit which supplies impressed current to anodes and monitors the same. While in operation, the copper anode produces ions, which are carried away by water into the piping and machinery system. Concentration of copper in the solution is less then 2 parts per billion but enough to prevent marine life from settling. • Due to the impressed current, the aluminum/ferrous anode produces ions, which spread over the system and produce a anti corrosive film over the sea water system’s pipes, heat exchanger and valves etc, internally.
  64. 64. image Credit: Fig: MGPS
  65. 65. Marine Growth: Sea water contains both macro and micro marine organisms such as sea worm, molluscs, barnacles, algae, hard shells like acorn barnades etc. These organisms stick to the surface of the ship and flourish over there, resulting in marine growth. Effects of Marine Growth As the marine organisms flourish they block and narrow the passage of cooling water in the ship’s system resulting in the following factors: – Impairing the heat transfer system. – Overheating of several water-cooled machineries. – Increase in the rate of corrosion and thinning of pipes. – Reduced efficiency which can lead to loss of vessel speed and loss of time. Fighting Marine Growth: To avoid formation of marine growth MGPS or marine growth preventive system (MGPS) is used onboard ship.
  66. 66. References & Websites: 1. 2. pdf 3. 4. 5. 6. 7. 8. Protection/cathiccp.pdf Thank you!
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