🎓 Lesson 12
D5
Comparing 2205, 2507, and 825 in Sulfuric Acid: Thermodynamic vs. Kinetic Resistance
Thermodynamic resistance tells us *if* a metal will corrode in sulfuric acid, while kinetic resistance tells us *how fast* it will corrode — and real-world alloy selection depends on both.
🎯 Learning Objectives
- ✓ Explain the distinction between thermodynamic driving force and kinetic barrier using Pourbaix diagrams and polarization curves
- ✓ Analyze corrosion rate data (mm/yr) for UNS S32205, S32750, and N08825 in 20–70 wt% H₂SO₄ at 40–80 °C to rank relative performance
- ✓ Calculate critical pitting temperature (CPT) and compare against ASTM G150 test results for each alloy
- ✓ Design material selection criteria for sulfuric acid leaching tanks by integrating thermodynamic stability limits with kinetic corrosion thresholds
📖 Why This Matters
In copper and nickel hydrometallurgical plants, sulfuric acid leaching circuits operate at 30–70 wt% H₂SO₄ and 60–85 °C — conditions that rapidly degrade carbon steel and even many stainless steels. Choosing between duplex 2205, super duplex 2507, and nickel-based alloy 825 isn’t about cost alone: misjudging whether corrosion failure is thermodynamically inevitable or merely kinetically delayed can lead to catastrophic tank breaches, unplanned shutdowns, and hazardous acid releases. This lesson equips you to distinguish *why* an alloy resists corrosion — not just *that* it does.
📘 Core Principles
Thermodynamic resistance is assessed via Pourbaix diagrams: if the operating pH–potential point lies outside the immunity or passivation region, corrosion is thermodynamically spontaneous. However, real alloys rarely corrode at predicted rates because kinetic barriers — such as Cr₂O₃/FeOOH/MoO₂-rich passive films — suppress dissolution. Alloy 2205 relies on ~22% Cr + 3% Mo for passivation but lacks sufficient Ni/Cu to stabilize films above 40 °C in >30% H₂SO₄. Alloy 2507 adds ~4% Mo and 7% Ni, widening the passivation domain and improving repassivation kinetics. Alloy 825 (42% Ni, 14–16% Cr, 1.5–3% Mo, 1.5–3% Cu) shifts the entire electrochemical behavior into the Ni–Cu-dominated immunity zone, where thermodynamic stability *and* kinetic resilience coexist across wide concentration–temperature ranges.
📐 Critical Pitting Temperature (CPT) Prediction
CPT correlates with alloy composition and predicts the upper temperature limit for stable passivity in chloride-containing sulfuric acid environments (e.g., impure leach liquors). The CPT estimation formula accounts for synergistic effects of Cr, Mo, and N — key for duplex grades — but requires empirical correction for high-acid, low-chloride service.
💡 Worked Example
Problem: Calculate estimated CPT for UNS S32205 (22.2% Cr, 3.2% Mo, 0.17% N) and UNS S32750 (25.3% Cr, 4.2% Mo, 0.28% N) using the modified CPT model: CPT (°C) = 20 + 19.5×Cr + 25.5×Mo + 260×N − 0.5×(Cr×Mo).
1.
Step 1: For 2205: Cr = 22.2, Mo = 3.2, N = 0.17 → CPT = 20 + (19.5×22.2) + (25.5×3.2) + (260×0.17) − (0.5×22.2×3.2)
2.
Step 2: Compute terms: 19.5×22.2 = 432.9; 25.5×3.2 = 81.6; 260×0.17 = 44.2; 0.5×22.2×3.2 = 35.52 → Sum = 20 + 432.9 + 81.6 + 44.2 − 35.52 = 543.2 °C
3.
Step 3: Apply empirical derating for 30% H₂SO₄ + 100 ppm Cl⁻: subtract 220 °C → Estimated CPT ≈ 323 °C (non-physical; indicates model overpredicts — real CPT is ~35–45 °C per ASTM G150). For 2507: repeat with Cr=25.3, Mo=4.2, N=0.28 → raw CPT = 20 + 493.4 + 107.1 + 72.8 − 53.2 = 640.1 → derated ≈ 45–55 °C.
Answer:
The raw calculation overestimates CPT due to absence of acid-specific terms; measured CPT values are 38 °C (2205) and 52 °C (2507) in 3.5% NaCl per ASTM G150 — confirming 2507’s superior kinetic stability. In 40% H₂SO₄, CPT drops further: 2205 fails at >35 °C, while 2507 remains passive up to 65 °C.
🏗️ Real-World Application
At the Tenke Fungurume copper mine (DRC), sulfuric acid leach tanks initially used 2205 duplex steel but suffered localized corrosion at weld heat-affected zones after 18 months of operation at 45 wt% H₂SO₄ and 72 °C. Post-failure analysis (ASTM G31 immersion + ASTM G102 electrochemical testing) revealed thermodynamic passivity was maintained (Ecorr within passive region), but kinetic breakdown occurred due to slow repassivation in hot, high-acid, low-oxidant conditions. The plant upgraded to UNS N08825 (alloy 825), which showed <0.02 mm/yr uniform corrosion at 75 °C in 50% H₂SO₄ (per ISO 15156-3 Annex D data), extending service life to >15 years — validating the necessity of coupling thermodynamic assessment with kinetic validation.
🔧 Interactive Calculator
🔧 Open Corrosion-Resistant Alloys Calculator📋 Case Connection
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