Research progress of nickel-based corrosion-resistant alloys
Nickel-based alloys not only have unique corrosion resistance and even high-temperature corrosion resistance in many industrial corrosion environments, but also have high strength, good plasticity and toughness, and can be smelted, cast, cold and hot deformed, processed and welded. They are widely used in petrochemical, energy, marine, aerospace and other fields.
1 Composition and classification
Nickel is completely stable in alkaline solutions and has excellent corrosion resistance to active gases (such as fluorine, chlorine, bromine and their hydrides), hydroxides (such as NaOH, KOH), and some organic matter. .
Studies have shown that the effect of alloying elements on improving the corrosion resistance of alloys comes from the stability of the passivation film formed on the surface of nickel alloys. The nickel content in nickel-based alloys is above 30%, among which W (Ni+Fe) ≥50% is called iron-nickel-based corrosion-resistant alloys, and W (Ni) ≥50% is called nickel-based corrosion-resistant alloys. In actual use, they can all be considered nickel-based corrosion-resistant alloys.
2 Performance and Application
2.1 Ni-Cu System
Ni and Cu can be mixed in any proportion to form a solid solution alloy, called Monel alloy. Ni-Cu has good resistance to HF, seawater and crevice corrosion. Monel alloy Ni70Cu30 is the earliest nickel-based corrosion-resistant alloy. It has both high strength and toughness, and excellent resistance to corrosion by reducing acids, strong alkali media and seawater. It is usually used to manufacture equipment for transporting hydrofluoric acid (HF), brine, neutral media, alkali salts and reducing acid media.
Monel alloys are divided into two categories: cast alloys and deformed alloys (rolled products). The most commonly used Monel alloys are Monel 400 and Monel K500
Monel alloys are widely used in corrosion-resistant valves, naval ships, etc. abroad. For example, the bolts and nuts used in the North Sea submarine oil pipeline and the Thames Bridge in the UK are all made of Monel 400 and Monel K500; the key shafts and pump parts of the propellers of the British Class I ships are all made of Monel alloy; many equipment and systems of the French "Charles de Gaulle" aircraft carrier are made of Monel materials. Even some roofs of buildings in the United States use Monel alloy, such as Pennsylvania Station, Brooklyn Museum of Art and Science, National Gallery of Art, Pentagon, etc.
2.2 Ni-Cr, Ni-Fe-Cr system
In the Ni-Cr-Fe system, the alloy with lower Fe content is Inconel alloy, and the alloy with higher Fe content is Incoloy alloy. Incoloy alloy is actually Fe-Ni-Cr alloy in nickel-based alloy because of its high Fe content of 45%. The casting alloy of Inconel alloy is CY240, and the rolled material grade is Ineonel 600. The rolled material grade of Ineoloy alloy is Incoloy 800 alloy. Among the current corrosion-resistant nickel-based alloys, their application in valve production is second only to Monel alloy.
Typical Ni-Cr corrosion-resistant alloys include Inconel 600 (domestic grade 0Cr-15Ni75Fe). It is both resistant to high temperatures and corrosion. Inconel 690 (0Cr30Ni60Fe) has improved stress corrosion and intergranular corrosion resistance and is widely used in the chemical industry.
2.3 HasteIloy series high corrosion-resistant alloys
The ultra-low carbon Ni-Cr-Mo series with Cr and Mo as the main alloying elements is also called Hastelloy series (Hastelloy) alloy. Hastelloy is a trademark of Haynes International, USA, composed of HA in HAYNES, STELL in STEI.LITE, and OY in ALLOYS. As an advanced nickel-based alloy, Hastelloy has excellent corrosion resistance to a variety of harsh corrosive environments, such as wet oxygen, sulfurous acid, and strong oxidizing salt media.
As shown in Table 5, Hastelloy alloys mainly include A, B, C, D, F, G, N, W, and X series. The major international companies producing Hastelloy alloys include Haynes International Inc. of the United States, Special Metals (Super Alloy Group) and ThyssenKrupp of Germany.
The Hastelloy alloys used in chemical plants are mainly B, C, and G. The mechanical properties of these three types of Hastelloy alloys are shown in Table 6.
(1) Hastelloy B
The B series includes B, 132, and B3. The corrosion resistance of this type of Hastelloy alloy under typical conditions is shown in Table 7. Hastelloy B alloy has the best hydrochloric acid corrosion resistance. Reducing the carbon content of Hastelloy B alloy to 0.02% and the Si content to 0.1% will result in Hastelloy B-2 alloy. It can be used in hydrochloric acid media of any concentration at boiling temperature, and improves the intergranular corrosion resistance in the sensitized state and the post-weld state.
In the 1990s, the invention of Hastelloy B-3 completely changed the disadvantage of B-2 that Ni-Mo precipitation phase is easy to precipitate when passing through the intermediate temperature, and improved the processing performance. However, Hastelloy B and Hastelloy B-2 alloys contain very low Cr and cannot be used in oxidizing environments.
(2) Hastelloy C
Hastelloy C is a Ni-Cr-Mo system and is a general-purpose corrosion-resistant material that can adapt to various environments. The C series has a high degree of alloying. The Cr element and the Mo element play the role of resisting corrosion in oxidizing media and reducing media respectively, and together play the role of resisting local corrosion (pitting corrosion and crevice corrosion).
The C series includes C, C-276, C-4, C-22, and C-2000. The first alloy of the C series, Hastelloy C, was produced in the 1930s. In the 1960s, there was C-276; in the 1970s, there was C-4; in the 1980s, there was C-22; in the 1990s, there were alloys 59, 686, and C-2000.
Electrochemical methods were used abroad to study the intergranular corrosion, localized corrosion, stress corrosion, and passivation behavior of Ni-Cr-Mo alloys. S Ghosh's research showed that in 95°C NaCl and H3PO4 solutions, C-276 had better corrosion resistance than UNS N08367 and UNS N08028 alloys.
(3) Hastelloy G
Hastelloy G is a Ni-Cr-Cu-Mo system. The Cu-containing Ni_Cr_M0-Cu alloy is more resistant to corrosion by sulfuric acid, phosphoric acid, and other media than the Ni-Cr-Mo alloy. Ni-Cr-Mo-Cu alloys with high Cr content have good corrosion resistance in media such as sulfuric acid, phosphoric acid, sulfuric acid containing F-, Cl-, phosphoric acid and wet phosphoric acid. Adding Si and B to Ni-Cr-Mo-Cu alloys can resist hot concentrated sulfuric acid corrosion and have the characteristics of high hardness and good wear resistance.
The G series includes G, G-3, G-30, G-50, etc. The corrosion resistance of G alloys in typical environments is shown in Table 9. The advent of Hastelloy G-3 and G-30 alloys solves the application in strong oxidizing or mixed acid environments. The new Hastelloy G-35 is essentially a Ni-Cr-Mo alloy with a composition close to that of the C series alloy.
Because its invention is still designed for strong oxidizing and mixed acid environments, the commercial brand is still classified as the G series.
