Assignments. 1. Prepare Galvanic series for metals and alloys in flowing sea water. Compare this with the series available for stagnant sea water.

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1 Assignments 1. Prepare Galvanic series for metals and alloys in flowing sea water. Compare this with the series available for stagnant sea water. 2. Construct the Eh ph diagram for the Zn H 2 O O 2 system and comment on its electrochemical behavior with respect to corrosion Zn e = Zn Zn (OH) 2 + 2H + + 2e = Zn + 2 H 2 O HZnO H + + 2e = Zn + 2 H 2 O ZnO H + + 2e = Zn + 2H 2 O F 0 f in KJ / mol Zn 0.00 Zn(OH) HZnO Zn ZnO H OH H 2 O Comment on the corrosion possibilities in the following cases (all exposed to sea water). a) Steel propeller shaft in bronze bearing. b) Steel screw in brass marine hardware c) Lead antimony solder around copper cable. 4. What are the recommendations you would offer to minimize galvanic corrosion? 5. A new heat exchanger is to be installed under marine conditions as part of a condenser system. Sea water need be used to pass through condenser plates. Comment on the types of materials that you will recommend with reasons. 1

2 6. You have to design a cathodic protection system for protection of oil drilling platform structures in a marine environment. There is the possibility of calcareous deposit formation on the electrode and protected surfaces. Bring out your choice of the protection method and design aspects with respect to current requirements. 7. Can you design a biofilm sensor using electrical resistance concept in a marine environment? 8. a) A bivalent metal M is corroding in an acid solution. Exchange current for anodic reaction is 10-6 A / cm 2 at E = -0.40V (SHE) with the anodic Tafel slope at 0.050V. Assume a suitable cathodic curve. Plot a general polarization curve and comment on electrochemical behavior of the metal. b) If you change the acid concentration, how will the corrosion rate be influenced? 9. Clearly define and explain the following: a) Exchange current b) Electrode polarization c) Double layer at electrode-solution interface d) Borderline passivity e) Depolarization 10. Mild steel is connected to a copper plate and immersed in a solution containing ferrous sulfate and copper sulfate at acidic ph. a) Represent the electrochemical cell. b) Calculate cell potential assuming any concentrations. c) Which electrode is anode? d) Which is cathode? 2

3 11. Represent an electrochemical cell having nickel and cadmium electrodes in equilibrium with solutions of their ions. Write down possible reactions and calculate the maximum potential attained from such a cell. 12. Qualitatively draw anodic polarization diagrams for an active passive alloy in acid solution for conditions under potentiostatic and galvanostatic polarization. a. Is there natural and induced passivity? b. How will you determine pitting potentials of an active-passive metal in the presence of chloride ions? c. What is the role of chloride on anodic critical current density? 13. Calculate the theoretical corrosion tendency of zinc in an acid solution containing 0.5M ZnCl 2 at ph Can you estimate the standard potentials at room temperature for: a) Copper in cyanide and ammoniacal solutions. b) Zinc in alkaline solution. 15. Calculate the half cell potential of the hydrogen electrode at ph 5 and 10 at partial pressures of hydrogen at 0.5 and 0.8 atm. 16. Construct an Eh ph diagram for titanium at two different activities (10-3 and 10-6 M ). Comment on the active-passive behavior. 17. Calculate the rate of zinc oxidation (amp/cm 2 ) if the measured potential is 1.0V (SHE) in 0.5M ZnSO 4 βa = 0.05V / decade. Refer i o value from standard tables. 18. For a overvoltage of 0.2 V for a metal, calculate the anodic exchange current density if the corrosion current is 1A/m 2 and βa =

4 19. Explain the electrochemical basis for criteria for cathodic protection. 20. Explain why corrosion rate of zinc is increased when coupled to a noble metal. 21. Through a potential-current diagram and mixed potential theory, explain corrosion processes for coupling two active metals. 22. Discuss spontaneous passivation for titanium, when coupled to a noble metal such as platinum. 23. Illustrate an industrial example for anodic protection with respect to design and efficiency of protection. 24. Explain as to how one can estimate polarization resistance experimentally. 25. Define microbially influenced corrosion (MIC). 26. How is biofouling linked to metallic corrosion? Explain with a model, bacterial attachment and biofilm formation on metals exposed to sea water. 27. Discuss with the help of models and polarization diagrams, the cathodic depolarization theory to explain MIC brought out by SRB. 28. What are the preventive methods for MIC in an industrial environment? 29. How can you effectively monitor the onset of MIC? 30. Discuss with appropriate examples corrosivity of the human body and the various medical implants suitable for a particular body function? 31. Discuss microbially influenced corrosion of concrete with respect to microbiology, mechanisms and monitoring techniques. 4

5 32. Critically assess corrosion prevention methods based on a. Design b. Inhibitor addition c. Surface coatings. 33. Distinguish between: a. Standard electrode potential, overpotential and corrosion potential. b. Corrosion current, exchange current and anodic current. c. Anode and cathode. d. Metallic conduction and electrolytic conduction. e. Immunity and passivity. f. Galvanic cell and electrolytic cell. 34. Calculate Eh-pH relationships and plot on an Eh ph diagram. a. 3Fe 2 O 3 + 2H + + 2e = 2Fe 3 O 4 + H 2 O (E 0 = +0.22V) b. Fe 2 O 3 + 6H + + 2e = 2Fe H 2 O (E 0 = +0.73V) 35. Calculate the theoretical tendency of nickel to corrode in deaerated water of ph = 9. Assume corrosion products as H 2 and Ni (OH) 2 and K sp = 1.6 x 10-16, E 0 Ni = V. 36. Calculate the pressure of hydrogen required to stop corrosion of steel immersed in 0.5M FeCl 2 at ph = 2. E o Fe/Fe+ = V, Assume γ Fe ++ = Calculate the potential of the oxygen electrode at ph = 10 and A current of 3.5 amperes are passed through an electrolyte of copper sulfate solution. How long it would take to deposit 15 grams of copper? 39. For corrosion of a carbon steel specimen in acid solution and aerated sea water, write down the probable anodic and cathodic reactions. 40. Calculate the value for RT/F for room temperature. 5

6 41. Calculate the cell potential for. a) the spontaneous reaction of nickel oxidation in an acid medium (hydrogen reduction). Assume unit activities at room temperature. Write down halfreactions and net cell reaction. b) For the above reaction, calculate the cell potential for conditions when nickel concentration is 10-2 M at a ph of With a platinum SCE couple, the redox potential of an aqueous solution was measured to be +600 mv. What is the potential versus SHE? versus Ag / AgCl? and versus Cu / CuSO 4? 43. Write down electrochemical reactions for the following conditions: a) Copper in acid solution containing ferric sulfate. b) Mild steel in aerated sea water. c) Zinc Iron couple in acid solution. d) Iron Tin couple in sea water. 44. On an Eh ph diagram, draw the lines corresponding to Zn = Zn e Zn + H 2 O = ZnO + 2H + + 2e Cu = Cu e 2H + + 2e = H 2 Cl 2 + 2e = 2Cl - O 2 + 4H + + 2e = 2H 2 O Ag + + e = Ag Predict the use of the various oxidizing and reducing agents for oxidation and reduction of the various metal metal ion species. 45. Inhibitors such as chromates, molybdates and tungstates are added to reduce corrosion rate of iron. Show on the Fe H 2 O O 2 diagram, the effect of a known concentration addition of the above inhibitors on the corrosion region of iron. 46. Clearly bring out the meaning and significance of the following: a) Flade potential b) Passivity index c) Electrical energy content of anode for cathodic protection d) Controlled potential cathodic protection e) Electrical shielding in cathodic protection f) Border line passivity g) Pitting Potential h) Protection range for anodic protection 6

7 47. Through graphical representations only explain the following: a) Effect of solution velocity on the corrosion rate of an active-passive alloy. b) Typical galvanostatic polarization curve depicting active-passive behavior. c) Cathodic protection by impressed current density for steel in neutral aerated water. d) Effect of chromium addition on the anodic polarization behavior of active-passive stainless steel. 48. Mention whether the statements below are TRUE or FALSE. Then justify your conclusion based on electrochemical principles: a) Objective of cathodic protection is to force entire structure to collect current from the environment. b) Potential measurements are used most commonly as a criterion for cathodic protection. c) Scale formation causes continuous decrease in limiting current for oxygen reduction on cathodic polarization of steel in sea water. d) In the active state corrosion rate is proportional to the anodic current density whether or not the alloy is of the active-passive type. e) Amount of oxidizer necessary to cause passivation is greater than that required to maintain passivity. f) Spontaneous passivation is dependent on the relative positions of cathodic reduction curve and the maximum of the anodic dissolution curve. g) Coupling with platinum produces spontaneous passivation of titanium even in the absence of oxidizers. h) Cathodic protection is recommended to resist corrosion of steel in an acid medium. 49. Both mercury (as dropping mercury electrode) and platinum are used as indicator electrodes in electrochemical investigations. Comment on their, electrochemical behavior with respect to hydrogen reduction and associated overpotential. 50. Draw a general polarization diagram for a corroding metal in an acid environment. Show effect of cathodic protection to bring down corrosion rate. Compare applied current requirements for anodic and cathodic control. 51. Estimate the potential to which copper need be polarized for complete cathodic protection in 0.5M copper sulfate solution. 52. Zinc metal is immersed in a solution containing copper sulfate (Cu ++ = 0.5M). What are the anodic and cathodic reactions and net cell reaction. At what concentration of cupric ions will the reaction stop? 7

8 53. Match the corrosion terms in left with the most appropriate fitting example from right: 1. Concentration Cell [ ] A. Manganic oxide 2. Polarization [ ] B. Overvoltage 3. Corrosion potential [ ] C. Oxygen gradient 4. Galvanic cell [ ] D. Dry battery 5. Depolariser [ ] E. Exchange current F. Mixed potential 54. A copper storage tank containing dilute sulfuric acid at ph 0.5 is blanketed with hydrogen at one atmosphere. Calculate maximum cupric ion contamination of the acid in moles/liter. What is the corresponding copper contamination if the hydrogen partial pressure is reduced to 10-3 atmosphere? 55. State as to whether the following statements are TRUE or FALSE. Then justify your conclusion convincingly a) Metallic zinc can be electrodeposited from an acid electrolyte without any significant interference from hydrogen. b) In corrosion, there is no necessity for discrete anodes and cathodes. c) Driving force for corrosion, in reality, is the overpotential d) Stray-current corrosion can be avoided by placing a low resistance metallic conductor between the two structures involved in current leakage and the affected one. e) Anodic protection is suitable for all types of metals and alloys irrespective of the environment. f) Test coupons of the same metal can be electrically connected to a cathodically protected structure as a better proof to assess protection. g) Coupling of zinc with platinum in acid solution results in reducing hydrogen evolution on zinc, shifting corrosion potential of zinc to more noble values. h) Galvanized steel tanks can be used for storing hot water without any significant corrosion. i) Neither the half-cell electrode potential nor the exchange current can be calculated from first principles. j) For two metals close to one another in the EMF series, the corrosion rates in the same acid solution could be very different. 8

9 56. In mixed electrode systems, bring out clearly the following: a) Zero current criterion. b) Multiple reaction processes involved in corrosion of iron with possible hydrogen, oxygen and ferric ion reduction as cathodic processes. c) Through a mixed potential diagram, show the effect of coupling zinc and platinum in a acid medium on the corrosion rate of zinc. Take into consideration area effect. 57. a) What is Tafel relationship? Bring out clearly all the electrochemical parameters involved. b) Through typical polarization plots explain Tafel extrapolation method for determination of corrosion current, corrosion potential and exchange current density (show both anodic and cathodic reactions). 58. From first principles, derive a relationship between cathodic overpotential and limiting current for diffusion controlled cathodic processes. Make all necessary assumptions. 59. Briefly explain as to how stray currents resulting from a cathodically protected structure can induce corrosion of an adjacent unprotected water pipeline. Suggest possible designs and combating methods for prevention of such stray current corrosion? 60. Show through appropriate polarization diagrams. a) Different conditions which can arise when three different activation controlled reduction processes having different exchange current densities are superimposed on an anodic active-passive polarization curve. Bring out the significance of spontaneous passivation and passivation-index in the development of corrosion resistant alloys. b) Schematically illustrate an Eh-pH diagram for iron with a cathodic protection criterion. 61. Define Microbially-Influenced Corrosion (MIC) and illustrate the following aspects. a) Typical Sulfate Reducing Bacteria (SRB) implicated in biocorrosion. b) Cathode depolarization theory to explain biocorrosion in presence of SRB. c) Model to understand bacteria biofilm formation on metals. d) Monitoring, Diagnosis, prevention and control of Microbially-Influenced Corrosion (MIC). 9

10 62. The EMF of cell made up of zinc and hydrogen electrodes immersed in 0.5M zinc chloride is V. What is the ph of the solution? 63. Calculate the EMF of a concentration cell made up of copper electrodes in 0.1 M CuSO 4 and 0.5M CuSO 4 solutions (separated from each other), neglecting liquid junction potentials. 64. What is the relationship between corrosion rate expressed in mpy with respect to current density, atomic weight of metal, equivalent weight and density of metal or alloy? a) Uniform corrosion current density for a nickel alloy (0.20 % C, 32% Cu, 1.5% Fe and 0.5% Si) having density of 8.8 was determined to be 25.0 µa/cm 2. Calculate the equivalent corrosion rate in mpy? 65. Calculate the pressure of hydrogen required to stop corrosion of iron immersed in 0.5M FeCl 2 at ph 2. What would be the required hydrogen pressure in deaerated water for the same purpose? (Ksp Fe (OH 2 ) = 1.8 x ). What is the solubility of Fe( OH) 2 precipitate? 66. How long will it take to plate out 6.0 grams of cadmium from its sulphate solution using a current of 0.30 ampere? What volume of oxygen (in ml) will be liberated? State the anodic and cathodic reactions. 67. State as to the whether the following statements are right or wrong and clearly bring out why with pertinent examples and arguments.. a) In differential aeration systems, oxygen - depleted regions serve as anodes while oxygen rich areas as cathodes. b) For two metals close to one another in the EMF series, the corrosion rates in the same acid could be very different. c) The larger the exchange current density then the smaller is the overpotential for a given net external current i a or i c. d) Kinetic processes are not indicated in Eh ph diagrams. e) The corrosion rate of active metals in contact with relatively nobler metals in a medium depends on the type (species, concentration )of the reduction reaction. f) Galvanic corrosion behavior cannot be predicted accurately on the basis of standard electrode potentials (EMF series ). g) Simple ammeter placed between the anode and cathode in a galvanic cell cannot accurately measure the galvanic current. 10

11 68. Using appropriate polarization diagrams, determine the effect of the following parameters on the corrosion potential and corrosion rate of a metal, corroding to form M ++ in an acid solution. a) Increasing i o for the anodic reaction b) Increasing i o for the cathodic reaction. c) Increasing H + concentration. d) Increasing the Tafel constant of the anodic reaction. 69. Through appropriate diagrams only, show and explain: a) Effect of velocity on the corrosion rate of an active passive metal, corroding under diffusion control. b) Effect of increasing velocity / concentration on the concentration polarization curve. c) Corrosion of a metal under reduction diffusion control. d) Effect of increasing chloride ion concentration on the active passive behavior of a metal. e) The change in potential of a passivated alloy, when the anodic polarization is interrupted. 70. Through appropriate equations only, represent a) Corrosion reactions for a metal immersed in aerated acid. b) Corrosion of nickel in deaerated sulphuric acid. c) Variation of diffusion current with bulk concentration of the electro - active species. d) Reactions involved in the corrosion of steel in aerated sea water of neutral ph? e) Dezincification of brass pipe used in transport of water. 71. Calculate the ratio of Sn ++ to Fe ++ at which the polarity of a Fe Sn couple could reverse sign. Explain the practical aspects of such a phenomenon. 72. The potential of an iron electrode when polarized as cathode at amp/cm 2 is -1.2V (SCE). The ph of the electrolyte is 3.0. What is the hydrogen overvoltage? 73. Will a steel container corrode in contact with a solution of ph 2.5? (Assume Fe ++ concentration of 10-4 M as the criterion). 11

12 74. a) With the help of theoretical models, discuss attachment, biofilm formation and corrosion of steels brought about by Sulfate Reducing Bacteria (SRB). Base your explanation on cathodic depolarization theory. b) What are aerobic iron bacteria involved in corrosion of steels? Explain tubercle formation in steel pipelines with probable mechanisms. c) What are hydrogenase positive and hydrogenase-negative SRB? Explain the significance with respect to MIC. How would you characterize the SRB for the above classification based on enzyme activity? d) What are the major factors and variables contributing to MIC by SRB? e) Compare mechanisms and relevance of MIC brought about by aerobes and anaerobes. 75. a) What are the forces in bacterial attachment to metals? b) Suggest a model to explain bacterial attachment and colonization of bacteria on a metal surface. c) Explain possible mechanisms in bacterial attachment? 12

Laboratory Experiments in Corrosion Engineering II

Lecture - 40 Laboratory Experiments in Corrosion Engineering II Keywords: Polarization Experiments, Pitting Potentials, Microbial Corrosion. A. Electrochemical tests in a given environment Polarization