Why Does SUS304 Rust? - 5 Corrosion Traps of Stainless Steel

Galvanic corrosion from dissimilar metals, chloride-induced SCC, weld sensitization, crevice corrosion, and passive film breakdown — the real reasons stainless steel corrodes and how to prevent it

"It's stainless, so it won't rust, right?" — You wouldn't believe how many field failures this single assumption has caused. SUS304 is not "rust-proof steel" but "rust-resistant steel." When the passive film created by 18% Cr breaks down under certain conditions, it can corrode even worse than plain carbon steel. Over 10 years in the field, 90% of corrosion incidents I've encountered fell into one of these five categories.

1. Galvanic Corrosion (Dissimilar Metal Contact)

When SS400 bolts are used on SUS304 parts, or SUS sheet is placed directly on a carbon steel frame, the potential difference causes the less noble metal to corrode rapidly. A potential difference exceeding 0.25V is a danger signal.

Countermeasure: Use insulating washers/sleeves, match bolt material to component (SUS bolts for SUS parts), apply epoxy coating at contact surfaces

2. Chloride Stress Corrosion Cracking (SCC)

In environments with high Cl⁻ concentration — swimming pools, coastal areas, wash-down lines — SUS304 develops intergranular cracks along stressed areas. Temperatures above 60°C with Cl⁻ above 25ppm enter the SCC danger zone.

Countermeasure: Upgrade to SUS316L (Mo-added), relieve residual stress (stress-relief annealing), use double-wall construction

3. Weld Heat-Affected Zone Sensitization

Welding SUS304 exposes the HAZ to the 500–850°C range, precipitating Cr₂₃C₆ carbides at grain boundaries. When local Cr drops below 12%, the passive film cannot form and intergranular corrosion begins.

Countermeasure: Use SUS304L (C ≤ 0.03%), perform solution heat treatment post-weld (1050°C + rapid quench), minimize heat input

4. Crevice Corrosion

In narrow gaps — gasket faces, lap welds, under bolt washers — trapped solution depletes oxygen, locally drops pH, and destroys the passive film. Gap widths of 0.025–0.1mm are most dangerous.

Countermeasure: Design crevice-free weld joints (butt welds), fill gaps with sealant, ensure drainage slope

5. Mechanical Destruction of Passive Film

Using carbon steel wire brushes after grinding, or storing SUS parts on the same rack as carbon steel, embeds iron particles that become nucleation sites for pitting corrosion.

Countermeasure: Dedicate tools for SUS only, acid passivation (10–20% nitric acid, 50°C, 30 min), electropolishing

💡 Expert Advice
"Stainless steel costs more, but if you ignore these traps, it can have a shorter life than carbon steel with plating. First assess the environment (temperature, Cl⁻, pH), then select the right grade. 'Just use SUS304' is the most dangerous phrase in material selection."

Stainless Corrosion — US Industry Awareness

Common US stainless corrosion failures: (1) chloride-induced pitting in 304 near coastlines or pool environments — 316 minimum required; (2) crevice corrosion under gaskets — eliminate with butt joint or specify continuous welds; (3) iron contamination during fabrication (carbon steel grinding wheel, drill bit) — passivation per ASTM A967 removes residual iron. NACE MR0175 covers H2S service requirements.

Passivation Requirements

ASTM A967 (or AMS 2700 for aerospace) requires passivation of stainless after machining/welding to remove free iron and restore the chromium-rich passive layer. Common procedures: nitric acid (Type 2), citric acid (Type 8). Citric is increasingly preferred — environmentally safer, equally effective. Failure to passivate causes "rust spotting" within weeks in high-humidity environments.