Hastelloy C-22 Superior Corrosion Resistance in UNS N06022 W.Nr.2.4602 Bolts

Fasteners in acid service usually fail before the pipe wall does, and a joint is only as sound as its flange gaskets, its machined faces and its bolt load. Hastelloy C-22 bolts, ordered as UNS N06022 or W.Nr. 2.4602, are nickel-chromium-molybdenum-tungsten fasteners that hold up in oxidizing and reducing media alike.

Alloy Identity and Corrosion Resistance

Hastelloy C-22 is a nickel-chromium-molybdenum-tungsten alloy. Chromium keeps oxidizing acids under control, molybdenum and tungsten raise resistance to chloride pitting and reducing media, and that balance lets one bolting grade cover mixed acid lines where standard austenitic stainless steel is attacked quickly.

A bolt is only one part of the joint. The pipe flanges, the gasket and the fasteners share the same medium, so C-22 studs fitted against carbon steel flange faces still face the attack at the interface. Specify the bolting alloy together with the flange, the gasket and the medium.

Two designations turn up on drawings and specifications, and both describe the same material.

• UNS N06022 — the unified numbering system entry shown on the material test report.
• W.Nr. 2.4602 — the Werkstoffnummer used in European specifications.
• Product forms — bolts, stud bolts, nuts and washers machined from bar or forged stock of the same alloy.

This resistance is what buys service life. A bolt that keeps its cross-section keeps its preload, and preload is what keeps a joint tight; ordinary grades lose section to pitting first, and the joint starts to leak.

Where C-22 Bolting Is Used

C-22 bolting earns its cost wherever the medium attacks standard stainless steel.

• Chemical processing — reactors, transfer lines and storage vessels carrying acids, solvents and caustic solutions, where a lost bolt stops production and releases hazardous media.
• Oil and gas — sour gas service with hydrogen sulfide, plus offshore platforms, pipelines and drilling equipment, where fastener failure puts pressure containment at risk.
• Power generation — turbine casings, heat exchangers and boiler components working in steam and seawater.
• Aerospace and defense — propulsion and structural hardware that has to keep its strength at elevated temperature.

The shared factor is not the industry but the duty: hot media, chlorides, wet hydrogen sulfide, or a mix of oxidizing and reducing conditions inside one system.

Specifying, Installing and Inspecting Bolts

Most field problems with high-alloy bolting come from how the material is ordered and how the joint is assembled, not from the alloy itself.

• Material — specify by the UNS or W.Nr. designation and require the matching material test report for the heat.
• Pairing — use nuts and washers of the same or a compatible alloy; a carbon steel nut on a C-22 stud adds a weak point and a galvanic couple.
• Threads and galling — nickel alloys gall under pressure, so clean the threads, apply a lubricant or coating, and start every nut by hand.
• Tightening — work in a cross pattern over at least three passes so the gasket seats evenly, and set the final load for the flange class instead of guessing from diameter.
• Inspection — check gasket seating and bolt condition at every shutdown, and keep the fasteners away from carbon steel handling so embedded iron particles cannot start pitting.

Handled this way, C-22 bolting gives long and predictable intervals between shutdowns and avoids the most expensive outcome for a joint: a leak that stops the unit.

FAQ

Answers to the technical questions that come up before this alloy is specified.

Why specify Hastelloy C-22 bolts instead of stainless steel bolts?

C-22 keeps its corrosion resistance in oxidizing and reducing media, while standard austenitic stainless steels suffer pitting and crevice corrosion in chlorides and mixed acids. The choice pays off whenever a stainless bolt would lose section or preload within one shutdown cycle.

Are UNS N06022 and W.Nr. 2.4602 the same material?

Yes. Both numbers designate the same nickel-chromium-molybdenum-tungsten alloy: UNS N06022 is used in North American documentation and W.Nr. 2.4602 in European specifications.

Can Hastelloy C-22 bolts be fitted to carbon steel flanges?

They can be fitted, but the mixed pair invites galvanic attack at the interface and the flange stays the weak element. In aggressive media, matching flange, gasket and bolting materials is the sounder design.

How should C-22 bolts be tightened?

Nickel alloys gall easily, so clean the threads, apply a consistent lubricant or coating, start the nuts by hand and tighten in a cross pattern over at least three passes to the target load for the flange class.

What should the material documentation show?

The grade designation (UNS N06022 or W.Nr. 2.4602), the heat number, and the chemical and mechanical test results for the lot, so every bolt can be traced back to its melt.