Selective corrosion (dealloying)

Dr. Dmitri Kopeliovich

Selective corrosion (dealloying, selective leaching) is a preferential attack of a particular component of an alloy in presence of electrolyte as a result of an electrochemical oxidation-reduction (redox) process.

Selective corrosion may occur in a single-phase and in multi-phase alloys.

Alloys prone to selective corrosion are composed of components with a great difference between their Electrode potentials (eg.Cu-Zn).
The component having lower value of electrode potential (higher position in the table of Electrochemical series) will oxidize (anodic reaction) and dissolve in the electrolyte when the component having higher value of electrode potential will provide cathodic reaction (reduction).

Selective corrosion may be either uniform or plug type (localized).

Selective corrosion results in formation of nano-scale porous structure on the alloy surface.

• Alloys prone to selective corrosion
• Dezincification
• Graphitic corrosion

• Brasses (Cu-Zn): dezincification;
• Silicon bronze: desiliconification;
• Aluminum bronze: dealuminification;
• Tin bronze: destannification;
• Copper-nickels: denickelification;
• Grey cast irons: graphitic corrosion (leaching of iron ).

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Dezincification is the loss of zinc from *Brasses (copper-zinc alloys) as a result of Selective corrosion (dealloying).

Dezincification occurs in brasses containing more than 15% of zinc.
Zinc is chemically active element. Its standard electrode potential is very low (-0.763). Standard electrode potential of copper is much higher (+0.337).
The difference between the potentials is the driving force of dezincification.

Possible mechanisms of dezincification:

• Selective dissolution of zinc only according to anodic reaction:

Zn = Zn²⁺ + 2e^-.

• Parallel dissolution of both: zinc and copper according to anodic reactions: Zn = Zn²⁺ + 2e^- and Cu = Cu²⁺ + 2e^- followed by redeposition of copper by cathodic reaction: Cu²⁺ + 2e^- = Cu.

Means of preventing dezincification:

• Improve aggressive environment (lower chloride and Oxygen content, decrease acidity, avoid stagnant water);
• Use brass with low zinc content (less than 15%) or *Copper-nickels;
• Apply Cathodic protection;
• Use brasses containing inhibiting additives: 0.05% As, 0.05%Sb, 1% Sn, P.

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Graphitic corrosion is the selective leaching of iron from Grey cast irons.

Standard electrode potential of Graphite is higher than that of iron therfore graphite particles in grey cast irons act as cathodes in the galvanic cell graphite-iron.
Ferrite (solid solution of carbon in α-iron) dissolves in the electrolyte by anodic reaction: Fe = Fe²⁺ + 2e^- leaving porous network of graphite flakes behind.

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• Electrode potentials
• Polarization
• Pourbaix diagrams
• Galvanic corrosion
• Pitting corrosion
• Crevice corrosion
• Intergranular corrosion
• Stress corrosion cracking
• Corrosion fatigue
• Cathodic protection
• Corrosion protection coatings
• Corrosion inhibitors
• Volatile corrosion inhibitors (VCI)
• High temperature oxidation of metals

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