🎓 Lesson 22
D5
Corrosion-Resistant Alloys Mastery Quiz
Corrosion-resistant alloys are special metal mixtures designed to resist rust and chemical damage when exposed to harsh environments like acidic groundwater, saltwater, or aggressive mining chemicals.
🎯 Learning Objectives
- ✓ Calculate the Pitting Resistance Equivalent Number (PREN) for a given alloy composition
- ✓ Analyze corrosion test data (e.g., ASTM G48 mass loss) to classify alloy suitability for sulfide-rich mine water
- ✓ Design material selection criteria for underground leaching systems using ISO 21457 risk-based methodology
- ✓ Explain the role of microstructural phases (e.g., sigma, chi) in compromising CRA integrity during welding or heat treatment
- ✓ Apply NACE MR0175/ISO 15156 requirements to qualify alloys for sour service in acid mine drainage applications
📖 Why This Matters
In mining operations—especially heap leaching, underground dewatering, and tailings management—equipment fails not from mechanical overload, but from insidious corrosion. A single failed stainless-steel liner in an acid sulfate solution can trigger unplanned shutdowns costing $500k/day. CRAs prevent catastrophic failure, extend asset life beyond 20 years, and ensure regulatory compliance with EPA and ICMM sustainability standards. Mastering their selection isn’t optional—it’s foundational to safety, economics, and ESG accountability.
📘 Core Principles
Corrosion resistance arises from thermodynamic stability and kinetic passivation: Cr ≥10.5 wt% enables spontaneous Cr₂O₃ layer formation; Mo and N synergistically raise the breakdown potential in chlorides; Ni stabilizes austenite and reduces stress corrosion cracking susceptibility. Alloy families are classified by crystal structure (austenitic, duplex, super-duplex, super-austenitic, nickel-based) and corrosion resistance hierarchy (e.g., 304 < 316 < 2205 < 2507 < AL-6XN < Alloy 59). Degradation mechanisms include uniform corrosion, pitting, crevice corrosion, stress corrosion cracking (SCC), and microbiologically influenced corrosion (MIC)—each governed by distinct electrochemical thresholds and microstructural sensitivities.
📐 Pitting Resistance Equivalent Number (PREN)
PREN is a semi-empirical index used to rank relative pitting resistance of CRAs in chloride environments. Higher PREN values correlate strongly with higher critical pitting temperature (CPT) and improved performance in ASTM G48 testing. It guides preliminary alloy screening before costly lab validation.
PREN
PREN = %Cr + 3.3 × %Mo + 16 × %NQuantitative index estimating relative resistance to chloride-induced pitting corrosion
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| %Cr | Chromium content | wt% | Mass percent of chromium in alloy |
| %Mo | Molybdenum content | wt% | Mass percent of molybdenum in alloy |
| %N | Nitrogen content | wt% | Mass percent of nitrogen in alloy |
Typical Ranges:
304 stainless steel: 18–20
316 stainless steel: 24–26
Super-duplex S32750: 38–45
Super-austenitic AL-6XN: 48–52
💡 Worked Example
Problem: Calculate PREN for UNS S32750 (super-duplex stainless steel) with nominal composition: Cr = 25.0%, Mo = 4.0%, N = 0.28%.
1.
Step 1: Identify composition values from ASTM A890/A182 or manufacturer datasheet.
2.
Step 2: Apply PREN = %Cr + 3.3 × %Mo + 16 × %N = 25.0 + (3.3 × 4.0) + (16 × 0.28)
3.
Step 3: Compute: 25.0 + 13.2 + 4.48 = 42.68
Answer:
The result is 42.7, which falls within the safe range of 40–45 for seawater-influenced mine drainage applications.
🏗️ Real-World Application
At the Cerro Verde copper mine (Peru), super-duplex UNS S32750 pumps and piping replaced 316L stainless steel in sulfuric acid leach solution (pH 1.8, [Cl⁻] = 1200 ppm, T = 45°C). Within 6 months, 316L suffered severe crevice corrosion at flange gaskets (corrosion rate >0.5 mm/year); S32750 showed no measurable loss after 36 months (rate <0.02 mm/year per ASTM G102). Root cause analysis confirmed PREN >40 was essential to exceed the CPT threshold of 42°C measured in-situ per ISO 18069 Annex B.