corrosion - salford 1

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CORROSION IN METALS (ERASMUS PROGRAMME-SALFORD UNIVERSITY)Professor Fevzi Ylmaz / Istanbul - TURKEY

Fatih Sultan Mehmet Vakif UniversityFaculty of Engineering and ArchitectureHalic Campus, Stlce Mah. Karaaa Cad. No:12 Beyolu / Istanbul TURKEY Phone: +90 212 369 8162 Fax: +90 212 369 8164E_mail: [email protected]

Erasmus Lecture 2013, March 07, Salford University

CORROSION IN METALSThe composition and physical integrity of a solid material are altered in a corrosive environment. Dissolving of MaterialFormation of oxides or other compounds.

The atom gives up electrons

The Electrochemical Cell

An electrochemical cell is formed when two pieces of metal in contact with one another are placed in a conducting liquid or electrolyte.Electrochemical cell leads either electrochemical corrosion or electroplating.

The Electrochemical Cell

The Electrochemical CellAnode: The anode gives up electrons to the circuit and corrodes.Cathode: The cathode receives electrons from the circuit Physical contact: To permit the electrons to flow from anode to the cathode.Electrolyte: A liquid electrolyte is conductive, thus completing the circuit.Metallic ions leave the anode surface and assures that ions move to the cathode to accept the electrons.

The Electrochemical CellAnode Reaction :Oxidation reaction The metal ions enter the electrolytic solutions electrons leave the anode through the electrical connection. Because metal atoms leave the anode, the anode corrodes.

The Electrochemical CellCathode Reaction in ElectroplatingA reduction reaction, which is the reverse of the anode reaction, occurs at the cathode.

The metal then plates out and cover the cathode surface.Cathode reaction in corrosionThe reduction reaction forms a gas, liquid or solid by-product (e.g. rust) at the cathode.

The Electrochemical CellExample of Anode Cathode Reaction (Corrosion of Zinc)Consider zinc metal immersed in an acid solution containing H+ ions.Zinc will experience oxidation or corrosion.

Electroplating (electrorefining)

AnodeCathode

Anode: Ag e- Ag+Cathode: Ag+ + e- Ag

Silver coatingElectroplating (electrorefining)

The Electrochemical CellProtective Oxide LayerMany metals form protective oxide layers (e.g. Al2O3, Cr2O3 and TiO2 etc.)If the oxide layer becomes scratched and aluminium metal exposed, more oxide is formed and the protected layer is renewed.

Iron oxidizes and forms the familiar red-orange rust. However, rust takes up a greater volume than the iron. Consequently layer splits and cracks.

Types of Electrochemical Corrosion (Galvanic Attack)

Galvanic attack occurs when certain areas always act as anodes, while other areas always act as cathodes.Uniform attackComposition cellsStress cellsConcentration cellsErosion corrosion

Types of Electrochemical Corrosion (Galvanic Attack)Uniform AttackWhen a metal is placed in an electrolyte, some regions are anodic to other regions.Anode and cathode regions continually shiftThe extent of uniform attack can be given as weight loss per unit area or by the average penetration.

Types of Electrochemical Corrosion (Galvanic Attack)Rusting of iron or tarnishing of silver. Fogging of nickel and high-temperature oxidation of metals are also examples of uniform attack.Rusting of iron


Types of Electrochemical Corrosion (Galvanic Attack)Rate of uniform attack is reported in various units, an accepted terminology being, for example;

Millimetres penetration per year (mm/y)Micrometer penetration, m, per year i.e. m/yInches penetration per year (ipy)Grams per square meter per day (gmd)Grams per square meter per year (gmy)Milligrams per square decimetre per day (mdd)



Composition Cells (bi-metallic or dissimilar metal corrosion)In a corrosion cell having two different metal electrodes the more noble metal will be the cathode and the less noble the anode.Corrosion cell in bimetallic corrosion

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Examples of dissimilar metal corrosionMagnesium shell that was cast around a steel coreMagnesium alloy aircraft fuel pipe coupling which seized after it had been lubricated with graphite grease

Types of Electrochemical Corrosion (Galvanic Attack)Precautions (to avoid bimetallic corrosion)Chose metals that are close together in galvanic seriesWhen required insulate, if possible, dissimilar metals from each other with age-resistant plastic or rubber

Bimetallic CorrosionTypes of Electrochemical Corrosion (Galvanic Attack)

Types of Electrochemical Corrosion (Galvanic Attack)Precautions (to avoid bimetallic corrosion)

Design the construction so that moisture cannot collect and remain at the point of contact Coat the area corrosion-preventing paint or equivalent.Electrically connect a third, anodic, metal to the other two(Cathodic protection).

Types of Electrochemical Corrosion (Galvanic Attack)Stress CellsLocal Stresses. The most highly stressed, or high energy, regions act as anodes.


corrosion - salford 1

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