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

CONFIDENTIAL

Protective α-PbO2

Brownish, Hard, Dense, Adherent

CONFIDENTIAL

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.

CONFIDENTIAL

Uniform Corrosion

CONFIDENTIAL

Penetrating Corrosion

Penetration Corrosion Uniform Corrosion

CONFIDENTIAL

Penetrating Corrosion

Sulfate deposits enhance penetration corrosion

CONFIDENTIAL

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

CONFIDENTIAL

MnO2 Disruption of PbO2

CONFIDENTIAL

MnO2 Corrosion

Formation of large, soft, loosely adherent layers of PbO2

CONFIDENTIAL

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

CONFIDENTIAL

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

CONFIDENTIAL

Organics are “Not” my Friend

CONFIDENTIAL

Solution Line Corrosion

Corrosion at Solution Line by Organics

CONFIDENTIAL

Hoods

CONFIDENTIAL

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

CONFIDENTIAL

Hanger Bar Design

Corrosion of Hanger Bar due to immersion or seepage

Progressive evolution in hanger bar design

CONFIDENTIAL

Uneven Electrode Spacing

Risk of shorts due to dendrites at narrow electrode spacing

CONFIDENTIAL

Housekeeping

Risk of shorts due to housekeeping

CONFIDENTIAL

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

CONFIDENTIAL

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

CONFIDENTIAL