🎓 Lesson 3
D2
Calculating Galvanic Current Using Polarization Resistance
Galvanic current is the electric current that flows when two different metals are connected in a corrosive environment—like seawater or wet rock—causing one metal to corrode faster.
🎯 Learning Objectives
- ✓ Calculate galvanic current density using polarization resistance and measured potential difference
- ✓ Analyze the effect of alloy composition and surface area ratio on galvanic corrosion rate
- ✓ Explain how polarization resistance relates to corrosion rate using the Stern-Geary equation
- ✓ Apply ASTM G59 and G102 standards to design valid electrochemical testing protocols
📖 Why This Matters
In mining infrastructure—such as ore chutes, slurry pipelines, and blast-hole casings—dissimilar metal contacts (e.g., stainless steel bolts on carbon steel liners) create galvanic couples accelerated by acidic mine water or sulfide-rich groundwater. Unchecked, this causes premature failure, unplanned downtime, and safety hazards. Quantifying galvanic current via polarization resistance enables predictive maintenance and material selection—turning corrosion from a hidden cost into a design parameter.
📘 Core Principles
Galvanic corrosion occurs when two electrodes with different equilibrium potentials form a short-circuited electrochemical cell in an electrolyte. The magnitude of current depends on the driving voltage (ΔE_corr), the total circuit resistance—including polarization resistance (R_p), which reflects the kinetics of anodic dissolution and cathodic reduction—and ohmic drop (R_Ω). Polarization resistance (R_p) is defined as the slope of the potential-current curve near the open-circuit potential (OCP) and inversely correlates with corrosion rate. For low-polarization systems (e.g., passive alloys in chloride media), R_p must be corrected for solution resistance using electrochemical impedance spectroscopy (EIS) or current interrupt techniques per ASTM G102.
📐 Key Calculation
The galvanic current (I_galv) is derived from Ohm’s law applied to the galvanic couple: I_galv = ΔE_corr / R_total, where R_total ≈ R_p (when solution resistance is compensated). R_p itself is obtained experimentally via linear polarization resistance (LPR) measurement around OCP, then related to corrosion current (i_corr) using the Stern-Geary equation. This forms the basis for predicting galvanic attack rates in mixed-metal systems common in blasting equipment and ground support.
💡 Worked Example
Problem: A stainless steel 316 (E_corr = −0.18 V vs. SCE) is electrically coupled to carbon steel (E_corr = −0.65 V vs. SCE) in acidified mine water (pH 3.2). Measured polarization resistance of the coupled system is 420 Ω·cm². Solution resistance R_Ω = 15 Ω·cm² (measured via EIS). Use Stern-Geary constant B = 0.026 V for mixed-metal interface.
1.
Step 1: Calculate driving potential: ΔE_corr = |−0.18 − (−0.65)| = 0.47 V
2.
Step 2: Correct R_p for solution resistance: R_total = R_p − R_Ω = 420 − 15 = 405 Ω·cm²
3.
Step 3: Compute galvanic current density: i_galv = ΔE_corr / R_total = 0.47 V / 405 Ω·cm² = 1.16 × 10⁻³ A/cm² = 11.6 µA/cm²
4.
Step 4: Convert to mass loss rate using Faraday’s law: for Fe (M = 55.85 g/mol, n = 2), CR = (i_galv × M × K) / (n × F) ≈ 0.13 mm/yr (K = 3272 mm·g/(A·cm·yr), F = 96485 C/mol)
Answer:
The galvanic current density is 11.6 µA/cm², corresponding to ~0.13 mm/yr corrosion rate on the carbon steel—an unacceptable rate for underground mine conveyors per ISO 15156-2, requiring insulation or material redesign.
🏗️ Real-World Application
At the Red Dog Mine (Alaska), galvanic coupling between 304 stainless steel anode bags and carbon steel blast-hole collars in sulfide-laden groundwater led to collar wall thinning >0.8 mm/yr—exceeding NACE SP0169 limits. Engineers deployed LPR probes per ASTM G59 to quantify R_p in situ, revealing R_p values <200 Ω·cm² at collar interfaces. By switching to duplex stainless steel (UNS S32205) couplings and adding dielectric sleeves, R_p increased to >2500 Ω·cm², reducing i_galv by 92% and extending service life from 18 to >60 months.