Two Nickel Alloy Pipe Grades for Aggressive Service
UNS N06625 and UNS N06686 are both nickel-chromium-molybdenum pipe materials used where carbon steel and stainless steel cannot survive the process medium. The two grades are frequently specified against each other, yet they are engineered around different priorities: N06625 is the broader workhorse grade with balanced corrosion resistance, strength and weldability, while N06686 pushes molybdenum and tungsten additions upward to handle the most severe chloride and reducing acid service.
Both are supplied as seamless and welded pipe and tube, and the correct choice depends on the medium, the operating temperature and the amount of field welding the installation requires.
Chemical Composition Comparison
| Element | UNS N06625 | UNS N06686 |
| Nickel (Ni) | 58.0 % minimum | Balance |
| Chromium (Cr) | 20.0-23.0 % | 19.0-23.0 % |
| Molybdenum (Mo) | 8.0-10.0 % | 15.0-17.0 % |
| Tungsten (W) | Not specified | 3.0-4.4 % |
| Niobium plus tantalum (Nb+Ta) | 3.15-4.15 % | Not specified |
| Titanium (Ti) | 0.40 % maximum | 0.02-0.25 % |
| Carbon (C) | 0.10 % maximum | 0.010 % maximum |
| Iron (Fe) | 5.0 % maximum | Low residual level |
The decisive difference is the molybdenum content, supported by tungsten in N06686 and by niobium in N06625. Grade data circulating for these pipes sometimes quotes a molybdenum content near 9 % together with an iron content above 18 % for the high molybdenum grade; those numbers describe neither alloy, and purchasing decisions should be based on the ranges in the table above and on the mill certificate.
Corrosion Performance in Reducing, Oxidising and Nitriding Media
The N06625 grade performs well in reducing, oxidising and nitriding media and is highly resistant to chloride stress corrosion cracking. It also withstands dry chlorine and hydrogen chloride gas, which is unusual among engineering alloys, and it keeps useful strength, formability and weldability below about 650 degrees Celsius.
The N06686 grade raises molybdenum and adds tungsten specifically to resist pitting and crevice corrosion in chloride bearing environments and to hold up in strongly reducing acids. It maintains structural integrity and functional stability in harsh duties such as chemical processing and pollution control, and it shows good wear and fatigue resistance in high temperature flue gas, which is why it is often selected for heat exchangers and reactor internals in scrubber circuits.
High Temperature Strength and Pipe Fabrication
N06625 pipe retains high strength together with good formability, which allows cold bending and field welding without a post-weld precipitation treatment. Fabrication is nevertheless demanding: the alloy work hardens rapidly and conducts heat poorly, so cutting, bevelling and machining need sharp tooling, rigid clamping and controlled heat input.
N06686 pipe is specified for operation from sub-zero conditions through room temperature to elevated temperature, and its higher alloy content makes welding control more critical. Low heat input, matching filler metal and clean, controlled interpass conditions are needed to preserve the corrosion resistance of the weld and heat affected zone. For both grades, allowable stress and the maximum design temperature must be taken from the applicable pressure equipment design code rather than extrapolated from room temperature data.
Applications and Selection Guidance
Select N06625 when one grade must cover a mixed duty of chemical, marine and elevated temperature service and when welded fabrication must be completed without post-weld heat treatment.
Select N06686 for severe chloride pitting and crevice corrosion, strongly reducing acid streams, and pollution control equipment such as flue gas desulphurisation ductwork, dampers and heat exchanger tubing.
Check product form availability and tolerances. Seamless and welded pipe, and tube for heat exchangers, are covered by different specification requirements for dimensions, testing and surface condition.
Qualify the welding procedure for the specific grade before fabrication begins, because the higher molybdenum grade tolerates less heat input than the balanced grade.
Review the complete wetted circuit. Mixing the two grades in one system is acceptable only when the corrosion behaviour of the joint and of any galvanic couple has been assessed.
FAQ
Q: What is the main difference between N06625 and N06686 pipe?
N06686 contains substantially more molybdenum and a deliberate tungsten addition, while N06625 relies on molybdenum plus niobium. The result is better resistance to chloride pitting and crevice corrosion in N06686, and broader balanced performance in N06625.
Q: Which pipe grade resists dry chlorine gas?
The N06625 grade is noted for resistance to dry chlorine and hydrogen chloride gas, together with excellent resistance to stress corrosion cracking in chloride environments.
Q: Can N06686 pipe be welded in the field?
It can be welded, but the procedure must control heat input and interpass temperature, and matching filler metal should be used so that the corrosion resistance of the joint matches the parent pipe.
Q: Are both grades suitable for heat exchanger tubing?
Yes. Both are used for heat exchangers and reactors, with N06686 preferred for the most aggressive chloride and reducing acid duties and N06625 preferred where a single versatile grade is required.
Q: What temperature limit applies to these pipes?
The usable range is set by the applicable design code, the medium and the required strength. The N06625 grade keeps good strength and weldability below about 650 degrees Celsius, and N06686 serves from sub-zero conditions to elevated temperature duties.
Q: How should the grade be specified on a purchase order?
State the UNS number, the product form, the dimensions, the applicable pipe specification, the heat treatment condition and the required testing, so that the mill certificate can be checked line by line on receipt.
