Chloride Pitting in 316L Sample Probes: Temperature, Concentration, and Upgrade Paths
Engineering chart of chloride pitting limits for 316L sample probes. The temperature × concentration safe envelope, when to upgrade to duplex or nickel alloy, and how to read a Tsujikawa-style pitting diagram.
The Practical Limit for 316L
316L is reliable in chloride service up to about 200 ppm Cl⁻ at 60 °C, or about 1000 ppm at room temperature. Beyond either threshold, pitting initiation becomes likely within months. Above ~60 °C and ~1000 ppm, the probability is high enough that 316L should be replaced with duplex 2205 or a nickel-base alloy regardless of stress condition.
This rule is conservative and gland-installation friendly. It collapses the full Tsujikawa pitting diagram into one number a maintenance engineer can carry in their head.
The Mechanism in 60 Seconds
316L resists corrosion because of a thin, self-healing chromium oxide passive film. Chloride ions, when concentrated locally (in a crevice, under a deposit, or at a grain boundary), out-compete oxygen for the metal surface and locally break down the passive film. Once the film is broken, the underlying metal corrodes anodically and the pit grows autocatalytically because the local chemistry inside the pit becomes more acidic and more chloride-rich than the bulk.
Two engineering implications:
1. Pits are local, not uniform — a probe can fail by perforation while the bulk wall is essentially untouched.
2. Pit initiation is stochastic; pit propagation is deterministic. The cure is to prevent initiation.
Variables That Matter
| Variable | Effect |
| Chloride concentration ↑ | Pit initiation faster |
| Temperature ↑ | Pit initiation much faster (Arrhenius) |
| pH ↓ | Pit initiation faster |
| Oxygen ↑ | Pit propagation faster |
| Surface finish (rough) | Pit initiation easier |
| Crevices (under gland sealant) | Pit initiation dramatically easier |
| Tensile stress | Adds chloride SCC risk on top of pitting |
The crevice column is the one most often missed. Under a PG packing gland, the sealant-probe contact line is a long, thin crevice — and crevice corrosion onsets at temperatures 20-30 °C lower than open-surface pitting.
Upgrade Path
When the chloride/temperature combination violates the 316L envelope:
| Upgrade | Practical Cl⁻ × T limit |
| 316L | 200 ppm × 60 °C |
| 904L super-austenitic | 5000 ppm × 80 °C |
| 6Mo (254 SMO, AL-6XN) | 25000 ppm × 90 °C |
| Duplex 2205 | 5000 ppm × 100 °C |
| Super duplex 2507 | 25000 ppm × 120 °C |
| Inconel 625 | 50000 ppm × 150 °C |
| Hastelloy C276 | Seawater + hypochlorite |
| Titanium Gr 2 | Seawater up to 200 °C; not free chlorine |
Surface Finish Helps
Electropolishing a 316L probe to Ra ≤ 10 µin raises the practical pitting limit by roughly 30% in concentration and 10 °C in temperature. The mechanism: smoother surfaces have fewer initiation sites and the passive film re-forms faster after disturbance.
For trace-analysis applications, electropolishing is often paired with SilcoNert 2000 coating to combine pitting resistance with chemical inertness.
Configurator Behavior
When the user enters a chloride concentration and temperature in the process conditions step of the SPA Configurator, the material recommender:
1. Checks 316L against the conservative envelope above
2. Adds a margin for the gland crevice
3. If outside the envelope, recommends the next acceptable upgrade
4. Surfaces this blog as the explanation