Technical knowledge
Galvanic corrosion when nickel alloys meet carbon steel
How galvanic corrosion works in a flanged joint, why area ratio matters more than the alloy pair, and the isolation, coating and material measures that prevent the carbon steel from becoming the sacrificial anode.
How the cell works in a flanged joint
When two metals with different electrode potentials are connected electrically and both are wetted by a conductive fluid, current flows between them. The less noble metal becomes the anode and corrodes; the more noble metal is protected. Nickel alloys, titanium and stainless are more noble than carbon steel, so in a mixed joint the carbon steel carries the corrosion.
Area ratio: the factor that decides severity
| Arrangement | Severity | Interpretation |
|---|---|---|
| Small steel area, large nickel area | Severe | Corrosion current is concentrated on a small anode |
| Large steel area, small nickel area | Usually tolerable | Current is spread over a large anode |
| Similar areas | Moderate | Assess by environment and consequences |
| Bolts smaller than the flange | Bolts at risk | Classic cause of seized or wasted fasteners |
Tip Look at the joint and ask which component is the small one. That component is where the damage will appear, regardless of which alloy name appears larger on the drawing.
Controls that work
- Insulating gasket kits. Insulating gasket, sleeves and washers break the electrical path through the joint.
- Coating strategy. Coat the more noble surface rather than thesteel alone, since coating only the anode leaves a small defect area carrying all the current.
- Material matching. Where practical, keep the wetted materials in the same galvanic group.
- Sacrificial anodes or cathodic protection where the system already includes them.
- Avoid small steel members such as thin bolting or small bore branches coupled into a large alloy system.
Joint specific notes
Bolting deserves special attention. A set of carbon steel studs in a nickel alloy flange presents a small anodic area inside a large cathodic assembly, so the studs corrode and seize, and the joint loses preload. Specify bolting in a compatible grade, or isolate it. Where carbon steel piping connects to alloy equipment, an insulating kit or a transition piece is normally the most economical long term answer.
Questions about this topic
Is carbon steel to stainless always a problem?
It depends on the area ratio and on the conductivity of the fluid. A large steel flange against a small stainless component is usually acceptable, while the reverse arrangement corrodes the steel quickly.
Should I coat the carbon steel to protect it?
Coating only the steel is risky, because any defect in the coating concentrates the corrosion current at that spot. Coating the more noble surface, or using an insulating kit, is generally more reliable.
Do insulating gasket kits actually work?
Yes, when they are installed completely and correctly. The gasket, the sleeves and the washers must all be insulating; a single metal path bypasses the whole kit.
Which bolting should I use on a nickel alloy flange?
A compatible alloy stud with the correct strength and hardness, plus a suitable anti-seize compound. Carbon steel studs in a nickel alloy joint are a frequent cause of seized and wasted fasteners.
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 [email protected] 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.