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Stainless steel

Corrosion resistance: Due to the chromium oxide layer, stainless steel resists rust and staining better than regular steel.

Strength & toughness: Maintains mechanical strength across a wide temperature range.

Hygiene: Smooth, non-porous surface makes it ideal for food processing, medical, and cleanroom environments.

Aesthetic appeal: Polished finish and luster make it popular in architecture and consumer products.

Recyclability: Fully recyclable without losing quality.

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1790s–1800s: Early hints—chromium added to iron resists rust, but alloys are brittle due to high carbon.

1913: Harry Brearley (Sheffield) makes a low-carbon ~13% Cr steel that doesn’t rust → birth of stainless steel.

1920s–40s: Major families emerge—austenitic (304/316), martensitic, ferritic—spurring industrial and wartime use.

Post-1950s: Expands into architecture, food/chemical processing, and energy; becomes mass-produced.

1960s–today: 316L becomes a biomedical workhorse (instruments, temporary implants); new grades keep evolving.

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Type

Composition (typical)

Structure

Key Properties

Common Grades

Applications

Austenitic

16–26% Cr, 6–12% Ni, sometimes Mo

FCC

Excellent corrosion resistance, non-magnetic (annealed), good ductility & weldability

304, 316

Food industry, chemical plants, medical devices, architecture

Ferritic

11–17% Cr, little/no Ni

BCC

Magnetic, moderate corrosion resistance, low cost

409, 430

Automotive exhausts, appliances, decorative trim

Martensitic

12–18% Cr, higher C

BCC (hardened)

High strength & hardness, magnetic, lower corrosion resistance

410, 420

Cutlery, surgical tools, turbine blades

Duplex

~22% Cr, 3–5% Ni, Mo, N

Mixed (FCC + BCC)

Very strong, excellent resistance to stress corrosion cracking, lower Ni cost

2205, 2507

Marine, offshore, petrochemical, pipelines

Precipitation-Hardening (PH)

Cr, Ni, Cu, Al, Ti, Nb

Austenitic or Martensitic + precipitates

Ultra-high strength, good corrosion resistance, can be aged

17-4 PH

Aerospace, defense, high-performance engineering

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Chromium Content (≥10.5%)

  • When chromium is alloyed with iron, it reacts with oxygen in the environment.
  • This forms a very thin, invisible chromium oxide (Cr₂O₃) film on the surface.

Passive Film

  • The chromium oxide layer is stable, tightly adherent, and only a few nanometers thick.
  • It blocks oxygen and moisture from penetrating deeper, preventing rust (iron oxide) from forming.

Self-Healing Property

  • If the surface is scratched, chromium in the steel reacts quickly with oxygen and repairs the oxide layer.
  • This “self-passivation” is what makes stainless steel unique compared to plain carbon steel.

Alloying Additions

  • Nickel (Ni): Stabilizes the austenitic structure, improves resistance to acids.
  • Molybdenum (Mo): Increases resistance to chloride attack (pitting, crevice corrosion).
  • Nitrogen (N): Strengthens the passive film, improves pitting resistance.

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  • Super-austenitic: 904L, 254SMO — very high Ni/Mo → elite corrosion resistance; ~3–6× 304 cost.
  • Super-duplex: 2507, Zeron 100 — high strength + chloride resistance; ~4–6× 304.
  • Precipitation-hardening: 17-4 PH, Custom 465 — aerospace-level strength; ~5–10× 304.
  • Drivers of price: Ni, Mo, N content + complex processing.

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Property

Stainless Steel (STS)

Titanium (Ti)

Density

~7.8 g/cm³ (heavier)

~4.5 g/cm³ (much lighter)

Strength

High (esp. martensitic, PH steels); good fatigue strength

High strength-to-weight ratio; excellent fatigue performance

Corrosion Resistance

Very good (due to Cr₂O₃ passive film), but can pit in chlorides

Exceptional (TiO₂ passive film); highly resistant to seawater and body fluids

Biocompatibility

Good (316L used in biomedical), but may release Ni/Cr ions

Excellent; very biocompatible, no nickel; preferred for permanent implants

Cost

Cheaper; widely available

Expensive (raw material + processing)

Machinability

Easier to machine & weld

Harder to machine (gummy, requires special tools)

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Factor

Titanium (Ti)

Effect on Biocompatibility

Surface Film

Forms stable TiO₂ layer

Inert, prevents corrosion, non-toxic

Ion Release

Minimal (no Ni, Cr)

No harmful ions → low allergy risk

Corrosion Resistance

Excellent in body fluids

Prevents degradation in blood/saline

Bone Bonding

Supports osseointegration

Promotes direct attachment of bone cells

Elastic Modulus

~110 GPa (closer to bone than steel)

Reduces stress shielding → healthier bone

Magnetic Behavior

Non-magnetic

Safe for MRI and medical imaging

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Titanium