Technical knowledge
Crevice corrosion under gaskets and deposits: the hidden failure mode
Why crevice corrosion starts at gasket faces, under deposits and at tube joints, how it differs from pitting, and the design, material and maintenance measures that prevent it.
Why a gap is worse than an open surface
In a crevice, the solution becomes isolated. Metal ions accumulate, chloride migrates in to maintain charge balance, and the pH drops. The passive film that protects the open surface cannot be maintained inside the gap, so attack proceeds even though the surrounding metal looks untouched.
Where it starts in real systems
| Location | Why it crevices | Control |
|---|---|---|
| Gasket faces and bolt holes | Confined geometry under compression | Select a higher alloy, use a compatible gasket, reduce the crevice |
| Under deposits and scale | Shielded surface, stagnant fluid | Filtration, cleaning, flow velocity |
| Tube to tubesheet joints | Tight annular gap | Proper expansion, seal welding, material upgrade |
| Lap joints and slip on flanges | Inherent crevice at the lap | Weld where possible, seal the crevice |
| Under insulation | Moisture retention and chlorides | Weatherproofing, coating, inspection |
Material measures
- Raise the alloy. Crevice corrosion temperature is typically well below pitting temperature, so select on crevice data.
- Consider titanium or nickel alloys where chlorides and stagnation coexist.
- Use a compatible gasket. Some gasket materials extract chloride or create a larger crevice.
- Avoid graphite in oxidising service where galvanic effects with the flange are a concern.
Design and maintenance measures
- Prefer butt welded joints over threaded or lap joints in corrosive service.
- Design for drainage and avoid dead legs where fluid can stagnate.
- Keep velocity high enough to prevent deposition, but below the erosion threshold.
- Inspect under insulation at defined intervals, especially in marine and coastal plant.
- Use low chloride wash water and dry thoroughly after hydrotesting.
Questions about this topic
Why does crevice corrosion start at lower temperature than pitting?
The confined chemistry inside a crevice becomes more aggressive than the bulk fluid, so the passive film breaks down earlier. Always compare crevice data rather than pitting data when crevices are present.
Can a gasket itself cause corrosion?
Yes. Some gasket materials contain leachable chlorides or create a wider crevice. Select a gasket compatible with both the fluid and the flange material, and avoid graphite where galvanic coupling to the flange is a concern.
Is 316L acceptable under insulation outdoors?
It is at risk where chlorides and moisture are present. Weatherproofing, coating and a defined inspection regime are needed, and duplex or a nickel alloy is often justified for coastal or marine plant.
Questions about our products
What products does Hangbo Alloy supply?
Four product families, all available in nickel-based alloys, titanium, duplex and special stainless steel: flanges, butt weld pipe fittings, elbows, and seamless tube and pipe. Flanges cover weld neck, long weld neck, slip-on, blind, socket weld, threaded and lap joint patterns. Fittings and elbows cover concentric and eccentric reducers, equal and reducing tees, caps, stub ends, and 45°, 90° and 180° returns in both long and short radius, plus 3D and 5D bends for piggable lines. Seamless tube and pipe is supplied cold drawn or hot finished from 6 mm to 630 mm outside diameter.
Which alloy grades are available for these products?
More than 90 grades are published in our grade library. The most requested are Inconel 625, 718, 600 and X-750, Hastelloy C-276, C-22 and B-2, Monel 400 and K-500, Incoloy 800H, 825 and 925, duplex 2205 and super duplex 2507, 316L, 904L and 254SMO stainless steel, and titanium Grade 2 and Grade 5. Nickel-based grades can also be supplied with VIM plus ESR double melting where cleanliness or fatigue life is critical.
What sizes, wall thicknesses and pressure classes can you supply?
Flanges run from NPS ½ to NPS 24 to ASME B16.5 and from NPS 26 to NPS 60 to ASME B16.47 Series A or B, in pressure classes 150 through 2500. Butt weld fittings and elbows run from NPS ½ to NPS 48 (DN 15 to DN 1200) in wall thickness Schedule 5S through XXS. Seamless tube and pipe covers 6 mm to 630 mm outside diameter, in random lengths of about 5 to 7 metres or fixed lengths up to 12 metres. Non-standard and intermediate sizes are quoted on request.
Which material and dimensional standards do you work to?
Forged flanges to ASTM B564, B381 and A182 with dimensions to ASME B16.5 and ASME B16.47. Butt weld fittings and elbows to ASTM B366 and A403 with dimensions to ASME B16.9 and MSS SP-43. Seamless tube and pipe to ASTM B444, B423, B622, B163, B167, B861, A312, A213 and A790. Marking follows MSS SP-25 and the applicable product standard. If your project is governed by EN, DIN, JIS or a company specification, send the document and we will confirm equivalence before quoting.
Is there a minimum order quantity?
There is no minimum for standard sizes and grades held in stock, and prototypes, spares and small trial batches are accepted. Made-to-order melts and special forgings carry a minimum weight that depends on the alloy and the route, typically from about 500 kg for special grades.
What inspection and certification is issued with the goods?
Every shipment carries an EN 10204 3.1 mill test certificate quoting the actual chemical and mechanical results for the specific heat lot, together with dimensional records and heat treatment data where applicable. Testing is carried out in house by optical emission spectrometry, tensile, impact and hardness testing, plus ultrasonic and eddy current examination, with hydrostatic pressure and intergranular corrosion testing to ASTM G28 or A262 Practice E where the specification requires it. Third party inspection by SGS or another recognised agency can be arranged, and their report is issued alongside the shipment.
What are typical lead times?
Standard stock items ship within days. Common production grades such as Inconel 625 or duplex 2205 are typically a few weeks. Special chemistries, larger forgings and items that require third party inspection take longer and are quoted individually against the requirement.
How are the products packed for export?
Flanges and fittings receive a rust preventive coating, are individually wrapped and are packed in plywood cases secured with steel banding. Tube ends are capped and bundles are strapped. Packing lists quote heat numbers so every case can be cross-checked against the mill certificate on arrival. We quote EXW, FOB, CFR, CIF and DAP, and mixed-alloy orders can be consolidated into one shipment with a single documentation package.
How do I request a quotation or a sample?
Send the grade or UNS number, the dimensions including schedule or wall thickness, the governing material and dimensional standards, quantity, any heat treatment or NDT scope and the certificate type you need; a drawing is ideal for special flanges and fittings. Email demi@nickel-alloy.com or message us on WhatsApp +86 15618279078 and we normally issue a formal quotation within 24 hours on working days. Samples and previous test reports can be arranged before the first order.
Talk to an engineer
Share your duty conditions and we will shortlist candidate alloys within one working day.