What Is nickel-molybdenum alloy G-30 (UNS N06030)?
nickel-molybdenum alloy G-30 is a nickel-chromium-iron-molybdenum alloy, UNS N06030, engineered for severe oxidizing acid service. Its defining feature is a very high chromium content of about 28.5-31.5%, combined with molybdenum 4.0-6.0%, tungsten 1.5-4.0%, copper 1.0-2.4%, and iron 13.0-17.0%, with cobalt limited to 5.0% maximum and niobium plus tantalum 0.30-1.50% for stabilization. It is supplied as seamless and welded pipe, tube, plate, bar, and fittings under the ASTM B619, B622, B581, B582, and B366 specification families, and it was created to bridge the gap between conventional stainless steels and the most expensive nickel alloys in oxidizing, mixed-acid environments.
G-30 versus G-3: Design Philosophy
The predecessor grade nickel-molybdenum alloy G-3 (UNS N06985) is a nickel-chromium-iron-molybdenum alloy with chromium around 21.0-23.5%, molybdenum 6.0-8.0%, low carbon, and niobium stabilization. It resists wet-process phosphoric acid and sulfuric acid well and is a solid choice for established processes with low to moderate chlorides and mild oxidizing conditions. G-30 raises the chromium level by about ten percentage points and adds a careful balance of tungsten and copper, which fundamentally changes its behavior: it resists highly oxidizing acid mixtures that quickly defeat G-3, including phosphoric acid containing fluorides, chlorides, and solids, nitric acid blends, and concentrated sulfuric acid. For pipe selection, G-3 suits stable sulfuric and phosphoric acid service with controlled chloride levels, while G-30 is specified for severe oxidizing mixtures, high-chloride environments, and streams with variable chemistry where a safety margin is required.
Applications in Severe Oxidizing Acid Service
G-30 pipe is a recognized solution in the most aggressive segments of chemical processing and metal production. Its flagship application is wet-process phosphoric acid production and recovery, where evaporator piping, filter feed lines, and acid concentrators handle hot acid laden with fluorides, chlorides, and solids. It is also widely used for nitric-hydrofluoric acid mixtures in stainless steel pickling lines and semiconductor etching, for nitric-hydrochloric acid mixtures (aqua regia) in precious metal refining, for concentrated sulfuric acid around 93-98% at moderate temperatures, for flue gas desulfurization systems in severe downstream sections, and for nuclear fuel reprocessing lines carrying nitric acid-based streams with fission product contaminants.
Welding and Fabrication Considerations
G-30 welds reliably with conventional processes, but the filler metal is critical: a matching ERNiCrMo-11 (G-30 filler) must be used to preserve the alloy's unique corrosion resistance in the weld zone, whereas G-3 is often welded with FM 82 or ENiCrMo-3, and C-276 with ERNiCrMo-4. Because of the high chromium content, good shielding gas practice is essential to prevent chromium oxide formation. General rules for high-performance nickel alloys apply: low heat input, interpass temperature control typically below 150°C (300°F), and immaculate joint cleanliness. The alloy work-hardens rapidly, so cold bending requires powerful equipment and intermediate annealing may be needed for severe bends; thermal cutting by plasma or laser is preferred and oxy-fuel cutting is prohibited.
Critical Quality Tests for G-30 Pipe
Standard mill certification to ASTM B619 or B626 provides a material test report confirming chemistry to UNS N06030, mechanical properties, and non-destructive examination of welds. For oxidizing acid duty, performance-validation tests add metallurgical proof. ASTM G28 Method A (ferric sulfate-sulfuric acid test) is the baseline check for proper solution annealing and resistance to oxidizing media; a properly processed G-30 pipe typically shows a corrosion rate below 0.5 mm/year in this test, with the exact acceptance value set by the purchasing specification. Application-specific tests include immersion in hot, contaminated wet-process phosphoric acid and in mixed acid solutions such as synthetic Green Death or nitric-hydrofluoric acid blends, with acceptance based on corrosion rate in mm/year agreed between buyer and supplier. Incoming inspection should also include positive material identification by X-ray fluorescence or optical emission spectroscopy, ultrasonic wall thickness verification, and hydrostatic testing where required, to guard against material mix-ups and manufacturing defects.
Frequently Asked Questions
What is the difference between nickel-molybdenum alloy G-3 and G-30? G-30 carries much higher chromium (about 28.5-31.5% versus 21.0-23.5%) plus tungsten and copper additions, giving it markedly better resistance to oxidizing acids such as phosphoric-nitric mixtures and aqua regia, at the cost of higher price. G-3 suits less severe sulfuric and phosphoric acid service.
Which filler metal is used to weld G-30? ERNiCrMo-11, the matching G-30 filler, is required to reproduce the alloy's corrosion resistance in the weld deposit. Using a different filler such as ERNiCrMo-4 or ENiCrMo-3 can leave the weld zone vulnerable to attack.
What does the ASTM G28 Method A test show? It measures resistance to intergranular attack in boiling ferric sulfate-sulfuric acid and indicates whether the material is properly solution annealed. Well-processed G-30 typically exhibits a corrosion rate below 0.5 mm/year, with the acceptance limit defined by the applicable specification.
Can G-30 pipe handle hydrochloric acid? No. Like other high-chromium alloys, G-30 is not intended for hot hydrochloric acid. For reducing HCl service, a molybdenum-rich alloy such as nickel-molybdenum-chromium alloy UNS N10276 or B-3 is the appropriate choice.
Why is G-30 preferred for phosphoric acid plants? Wet-process phosphoric acid contains fluorides, chlorides, and solids that attack most stainless steels. G-30 resists this combination of oxidizing acid and aggressive impurities, giving long service life in evaporators, filter lines, and concentrators.
Is welded G-30 pipe acceptable for critical service? Yes, when the weld is made with ERNiCrMo-11 filler under a qualified procedure and the seam passes full volumetric examination such as radiographic or automated ultrasonic testing. Seamless pipe remains preferred for the highest pressures and most hazardous duties.
How does G-30 compare economically with C-2000 or 904L? 904L stainless is far cheaper but fails rapidly once the stream becomes oxidizing or chloride-rich. C-2000 and C-276 resist a broader range of media, including chlorides and hydrochloric acid, but cost significantly more. G-30 is the cost-effective specialist when the primary threat is oxidizing phosphoric or nitric acid mixtures and mid-range sulfuric acid.
