High-Tech UNS S32205 Bolts Revolutionizing Desalination Equipment

Desalination plants keep every wetted surface in permanent chloride service, and the bolting decides whether the joint survives. UNS S32205 duplex bolts — roughly half austenite, half ferrite — resist pitting, crevice attack and stress cracking far better than standard austenitic grades, and they work as one joint with the flange gaskets and the sealing faces.

What UNS S32205 Actually Is

UNS S32205 — commonly called 2205 and covered by EN 1.4462 — is a duplex stainless steel with a mixed microstructure of roughly equal parts austenite and ferrite. The austenite carries toughness and weldability, the ferrite carries strength and chloride resistance. For fasteners the consequence is direct: a 2205 stud can be smaller than a 316 stud for the same load, which matters on the pipe flanges and covers it must clamp.

Typical composition is around 22% chromium, 3% molybdenum and 0.18% nitrogen. The nitrogen matters more than its small percentage suggests: it stabilises austenite during cooling and contributes heavily to pitting resistance.

• Two-phase microstructure — austenite for toughness, ferrite for strength and resistance to chloride attack.
• Pitting resistance — the nitrogen addition pushes the pitting resistance equivalent well above the 316L range.
• Bolt sizing — roughly twice the yield strength of 316L means a smaller stud for the same preload, or more margin in the same size.

Chlorides attack stainless steel in two distinct ways. Pitting and crevice corrosion start where the passive layer breaks down — under a gasket, in a thread root, in the gap between flange faces — and chloride stress corrosion cracking adds a sustained tensile stress to that environment. Seawater carries chlorides around 19,000 mg/L, and a tightened bolt lives under permanent tensile stress with crevices in its own threads: both mechanisms are aimed at it.

Reading PREN to Compare Grades

The pitting resistance equivalent number is a composition-based ranking: PREN = %Cr + 3.3 × %Mo + 16 × %N. It is a screening tool, not a guarantee, but it separates grades reliably.

The gap between 23 and 35 is what makes 2205 the entry point for genuinely wet seawater service; a 316L bolt in a seawater-wetted joint is a scheduled replacement. Temperature sets the other boundary: austenitic grades become vulnerable to chloride stress cracking above roughly 60°C in seawater, while duplex is limited to about 300°C before embrittlement risk grows.

Specifying and Installing 2205 Bolts

The minimum specified yield strength of 2205 is around 450 MPa against roughly 170 MPa for 316L, so the same bolt carries more preload. Duplex is also less forgiving during tightening: it work-hardens quickly and has less ductility than austenitic grades.

• Torque and sequence — respect specified values and cross-pattern tightening; improvisation cracks high-strength duplex faster than it loosens it.
• Manufacturing route — ask for rolled threads, solution annealing after any hot forming, and pickling or passivation of the finished part.
• Specification — state the UNS number (S32205 and S31803 both trade as 2205 but differ at the edges), and request the heat analysis with PREN calculated from that heat.
• Inspection and service — dye penetrant or eddy current checks on finished threads catch crack starters; watch galvanic pairing with carbon steel or copper alloys and hydrogen effects under cathodic protection.

In a desalination plant 2205 is the practical baseline for wet-side bolting: brine heaters, evaporator internals and seawater pump joints. Specify super duplex 2507 where warm brine sits in tight crevices or seawater runs above roughly 40-50°C, and nickel alloys where duplex genuinely cannot hold.

Frequently Asked Questions

Is 2205 duplex better than 316 for seawater bolting?

Yes, decisively. 2205 has a PREN near 35 against roughly 23 for 316L, resists pitting and crevice corrosion in chlorides far better, and withstands the chloride stress cracking that makes 316 risky in warm seawater.

Can 2205 bolts be used at high temperature?

Only in the moderate range. Duplex grades are limited to roughly 300°C in continuous service because higher temperatures cause embrittlement; for hot concentrated media, nickel alloys are the correct choice.

What is the difference between 2205 and super duplex 2507?

2507 carries more chromium, molybdenum and nitrogen, giving a PREN of about 42-45 against 34-36 for 2205. It is specified for hotter, more concentrated brine and severe crevice conditions.

How should 2205 fasteners be specified?

State the UNS number, request the heat analysis with PREN calculated from it, confirm rolled threads, solution annealing and passivation, and inspect finished threads by penetrant or eddy current methods.