A Comprehensive Guide to Nickel-based Alloy: Hastelloy G3 (UNS N06985)

What is Hastelloy G3?

Hastelloy G3 (also known as “Alloy G3”) is designated UNS N06985 or DIN 2.4619 and is a nickel-chromium-iron alloy with added molybdenum and copper. It has good weldability and has the ability to resist intergranular corrosion under welding conditions. The low carbon content helps prevent sensitization of the welding heat-affected zone and subsequent intergranular corrosion. Hastelloy G3 is commonly used in flue gas scrubbers and processing reducing acids, such as phosphoric acid and sulfuric acid.
Hastelloy G3 is a nickel-based high-temperature alloy composed of nickel, chromium, molybdenum, cobalt and other elements, with a nickel content of about 44%. Hastelloy G3 alloy, referred to as G3, is a nickel-based corrosion-resistant alloy with superior performance. It belongs to the Ni-Cr-Fe series containing Mo and Cu. It has excellent resistance to oxidation, atmospheric corrosion and stress corrosion cracking, and has relatively High resistance to local corrosion (pitting corrosion, crevice corrosion). Due to the high content of Fe in the alloy, it has the characteristics of low cost compared to other nickel-based corrosion resistant alloys. G3 alloy is often used in flue gas desulfurization systems, papermaking, phosphoric acid production steam generators and heat exchangers. The oil well pipe made of this alloy has excellent resistance to H₂S, CO₂, Cl-corrosion, and is the best material for sour gas field oil well pipe.

Hastelloy G3 Weld Neck Flanges

Characteristics of Hastelloy G3 (UNS N06985)

• 1.Good resistance to heataffected-zone (HAZ) corrosion and weldability;
• 2.Excellent corrosion resistance to oxidizing chemicals and atmospheres;
• 3.Good resistance to reducing chemicals;
• 4.Exceptional stress-corrosion-cracking resistance in chloride-containing environments;
• 5.Good resistance to pitting and crevice corrosion.
• 6.Good resistance to intergranular corrosion.

Hastelloy G3 alloy is a nickel-based corrosion-resistant alloy with superior performance. Among the cold-work-strengthened nickel-based corrosion-resistant alloys, Hastelloy G3 alloy has better corrosion resistance than 825 and 028 alloys. Hastelloy G3 alloy has a temperature of 220°C, pH=3.3, and Cl ion concentration of 15.175%. H₂S and CO: In a corrosive environment with a partial pressure of 2.1 MPa, it still shows good corrosion resistance.
In addition, the grain size of G-3 alloy affects its resistance to stress corrosion cracking and intergranular corrosion in simulated acidic solutions in the Gulf of Mexico (25%NaCl + 1.03 MPa H₂S+1.03 MPa CO₂, temperature is 218 ℃). Slow strain rate corrosion test results show that the reduction of area and elongation of G-3 alloy are greater than 92%, and no secondary cracks appear. G-3 alloy shows good resistance to stress corrosion cracking. When the grain size changes from grade 6-7.5 to grade 4-5.5, it has little effect on its resistance to stress corrosion cracking. The intergranular corrosion test shows that the corrosion rate of G-3 alloy is about 0.27-0.36 mm/a, which is significantly lower than the maximum allowable corrosion rate (0.61 mm/a) in the chemical process, and the grain size has little effect on the intergranular corrosion. , Thompson et al. studied the pitting corrosion behavior of G-3 alloy in an acidic solution with a C1 ion concentration of 100 g/L and a temperature of 50 ℃ by using the cyclic potential scanning method. The results show that the pitting potential of G-3 alloy is 0.59 V. When the potential exceeds this value, the corrosion current increases rapidly and the corrosion resistance is greatly reduced.

Production process of Hastelloy G3 alloy:

• 1. Hot rolled forming;
• 2. Hot extrusion molding.

G3 alloy has poor high-temperature plasticity, narrow hot forming temperature range, and greater resistance to deformation. At about 1150°C to 1220°C, the alloy has the best thermoplasticity. Therefore, the production of G-3 alloy pipes is mainly formed by hot extrusion. The thermal deformation of the blank in the extrusion cylinder is a key technology in hot extrusion molding, and it is also a bottleneck in the production of G-3 alloy pipes.

Types of Hastelloy

• Hastelloy B-3 nickel molybdenum alloy has excellent corrosion resistance in reducing environment
• The upgraded version of Hastelloy B-3: B-3 has excellent corrosion resistance to hydrochloric acid at any temperature and concentration
• Hastelloy C-4: good thermal stability, good toughness and corrosion resistance at 650-1040 ℃
• Hastelloy C-22: has better uniform corrosion resistance than C-4 and C-276 in oxidizing medium and excellent local corrosion resistance
• Hastelloy C-276: good resistance to oxidizing and moderate reducing corrosion, excellent resistance to stress corrosion
• Hastelloy C-2000: the most comprehensive corrosion resistant alloy with excellent uniform corrosion resistance in oxidation and reduction environments
• Hastelloy G-35: the upgraded product of G-30 has better corrosion resistance and thermal stability, and has excellent performance in phosphoric acid and other strong oxidizing mixed acid media with high chromium content
• Hastelloy X: combined with the characteristics of high strength, oxidation resistance and easy processing, each of the above grades has its own specific chemical composition, mechanical properties and strong points, so we can’t generalize the characteristics of Hastelloy.

Hastelloy alloy is mainly divided into three series B, C and G. it is mainly used in iron-based Cr Ni or Cr Ni Mo stainless steel, non-metallic materials and other occasions with strong corrosive medium.
In order to improve the corrosion resistance and cold and hot working properties of Hastelloy, three major improvements have been made to Hastelloy:

• Series B: B → B-3 (00ni70mo28) → B-3
• Series C: C → C-276 (00cr16mo16w4) → C-4 (00cr16mo16) → C-22 (00cr22mo13w3) → C-2000 (00cr20mo16)
• G Series: G → G-3 (00cr22ni48mo7cu) → G-30 (00cr30ni48mo7cu)
• The most widely used materials are N06985 (B-3), N10276 (C-276), N06985 (C-22), N06455 (C-4) and N06985 (G-3)

Chemical Composition of Hastelloy G3 (UNS N06985)

Ni	Balance
Cr	21.0-23.5
Fe	18.0-21.0
Mo	6.0-8.0
Cu	1.5-2.5
Nb+Ta	≤0.50
C	≤0.015
W	≤1.5
Si	≤1.0
Mn	≤1.0
P	≤0.04
S	≤0.03
Co	≤5.0

Physical Properties of Hastelloy G3 (UNS N06985)

Density	Melting Range		Specific Heat		Electrical Resistivity
g/cm3	°C	°F	J/kg. k	Btu/lb.°F	µΩ·m
8.14	1260-1345	2300-2450	452	0.108	1180

Mechanical Properties of Hastelloy G3 (UNS N06985)

Density	Melting Range		Specific Heat		Electrical Resistivity
g/cm3	°C	°F	J/kg. k	Btu/lb.°F	µΩ·m
8.14	1260-1345	2300-2450	452	0.108	1180

Annealing of Hastelloy G3 (UNS N06985)

• Anneal at 2100 °F and rapid air cool or water quench.

Forging of Hastelloy G3 (UNS N06985)

• Forge in the range of 2100 °F to 1700 °F and be sure to anneal after forging to restore corrosion resistance.

Cold Working of Hastelloy G3 (UNS N06985)

Cold forming may be done using standard tooling although plain carbon tool steels are not recommended for forming as they tend to produce galling. Soft die materials (bronze, zinc alloys, etc.) minimize galling and produce good finishes, but die life is somewhat short. For long production runs the alloy tool steels ( D-2, D-3) and high-speed steels (T-1, M-2, M-10) give good results especially if hard chromium plated to reduce galling. Tooling should be such as to allow for liberal clearances and radii. Heavy duty lubricants should be used to minimize galling in all forming operations. Bending of sheet or plate through 180 degrees is generally limited to a bend radius of 1 T for material up to 1/8″ thick and 2 T for material thicker than 1/8″. In order to avoid “orange peel” surface effect the cold work reduction of area should be greater than 15% at any given operation. Intermediate annealing may be done, to restore ductility, during the sequence of cold forming operations.

Machinability of Hastelloy G3 (UNS N06985)

Conventional machining techniques used for iron based alloys may be used. Machining characteristics are somewhat similar to those for the austenitic (300 Series) stainless steels. This alloy does work-harden during machining and has higher strength and “gumminess” not typical of steels. Heavy duty machining equipment and tooling should be used to minimize chatter or work-hardening of the alloy ahead of the cutting. Water-base coolants of premium quality are preferred. Rigid mounting of tooling and the workpiece are important to avoid “chatter” (work hardening ahead of the cut). Both carbide tools and high-speed tools may be used successfully. Carbide tooling generally permits twice, or better, the feed rate of high-speed tooling for the same depth of cut or drilling. Turning: For roughing cuts the tools should have -5 degree back rake for carbide and -10 degree back rake for high-speed steel. Normal and/or finish turning call for positive rake angles of about +10 degrees for both carbide and hig-speed cutters. Cutting speeds and feeds are in the following ranges: For High-Speed Steel Tools For Carbide Tooling Depth Surface Feed Depth Surface Feed of cut speed in inches of cut speed in inches inches feet/min. per rev. inches feet/min. per rev. 0.040″ 0.040″ 0.250″ 0.250″ Drilling: Steady feed rates must be used to avoid work hardening due to dwelling of the drill on the metal. Rigid set-ups are essential with as short a stub drill as feasible. Conventional high-speed steel drills work well. Feeds vary from 0.001 inch per rev. for holes of less than 1/16″ diameter, 0.002 to 0.003 inch per rev. for 1/4″ dia., 0.004 to 0.010 inch per rev. for holes of 7/8″diameter. Speeds of 10 to 25 surface feet/minute, are best for drilling. Milling: To obtain good accuracy and a smooth finish it is essential to have rigid machines and fixtures and sharp cutting tools. High-speed steel cutters such as M-2 or M-10 work best with cutting speeds of 30 to 50 surface feet per minute and feed of 0.002-0.007 inch per cutting tooth. Grinding: The alloy should be wet ground and aluminum oxide wheels or belts are preferred.

Welding of Hastelloy G3 (UNS N06985)

The commonly used welding methods work well with this alloy. Matching alloy filler metal should be used. If matching alloy is not available then the nearest alloy richer in the essential chemistry (Ni, Co, Cr, Mo) should be used. All weld beads should be slightly convex. It is not necessary to use preheating. Complete removal of slag is important after every weld pass and upon completion of welding. Usually this is accomplished by use of a wire brush (hand or powered). Surfaces to be welded must be clean and free from oil, paint or crayon marking. The cleaned area should extend at least 2″ beyond either side of a welded joint.

• Gas Tungsten Arc Welding (TIG): DC straight polarity (electrode negative) is recommended. Keep as short an arc length as possible and use care to keep the hot end of filler metal always within the protective atmosphere. Arc voltage should be in the range of 9 to 13 volts with current of 20-60 amps for thin material, 60-150 amps for material 1/8″ thick or so, and 100-150 amps for material 1/4″ thick.
• Shielded Metal-Arc Welding (SMAW): Electrodes should be kept in dry storage and if moisture has been picked up the electrodes should be baked at 600 F for one hour to insure dryness. Use electrode positive polarity. Current settings vary from 60 amps for 3/32″ dia. rods up to 180 amps for 3/16″ dia. rods. It is best to weave the electrode slightly as this alloy weld metal does not tend to spread.
• Metal-Arc Welding (MIG): Electrode positive polarity should be used and best results are obtained with the welding gun at 90 degrees to the joint. For Short-Circuiting-Transfer GMAW a typical voltage is 18-22 with a current of 75-150 amps and a wire feed of 8-10 inches per minute.
• Submerged-Arc Welding: Generally submerged-arc welding should be avoided. This weld process involves high heat input and may lead to cracking of the alloy workpiece.

Heat treatment of Hastelloy G3 (UNS N06985)

• The alloy may be annealed, but does not respond to age-hardening. It is normally furnished in the annealed condition. It is important to anneal after hot or cold working in order to restore optimum corrosion resistance.

Hardening of Hastelloy G3 (UNS N06985)

• Hardens due to cold working only.

ASTM Specifications

Pipe Smls	Pipe Welded	Tube Smls	Tube Welded	Sheet/Plate	Bar	Forging	Fitting	Wire
B622	B619	B622	B626	B575	B574	B564	B366

Application areas of nickel-based alloy Hastelloy G3 (UNS N06985)

G series nickel-based alloys (G3, G30, G35) are mainly used in the petrochemical industry such as oil well pipes, evaporators used in the production of wet phosphoric acid, nuclear fuel regeneration equipment in the nuclear industry, and pickling equipment in steel plants. C series alloys (C276, C22, C4) are one of the most used corrosion-resistant alloys, and they have good corrosion resistance in oxidizing or reducing environments. Therefore, it is widely used in various places with complex corrosive environments, such as the nuclear industry and the pharmaceutical industry. Alloy 690 is a very important nuclear material. It is an irreplaceable material for the steam pipe of nuclear power generation in nuclear power plants. It is a core component and has good resistance to stress corrosion cracking.
When drilling for oil and natural gas, in addition to drilling machinery and equipment, special pipes are also needed, namely drill strings, casings, tubing, etc., collectively referred to as “oil well pipes.” Oil well pipe accounts for about 40% of the total steel used in the oil and gas industry, and is an important part of oil and natural gas exploitation. According to the particularity of China’s oil and gas production environment, experts believe that ultra-high-strength tubing, high-strength tubing, tubing resistant to corrosion in acid environments, and special threaded tubing are high-performance tubing that China urgently needs today and in the future.
For a long time, China’s oil well pipe materials have mainly been 13Cr, 22Cr, 25Cr, 316 stainless steel and so on. These stainless steels have high strength and high Cr content. A dense Cr₂O₃ passivation film is easily formed on the surface of the alloy, which can effectively resist the corrosion of CO, and with the increase of Cr content, it is resistant to CO₂ corrosion. The ability to gradually increase oil. However, with the gradual discovery and exploitation of deeply buried high-acid oil and gas fields, the content of H₂S, CO₂, S, and C1 in the mining environment is high, and the commonly used stainless steel pipes can no longer meet the mining needs. Therefore, highly alloyed nickel-based corrosion-resistant alloys (600, 825, G-3, 2550, 050, 625, C276) are gradually used in oil well pipes. G3 alloy is a nickel-based corrosion-resistant alloy with superior performance. The Ni-Cr-Fe system containing Mo and Cu has excellent resistance to oxidation, atmospheric corrosion and stress corrosion cracking. Due to the high content of Fe in the alloy, it has the characteristics of low cost compared to other nickel-based corrosion resistant alloys. The oil well pipe made of this alloy has excellent resistance to H₂S, CO₂, and Cl corrosion, and is the best choice for oil well pipe in sour gas fields. With the gradual development of sour oil and gas fields, the demand for nickel-based alloy oil well pipes continues to rise. Related products have not been fully mastered by a few foreign manufacturers of nickel-based corrosion-resistant alloy pipe manufacturing technology, which seriously threatens the country’s energy security. Therefore, it is imperative to localize nickel-based alloy pipes.
Alloy G3 is an improved version of Alloy G. The alloy also has excellent corrosion resistance, but its resistance to HAz (heat affected zone) corrosion is stronger, and it has good weldability. The lower carbon content of the alloy can delay the precipitation kinetic behavior of carbides. And its slightly higher aluminum content provides excellent local corrosion resistance. G3 alloy has replaced G alloy in almost all industrial applications. At the same time, it replaces 825 alloy in many applications that require local corrosion resistance. Commonly used downhole tubing materials are 825, G3, G50, C276 and 028 alloys. According to the fluid properties of the high acid gas fields in the Sichuan-Chongqing area, the material must meet the following three requirements: high temperature resistance, high pressure resistance, and strong corrosion resistance.

It is used in flue gas desulfurization systems (scrubbers), especially in quencher, damper, and outlet ducting areas. It can be used in other air pollution control systems in the chemical and pulp and paper industries. It is a good candidate for evaporators, heatexchangers, tank liners, and other equipment in phosphoric acid manufacturing plants. Some of the more common uses of Hastelloy G3 include:

• Wire wound resistors.
• Bimetal contacts.
• Electric and electronic applications.
• Marine engineering.
• Chemical and hydrocarbon processing equipment.
• Gasoline and freshwater tanks.
• Crude petroleum stills.
• De-aerating heaters.
• Boiler feed water heaters and other heat exchangers.
• Pumps, shafts and fasteners.
• Industrial heat exchangers.
• Chlorinated solvents.
• Crude oil distillation towers.
• Meter and valve parts.
• Screw machine products.
• Oil refinery piping.
• Heat exchangers.
• Nuclear fuel production.
• Generator tubing.
• High temperature heating coils.
• Crude oil transfer piping.
• Propeller and pump shafts.
• Piping system.
• Heat exchange tubes.
• Pipe fittings.
• Flanges.
• Valves.

Variety specifications and supply status of Nickel-based super alloy: Hastelloy G3 (UNS N06985)

Variety classification:

Yaang Pipe Industry can produce various specifications of Hastelloy G3 seamless pipe, Hastelloy G3 steel plate, Hastelloy G3 round bar, Hastelloy G3 forgings, Hastelloy G3 flange, Hastelloy G3 pipe fittings, Hastelloy G3 welded pipe, Hastelloy G3 steel strip, Hastelloy G3 wire and supporting welding materials. 

Delivery status:

• Seamless pipe: solid solution + acid white, length can be set;
• Plate: solid solution, pickling, trimming;
• Welded pipe: solid solution acid white + RT% flaw detection;
• Forging: annealing + car polish; Bars are forged and rolled, surface polished or car polished;
• Strips are delivered after cold rolling, solid solution soft state, and deoxidized;
• Wire rods are finely ground in solid solution pickled disk or straight strips, solid solution straight strips Delivery in light state.

Effect of heat treatment process on the corrosion resistance of Hastelloy G3 alloy

In the oilfield equipment assets, oil well pipe accounts for 60%, is the largest dosage and spends the most on oil materials; its quality directly affects the oil company’s exploration and development benefits, as well as the safety and reliability of oil and gas wells and service life. In recent years, China’s oil well pipe imports have been maintained at 15%-20% of the total domestic demand, basically high technology content, high value-added high-end oil well pipe, such as high toughness and ultra-high strength oil well pipe, stainless steel and nickel-based alloy casing. Corrosion of metal pipelines seriously affects the normal operation of oilfield production; how to effectively prevent and slow down the metal failure and reduce the resulting economic losses for the oilfields is a crucial research topic. Composite pipe can effectively reduce the material cost by 30% -50% in the chemical industry, petroleum, desalination, shipbuilding industry and other fields has a wide range of application prospects. The bimetallic composite pipe consists of two different metal materials of metal pipe composition: the external pipe for the lower cost of high-strength carbon steel and the internal for the corrosion resistance performance excellent corrosion-resistant alloy.
Hastelloy G3 alloy belongs to a new type of nickel-based corrosion-resistant alloy that can be used as a composite tube attached to the inner layer to improve the corrosion resistance of the pipe. As it contains Ni, Cr, Mo and other elements of the composite strengthening effect, it can be used for stainless steel cannot be competent in the work of the bad situation. However, in the heat treatment process, the austenite crystal or grain boundaries will have carbides and other precipitation phases and the generation of precipitation phases will affect the corrosion resistance of the alloy. This study is aimed at the Hastelloy G3 alloy bimetallic composite pipe internal attached layer in the heat treatment that may occur after the decline in corrosion resistance to the P110 grade oil well pipe conventional heat treatment process as the goal, to carry out the quantitative study of the corrosion resistance of the internal attached layer, so as to provide a basis for the development of the heat treatment process of the G3/P110 composite pipe.

1. Test materials and methods

1.1 Heat treatment process
Test materials for commercial Hastelloy G3 alloy, the alloy for nickel-based corrosion-resistant alloys, the national standard grade 00Cr22Ni48Mo7Cu2-WNb (NS3403), the U.S. standard for the Hastelloy G3 (No6985), the German standard for the 2.4619. The Hastelloy G3 alloy chemical composition of the test is shown in Table 1. Hastelloy G3 alloy specimens were 850, 880, 910 ℃ heat preservation 45 min water-cooled quenching, and then were 560, 600 ℃ heat preservation 45 min air-cooled tempering heat treatment test.
Table.1 Chemical composition of Hastelloy G3 alloy (mass fraction, %)

C	S	P	Si	Mn	Cr	Mo	Cu	Nb	Fe	Ni
0.0084	0.0013	0.012	0.2	0.85	22.19	6.82	1.98	0.14	20.15	allowance

1.2 Critical Pitting Temperature (CPT) test method
In accordance with GB/T 32550-2016 “Determination of critical pitting temperature under constant potential control of corrosion of metals and alloys”, CPT determination was carried out by the constant potential method. The working electrode was immersed in 1MNaCl solution, and a constant potential of 750 mV/SCE was applied. In contrast, the solution was warmed up at a rate of (1±0.3) °C/min starting from 10 °C to test the change of corrosion current density with temperature. When the corrosion current density continued to rise to 100 μA/cm², the test was stopped after keeping the corrosion current density continued to grow for 60s. The corresponding temperature was the critical pitting temperature. The test apparatus consists of a CS electrochemical workstation, a controllable constant temperature water bath and a standard three-electrode electrolytic cell (the working electrode is the G3 test specimen, the reference electrode is the AgCl electrode, and the auxiliary electrode is platinum electrode).
1.3 Intergranular corrosion test method
In accordance with GBT29088-2012 “Corrosion of metals and alloys double-loop electrochemical kinetic potential reactivation measurement method” for nickel-based alloy G3 kinetic potential reactivation method, the working electrode was immersed in 0.5 mol/L H2SO4 + 0.01mol/L KSCN mixed solution, the solution temperature was 30 ℃, to be self-corrosive potential stabilized for 10 min, and then the working electrode was scanned from the self-corrosive potential at a scanning rate of 1.67mV /s scanning rate from the self-corrosion potential began anodic forward scanning polarization. When the scanning to the passivation potential of 400mV, the same scanning rate for reverse scanning to the self-corrosion potential.
1.4 Electrochemical Impedance Spectroscopy (EIS) Test Methods
Impedance spectroscopy tests were carried out on Hastelloy G3 alloy in 3.5% NaCl solution under different heat treatment processes. The corresponding Nyquist curves were obtained, and the corrosion resistance of the materials was evaluated accordingly by curve characterization and data fitting. The open circuit potential was stabilized for 1h before the impedance spectroscopy test was carried out and the frequency was set in the range of 0.01-10000Hz.

2. Results and analysis

2.1 CPT test results and analysis
The current density versus time curves of Hastelloy G3 alloy in different heat treatment states were tested by numbering the different heat treatment processes sequentially using the constant potential method, see Fig. 1. From Fig. 1, it can be seen that the critical pitting temperature of Hastelloy G3 alloy in the pristine state is the highest, and the critical pitting temperature of heat-treated alloy varies with the heat treatment process. Through data processing, the critical pitting temperature (CPT) of Hastelloy G3 alloy in different heat treatment states is obtained, which is shown in Table 2. From the results in Table 2, it is easy to see that when the tempering temperature is equal to 560 ℃, the critical pitting temperatures of G3-1#, G3-2#, and G3-3# are 41, 42, and 36 ℃, respectively. The quenching temperature has a certain influence on the critical pitting temperature of Hastelloy G3 alloy, especially at 960 ℃. There is a certain effect on the critical pitting temperature of Hastelloy G3 alloy, especially in the 910 ℃ critical pitting temperature when the maximum decline, compared with the original state reaches 15 ℃. Tempering temperatures of 600 ℃, G3-4 #, G3-5 # and G3-6 # critical pitting temperatures of 42, 43 and 46 ℃, respectively, three kinds of quenching temperature under the critical pitting temperature change is not obvious, 850, 880 ℃ critical pitting temperature is basically the same, the critical pitting temperature in the 910 ℃ when the critical pitting temperature there is a small increase in the quenching temperature at this time the impact of critical pitting temperature on G3 alloy is not significant. In order to carry out further comparative analysis, the results in Table 2 are plotted in Figure 2 for detailed analysis.

Fig.1 Current density-time curve for Hastelloy G3 alloy
Table.2 Critical pitting temperature of Hastelloy G3 alloy for different heat treatment states

Sample number	G3-1#	G3-2#	G3-3#	G3-4#	G3-5#	G3-6#	Primitive state
CPT/°C	41	42	36	42	43	46	51

As can be seen from Figure 2, the quenching temperature of 850, 880 ℃ quenching, 560, 600 ℃ tempering state Hastelloy G3 alloy critical pitting temperature difference is not significant, the latter is slightly higher than the former; however, when the quenching temperature of 910 ℃, the tempering temperature will lead to a large change in the critical pitting temperature, 600 ℃ tempering state of the Hastelloy G3 alloy CPT than the 560 ℃ tempering 10 ℃ higher, indicating that the pitting resistance is better at this time.

Figure.2 Critical pitting temperature comparison of Hastelloy G3 alloy in different heat treatment states
Hastelloy G3 alloy matrix organization for austenite its corrosion resistance is largely determined by the types of various types of precipitation phases, the number, size, distribution of state and the diffusion of alloying elements in the austenite grain distribution state. It can be concluded that different heat treatments will change the distribution of alloying elements, types and sizes of precipitated phases in the pristine Hastelloy G3 alloy, which in turn will cause changes in its properties, especially the corrosion resistance. This is basically proved by the fact that the critical pitting temperatures of heat-treated Hastelloy G3 alloys in this study are lower than those of the pristine state (solid solution state). An in-depth study of the microstructure of Hastelloy G3 alloys with different heat treatments will be discussed in a separate paper.
2.2 Intergranular corrosion test results of Hastelloy G3 alloy
The EPR curves of Hastelloy G3 alloy under different heat treatment states were tested, and the results are shown in Fig.3. From Fig.3, it can be seen that each curve has obvious activation-dissolution zone, activation-passivation zone, passivation zone, and re-activation zone, but the maximum activation current and the re-activation current have obvious differences. The maximum activation current Ia (maximum anodic current during forward scanning) and the reactivation current Ir (maximum anodic current during reverse scanning) were obtained from the EPR curves, and the ratio of the reactivation current to the maximum activation current Ra=Ia/Ir×100% was calculated for Hastelloy G3 alloy in each state, and the results are shown in Figure 4.

Fig.3 EPR test curves of Hastelloy G3 alloy in different heat treatment states

Fig.4 Ra value of Hastelloy G3 alloy in different heat treatment states
The EPR method is mainly used to study the sensitization behavior of the alloy. Under the action of a certain electrolyte and applied potential, a dense passivation film is formed on the surface of the alloy, while the passivation film formed on the sensitized specimen is incomplete due to the depletion of Cr at the grain boundaries. When the applied potential swept back to the reactivation zone, the incomplete passivation film will be corroded preferentially, in the polarization curve for a larger activation peak. Corrosion-resistant alloys in the passivated state, the morphology and structure of the passivation film depend largely on the content of Cr and Mo in the solid solution. As can be seen from Figure 4, when the tempering temperature of 560 ℃, Hastelloy G3 alloy 850 ℃ quenched Ra value is the largest, indicating that there is a more serious intergranular corrosion susceptibility in this heat treatment state, and in the 880 ℃ quenched Ra value is the smallest, indicating that this state has relatively good resistance to intergranular corrosion; when the tempering temperature of 600 ℃, Hastelloy G3 When the tempering temperature is 600 ℃, Hastelloy G3 alloy in 910 ℃ after quenching Ra value is the largest, indicating that in this heat treatment state also has serious intergranular corrosion sensitivity, relatively speaking, in 880 ℃ after quenching Ra value is the smallest. Comparison can be found, regardless of the tempering process, in the 880 ℃ quenching process under the Ra value is smaller, indicating that 880 ℃ is the preferred quenching temperature to improve the resistance to intergranular corrosion of Hastelloy G3 alloy. Comparatively speaking, the intergranular corrosion resistance of Hastelloy G3 alloy is better when tempered at 560 ℃ after quenching.
2.3 EIS curves of Hastelloy G3 alloy under different heat treatment processes
The impedance spectroscopy of Hastelloy G3 alloy specimens under different heat treatment processes was carried out in 3.5% NaCl solution, and the obtained Nyquist curve is shown in Fig. 5. It can be seen from Fig. 5 that the Nyquist curve characteristics of Hastelloy G3 alloy specimens in different heat treatment states are composed of individual capacitive resistance arcs. This indicates that the process of corrosion is only a function of time, and that its corrosion process is a charge transfer process, controlled by the kinetics of electrochemical reactions. In general, the corrosion resistance of an alloy can be qualitatively evaluated using the radius of curvature of the capacitive resistance arc.

Fig.5 Impedance spectrum of Hastelloy G3 alloy in 3.5% NaCl solution
Zview fitting software was used to fit the curves and the corresponding equivalent circuit diagrams were obtained and the results are shown in Fig. 6. where Rs is the solution resistance from the reference electrode to the working electrode, CPE is the constant phase angle original associated with the double layer capacitance at the interface of the working electrode and the solution, and Rct is the charge transfer resistance of the electrode. Rct is usually used to characterize the difficulty of transferring charge to the surface of the electrode by electrochemical reaction. The smaller the Rct, the higher the charge transfer rate, which means that the metal is more susceptible to corrosion, and thus the Rct size can be used to evaluate the corrosion resistance of the metal. The change of radius size and Rct parameter can characterize the change of the Cr-poor zone during sensitization treatment. The Rct parameters of Hastelloy G3 alloy in different heat-treated states were given as Fig.7 and analyzed by data processing.

Fig.6 Equivalent circuit diagram of electrochemical impedance spectrum

Fig.7 Comparison of Rct values of Hastelloy G3 alloy for different heat treatment states
From Fig.5 and 7, when the heat treatment process is 850 ℃ quenching + 560 ℃ tempering, Hastelloy G3 alloy has the largest size of the capacitive arc semicircle and the largest Rct, which is even better than that of the original state (solid solution state), which indicates that Hastelloy G3 alloy has a high corrosion resistance under this process. And when the heat treatment process for 880 ℃ quenching + 560 ℃ tempering, 910 ℃ quenching + 600 ℃ tempering, Hastelloy G3 alloy tolerance arc radius and Rct are much smaller than the original state, indicating that the charge transfer rate is higher, electrochemical corrosion kinetics of the blocking performance of the weakening of the alloy the more prone to corrosion.
After three kinds of electrochemical testing methods comprehensive analysis can be seen, when the heat treatment process for 880 ℃ quenching + 600 ℃ tempering process, Hastelloy G3 alloy critical pitting temperature is higher, in this process has a good resistance to pitting corrosion performance. From the EPR method can also be seen, this state of the alloy’s Ra is relatively small, the resistance to intergranular corrosion susceptibility performance is superior. At the same time, EIS analysis can be obtained from the alloy’s tolerance arc semicircle size and Rct are larger, Hastelloy G3 alloy at this time has good corrosion resistance. Therefore, the final determination of 880 ℃ quenching + 600 ℃ tempering process can be done as a G3/P110 composite pipe recommended heat treatment process.

3. Conclusion

• (1) 910 ℃ quenching + 600 ℃ tempering, Hastelloy G3 alloy has good pitting resistance; heat treatment process for 910 ℃ quenching + 560 ℃ tempering, the critical pitting temperature is the lowest, the worst pitting resistance.
• (2) 880 ℃ quenching + 560 ℃ tempering process under the EPR method Ra value is the smallest, Hastelloy G3 alloy shows good resistance to intergranular corrosion; 850 ℃ quenching + 560 ℃ tempering process Hastelloy G3 alloy EIS tolerance arc radius and Rct are the largest, showing good corrosion resistance.
• (3)The comprehensive corrosion resistance of Hastelloy G3 alloy is better under 880 ℃ quenching + 600 ℃ tempering process, and this process can be used as the recommended heat treatment process for G3/P110 composite pipe.

Author: Niu Jing