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Lead Anode Failure Mechanisms
Abbas Mirza, RSR Technologies, Inc., Dallas, TXRicardo Cabrejas, Quemetco Metals Limited, Casa Grande, AZLarry Webb, Quemetco Metals Limited, Casa Grande, AZ
Lead Anode Corrosion in Sulfuric Acid
α-PbO2
β-PbO2
Lead
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Protective α-PbO2
Brownish, Hard, Dense, Adherent
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Lead Corrosion Films
α-PbO2 / β-PbO21.77
1.69
1.18
0.25
0.00
-0.30
Electrode Potential(Volts, SHE)
Metal InterfaceElectrolyte/Anode
Interface
Pb
Pb(OH)2
PbO / PbSO4
PbSO4
PbSO4
PbSO4 / β-PbO2α-PbO2 / t-PbO
α-PbO2 / β-PbO2
t-PbO
β-PbO2
Lead corrosion films; adapted from BurbankBurbank, J., “Anodization of Lead in Sulfuric Acid”, J. Electrochemical Society,Volume 103, Issue 2, 87-91, 1956.
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Uniform Corrosion
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Penetrating Corrosion
Penetration Corrosion Uniform Corrosion
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Penetrating Corrosion
Sulfate deposits enhance penetration corrosion
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Manganese attacks PbO2 layer and causes MnO2
to build up in the layer.
MnSO4 + PbO2 -> PbSO4 +MnO2
The PbSO4 is converted back to PbO2, but it isnow deposited in loose layers.These layers are easily spalled off causingcontamination of the copper cathode.
Must control Mn at SX
Manganese Problem
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MnO2 Disruption of PbO2
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MnO2 Corrosion
Formation of large, soft, loosely adherent layers of PbO2
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Reduction Potential ElectrodepositionCu2+ +2e- ↔ Cu E0 = 0.34 VPbSO4 + 2e- ↔ Pb + SO4
2- E0 = -0.36 VPb2+ +2e- ↔ Pb E0 = -0.13 VNo co-reduction of Pb ionsOnly physical occlusion of particulate Pb species
PbO2 reduction to PbSO4 or PbO possiblePbO2 + SO4
2- + 4H+ 2e- ↔ PbSO4 + 2H2O E0 = 1.69 VPbO2 + H2O + 2e- ↔ PbO + 2OH- E0 = 0.25 V
No further reduction of PbOPbO + H2O + 2e- ↔ PbO + 2OH- E0 = -0.58 V
Lead Contamination in Copper Deposit
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PbO2 Particle Encapsulated
• Contamination of cathode metal
• Particle are mechanicallyentrapped
• Issues• Excessive organics• Cell mud accumulation• Uneven anode side wall
spacing, non-uniform faceflow patterns
• Flow eddys around isolators
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Organics are “Not” my Friend
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Solution Line Corrosion
Corrosion at Solution Line by Organics
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Hoods
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Corrosion of Lead over Hanger Bar
Corrosion of Lead by CuSO4:5H2O
2CuSO4:5H2O (Blue) + Pb Cu2SO4 (Green) + PbSO4 (White) + 10 H2OΔGrkn @ 40 C = -21.72 kcal/mole
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Hanger Bar Design
Corrosion of Hanger Bar due to immersion or seepage
Progressive evolution in hanger bar design
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Uneven Electrode Spacing
Risk of shorts due to dendrites at narrow electrode spacing
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Housekeeping
Risk of shorts due to housekeeping
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Why Lead Anodes? Lead anodes are the preferred material for EW
Anodes from Acidic Sulfate Solution
• Insoluble• Ability to form a protective PbO2 layer• Corrosion resistant• Robust in tankhouse environment• Economical• Acceptable Operating Voltage
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Oxygen evolution reaction at anodes is necessary to protectthe anodes from corrosion.
Establish and maintain adequate cobalt content in electrolyte
Clean anodes and cells regularly. Do not expose anode to baremetal during washing
Anodes must be straightened before inserting into cell
Suspended solids /Precipitates can lead to concentration cellcorrosion
Organics in cells will lead to solution line corrosion
Temperature fluctuations could result in shedding
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