IEEE STNP Task ForceCorrosion Effects on Subsurface Transformers &

Network Protectors

Introduction to Field Corrosion Measurements

Will Elliott – General Electric, Shreveport LA

Recall:

Galvanic Potential of Metals

• Galvanic (voltage) potentials exist between dissimilar

metals in an electrolyte (“water”)

• Potentials listed in Galvanic Series shown are versus a

common reference electrode

• The corrosiveness of the electrolyte will vary with

location

• Each location or generalized area would have its own

galvanic series

• Reference Galvanic Series shown in ASTM G82-98 in

flowing seawater

Corrosion PotentialTypes of Field Measurements

Corrosion Potential• Corrosion Potential measurement is a DC voltage measurement of

the galvanic potential between a metal sample and a reference electrode

• ASTM C876-15 gives a similar procedure for testing steel rebar in concrete

• Common connected to porous electrode.

• Also uses electrolyte solution, sponge, and porous electrode(not applicable for subsurface applications electrolyte already present)

• Positive connected to exposed section of rebar imbedded in concrete

• Basic Requirements

• Voltmeter (high impedance, mV range)

• Electrical Equipment Enclosure

• Anode(s), if present

• Reference electrode

• Subsurface vault (or manhole) with standing water

Corrosion Potential• Key parameters to document & measure:

• Materials:

• Equipment enclosure material

• Anode material

• Reference electrode material

• Use a consistent electrode material

• Water (electrolyte) temperature

• Water depth in vault

• Depth of electrode in water

• Measured voltage

• Record steady-state meter readings

Corrosion Potential• Reference Electrode:

• Copper/Copper Sulfate electrode

• https://www.certifiedmtp.com/m-c-miller-ionx-

submersible-copper-copper-sulfate-electrode/

• Electrode should be designed for immersion in

liquid electrolytes

• https://www.mcmiller.com/assets/site/docs/Articles/mp

%20article%20sub.pdf

• Equipment Electrode(s):

• Connect to equipment ground

• The electrolyte between locations should be

the variable!

Electrolyte

Corrosion Potential• Development of Galvanic Series &

Corrosion Risk Map

• Measure half-cell potential from equipment

ground to reference electrode

• If present, measure half-cell potential from

anode to reference electrode

• Depending on resolution and consistency of

the data, a map can be generated based on

the scale of data

Anode Potential Measurement

Equipment Potential Measurement

Corrosion Potential• Possible Data Sets:

• Number of measurements in a vault

• Each corner or side of vault/manhole?

• Fixed spacing between measurements?

• Average data or map results

• Periodic (sample) measurements

• Perform on a fixed schedule

• Discrete data points

• Easier & cheaper but low resolution data

• Continuous measurements

• Continuous data measures dynamic changes

• Provides better data for changing

temperatures, water levels, etc.

• More complicated & expensive, but high

resolution data