What Are Nickel Alloy Tubes?
Nickel-based alloy tubes are used where stainless steels no longer survive: strong reducing acids, chloride-bearing solutions, wet chlorine, hot phosphoric acid and other aggressive chemical environments. Unlike the iron-based Cr-Ni and Cr-Ni-Mo stainless steels, these alloys keep a stable, corrosion-resistant surface in both oxidising and reducing conditions, and they retain useful strength and ductility from cryogenic temperatures up to high process temperatures. In chemical equipment they fall into three practical families, and the family determines which chemistry the tube can tolerate.
Three Families and Their Grades
| Family | UNS grade | Nominal composition | Typical duty |
|---|---|---|---|
| Ni-Mo | N10001, N10665, N10675 | About 26-30% Mo, balance Ni | Hydrochloric and sulphuric acid in reducing conditions |
| Ni-Cr-Mo | N10276, N06455, N06022, N06200 | About 15-17% Mo, 15-23% Cr, balance Ni | Mixed oxidising and reducing acids, wet chlorine, chlorides |
| Ni-Cr-Fe-Mo-Cu | N06985, N06975, N06030 | About 21-31% Cr with Fe, Mo and Cu additions | Hot sulphuric and phosphoric acid, mixed acid attack |
Within the Ni-Mo family the older N10001 grade resists hydrochloric acid over the full range of concentrations up to boiling point but is prone to weld heat-affected zone corrosion and needs post-weld heat treatment. The later N10665 grade limits iron, carbon and silicon so that it can be used in the as-welded condition, but it becomes brittle after long exposure between roughly 600 and 800 degrees Celsius because of ordered intermetallic phases. The N10675 grade was developed to improve that thermal stability. In the Ni-Cr-Mo family, chromium was added to give resistance to oxidising acids as well: N10276 offers broad resistance to oxidising and reducing media, N06455 trades some high-temperature stability for better weldability, and N06022 gives improved resistance to pitting and crevice corrosion together with a more stable microstructure in the heat-affected zone. The Ni-Cr-Fe-Mo-Cu family, with roughly 21 to 31% Cr, was created for hot acid service, and the high-chromium N06030 grade is the established choice for industrial phosphoric acid.
Chemical Composition and Mechanical Property Control
Composition is controlled tightly because small changes in carbon, silicon and iron decide whether a weld zone survives. For example, a typical Ni-Cr-Mo tube grade holds carbon at 0.015% or below and silicon at 0.08% maximum to limit grain-boundary precipitation, with about 14.5 to 16.5% Cr, 15.0 to 17.0% Mo and 3.0 to 4.5% W. The Ni-Mo grades contain little or no chromium but 26 to 30% Mo, and the as-welded variants hold carbon below 0.02% and silicon at or below 0.10%. Tubes are supplied in the solution-annealed condition and are tested for tensile strength, yield strength and elongation, with minimum values stated in the applicable ASTM product specification; the corrosion-resistant grades are typically specified with a minimum tensile strength of the order of 690 MPa and a minimum yield strength near 280 MPa in that condition. Corrosion testing, grain size and hardness requirements are ordered by agreement for critical services.
Standards and Product Forms
Seamless nickel alloy pipe and tube is ordered to ASTM B622, while welded tube is covered by ASTM B626 and welded pipe by ASTM B619; general requirements for nickel alloy pipe and tube are consolidated in ASTM B775, and forged fittings in ASTM B564. The ASME equivalents, such as SB-622 and SB-626, apply for pressure equipment. Size range, wall thickness tolerance, surface finish, eddy current testing and hydrostatic testing are all specified in the standard, so the purchaser should state outer diameter, wall thickness, length, grade, condition and inspection level on the enquiry.
Fabrication, Welding and Applications
These alloys work harden rapidly, so tube bending and flaring should be done in the annealed condition with generous radii and low forming speeds. Welding is normally performed with matching filler metal, low heat input, controlled interpass temperature and scrupulously clean surfaces, because sulphur, lead and other low-melting contaminants cause cracking. The Ni-Mo grades in particular demand matched chemistry and, for the older grade, post-weld heat treatment. Typical applications include heat exchangers and condensers handling chlorides, reactor and column internals in chemical plants, flue gas desulphurisation and pollution control equipment, pharmaceutical process piping and equipment for handling mixed acids. The correct grade is chosen from the specific medium, concentration and temperature rather than from a general corrosion table, because corrosion behaviour changes sharply with small changes in oxidising power.
FAQ
Q: How do I choose between a Ni-Mo and a Ni-Cr-Mo tube?
If the medium is a reducing acid such as hydrochloric or dilute sulphuric acid, a Ni-Mo grade is normally used. If the medium is oxidising, or alternates between oxidising and reducing conditions, or contains chlorides, a Ni-Cr-Mo grade gives the wider operating window.
Q: Which standard covers seamless nickel alloy tube?
ASTM B622 covers seamless nickel and nickel-cobalt alloy pipe and tube. Welded tube is covered by ASTM B626, welded pipe by ASTM B619, and general requirements by ASTM B775.
Q: Why is post-weld heat treatment sometimes required?
The older Ni-Mo grade forms a sensitised zone next to the weld and can suffer preferential attack there, so the welded joint is heat treated. Lower-carbon, low-silicon versions were developed specifically so that vessels and piping can be used in the as-welded condition.
Q: Are nickel alloy tubes resistant to chloride stress corrosion cracking?
Yes. Nickel-based alloys are markedly more resistant to chloride stress corrosion cracking than austenitic stainless steels, which is one reason they are selected for chloride-bearing and wet chlorine service.
Q: What should be specified when ordering nickel alloy tubes?
Grade by UNS number, product standard, outer diameter, wall thickness, length, delivery condition, corrosion test requirement and inspection level. Cleanliness and freedom from embedded iron or copper contamination should also be stated on the order.
