🎓 Lesson 16
D5
Biofilm Electrochemistry and Alloy MIC Susceptibility
Biofilm electrochemistry is how tiny layers of microbes on metal surfaces create electrical currents that speed up corrosion, especially in alloys used underground or in water.
🎯 Learning Objectives
- ✓ Explain the role of electrochemical gradients across biofilms in initiating pitting and crevice corrosion on CRAs
- ✓ Analyze polarization resistance data to quantify MIC-driven corrosion rates in alloy systems
- ✓ Apply NACE SP0775 and ISO 21876 test protocols to assess alloy susceptibility to sulfate-reducing bacteria (SRB) biofilms
- ✓ Design mitigation strategies (e.g., biocide dosing, cathodic protection potential windows) based on biofilm redox potential measurements
📖 Why This Matters
In mining infrastructure—slurry pipelines, leach pads, dewatering systems, and underground ventilation ducts—biofilm-driven corrosion causes unexpected failures in corrosion-resistant alloys (CRAs) even when chemical corrosion models predict long service life. A single SRB biofilm can reduce the safe operating life of a duplex stainless steel pipeline by >70% due to localized pitting under deposits. Understanding biofilm electrochemistry isn’t academic—it’s essential for preventing unplanned shutdowns, environmental releases, and catastrophic structural loss.
📘 Core Principles
MIC begins with microbial adhesion forming a structured biofilm that creates heterogeneous microenvironments: oxygen-depleted zones beneath EPS foster anaerobic respirers (e.g., Desulfovibrio), while aerobic bacteria (e.g., Pseudomonas) colonize outer layers. This stratification establishes localized electrochemical cells: the biofilm acts as a 'biological battery'—cathodic reactions (e.g., SO₄²⁻ + 8e⁻ + 10H⁺ → H₂S + 4H₂O) occur at SRB sites, while adjacent bare metal becomes anodic and dissolves. Alloy passivity breaks down when critical local pH drops below ~4.5 or Eh falls below −150 mV vs. SHE, destabilizing Cr₂O₃ films. Duplex stainless steels resist general corrosion but remain vulnerable at phase boundaries (ferrite/austenite interfaces) where biofilm-induced differential aeration accelerates selective attack.
📐 Biofilm-Induced Corrosion Rate Estimation
The corrosion current density (i_corr) under biofilm influence can be estimated from electrochemical impedance spectroscopy (EIS) data using the Stern–Geary relationship, adjusted for biofilm ohmic resistance. This quantifies how much faster corrosion proceeds under biofilm vs. abiotic conditions.
Stern–Geary Biofilm Correction
i_corr = B / RₚEstimates corrosion current density from polarization resistance (Rₚ) and empirical Stern–Geary constant (B), corrected for biofilm-influenced electrochemistry.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| i_corr | Corrosion current density | A/cm² | Electrochemical corrosion rate per unit area |
| B | Stern–Geary constant | V/decade | Empirically determined slope relating overpotential to log(current); typically 20–30 mV/decade for SRB-influenced CRAs |
| Rₚ | Polarization resistance | Ω·cm² | Measured resistance to electrochemical charge transfer; inversely proportional to corrosion rate |
Typical Ranges:
SRB biofilm on duplex stainless steel: 0.5 – 5.0 kΩ·cm²
Abiotic passive film (sterile water): 10 – 100 kΩ·cm²
💡 Worked Example
Problem: A field-deployed UNS S32205 duplex stainless steel coupon in a mine water recirculation line shows Rₚ = 1.2 kΩ·cm² via EIS. Measured B value = 26 mV/decade (biofilm-modified). Abiotic Rₚ for same alloy in sterile water = 25 kΩ·cm². Calculate i_corr,biofilm and compare to abiotic rate.
1.
Step 1: Apply Stern–Geary: i_corr = B / Rₚ
2.
Step 2: Compute biofilm i_corr = 0.026 V / 1200 Ω·cm² = 2.17 × 10⁻⁵ A/cm² = 21.7 µA/cm²
3.
Step 3: Compute abiotic i_corr = 0.026 V / 25,000 Ω·cm² = 1.04 × 10⁻⁶ A/cm² = 1.04 µA/cm²
4.
Step 4: Ratio = 21.7 / 1.04 ≈ 20.9× acceleration due to biofilm
Answer:
The biofilm increases corrosion current density by ~21×, corresponding to an estimated penetration rate of ~0.18 mm/yr — exceeding NACE SP0775's 0.025 mm/yr threshold for 'severe MIC'.
🏗️ Real-World Application
At the Escondida copper mine (Chile), a duplex stainless steel (UNS S32205) slurry transfer pipeline failed after 18 months due to deep, irregular pits (up to 3.2 mm depth) beneath biofilm deposits. Post-failure analysis revealed dense SRB biofilms (confirmed by qPCR and FISH) with localized pH = 3.1 and Eh = −210 mV vs. SHE at pit bases. Electrochemical noise monitoring showed 10× higher current transients during wet-dry cycles—correlating with biofilm rehydration and metabolic reactivation. Mitigation required switching from intermittent chlorine dosing to continuous low-dose glutaraldehyde + controlled cathodic protection (−0.55 V vs. Cu/CuSO₄), extending service life to >5 years.
🔧 Interactive Calculator
🔧 Open Corrosion-Resistant Alloys Calculator📋 Case Connection
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