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Suez Canal University. Corrosion Part 1. Dr. Eng. Hamid A. Nagy. Corrosion. Driving force. Every Process to take place, we should have some driving force. The driving force depends on the energy of the first state and that of the final state. Driving force. A. Barrier. Energy.

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Corrosion Part 1


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    1. Suez Canal University Corrosion Part 1 Dr. Eng. Hamid A. Nagy WELD-INSPECTA CO.

    2. Corrosion

    3. Driving force • Every Process to take place, we should have some driving force. • The driving force depends on the energy of the first state and that of the final state.

    4. Driving force A Barrier Energy Driving Force B

    5. Thermodynamics • Every material has two sources of its energy • Heat content, Enthalpy • Content depending on its randomness, Entropy. • We can not measure this energy directly. • So we have a reference zero value which is the hydrogen molecule formation.

    6. Thermodynamics • Now consider the transfer of a metal from state A to state B. • This can only be done if the state A has higher energy than state B. DGA > DGB DG is the free energy of material state.

    7. Thermodynamics • Now consider the reaction between two materials a and B to produce C. • The same law applies. DGA + DGB > DGC For the reaction to proceed in the direction A + B → C And vice versa.

    8. Effect of Concentration • Consider the following Reaction M++ + 2e → M • As the M ions concentration increases the reaction tends to go the left. • Oxidation reaction increases. • More anodic tendency. • Potential decreases.

    9. Kinetics • Rate of reaction depends on what is called mechanism. • There should be some energy done to activate the first stage. • This is called the energy barrier. • This energy barrier could be high or low depending on the mechanism.

    10. Kinetics • Overcoming energy barrier may consist of several steps. • The rate of occurrence of this reaction depends on the interaction of steps to overcome energy barrier. • There is usually what is called rate determining step. • Determining the rate of the reaction is what is called KINETICS.

    11. Metals • Corrosion is a chemical reaction. • What is considered in corrosion? • Feasibility. • Rate. • The answer is both. • You can even protect metals if you impair the feasibility or slow down the rate.

    12. Metals • Now consider the structure of metal atoms and their mutual relation. • Metal atoms have some free electrons. • In the matrix of metals, free electrons do not relate specifically to a certain atom. • They are just Free ELECTRONS. • This is what provides the metals with their Characteristic Properties.

    13. Metals • If the metals are going to react, What is better for the atoms? • To share these electrons. • To loss these electrons. • The answer, for sure is the second option. • So they are going to exchange ions with other reactants. • Ionic Bond.

    14. Corrosion Potential • Now, reaction of metals depends on DG. • But this reaction involves transfer of electrons. • DG is a measure of chemical energy. • But this energy is transferred to electric energy for the reaction to take place. • Is it possible to measure this energy using the electric parameters.

    15. Corrosion Potential • Now Recall the definition of volt. • The voltage between two points is 1 volt if the amount of energy required to transfer 1 unit charge is 1 unit of energy. • This is why P = V I • Or Energy/ time = Volt X (Charge/ Time) • Energy = Volt X Charge.

    16. Normality • If you dissolve one mole (atomic weight) in one liter of water, concentration is called morality (1 molar solution). • If you dissolve one equivalent weight in one liter of water, concentration is called Normality (1 Normal solution). • So, for one and the same material • 1 N solution = 1M solution if n =1. • 1 N solution = ½ M solution if n = 2.

    17. Normality • For explanation let us consider sodium chloride (NaCl) • At. Wt. for Na = 23 • At. Wt. for Cl = 17. • At. Wt. for NaCl = 40. • 1 M solution means 40 gram NaCl in 1 liter water. • 1 N solution is the same. • Concentration of NaCl in sea water is about 3.5% (35 grams in 1 liter).

    18. Normality • Another example is (FeCl2) • At. Wt. for Fe = 56 • At. Wt. for Cl = 17. • At. Wt. for FeCl2 = 90. • 1 M solution means 90 grams FeCl2 in 1 liter water. • 1 N solution means 45 grams FeCl2 in 1 liter water. • Normality is more expressive than molality.

    19. Cu –ve potential Reduction prevails (Cathode) Cu +ve potential Oxidationprevails (Anode) Electrolytic Cell • Now consider Electro-refining of Cu.

    20. ECu = 0.34 V Reduction prevails Positive Electrode (Cathode) EZn = -0.76 V Oxidation prevails Negative Electrode (Anode) Galvanic Cell • Now consider a cell having Cu in 1N CuSO4 solution in half cell and Zn in 1N ZnSO4 solution in the other half cell. V Electrons Current O.C.P. = 1.1 Volts

    21. Anode/ Cathode • Now if you consider Zn Zn++ + 2e → Zn • This called reduction. Or Zn → Zn++ + 2e • This is called oxidation. • As a convention we use the potential measurement for the reduction reaction. • If V increases, DG decreases, more reduction takes place, more protection.

    22. Corrosion Potential • Back to Cu + Zn • Cu will not be dissolved (protected). Cu++ + 2e → Cu • Zn will dissolve (Corroded). Zn → Zn++ + 2e • As the difference in potential increases more current takes place but not necessarily.

    23. Corrosion Rate • As we know OC potential is higher than short circuit potential. • How much is the highest current provided by any galvanic cell, let us see.

    24. Concentration Polarization • 3- Concentration Polarization: • Consider the case of placing steel part in aerated water. • Anode: Fe dissolution • Cathode: Oxygen Reduction (Oxygen). 1- Transport of oxygen to steel by diffusion. 2- Reduction of Oxygen O2 + 2H2O +4e → 4OH- Steel Low diffusion

    25. High Oxygen Diffusion Potential Low Oxygen Diffusion Fe Current Concentration Polarization Factors affecting this phenomenon: 1- Temperature. 2- Agitation. 3- Pressure 4- Flow Rate. 5- Concentration. O2

    26. Control of Rate • Decrease the metal conductance. • Lower the electrolytic conductance. • Control one of the surfaces (Larger Anode is Better. • Control one of the two reactions (anodic and Cathodic).

    27. Control of Cathodic Reactions 2H+ + 2e → H2 • Increase or Control pH. • Increase Pressure (not a solution) • Decrease Pubbling rate (not a solution in tanks and pipelines.

    28. Control of Cathodic Reactions O2 + 2 H2O + 4 e → 4 OH- • Increase or Control pH. • Use Scavengers. • In open vessels, temperature lowers the reaction rate. • In closed vessels, temperature increases rate.

    29. Uniform Attack • If we have one steel plate, corrosion will take place. • The anode and cathode will alternate from a point at the surface to another. • As the polarization of hydrogen increases at a certain point. • The other point will act as a cathode. • Uniform corrosion is not very severe usually.

    30. Measurement of Corrosion • Conversion of Current to Corrosion Rate: • If i A/cm2 is the current density, i Coulombs/cm2 transfer per second. • OR (iX365X60X60X24) = (31,536,000Xi) Coulombs per year. • OR 31,536,000 (i/96,500) = 326.79 (i) Farads per year per cm2. • If the equivalent weight of the metal is (EW), this means that (326.79XiXEW)gms/year/cm2 • If the density of the metal is (r) grams/cm3, this means that (326.79XiXEW/r) cm3 of metal corrode in one year from 1cm2 OR the metal loses (326.79XiXEW/r)cm/year. • Metal loses (13.617/2.54)X(iXEW/r) or (128.66XiXEW/r)in./year. • This means that the metal lost (128,660XiXEW/r)mils/year {mpy}. NOTE THAT THIS IS VALID ONLY FOR UNIFORM CORROSION

    31. Galvanic corrosion • If you place two dissimilar metals beside each other, the more negative potential will corrode. • Corrosion effect will increase as • Ratio of anode to cathode decreases. • Resistance of electrolyte deceases. • Criticality of corroded part increases. • Some notes about painting.

    32. Electrochemical Series • Metals are ranked in accordance with their potential in 1 N solution of their solutions. • Hydrogen is zero reference: 2H+ + 2e → H2 • If metal has positive value (Au, Ag, Pt), it is called noble metal or semi-noble (Cu). • If metal has negative value (Fe, Al, Mg, Zn), it is called active metal. • Such Ranking is called Electrochemical Series.

    33. Galvanic Series • From all the discussion, it can be noticed that every metal will have different potentials in different media. • The behavior in different media depends on many different correlated factors. • This is why Electorchemical series can be not indicative of the corrosion state. • So, Galvanic Series is more practical.

    34. Passivity • But what about if a product of corrosion is formed. • The rate of generation of product increases with current. • At a certain amount of product, it could hinder ions from dissolution into solution. • This makes the rate of corrosion very slow. • This takes place in a few metals only.

    35. Pitting and Crevice • At a certain value passivity breaks down to start the transpassivity stage. • The presence of chloride ions was found to decrease as the chloride content in the solution increases. • Chloride ions were expected to attack the passive layer leaving unprotected area. • This case represents high cathode to anode area.

    36. Pitting and Crevice • As resistance of the material increases the Epit is expected to increase. • So it can be taken as a measure of resistance to pitting. • Chromium is added to iron to increase passivity. • At 12% Cr the surface is expected to be covered with Cr2O3. • However further increase in Cr will increase the passive layer thickness and increase resistance to damage by chloride ions.

    37. Pitting and Crevice • As a rule of thumb those steels covered with 100% passive layer are called stainless steels. • Cr and Mo increases both thickness and stability of passive layer. • However, Fe++ formed in the pit will hydrolyze according to the reaction: Fe++ + H2O + 2Cl- → Fe(OH)2 + 2HCl HCl is a strong acid leading to decrease of the pH.

    38. Fe+++ Transported Cl- Fe++, H+, Cl- Probability Pitting and Crevice P/D Surface Area

    39. Pitting and Crevice • This is why pits more corrosive environment takes place within pits as they grow leading to autocatalytic action. • Nitrogen in steel was found to react with H+ in the pits to form NH3 and reduce the autocatalytic action. • For stainless steels, pitting resistance equivalent number (PREN) is equal to: PREN = Cr + 3.3 (Mo + 0.5 W) + 16N

    40. Pitting and Crevice • How to measure the resistance of material to pitting: 1- PREN will identify the grade of stainless steel. 2- Pitting potential. 3- the potential at which the anodic polarization curve intersects with the passive zone again (Eprot). • However, the difference of Epit-Eprot is more indicative of the resistance.

    41. Pitting and Crevice • Pitting is expected to grow more downward or at the upstream especially encountering elbows. • Now how to measure the intensity of pitting: • Density. • Diameter. • Depth. • Pitting factor is a measure of the prevailage of pitting against general corrosion • P/d tends to zero for general corrosion.

    42. Pitting and Crevice • P-d could be measured by: • 1- Metallography. • 2- Machining • 3- Micrometer. • 4- Microscopy. d P

    43. Pitting and Crevice

    44. Pitting and Crevice

    45. Pitting and Crevice • Crevice attack is similar to pitting in a way or another. • Inside the crevice lack of oxygen and increase in the chloride content take place leading to break down of passivity. • Thermal insulation and carbonate deposits may lead to the dame situation. • Filliform corrosion is an example of crevice attack.

    46. Pitting and Crevice Inert Washer Stainless Steel

    47. Pitting and Crevice • Even in bolts, which after rain contains some corrosive media in their crevices (does not dry easily). • Solutions include: • 1- Use larger and less number of bolts. • 2- Tighten the bolts as possible. • 3- Use ductile caulking. • 4- Use sealing compounds. • 5- Use Weathering Steel (A HSLA containing copper, phosphorous and nickel in controlled amounts).

    48. Differential Aeration • can take place even if there is no passivity. • Consider immersion of pipe in the earth (soil to air interface). • There will be difference in the mixed potential due to different values of oxygen Concentration.

    49. Pourbaix Diagram

    50. H2 Hg Platinized Platinum H2SO4 H2 Reference Electrodes • Our zero arbitrary reference electrode. • Potential =0 at STP. H+ + 2e → H2 Standard Hydrogen Electrode (SHE)