Corrosion

DefinitionGradual attack on a metalChemical or electrochemical reaction by its surroundingsMetal is converted into its oxide, salt or some other compound that is, deterioration and loss of material due to chemical attack

Factors influencing corrosion1. Chemical nature of the metal2. Internal structure3. Working conditions or environment like stresses, temperature or concentration4. Presence of dust, dirt or foreign matter5. Surface film

Contd.6. Blow holes, inclusions and trapped gases7. Distribution of secondary phases8. Nature of engineering applications9. Eddy electric currentsTypes of corrosion: (i) general types (ii) specific types

General types1. Direct corrosion2. Electrochemical corrosion3. Galvanic corrosion4. high-temperature oxidation

Specific types1. uniform corrosion2. Pitting corrosion3. Intergranular corrosion4. Atmospheric corrosion5. Stress corrosion6. Corrosion fatigue7. Erosion corrosion

Contd.8. Fretting & selective corrosion9. Soil corrosion10. Microbial corrosion11. Liquid metal corrosion12.Crevice corrosion

Direct corrosionEssentially ordinary chemical attackHappens due to chemical reactions between the metals and corrosive solutions such as acidsMetal is dissolved; no protective layer that could inhibit corrosion is formed Corrosion is uniform, relatively high and can be measured in standard units, e.g., milligrams per square decimeter per day (mdd)

Examples for direct corrosionAcid pickling used to clean metal surfaces and etching (involves chemical attack)Reactions of dry chlorine, H2S, O2, other dry gases with dry metals- these are examples for direct chemical attack and also considered as redox reactions

ControlBy adding some inhibiting chemicals in the corrosive solutionThey act as barriers between the metal involved and the environment

Electrochemical corrosionTakes place at or near room temperatureTakes place due to reaction of metals with water or with aqueous solutions of salts, acids or basesInvolves transfer of electronsFlow of electricity from certain areas of the metal surfaces to other parts through a medium which conducts electricity

Contd.Potential difference must exist between different metals or between two areas on the surface of metalMetal surfaces develop anodic and cathodic sitesAnode is the portion of the metal surface that is corroded and dissolved as ion electron leaves from that site and enter the environmentThis is called anodic oxidation

Contd.Cathode is that portion at which current returns to the metal from the environmentReduction takes place at cathode and the metal is not affected at cathodeHighly pure single phase metals corrode at a slower rate than impure metals and alloys containing many phases

Electrochemical SeriesMetals are arranged in the order of their standard electrode potentialsThe ability of metals to resist corrosion is to some extent dependent upon their position in the electrochemical series

Galvanic SeriesThe various standard electrode potentials listed in the electrochemical series were measured under certain chemical conditions at a particular concentration and temperature The electrodes were completely clean and free from oxide filmsTherefore, not always possible to predict which will be anode and which will be cathode

Contd.For practical purposes, electric potentials of many materials are obtained in a single environment, viz., sea waterThese when arranged in the decreasing order of oxidation potential is called galvanic seriesPair of metals from this series connected together in sea water, the metal which is higher in the series would be anodeFarther they are apart, greater the corrosion tendency

Galvanic seriesAnodic end (corroded end)Mg>Zn>Al>Cd>Al alloys>mild steel>alloy steel>cast iron>stainless steel>muntz metal>yellow brass>Al brass>red brass>Al bronze>CuNi alloys>inconel>Ni>Ag>Stainless steel(passive)>monel>titaniumCathodic end(protected)

Origin of potential difference on metal surfacesFactors: (i) internal (ii) externalInternal factors: Micro structure and composition of metal such as:1. Different crystal orientation (metal is polycrystalline)2. presence of small amounts of minute impurities within the microstructure, often concentrated at the grain boundaries

Contd.3. Change in concentration of alloying components in different points of the metal (that may occur as a result of the original casting technique or as a consequence of the precipitation of very fine particles)4. Presence of more than one phase in the metal as in the case of multiphase metals

External factorsAssociated with metals as well as corrosive environments1. Residual and applied stress in the metal particularly after mechanical working2. Presence of dust, dirt, soot and other foreign bodies on the metal surface

External factors3. Presence of protective oxide films on metal surface before their contact with a corrosive environment4. Presence of adsorbed gas on the metal surface

External factors5. Local changes in the concentration of an electrolyte, the presence of dissolved gases, the presence of some agents influencing polarization and depolarization of the anodic and cathodic areas distributed on the surface of the corroding metal6. Temperature differences in the body of the electrolyte leading to the formation of thermal gradients , the degree of agitation of a corrosive environment, and the differential lighting of some portion of a metal surface

Polarization at the electrodesIndividual electrode potentials, for ex. Cu and Zn electrodes, change from normal value when current flows between two electrodesChange in potential as a result of flow of current is known as the polarization of the electrode

Polarization at the electrodesPolarization causes the solution potential (difference in potential between metal and solution) of the anode to become more cathodic and that of the cathode to become more anodic (i.e. Direction reversed and reverse current is produced)

Polarization at the electrodesHence, the potential difference between the electrodes steadily decreases and the amount of current in the closed circuit diminishesPolarization at the electrodes is due to concentration polarization, over voltage, or the presence of surface film on the electrodes

cells(i) Electrochemical cells (ii) galvanic cellsElectrochemical cells electrolysis takes place; anode is +ve and cathode is veElectrical chemicalGalvanic cells Chemical electricalAnode is ve (oxidation); cathode is +ve (reduction) (-) (+)Anode cathode

Galvanic cellDaniel cell

Zn Zn2++2e-

Cu2++2e- Cu

ZnSO4CuSO4+ -Cu Zn e-current oxidationReduction

Concentration polarizationAs reaction proceeds near electrodes, ionic concentration near each electrode becomes different from that of the main body of the electrolyteReason: slowness in diffusion of ions in the immediate vicinity of the electrodesAt anode (-ve electrode), concentration of the metal ions will gradually increase and the anode potential will be more +ve, while at cathode (+ve electrode), concentration of the +ve ions will decrease and the cathode potential will be more negative

Overvoltage This is the excess voltage than the actual voltage of a single electrode while current passes through it.Ex: hydrogen over voltage hydrogen evolution is slow therefore, more H+ ions will be near the electrode more emf This will be opposite to the normal emfH+ + e- H; H+H H2(adsorbed)H2(adsorbed) H2(evolved)Plays an important role in corrosion processes

Presence of filmsProtective films are formed on the electrodes both at the cathode and the anode as a result of the combined effect of the electrode reactions and the action of the environmentResistance between cathode and anode increases and the rate of diffusion decreases

Galvanic cellsMetal of high electrode potential is placed in water and is electrically connected to another metal of much lower electrode potential A current will flow through the wire connecting the electrodes called galvanic cellsElectrochemical corrosion involves a transfer of electrons

Factors increasing the rate of corrosion in a galvanic cell1. A large difference in electrode potential between anode and cathode2. increased acidity of the electrolyteIncreased oxygen concentration in the electrolyte around the cathode

Summary of electrode reactions in galvanic cellsAnode: M Mn++ne- (metal dissolves)Cathode: H2O H+ + OH- 2H+ + 2e- H2 H2O + O2 + 2e- 2OH-Oxygen absorption at the cathode uses more electrons; hence rate of corrosion increases

Mechanism of electrochemical corrosionInvolves transfer of electronsElectrons must flow between anodic and cathodic areasAnodic reaction: always dissolution of metals and formation of ionsCathodic reaction: may involve two different processes depending on the nature of corrosive environment evolution of hydrogen gas, absorption of oxygen

Hydrogen evolution mechanismWhen corrosion environment is acidic such as acid industrial waters and the concentration of dissolved oxygen is low, hydrogen evolution type of corrosion occursExample: steel tank containing acid industrial wastes and a small copper scrap in contact with the steel plate

Hydrogen evolution mechanismContact between the piece of copper and steel vessel in the presence of acid electrolytes shown in the figure forms a corrosion cell; Cu becomes cathodic and steel anodicHence steel portion in contact with Cu is corrodedFe Fe2+ + 2e-H+ ions present due to dissociation of solution take up these electrons and bubbles of hydrogen gas is evolved from the cathode as follows:2H+ + 2e- H2 Therefore, hydrogen evolution type corrosion is simply the displacement of hydrogen ions from the solution by metal ions

Acid wasteSteel tankSmall Cu scrapH2Fe2+Fe2+

H+H+Electron flow from anodes

Oxygen absorption mechanismCorrosion mechanism is most likely to occur when dissolved oxygen is present in the electrolyteEx: Fe is corroded by a neutral aqueous solution of electrolytes in the presence of oxygenSteel plate lying on the ground and is exposed to atmosphere

Drop of water crackSteel plateAnode at crackFilm or scaleFe2+ Fe2+ e- e-cathoderustrust

Contd.In course of time, oxide layer will form on the surface of the steel plate and drops of water may collect over a small crack in the oxide film present on the surface of the steel (figure)Water acts as the electrolyte, crack as the anode and the scale protected steel as cathode

Contd.At the anode(i.e. crack), the steel goes into solution as ions according to the following equation: Fe Fe2+ + 2e-At the large cathodic area (scale protected steel plate) the electrons flow from the anode and are intercepted by the oxygen atoms present in the atmosphere, and in the presence of water drop.OH- ions are formed according to the following reaction:H2O +1/2 O2 + 2e- 2OH- Fe2+ ions migrate to the edge of the water drop, combine with OH- ions, form ferrous hydroxide, Fe(OH)2 or brown rustThen oxidised to ferric hydroxide, Fe(OH)3 or hard rustCorrosion will take place as long as there is oxygen supply available.