BSW NA16 Stud Bolt Corrosion-Resistant Solution for Challenging Environments

A stud bolt is a fully threaded rod with two nuts — no head — so preload is developed from both ends. In acid, alkali and seawater service it is the fastener form of choice for blind holes, large flanged joints and equipment that must be opened repeatedly, and it is normally installed with flange gaskets matched to the same medium.

What a Stud Bolt Is and Why Media Break It

That geometry matters in three situations: where the flange has a blind hole with no room for a bolt head; where the joint is large enough that a headed bolt of the required length is uneconomic or unavailable; and where the assembly is opened repeatedly and a damaged thread is fixed by renewing a nut instead of the whole fastener. The trade-off is that a stud relies on two nuts holding preload and has no shank to centre it, so fit and thread quality matter more than with a headed bolt.

In acid, alkali and seawater service fasteners rarely fail dramatically: they corrode at the thread root, lose section, and the joint starts to weep long before anything snaps. The attack concentrates wherever the stud touches a less noble part — carbon-steel pipe flanges or an aluminium bracket — and the other part corrodes instead.

• General corrosion thins the thread uniformly until the nut can no longer develop preload.
• Pitting and crevice corrosion attack locally — a thread is nothing but a stack of crevices.
• Galvanic corrosion takes over wherever two different metals meet in the same electrolyte.
• Hydrogen embrittlement under cathodic protection: absorbed hydrogen causes sudden brittle failure in a bolt that looks perfectly sound.

The Material Ladder

Choosing a grade is a matter of matching the medium and the temperature. Each step up the ladder buys resistance to a harsher combination of chloride, acid and heat, at a cost.

Table — typical service envelope for common stud materials. Confirm the actual chemistry, concentration and temperature before ordering.

Read the table as a budget. Over-specifying wastes money and can create new problems — high-strength grades bring their own embrittlement and galling risks. Under-specifying converts a capital item into a maintenance schedule. Where the medium is mixed or unknown, state the actual chemistry, concentration and temperature range and let the material be selected against real data rather than a grade name.

Threads, Installation and Care

BSW — British Standard Whitworth — uses a 55° thread angle with rounded crests and roots and is not interchangeable with the 60° Unified (UNC/UNF) or metric forms, even where diameters look deceptively similar. Mixing standards on one joint gives partial engagement, a false sense of tightness, far less preload than intended and stripped threads under load. A thread pitch gauge settles the question in seconds; a caliper does not.

A stud is tightened by holding one nut and turning the other, or by controlled pre-tensioning where preload is critical. The second nut is part of the load path, not a backup. Galling is the practical enemy of stainless threads: anti-seize compound, slower tightening and clean threads reduce the risk, and a galled pair is usually lost. Take torque values from the actual size, material and lubrication condition — a carbon-steel table value applied to a duplex stud can over-compress the joint and damage the gasket.

• Confirm the thread form before ordering replacement fasteners on older British-standard equipment — pumps, valves and vessels.
• Inspect the thread root rather than the shank: that is where corrosion starts and where the load concentrates.
• Do not reuse a stud that has carried load; it has taken a permanent set and lost its preload capability.
• Where a joint is opened repeatedly, treat studs and nuts as consumables and inspect the flange faces instead.

Where They Work and How to Specify Them

• Offshore and marine structures — deck equipment, pipeline supports and subsea frames under constant salt spray.
• Chemical and petrochemical plants — acids and alkalis, with risk concentrated at flanges, manways and column internals.
• Pharmaceutical and food plants — fasteners must not harbour residue or shed particles and must survive caustic and acid washdown.
• Water treatment, desalination and effluent plants — warm chloride service rules out standard austenitic grades at the margins.

In clean service the requirement goes beyond corrosion resistance: a smooth, pit-free surface, crevice-free details, and a length that leaves minimal exposed thread above the nut so residue cannot collect. To specify, give the medium, the concentration, the operating temperature range including cleaning cycles, and the flange standard; a grade name alone does not define a fastener. State the thread form — BSW, UNC or metric — with the dimensional standard. For pipeline flanges the governing documents are normally ASME B16.5 for the flange and ASME B16.20 for the gasket, with bolting to ASTM A193 or A320 depending on temperature. Ask for the heat analysis and a pickled, passivated surface free of embedded free iron.

Frequently Asked Questions

What is a stud bolt and why use two nuts?

A stud bolt is a fully threaded rod with a nut at each end and no head. Two nuts clamp the joint from both sides, let the fastener sit in a blind hole, and make repeated dismantling cheaper because a worn nut is replaced instead of the whole stud.

Is BSW thread interchangeable with metric or UNC?

No. BSW uses a 55° Whitworth form with rounded crests, while UNC and metric use 60°. Diameters at some sizes look similar, but the threads will not engage correctly, and forcing them produces low preload and stripped threads.

Which stainless grade suits acid and seawater service?

It depends on the acid, its concentration and the temperature. 316L covers many dilute acid and coastal applications; 2205 duplex is the practical baseline for seawater; 2507 super duplex handles hot concentrated chloride; strong reducing acids need nickel alloys.

Can a stud be reused after dismantling?

No. A stud that has carried load takes a permanent set and its thread is deformed by the nut, so its preload capability is no longer the catalogue value. Fit new studs and nuts on reassembly.