How to Match a Nickel Alloy to a Chemical Process
The decisive factors in chemical service are the acid or alkali involved, its concentration, the operating temperature, the presence of oxidizing or reducing impurities, and whether chlorides can concentrate in crevices or under deposits. A material that performs well in hydrochloric acid may fail quickly in nitric acid, and a grade that resists general corrosion may still crack if residual stress and chlorides are present. The practical approach is to classify the medium as oxidizing, reducing or mixed, then select the alloy family that is designed for that class.
Ni-Mo-Cr Grades for Mixed and Oxidizing Media
The nickel-molybdenum-chromium family, often called the C-type alloys, is the workhorse group for mixed acids, chlorides and oxidizing-reducing combinations. Chromium provides resistance to oxidizing species while molybdenum provides resistance to reducing acids, and tungsten further improves pitting resistance.
| Designation | Typical composition (wt. %) | Typical service |
|---|---|---|
| UNS N10276 | Ni balance, Cr 14.5 - 16.5, Mo 15.0 - 17.0, W 3.0 - 4.5, Fe 4.0 - 7.0, C 0.010 max | Wet chlorine, hypochlorite, mixed sulfuric and hydrochloric acid, chloride-bearing process streams |
| UNS N06022 | Ni balance, Cr 20.0 - 22.5, Mo 12.5 - 14.5, W 2.5 - 3.5, Fe 2.0 - 6.0, C 0.015 max | Oxidizing chlorides, ferric and cupric salt solutions, pulp and paper bleach plants |
| UNS N06455 | Ni balance, Cr 14.0 - 18.0, Mo 14.0 - 17.0, Fe 3.0 max, C 0.015 max | Reducing and mixed acids where low carbon and good weldability are required |
A useful comparative index is the pitting resistance equivalent number, calculated as Cr + 3.3 (Mo + 0.5 W). For UNS N10276 the mid-range composition gives a value of approximately 74, which is why the grade resists pitting and crevice attack in chlorides far beyond the range of conventional stainless steel. These alloys are supplied to ASTM B575 for plate, sheet and strip, ASTM B574 for bar and rod, and ASTM B622 or B619 for seamless and welded tube and pipe.
Ni-Mo Grades for Reducing Acids
Where the medium is strongly reducing and contains no oxidizing impurities, the nickel-molybdenum family is more economical and often more effective. UNS N10665 contains 26.0 - 30.0 % molybdenum with a maximum of 1.0 % chromium and 2.0 % iron, giving outstanding resistance to hydrochloric acid across a wide concentration and temperature range. The later UNS N10675 grade offers improved thermal stability and fabricability with 27.0 - 32.0 % molybdenum and small controlled chromium and iron additions.
The limitation must be understood clearly: these alloys cannot tolerate oxidizing species. Ferric ions, cupric ions, dissolved oxygen in significant concentration, nitric acid or hypochlorite will cause rapid attack. Where oxidants may be present, a C-type alloy must be selected instead. Plate, sheet and strip are specified under ASTM B333, bar under ASTM B335, and welded pipe under ASTM B619.
High-Purity Nickel for Caustic and Alkaline Service
Caustic soda and caustic potash are handled in commercially pure nickel because the metal forms a protective film in alkaline media and is essentially immune to the caustic stress corrosion cracking that attacks steel. UNS N02200 is the standard carbon grade, specified with a maximum of 0.15 % carbon, and it is used for evaporator tubes, caustic concentrators and storage vessels. Above roughly 315 C in concentrated caustic, the low-carbon grade UNS N02201, with a maximum of 0.02 % carbon, is preferred because it avoids graphitisation of the grain boundaries at high temperature. Both grades are supplied to ASTM B162 for plate, sheet and strip and ASTM B161 for tube.
Super-Austenitic Stainless Steels as an Economic Alternative
Before specifying a nickel-base alloy it is worth evaluating a highly alloyed austenitic stainless steel. UNS N08020, a nickel-iron-chromium alloy with 32.0 - 38.0 % nickel, 19.0 - 21.0 % chromium, 2.0 - 3.0 % molybdenum and 3.0 - 4.0 % copper, resists sulfuric acid over a wide concentration range and offers good resistance to chloride stress corrosion cracking at moderate cost. It is specified under ASTM B463 for plate, sheet and strip, ASTM B473 for bar and wire, and ASTM B729 for seamless tube. The 6 % molybdenum super-austenitic grades occupy a further intermediate position and should always be included in a cost comparison.
Fabrication and Specification Notes
Nickel alloys work harden rapidly, so machining requires low cutting speeds, heavy feeds and rigid tooling, and cold forming usually needs intermediate annealing. Welding is performed with matching or over-alloyed filler metals, and the heat-affected zone of the C-type alloys must be kept clean because copper, zinc and lead contamination causes cracking. Purchase orders should state the UNS number, the product form, the ASTM specification, the heat treatment condition, corrosion test requirements, and the applicable pressure vessel code design allowable for the intended temperature.
Frequently Asked Questions
Q: Which nickel alloy is best for hydrochloric acid?
A: For pure reducing hydrochloric acid service, the nickel-molybdenum grades UNS N10665 and UNS N10675 are the standard choice. If oxidizing impurities or dissolved oxygen are present, a Ni-Mo-Cr grade such as UNS N10276 is required instead.
Q: What is used for caustic soda evaporators?
A: Commercially pure nickel, UNS N02200 for lower-temperature service and the low-carbon UNS N02201 grade above roughly 315 C, is the traditional and most economical choice.
Q: Can one alloy handle both sulfuric and nitric acid?
A: Only the chromium-bearing families, such as UNS N06022 or UNS N08020, offer useful resistance to both, because chromium is required for oxidizing acids. The nickel-molybdenum grades are attacked rapidly by nitric acid.
Q: How do I compare pitting resistance between candidate grades?
A: Calculate the pitting resistance equivalent number as Cr + 3.3 (Mo + 0.5 W) using the mid-range composition. Higher values indicate better resistance to pitting and crevice corrosion in chloride media.
Q: What standards cover nickel alloy plate and tube?
A: Plate, sheet and strip are covered by ASTM B575 and B333, bar by ASTM B574 and B335, and tube and pipe by ASTM B622, B619 and B161 depending on the alloy family and the welded or seamless form.
Q: Is a nickel-base alloy always necessary?
A: No. A super-austenitic stainless steel such as UNS N08020 or a 6 % molybdenum grade handles many sulfuric acid and chloride duties at lower cost, and should be evaluated before a nickel alloy is specified.
