Chloride pitting and crevice corrosion are the leading causes of failure in seawater systems. The pitting resistance equivalent number, PREN, is the core selection tool, but it only reflects pitting resistance — temperature, velocity, crevice geometry and stagnation must be layered on top.
1. PREN tier table
| Tier | Typical grade | PREN | Verdict for seawater |
|---|---|---|---|
| 1 | 304 / 304L | ≈ 19 | Unsuitable — pits readily |
| 2 | 316 / 316L | ≈ 26 | Clean, cool, slow flow only; needs maintenance |
| 3 | 317L | ≈ 29 | Marginally better than 316L; limited use |
| 4 | 904L | ≈ 36 | Usable at ambient temperature; good value |
| 5 | 2205 duplex | ≈ 35 | Ambient seawater; high strength, SCC resistant |
| 6 | 2507 / 254SMO / Incoloy 926 | ≈ 40–47 | Warm seawater and chlorinated process water |
| 7 | Inconel 625 | ≈ 45–52 | Hot, high-velocity, heavily creviced service |
| 8 | C-276 / C-22 / titanium | ≈ 64–69 / — | Most severe duty; titanium virtually immune |
2. Decision path
- Fix the maximum temperature first: below 40 °C, superaustenitic and duplex are viable; 40–80 °C, use 6Mo or better; above 80 °C, evaluate nickel alloys or titanium
- Then judge velocity: 1–3 m/s favours passivation; below 0.3 m/s, deposits settle and crevice corrosion starts
- Then inspect crevices: flanges, threads, gaskets and welds are hot spots — upgrade one tier where crevices are severe
- Finally consider stagnation: shutdowns leave chloride liquid to concentrate, so frequently idle systems need a higher tier or a drain-and-flush procedure
3. Worked examples
| Application | Recommended material | Reason |
|---|---|---|
| Heat exchanger tube bundle | Titanium Gr2 / AL-6XN / CuNi 90-10 | Titanium is near-immune; CuNi resists fouling |
| Pump casings and impellers | Super duplex 2507 / nickel aluminium bronze | Balances strength and erosion resistance |
| Pump shafts and sleeves | Monel 400 / Inconel 625 / duplex | Resists impingement and cavitation |
| Seawater piping and flanges | Incoloy 926 / 254SMO / 625 | Resists crevice and pitting attack |
| Desalination evaporators | Titanium Gr2 / 254SMO / AL-6XN | Hot, high salinity — needs high PREN |
| Firewater and ballast systems | 2205 / 2507 / 926 | Intermittent duty; must resist stagnant corrosion |
Frequently asked questions
Is a higher PREN always better?
Not necessarily. PREN measures pitting resistance only and says nothing about stress-corrosion cracking, erosion-corrosion, strength or cost. Duplex 2205, for example, has PREN close to 904L but twice the yield strength and better SCC resistance. Evaluate every failure mode.
Why is low flow more dangerous in seawater piping?
Low velocity lets suspended solids and marine growth settle, creating oxygen concentration cells beneath deposits that trigger crevice corrosion and microbiologically influenced corrosion (MIC). It also hinders passive film repair. Designs therefore usually require a minimum velocity of 1 m/s, with draining and fresh-water flushing at shutdown.