Principles of Corrosion in Marine EnvironmentsElectrochemistry FactsSlide Number 3Corrosion in Iron with Oxygenated Water: A Galvanic CellSlide Number 5 Dual Use of Coatings and Sacrificial Anodes3. Impressed Current4. PassivationMassachusetts Institute of Technology 2.017Principles of Corrosion in Marine EnvironmentsReferences used:Oxtoby, D.W., H.P. Gillis, and N.H. Nachtrieb (1999). Principles of modern chemistry, 4th edition. New York: Saunders College Publishing.Chandler, K.A. (1985). Marine and offshore corrosion. London: Butterworths.Wranglen, G. (1985). An introduction to corrosion and protection of metals. New York: Chapman and Hall.Schweitzer, P.E., ed. (1996). Corrosion engineering handbook. New York: Marcel Dekker Inc.Massachusetts Institute of Technology 2.017Electrochemistry Facts• Elements can be categorized by their tendency to attract electrons: electronegativity• Oxidation: the loss of electrons, typically to oxygen anions (O-, O2-, etc.) but not always! E.g. 2Mg(s) + O2 (g) 2MgO(s) “Mg is oxidized”2+ 2-• Reduction: the gain of electrons• Reductions are associated with specific voltages; and the material ranking is similar to that of electronegativity, and the galvanic series.• A galvanic cell converts chemical into electrical energy; spontaneous by definition (e.g., battery discharge)• An electrolytic cell converts electrical into chemical energy; takes external power (e.g., battery charge)Massachusetts Institute of Technology 2.017Oxygen 1.229V (O2 ,H+/H2 O)Silver 0.800 (Ag+/Ag)Copper 0.340 (Cu2+/Cu)Hydrogen 0.000 (H+/H2 REFERENCE)Iron -0.036 (Fe3+/Fe)Iron -0.409 (Fe2+/Fe)Zinc -0.763 (Zn2+/Zn)Titanium -0.860 (TiO2 ,H+/Ti)Aluminum -1.706 (Al3+/Al)Magnesium -2.375 (Mg2+/Mg)Some Reduction Potentials in the Marine EnvironmentMore prone to lose electrons and to corrodeOxtoby et al.Massachusetts Institute of Technology 2.017Corrosion in Iron with Oxygenated Water: A Galvanic CellAnode: 2Fe(s) 2Fe2+ + 4e- (Iron oxidized: -0.41V)Cathode: 4H3 O+ (aq) + O2 (g) + 4e- 6H2 O(l) (Hydronium reduced: 1.23V)Also, iron cations are oxidized again, from Fe2+ to Fe3+ by oxygen in the water:4Fe2+(aq) + O2 (g) + 12H2 0(l) 2Fe2 O3 (s) + 8H3 0+(aq) andmaking RUST! Hydronium created in this second reaction can supply the cathode primary half-cell reaction. Cathode: Accretion of rustAnode: Pitting, crackingIronWaterCoatinge-Fe2+H2 0(l),O2 (g), H3 O+(aq)Fe(s)Adapted from Oxtoby et al.Massachusetts Institute of Technology 2.017Fe Zne-Anode: Zn(s) Zn2+(aq) + 2e--0.763VCathode: 4H3 0+(aq) + 4e- + O2 (g) 6H2 0(l) 1.230V(electrons move through the iron)The iron oxidation is suppressed because the reduction potential is only -0.41V: Zinc loses and is oxidized!Electrolyte: H2 0, O2 (g), H3 0+(aq)Other common anode materials:Magnesium, Aluminum1. Coatings (paint, rubber, grease, etc.)2. Sacrificial Anodes:Massachusetts Institute of Technology 2.017Dual Use of Coatings and Sacrificial AnodesPercentage of bare metal$0%100%Cost of coatingsCost of anodesCOMBINED COSTminimumWranglen, p. 179Massachusetts Institute of Technology 2.0173. Impressed CurrentM1M2Voltage source_+e-Anode:M1 (s) M1x+ + xe-CathodeUse of various materials for anode: high-silicon iron, lead alloys, platinised titaniumVoltage levels can be much higher than in a passive systemRecommended current densities are on the order of 100mA/m2 at the cathode (hull)Typical currentdensities are on the order of 500A/m2 at the anode ChandlerElectrolyteMassachusetts Institute of Technology 2.0174. Passivation• The material develops a protective barrier, that is sufficient to protect against corrosion. Examples:– Tin coating on steel cans– Galvanized steel: a coating of zinc– Aluminum oxide in atmospheric conditions– Chromium in stainless steel forms a layer • The barrier blocks oxygen from getting in and metal cations from getting out.MIT OpenCourseWarehttp://ocw.mit.edu 2.017J Design of Electromechanical Robotic SystemsFall 2009 For information about citing these materials or our Terms of Use, visit:
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