1. Copper (Cu) Alloying for Atmospheric Corrosion Resistance
Mechanism:
Cu (0.2-0.5%) forms a protective adherent oxide layer (CuO/Cu₂O) that slows rust progression.
Synergistic effect with P (0.07-0.15%) enhances passivation (per ASTM A710 standard).
Benefits:
✔ 50-100% longer lifespan in marine atmospheres vs. conventional steel.
✔ No loss of weldability (CEV remains <0.43).
ABS Compliance:
Cu content must not exceed 0.5% (ABS Pt.2, Ch.1) to avoid hot shortness.
2. Chromium (Cr) Addition for General/Pitting Corrosion Resistance
Mechanism:
Cr (0.5-1.2%) promotes Cr₂O₃ passive film formation.
Works synergistically with Mo (0.1-0.3%) for chloride resistance.
Performance Data:
Pitting resistance equivalent number (PREN) >18 (vs. 10 for plain C-Mn steel).
50% lower corrosion rate in splash zones (per ISO 9223).
Trade-offs:
Requires post-weld heat treatment (PWHT) for thickness >25mm to avoid HAZ cracking.
3. Nickel (Ni) Microalloying for Seawater Immersion Service
Mechanism:
Ni (1-3%) stabilizes austenite at low temperatures, reducing galvanic corrosion.
Particularly effective in LNG carrier containment systems (-162°C exposure).
Case Study:
9% Ni steel (ASTM A553) shows near-zero corrosion in cryogenic methanol cargo tanks.
Limitations:
High Ni (>3%) may require Schaeffler diagram control for weld metal phase balance.
4. Thermal Sprayed Nickel-Aluminum (Ni-Al) Coatings for Splash Zones
Process:
High-velocity oxy-fuel (HVOF) spraying applies 80Ni20Al coatings (150-300μm).
Advantages:
✔ Salt spray resistance >5,000 hrs (ASTM B117) vs. 300 hrs for bare steel.
✔ Cathodic protection (Al acts as sacrificial anode).
ABS Requirements:
Coating adhesion must exceed 10 MPa (per ABS Guide for Coatings).
5. Electroless Nickel-Phosphorus (Ni-P) Plating for Ballast Tanks
Technology:
Autocatalytic deposition of Ni-P (8-12% P) amorphous coating.
Key Properties:
Uniform thickness (25-75μm) even on complex geometries.
Hardness up to 600 HV (heat-treated) for abrasion resistance.
Performance:
<0.001 mm/yr corrosion rate in simulated ballast water (IMO Guideline MSC.215(82)).
Maintenance:
Requires ultrasonic thickness monitoring during dry-docking.
