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Galvanic corrosion.

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Galvanic corrosion

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Galvanic corrosion

  • Galvanic corrosion is either a chemical or an electrochemical corrosion. The latter is due to a potential difference between two different metals connected through a circuit for current flow to occur from more active metal (more negative potential) to the more noble metal (more positive potential)

  • Galvanic coupling is a galvanic cell in which the anode is the less corrosion resistant metal than the cathode


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  • Galvanic corrosion can be predicted by using the electromotive force (emf) or standard potential series for metal reduction listed in Table at the next slide

  • In selecting two metals or two alloys for a galvanic coupling, both metals should have similar potentials or be close to each other in the series in order to suppress galvanic corrosion


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Galvanic coupling can be used for cathodic protection purposes


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Other types of galvanic coupling are batteries and fuel cells

  • Lithium Ambient-Temperature Batteries (LAMBS)

  • Lead-Acid Battery

  • Dry-Cell Battery

  • Sintered Nickel Electrode in Alkaline Batteries


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MICROSTRUCTURAL EFFECTS

A mechanically deformed metal or alloy can experience galvanic corrosion due to

differences in atomic plane distortion and a high dislocation density


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  • Improper heat treatment can cause nonuniform microstructure and therefore, galvanic-phase corrosion is enhanced in corrosive media

  • Galvanic corrosion can occur in a polycrystalline alloys, such as pearlitic steels, due to differences in microstructural phases

  • In this case, pearlite consists of ferrite and cementite and when it is etched with a mild acid, which is the electrolyte, galvanic microcells between ferrite (cathode) and cementite (anode) are generated. Consequently, pearlite is revealed as dark cementite andwhite ferrite.


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LOCALIZED CORROSION

This term implies that specific parts of an exposed surface area corrodes in a suitable electrolyte. This form of corrosion is more difficult to control than general corrosion.

  • Crevice Corrosion which is associated with a stagnant electrolyte such as dirt, corrosion product, sand, etc. It occurs on a metal/alloy surface holes,underneath a gasket, lap joints under bolts, under rivet heads.


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  • Filiform Corrosion is basically a special type of crevice corrosion, which occurs under a protective film. It is common on food and beverage cans being exposed to the atmosphere.

  • Pitting Corrosion is an extremely localized corrosion mechanism that causes destructive pits.

  • Oral Corrosion occurs on dental alloys exposed to saliva.

  • Biological Corrosion due to fouling organisms non-uniformly adhered on steel in marine environments.

  • Selective Leaching Corrosion is a metal removal process from the base alloy matrix, such as dezincification ( Zn is removed) in Cu-Zn alloys and graphitization (Fe is removed) in cast irons.


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PITTING CORROSION

  • This form of corrosion is extremely localized and it manifests itself as holes on a metal surface.

  • The initial formation of pits is difficult to detect due to the small size, but it requires a prolong time for visual detection.

  • This form of corrosion can be found on aluminum and its alloys and automobile chromium-plated bumpers or body coated (painted) parts due to film/coating breakdown at isolated surface sites.

  • Pits vary in shape, but are very small surface holes due to the extremely localized anodic reaction sites


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  • The initiation of pits occurs at localized sites on a metal surface defects, which may be due to coating failure, mechanical discontinuities or microstructural phase heterogeneities such as secondary phases.

  • The depth of pits can be found: d =λtn


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