Titanium Flanges in Corrosive Service: Grade, Bolt Torque and Gasket Selection

Опубликовано Engineering Team, Shaanxi Qianyi Tuoda Technology Co., Ltd от 2026-10-19

Most leaks in corrosive service are not flange defects. The flange is usually the correct part; what fails is the interface around it — the wrong grade for the medium, the wrong bolt torque, or the wrong gasket for the facing. Any one of those three, wrong on its own, will leak a joint whose components were each individually correct.

This guide treats grade, bolt and gasket as one system, because in practice that is how they fail.

1. Grade Selection by Medium

The grade decision is made by the medium, not by the price list. Get it wrong and the failure appears at the sealing face first, because that is where the crevice is.

Medium / Condition	Grade	Reasoning
Dilute sulphuric acid, 5–10%, 60–150 °C	TA10 (Gr12)	0.3% Mo and 0.8% Ni stabilise the passive film in reducing acid. See the corrosion rate comparison below.
Concentrated sulphuric acid, 70%+, ambient	TA2 (Gr2)	At high concentration the acid is less aggressive to titanium; a stable passive film forms.
Sea water and chlorides, up to 80 °C, PN25	TA10 (Gr12)	Resists chloride pitting and crevice attack. TA2 works in clean flowing sea water but is vulnerable in stagnant or crevice conditions.
Organic acids — acetic, citric	TA2 (Gr2)	Adequate for long-term service in most concentrations at moderate temperature.
Hydrochloric acid, reducing acids	TA9 (Gr7)	Palladium-stabilised; resists reducing acid and crevice corrosion where unalloyed titanium does not.
Hydrofluoric acid, fluoride-bearing media	Not titanium — Monel 400 or Hastelloy C276	Titanium is attacked rapidly by fluoride in any concentration. This is a material-change decision, not a grade-selection one.

The single most important line in this table is the last one. Hydrofluoric acid and fluoride-bearing streams destroy titanium, and higher-strength titanium alloys such as TC4 are not more resistant — alloying improves strength, not fluoride resistance. If the medium contains fluoride, the flange material must change. No titanium grade solves it.

Why TA10 for dilute sulphuric acid

TA10 is titanium with 0.3% molybdenum and 0.8% nickel, with impurity content tightly controlled. In comparative testing at 5–10% sulphuric acid and 120 °C, the corrosion rate of TA2 runs at roughly 0.5 mm/year while TA10 stays under 0.05 mm/year — an order of magnitude, in the exact condition where TA2 is most often specified by default.

At 0.5 mm/year, a flange has no useful service life. It will pass commissioning and fail inside the first campaign, and the failure will be blamed on the supplier rather than on the grade choice.

Note the reasoning honestly: this is a comparison at one specific condition. In concentrated acid at ambient temperature, TA2 is the correct and cheaper choice. The grade follows the medium and its concentration — not the brand of titanium.

2. Bolt Material and Torque

Bolt material

Carbon steel bolts must never be used with Titanium Flanges in corrosive service. The potential difference between the two drives galvanic corrosion of the steel, and the bolt — being the smaller area — corrodes fastest. The joint then loses preload and leaks, and the flange gets blamed.

Duty	Bolt Material
Ambient, mildly corrosive	TA2 titanium
Medium to high temperature, high pressure, strongly corrosive	TC4 titanium
Hydrofluoric acid, strongly reducing media	Monel 400

The principle is galvanic compatibility, not strength. Match the bolt to the flange so the couple is eliminated, and match both to the medium.

Torque: how it is calculated

Torque is not a number to be copied from a table without understanding it, because the correct value depends on friction at the thread and under the nut. The standard relationship is:

T = K × d × F

where T is torque (N·m), d is nominal bolt diameter (m), F is the target preload (N) and K is the nut factor — a friction coefficient that depends entirely on the contact surfaces and lubrication.

Contact Condition	Nut Factor K
Titanium on titanium, dry	0.20 – 0.25
Titanium on graphite gasket	0.15 – 0.20
Molybdenum disulphide or fluorinated grease	0.12 – 0.16

The consequence is worth stating plainly: the same joint torqued dry versus lubricated needs materially different torque values to reach the same preload. A torque figure quoted without the K factor it assumes is not a specification. This is why torque tables differ between sources, and why copying one from a different context causes failures.

Practical design reference for TC4 bolts, RF facing and a graphite spiral wound gasket, at K = 0.20 with molybdenum disulphide lubrication:

Size	Class	Bolt	Torque Range (N·m)
DN50	PN16	M16	55 – 70
DN100	PN16	M16	55 – 70
DN200	PN16	M20	110 – 140
DN300	PN40	M24	200 – 250

Read this table as a design reference, not an instruction. Confirm against ASME PCC-1 or EN 1591 for the specific joint, and against the gasket manufacturer's compression requirement — the gasket, not the bolt, sets the upper limit. Over-torque necks the flange and deforms the sealing face; under-torque leaves the gasket uncompressed and the joint weeps.

Assembly sequence

Torque values are only half the procedure. The other half is how the load is applied:

1. Tighten in a cross pattern — opposite bolts in sequence, never around the circumference.
2. Three passes at 30%, 60% and 100% of the final torque. A single pass to full torque compresses one side of the gasket before the other and leaves a permanent cock.
3. Lubricate the threads in advance with molybdenum disulphide or fluorinated grease. Dry titanium threads gall — the thread surface welds and tears — which destroys the bolt and falsifies the torque reading.
4. Re-check after the first thermal cycle. Gaskets consolidate and bolts relax; a joint that was correct cold is frequently loose hot.

3. Gasket Selection by Facing

The facing determines which gasket types can be used, and the gasket must then be checked against the medium. Both conditions have to pass.

Facing	Gasket Type	Suits
RF — Raised Face	Flexible graphite spiral wound	General chemical service, medium pressure — the default combination
MFM — Male / Female	Spiral wound, confined	Higher pressure, gasket fully confined against blow-out
TG — Tongue and Groove	Spiral wound or metal jacketed	Higher pressure, aggressive media
RJ — Ring Joint	RTJ metal ring	Very high pressure, severe duty

The medium overrides the facing table

Graphite is not universal. It is the default filler for a reason — chemically broad, temperature-stable, cheap — but it has specific weaknesses that cause joint failures when overlooked:

• Hydrofluoric acid and fluoride media: graphite is attacked. Use PTFE.
• Strong oxidisers at elevated temperature: graphite can be consumed. Check before assuming.
• High-pressure severe duty: move to a metal ring joint gasket regardless of what the facing would otherwise allow.

For sulphuric acid and sea water, a titanium spiral wound gasket with graphite filler gives both corrosion resistance and sealing. For organic acids, graphite is generally acceptable. For fluoride-bearing streams, PTFE — and remember from Section 1 that the flange itself should not be titanium in that service either.

4. Three Failures Worth Recognising

TA2 specified for dilute sulphuric acid. Passes commissioning, pits at the flange neck within the first campaign, fails at the sealing face. The mechanism is grade selection, not manufacturing — which is why no amount of incoming inspection catches it.

Over-torque on a large high-pressure flange. The neck is stretched or the sealing face deformed, and the joint leaks continuously no matter how many times it is re-tightened. Once the flange is deformed, re-torquing cannot recover it — the part is scrap.

Graphite gasket in fluoride service. The gasket is consumed within a short period, the joint opens, and the leak appears to be a flange problem. It is a gasket-material problem, and the flange was never at fault.

The common thread: all three are selection errors that no inspection at goods-in would have caught, because the parts were made correctly to the wrong specification.

5. A Three-Step Selection Procedure

1. Match the grade to the medium. Dilute sulphuric acid — TA10. Sea water and chlorides — TA10. Concentrated sulphuric acid at ambient — TA2. Organic acids — TA2. Hydrochloric and reducing acids — TA9. Fluoride — not titanium at all.
2. Match the gasket to the facing, then check it against the medium. RF with graphite spiral wound as the default; PTFE wherever fluoride is present; metal ring joint for high pressure. State the Ra value on the drawing, not just the facing code.
3. Match the bolt to the flange and torque to the gasket. Titanium bolts with Titanium Flanges, Monel 400 for fluoride service; torque calculated from T = K × d × F with the K factor stated; three-pass cross-pattern tightening with lubricated threads.

Buy the three as a set from one source. When flange, bolts and gasket come from three suppliers, each one is individually correct and nobody is responsible for the joint.

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We supply Titanium Flanges in TA1, TA2, TA9, TA10 and TC4 together with matched titanium and Monel 400 bolting and gasket sets, in ASME B16.5, EN 1092-1 and HG/T 20592 configurations. Send us the medium, concentration, temperature and pressure class and we will confirm the grade, the bolt material, the torque value and the gasket type as one package before quoting.

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