Corrosion and its Control Lecture Notes PDF

Lecture No. 9 Corrosion and its control At the end of this lecture, students will be able to: Describe the process of Corrosion Identify the types of corrosion Explain Galvanic...

Lecture No. 9 Corrosion and its control At the end of this lecture, students will be able to: Describe the process of Corrosion Identify the types of corrosion Explain Galvanic series 1 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Corrosion Science Destruction and consequent loss of metal due to its reaction with surrounding Decay process in which metal exhibit their natural tendency to go back to their original state of existence as minerals (ex: oxides, sulphides, carbonates) Noble metals such as gold and platinum exist in nature as metals and are not susceptible to corrosion under ordinary atmospheric conditions Sliver does not rust, but tarnished It reacts with H2S found in food or water to form silver sulfide 2 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Corrosion Science  Metals have free energies greater than their compounds  Tend to become their compounds through the process of corrosion 3 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Corrosion Science Destruction of metals or alloys by the surrounding environment through chemical or electrochemical changes is known as corrosion Corrosion is a natural process Represents a return of metals to their more natural state as minerals (oxides) 4 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Why we have to study corrosion? Process products in chemical industry get contaminated with the corrosion product, the contaminated product becomes no more useful resulting in loss of products Collapse of building /bridge/other structures Instrument get deteriorated Annual loss due to corrosion is estimated to be 4 to 5% of the Gross National Product (GNP) Injuries to people arising from structural failure or breakdown 5 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Types of corrosion Dry corrosion or chemical corrosion Wet corrosion or electrochemical corrosion Dry Corrosion: Involves the direct attack of metals by dry gases mainly through chemical reactions Ex: i) Cl2, SO2 fumes comes in contact with any steel container ii) The attack of dry air on a metal to form oxide iii) CO2 attacking copper to form copper carbonate, a green scale 6 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Wet Corrosion Involves reactions in aqueous medium Conducting surface of the metal undergoes an electrochemical reaction with moisture and oxygen present in the atmosphere Ex: Rusting of iron 7 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Electrochemical Theory of Corrosion When a metal surface is exposed to the atmospheric air the following electrochemical changes occur gradually Formation of galvanic cells: Anodic and Cathodic areas are formed resulting in formation of minute galvanic cells Anodic reaction: oxidation or corrosion takes place M → Mn+ + ne- Ex: Fe Fe2+ + 2e- Cathodic reaction: The electron flows from anodic to cathodic areas and causes reduction 8 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Reactions at cathodic region: At cathode, depending on the surrounding medium the reaction is either a) liberation of hydrogen or b) absorption of oxygen a) Liberation of hydrogen (in the absence of oxygen) In acidic medium the reaction is In neutral or alkaline medium, the reaction is b) Absorption of oxygen (in the presence of oxygen) In acidic medium, the reaction is In neutral or alkaline medium, the reaction is 9 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Electrochemical Theory Of Corrosion Electrochemical Theory of Corrosion Metal ions formed combine with OH- ions to from the metal hydroxide and finally the hydrated metal oxide (Rust) Over all reaction: 2Fe2+ + 4OH- 2 Fe (OH) 2 2Fe (OH) 2 + O2 + (n-2) H2O Fe2O3.nH2O 10 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Galvanic series Necessity for the Galvanic series Electrochemical series is based on standard reduction potential for pure metals, doesn’t give a correct basis for predicting, in a particular environment, whether a metal will corrode or not Passivity of the metals is not considered Galvanic series is a more accurate representation of actual galvanic corrosion characteristics than the electrochemical series Corrosion of alloys is also accounted Galvanic series is built on corrosion studies of metals and alloys in sea water (determines the nobility of metals and semi-metals) 11 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Galvanic Corrosion 12 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Forms of Corrosion Stress corrosion Stress & corrosion Uniform Attack work together Erosion-corrosion Oxidation & reduction at crack tips. Break down of passivating occur uniformly over layer by erosion (pipe surface. elbows). Selective Leaching Pitting Preferred corrosion of Forms Downward propagation one element/constituent of of small pits & holes. (e.g., Zn from brass (Cu-Zn)). corrosion Intergranular Corrosion along grain boundaries, Galvanic often where special Dissimilar metals are Crevice Between two phases exist. physically joined. The pieces of the same metal. more anodic one g.b. corrodes. (see Table Rivet holes prec. Shown in previous slide) attacked Zn & Mg zones very anodic 13 13 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Galvanic Corrosion (Types of Corrosion) 1. Differential metal corrosion 2. Differential aeration corrosion 3. Stress corrosion Differential metal corrosion (Galvanic corrosion) For galvanic corrosion to occur, three conditions must be present: Electrochemically dissimilar metals must be present Metals must be in electrical contact Metals must be exposed to an electrolyte 14 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Differential Metal Corrosion Consider a galvanic cell where in iron and copper are in contact with aerated sodium chloride solution Reactions taking place in the cell are: At the anode: Fe Fe2+ + 2e At the cathode: O2 + 2H2O + 2e 4 OH- 2Fe2+ + 4OH- 2 Fe (OH) 2 2Fe (OH) 2 + O2 + (n-2) H2O Fe2O3.nH2O 15 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Differential Metal Corrosion Criteria for anode and cathode Metal which is above in the galvanic series act as anode and other metal which is below act as cathode More the reduction potential difference, more will be the rate of corrosion Iron (-0.44V) acts as anode to Sn (-0.14V), Cu (0.34V), Ni (-0.23V) hence iron undergoes corrosion Iron (-0.44V) acts as cathode to Mg (-2.37V) and Zn (-0.76V) as result Mg and Zn undergoes corrosion 16 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Differential Metal Corrosion It occurs when two dissimilar metals are in contact with each other in a corrosive conducting medium Metal with lower reduction potential acts as anode and gets oxidized and suffers from corrosion Metal with higher electrode potential acts as cathode and gets reduced Rate of galvanic corrosion depends upon how far the anode and cathode are placed in the galvanic series Larger the difference in the electrode potential, more will be the rate of corrosion 17 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Quiz Can corrosion occur in the absence of water? Why copper statues are green in color? If copper and zinc metals are connected which part can be corroded? If magnesium and zinc metals are connected which part can be corroded? 18 Faculty of Science and Humanities © Ramaiah University of Applied Sciences  Summary Corrosion is defined as destruction and consequent loss of metal due to its reaction with surrounding When a metal surface is exposed to the atmospheric air results in electrochemical changes with formation of galvanic cell Types of corrosion – Wet and dry corrosion 19 Faculty of Science and Humanities © Ramaiah University of Applied Sciences