Laser engraving is fast, clean, and extraordinarily precise—but what actually happens to stainless steel, aluminum, and titanium once the beam turns off? Below are the facts every engineer, shop owner, and maker should know before promising customers a “permanent” mark.

1. Stainless Steel – The “Forever” Myth

What you see: A crisp black serial number that looks indestructible.
What really happens:

• The laser heats the Cr-Ni surface fast enough to grow a chromium-oxide film ≈ 80–120 nm thick. That thin oxide is the black or dark-gray mark you see, not a deep groove .
• Corrosion resistance: If you use annealing parameters (low power, 30–60 kHz, 100–250 ns pulse) the protective chromium layer reforms and the mark will survive 1 000+ h salt-spray. Push the same spot into re-melt territory (>25 W, <20 ns) and you burn chromium away; rust can start within 48 h in coastal air unless you passivate after engraving .
• Wear life: On outdoor panels the oxide film will gradually polish off—about 5 µm per year in wind-blown sand environments. Expect 5–8 yr readability on unprotected 304; indefinitely if you clear-coat .

2. Aluminum – Fast Mark, Faster Fade

What you see: Brilliant white characters on anodized aluminum.
What really happens:

• On anodized parts the beam opens the dye-filled pores; the natural Al₂O₃ wall stays intact so the mark is actually inside the 10–25 µm hardcoat. Taber abrasion tests show <5 % contrast loss after 20 000 cycles—essentially lifetime durability .
• On bare aluminum the mark is nothing more than 1–2 µm of rough oxide created by surface melting. In marine or alkaline atmospheres that oxide converts to powdery Al(OH)₃; numbers can disappear in 12–18 months if the part is handled or washed regularly .
• Trick to keep it: Switch to a MOPA fiber, 200 kHz, 4 ns pulse. You build a black “mini-anodized” layer ≤3 µm thick that survives 500 h salt-spray with almost zero fade .

3. Titanium – Aerospace Favorite, Heat-Sensitive

What you see: Golden-blue logos on medical implants.
What really happens:

• Titanium needs far less energy than steel; 6–8 W fiber is enough. The colors come from controlled growth of a TiO₂ interference film <50 nm thick—beautiful but fragile .
• Structural risk: Overdose the energy and the α-phase near the mark can expand, producing micro-cracks. Fatigue tests show up to 8 % reduction in cycles-to-failure on 6Al-4V coupons when peak temperature exceeds 350 °C .
• Corrosion resistance: Proper annealing-type marks (5–8 W, 30 kHz, 100 mm/s) keep the surface biocompatible; ASTM F86 passivation is still recommended for implants .
• Wear life: The oxide film is only nanometres thick; repeated autoclaving or ultrasonic cleaning will strip colour in 30–50 cycles. Black “deep” marks (re-melt >25 µm) last longer but sacrifice fatigue strength .

Key Takeaways

• Stainless: Anneal, don’t gouge, and post-passivate if the mark sits in a corrosive zone.
• Aluminum: Anodize first; if you can’t, use MOPA nanosecond pulses to create a sealed black film.
• Titanium: Keep peak temperature <300 °C and expect coloured marks to fade under sterilization; choose deep, colour-free annealing for implants.

Laser engraving is still the most reliable non-contact way to mark these metals—just remember that “permanent” only applies if you match the beam parameters to the metal’s chemistry and service environment.