1. Hastelloy G-3 and G-30 are often mentioned together. What is the fundamental chemical difference between them, and what performance improvement does G-30 offer?
While both G-3 (UNS N06985) and G-30 (UNS N06030) are nickel-chromium-iron-molybdenum alloys designed for harsh environments, G-30 is a direct, chemically optimized upgrade of G-3. The key differences lie in the precise balancing of their key elements.
The G-3 Baseline: Hastelloy G-3 was developed to handle a wide range of chemical environments, particularly sulfuric and phosphoric acids. Its composition includes Nickel (base), ~22% Chromium, ~20% Iron, ~7% Molybdenum, and ~2% Copper, with additions of Niobium and Tungsten for stability.
The G-30 Advancement: Hastelloy G-30 was developed by reducing the iron content and increasing the levels of chromium, molybdenum, and tungsten. The most significant change is the increase in chromium content from ~22% in G-3 to ~29% in G-30. This recalibration was aimed at one primary goal: markedly improved resistance to oxidizing media and localized corrosion, while maintaining excellent resistance to reducing acids.
Performance Impact: The higher chromium content in G-30 dramatically boosts its performance in:
Oxidizing Acids: It shows superior resistance to nitric acid, chromic acid, and other strong oxidizers.
Mixed Acid Environments: In processes where reducing acids (e.g., sulfuric) and oxidizing contaminants (e.g., ferric or cupric ions) are present, G-30 performs far better than G-3. Its higher Pitting Resistance Equivalent Number (PREN) also gives it better resistance to pitting and crevice corrosion in chloride-bearing solutions.
In essence, G-30 is to G-3 what C-276 is to C-a more robust, versatile, and high-performance successor for the most challenging mixed-acid services.
2. In which specific, complex industrial applications is Hastelloy G-30 Plate the preferred material of choice?
Hastelloy G-30 plate excels in environments where "mixed chemistry" is the norm-where both reducing and oxidizing conditions exist, often with halide impurities. Its balanced composition makes it a versatile and reliable choice for heavy plate fabrications.
Phosphoric Acid Production (Wet Process): This is a flagship application. In the production of merchant-grade phosphoric acid, the process stream contains not only phosphoric acid but also highly corrosive impurities like sulfuric acid, hydrofluoric acid, chlorides, and solid silicates. G-30's resistance to this complex cocktail of corrosives, including its superior performance over stainless steels and G-3 in the evaporators, heat exchangers, and reactor tanks, makes it the premium material choice. Plate is used for the shells of these large vessels.
Sulfuric Acid Pickling and Recovery: In steel and metal finishing plants, pickling baths use hot sulfuric acid. These baths become contaminated with oxidizing metallic ions (Fe³⁺, Cu²⁺). G-30 handles this mixed reducing/oxidizing environment much more effectively than standard stainless steels or G-3, making it suitable for pickling tanks, heaters, and associated ductwork fabricated from plate.
Chemical Processing with Upset Conditions: Many chemical processes are designed to be reducing, but upset conditions (e.g., air ingress, catalyst contamination) can introduce oxidizing agents. G-30 provides a larger safety margin against such upsets. It is used in reactors, columns, and mixing tanks for complex organic synthesis and petrochemical processes where process stability is not guaranteed.
Pollution Control Scrubbers: Scrubbers that handle flue gases from waste incineration or chemical production can be exposed to a varying mix of sulfuric acid, hydrochloric acid, and other corrosive species. G-30 plate is specified for the critical shell and internal components of these scrubbers.
3. What are the key welding considerations for fabricating thick sections of Hastelloy G-30 Plate to ensure optimal corrosion resistance in the weldment?
The goal in welding G-30 is to preserve the carefully balanced chemistry of the base plate in the weld zone, preventing the formation of microstructures that are susceptible to corrosion.
Filler Metal Selection: The most critical decision is to use a matching G-30 filler metal (such as ERNiCrMo-11). Using a filler metal with a lower alloying content (e.g., a G-3 or stainless steel filler) would create a galvanic couple where the less-noble weld metal would corrode preferentially. The matching filler ensures the weld puddle solidifies with a composition and microstructure similar to the G-30 plate.
Controlling Heat Input and Interpass Temperature:
Moderate Heat Input: Use a stringer bead technique with moderate heat input. Excessive heat can cause excessive dilution from the base metal or lead to segregation of alloying elements during solidification.
Low Interpass Temperature: Maintain a strict maximum interpass temperature, typically below 150°C (300°F). This prevents the heat-affected zone (HAZ) from spending too much time in the sensitization temperature range (approx. 550-950°C / 1000-1750°F), where harmful chromium-carbides and other phases can precipitate.
Shielding and Cleanliness:
Back Purging: As with all high-performance alloys, proper back purging of the root side of the weld with high-purity argon is non-negotiable to prevent "sugaring" (oxidation) on the weld root bead.
Cleanliness: All surfaces must be free of contaminants like oil, grease, paint, and marking inks. Dedicated, clean tools must be used to avoid iron contamination, which can become initiation points for pitting corrosion.
4. How do the physical and mechanical properties of G-30 and G-3 Plate influence the design and fabrication of process equipment like reactors?
Understanding these properties is essential for engineers to design safe, efficient, and fabricable equipment.
Mechanical Properties (Typical for Annealed Plate):
G-30: Tensile Strength ~110 ksi (758 MPa), Yield Strength ~52 ksi (358 MPa), Elongation ~45%.
G-3: Slightly lower strength but similar high ductility.
The high strength allows for the design of pressure vessels with thinner walls, which can partially offset the high material cost. The excellent elongation (ductility) is crucial for cold forming plates into cylindrical shells and dished heads without cracking.
Physical Properties & Fabrication Impact:
Thermal Expansion: Both alloys have a thermal expansion coefficient similar to other nickel-chromium alloys but lower than standard stainless steels. This is a critical factor when designing systems with mixed materials to avoid excessive thermal stresses during startup/shutdown cycles.
Thermal Conductivity: Relatively low, about one-third to one-fifth that of carbon steel. This must be accounted for in heat exchanger design, as it impacts heat transfer efficiency.
Work Hardening: Both G-3 and G-30 work-harden rapidly during cold forming. Fabricators must account for this significant increase in strength and hardness during processes like rolling and bending. For severe deformations, intermediate annealing steps may be required to restore ductility and prevent cracking.
5. For a project engineer, what are the critical material certification and testing requirements when procuring G-30 Plate for a corrosive service application?
Procuring plate for a critical service requires verification beyond a standard Material Test Report (MTR) to ensure long-term performance.
Standard Mill Certification (MTR/CMTR): The plate must come with a certified MTR (or CMTR for international projects) that verifies the heat chemistry conforms to UNS N06030 for G-30 (or N06985 for G-3) and that the mechanical properties meet the specified minimums.
Corrosion Testing for Quality Assurance: For an alloy like G-30, which is often used in environments susceptible to intergranular attack, it is standard practice to specify an ASTM G28 Method A corrosion test. This test, which involves exposing a sample to a boiling solution of 50% sulfuric acid and 42 g/L ferric sulfate for 24 hours, is designed to detect susceptibility to intergranular corrosion. A passing result (e.g., a corrosion rate of <1.0 mm/month) confirms the plate is in the proper solution-annealed and quenched condition, free from harmful chromium-carbide precipitation at the grain boundaries.
Non-Destructive Testing (NDT): For pressure vessel applications, the purchase order will specify:
Ultrasonic Testing (UT): Performed to a standard like ASTM A578 Level II or III to detect internal laminations, inclusions, or voids that could compromise the integrity of the finished component.
Dye Penetrant Testing (PT): Often performed on the cut edges of the plate after shearing or machining to ensure no surface-breaking defects are present.
By specifying these tests, the project engineer ensures that the G-30 plate is not only of the correct grade but is also in the optimal metallurgical condition to deliver reliable, long-term service in aggressive chemical environments.
