Ni-Cr-Mo-Cu corrosion resistant alloy

Nickel-chromium-molybdenum-copper corrosion-resistant alloys [1-7] were developed by adding Cu to nickel-chromium-molybdenum alloys, mainly to improve their corrosion resistance in non-oxidizing acids, especially in phosphoric acid and sulfuric acid. Since the alloy contains both a high amount of Cr and Mo and Cu, the alloy is resistant to corrosion by reducing acids other than hydrochloric acid and hydrofluoric acid and oxidizing and reducing mixed acid. The chemical composition of the commonly used nickel-chromium-molybdenum-copper corrosion-resistant alloy is listed in Table 9-1.

Table 9-1 Chemical composition of common nickel-chromium-molybdenum-copper corrosion-resistant alloy

9.1 Effect of Cu on Corrosion Resistance of Nickel-Chromium-Molybdenum Alloy
Although many copper-containing nickel-chromium-molybdenum corrosion-resistant alloys have been developed, there are not many literatures on systematically studying the effect of Cu on the corrosion properties of nickel-chromium-molybdenum alloys. To Cr-Ni-Mo stainless steel and Fe-Ni-Cr-Mo (Ni mass fraction about 35%) iron-nickel based corrosion-resistant alloy, adding 1%~3% Cu, which is resistant to sulfuric acid, phosphoric acid, etc. The effect is beneficial. Table 9-2 lists the effect of 2% Cu on the corrosion resistance of Ni-Fe-Cr-Mo alloy with a Ni content of about 45%. Figure 9-1 and Figure 9-2 show the results obtained in the domestic study of Cii (mass fraction 1%~10%) for the corrosion behavior of 75Ni-15Cr-2Mo-lTi alloy in dilute hydrochloric acid and high temperature HF gas. Obviously, the mass fraction of 1% ~ 3% Cu is beneficial to the alloy’s resistance to dilute HC1 acid, while Cu has no significant effect on the alloy’s resistance to HF gas corrosion. Among the boiling HF acids, Cu has a significant benefit to the corrosion resistance of Ni-Cr-Mo alloys (Fig. 8-9).

Table 9-2 Effect of Cu on Corrosion Resistance of Ni-Fe-Cr-Mo Corrosion Resistant Alloy

9.2 Microstructure, Properties and Applications of Several Nickel-Chromium-Molybdenum-Copper Corrosion Resistant Alloys
In order to meet the corrosion resistance of H2so4, H3PO4 and HNO3 and H2SO4, H3PO4 or their mixed acid, nickel-chromium-molybdenum-copper alloy Cr21Ni60Mo4.5Cu6.5W2 was developed as early as 1915. On the basis of this alloy, some nickel-chromium-molybdenum-copper corrosion-resistant alloys have appeared in the past decades. The latest development of Ni-Cr-Mo-Cu alloys in the late 20th century is introduced in Chapter 8. ), the main alloys with wider application are shown in Table 9-1. 0Cr28Ni55Mo8. 5Cu5. 5 alloy is produced to solve the 98% thermal H2S04 corrosion. In order to improve the corrosion resistance of 0Cr28Ni55Mo8.5Cu5.5 alloy, the 0Cr28Ni50Mo8.5Cii5.5Si4B alloy which is added with Si and B can achieve this by aging heat treatment. Of the several grades listed in Table 9-1, the 0O21Ni68M05Cii3 alloy is the only deformed alloy. Due to the high content of Cr and Mo, other grades are difficult to achieve thermal deformation by thermal processing, so they are often used as castings.
9. 2.1 Chemical composition and structure of several alloys
The four nickel-chromium-molybdenum-copper alloys listed in Table 9-1 are the earliest developed and relatively widely used alloys. The solid solution treatment state of these alloys is generally a single-phase austenite structure, but the 0Cr50Ni28Mo8.5Cu5.5Si4B alloy has a complex Si content, whether it is a solid solution state or an aging state, because it contains a relatively high amount of Si and B. The precipitated phase of B precipitated. The presence of these phases hardens the 0Cr50Ni28Mo8.5Cu5.5Si4B alloy and improves its wear resistance and abrasion resistance.
9. 2.2 Mechanical properties
Table 9-3 shows the room temperature mechanical properties of several Ni-Cr-Mo-Cu corrosion resistant alloys.

Table 9-3 Mechanical properties of several nickel-chromium-molybdenum-copper alloys at room temperature

9.2.3 Corrosion resistance in various media 9. 2. 3.1 Seawater
In seawater, the four Ni-Cr-Mo-Cu corrosion-resistant alloys given in Table 9-1 generally have good seawater corrosion resistance, and the flow rate of seawater has no significant effect on them. For example, alloys still have some pitting resistance due to low flow rates, fouling of the alloy surface, or microbial adhesion. The experiment shows that the corrosion rate of 0Cr22Ni56Mo6.5Cu6.5 alloy is 0. 0075mm/a, 0Cr21Ni68Mo5Cu3 alloy is 0. 00125mm/a, and no corrosion occurs in both alloys.
9.2.3.2 Sulfuric acid
In sulfuric acid, dilute H2S04 is essentially reductive in the absence of air and oxidants; at 80% (room temperature), 40% more (boiling) and about 60% (60 to 95 °C). Under the same concentration and temperature conditions, the essence of H2S04 is oxidizing. The Ni-Cr-Mo-Cu alloys described in this section have nearly the same corrosion resistance in reducing H2S04; and the difference in H2S04 resistance between them is mainly in hot and concentrated sulfuric acid. The temperature is higher than 65 ft and the concentration is 70% to 85%. The ratio is 504, which is the most corrosive to the corrosion resistant alloy. Therefore, alloys resistant to this concentration of &H2S04 are few. However, the concentration is higher, and the corrosive property is weakened because the ionization tendency of the acid is lowered. High Ni-Cr-Mo-Cu corrosion resistant alloy, due to its chemical composition, can not only withstand different temperatures and concentrations of H2S04, but also can be used under oxidation, reduction and alternating conditions.

0Cr22Ni56Mo6. 5Cu6. 5 alloy is resistant to 60 °C: all the following concentrations of H2SO4 corrosion; except for 65% ~ 85% H2SO4, this alloy can be used to resist corrosion of other concentrations of H2SO4 at about 90 °C; in boiling H2SO4, This alloy is limited to use at <40% concentration. The Cr content and Mo content in the alloy are correspondingly increased, which expands the use range of the alloy, and has satisfactory corrosion resistance in all concentrations of sulfuric acid at <80 ° C, which is significantly better than UliumG alloy (Fig. 9-5, Fig. 9). -6), to make up for the lack of corrosion resistance of the IllinmG alloy in 65%~85% H2SO4.

The isocorrosion diagrams of three cast Ni-Cr-Mo-Cu corrosion resistant alloys in H2SO4 are shown in Figures 9-7 to 9-9. With the increase of Cr and Mo content in the alloy, the use temperature and the concentration range of the alloy in H2SO4 are expanded. However, the addition of Si only increases the use temperature in the concentration of >65% H2SO4 in the concentration <65% H2S04. Corrosion resistance decreases (Figure 9-10). In 100^H2SO4, with 0.51mm/a as the criterion, the boundary of the three alloys is shown in Figure 9-11. The corrosion resistance of the three alloys in 96% to 98% H2SO4 is shown in Figure 9-12. Illium B alloy has the best corrosion resistance, however, in this medium, high Si not pound steel and alloy 33 have better performance and are not competitive.

9.2.3.3 Hydrofluoric acid
Among hydrofluoric acid, the nickel-chromium-molybdenum-copper corrosion-resistant alloy is resistant to corrosion of many non-air-filled anhydrous hydrogen fluorides below 60 SC. However, once the temperature is higher or the acid is filled with air, the corrosion rate of these alloys increases. Table 9-4 lists the test results for the 0Cr21Ni68Mo5Cu3 deformed alloy. Obviously, the filling air has a significant effect of accelerating corrosion, and the corrosion rate of the partial immersion of the sample is significantly improved. Some tests have also shown that grades with high chromium content and high molybdenum content in nickel-chromium-molybdenum-copper alloys have better resistance to hydrofluoric acid corrosion.

Table 9-4 Corrosion resistance of modified 0Cr21Ni68Mo5Cu3 alloy in HF acid

Tests in HF + H2SO4 mixed acid (25% ~ 35% H2 SO4 + 4% – 8% HF acid, 50~80 °C) show that several nickel-chromium-molybdenum-copper alloys have fairly good corrosion resistance, see Table 9-5.

Table 9-S Corrosion resistance of several nickel-chromium-molybdenum-copper alloys in H2SO4+HF mixed acid

Hydrofluorosilicic acid is generally less corrosive than hydrofluoric acid. The foregoing nickel-chromium-molybdenum-copper alloys are resistant to corrosion by such acids. Table 9-6 is some experimental results.

Table 9-6 Test results of Ni-Cr-Mo-Cu alloy in hydrofluorosilicic acid

9.2.3.4 Hydrochloric acid
In hydrochloric acid, since the metal chloride has a high solubility in the solution and the chloride ion has a large concentration, increasing the concentration, temperature, flow rate, and air volume of the hydrochloric acid accelerates the corrosion of the alloy. Nickel chromium molybdenum copper alloy can only be used in hydrochloric acid which is near room temperature and is not filled with air, except for dilute hydrochloric acid (≤ 2%). For example, 0Cr22Ni56Mo6.5Cu6.5 alloy is limited to mass fraction ≤ 15%, used in room temperature hydrochloric acid, and some test results are listed in Table 9-7.

Table 9-7 Corrosion test results of 0Cr22Ni56Mo6.5Cu6.5 alloy in hydrochloric acid

9.2.3.5 Phosphoric acid
Among the phosphoric acid, the test results of 0Cr22Ni56Mo6.5Cu6.5 alloy are shown in Table 9-8. From the results in the table, it is known that the corrosion resistance of the alloy is about 50% and about 80% of H3PO4 at the boiling temperature under the conditions tested. In the actual wet production of phosphoric acid, there are also a certain amount of sulfuric acid, hydrogenophilic acid and hydrofluorosilicic acid and metal salts in the industrial acid, and the medium is oxidizing in nature. The corrosive nature of such media often depends on its F_ content. The plant content varies with the source of the phosphate rock. If the phosphate rock contains enough silicon and reacts with free HF acid to form hydrofluorosilicic acid, the corrosiveness of the medium is reduced. Table 9-9 lists the test results when a small amount (0.8%) of HF acid is added to 55% H3PO4. Obviously, the nickel-chromium-molybdenum-copper alloy with high chromium content and Mo and Cu composite has the best corrosion resistance. Table 9-10 shows the results of corrosion tests performed on two nickel-chromium-molybdenum-copper alloys under wet H3PO4 production conditions. It can be seen from the table that under the same test conditions, when the chromium content is high (for example, 0Cr28Ni55Mo8.5CU5.5 alloy), it has better corrosion resistance.

Table 9-8 Corrosion resistance of 0Cr22Ni56Mo6. 5Cu6.5 alloy cast in H3P04

Table 9-9 Comparison of Corrosion Resistance of Some Common Alloys in 55% H3P04 + 0.8% HF Medium

Table 9-10 Corrosion test results of two Ni-Cr-Mo-Cu alloys in the production process of wet H3PO4

9.2.3.6 Sulfite
In sulfurous acid, the nickel-chromium-molybdenum-copper alloy is resistant to moisture 302 and corrosion of most concentrations and temperatures of sulfurous acid. In the corrosion resistance of relatively pure sulfurous acid, the Cu-containing alloy is not superior to the alloy containing no Cu but the same content of Cr and Mo. Of course, when a certain amount of H2S04 is present in the sulfurous acid to form a mixed acid, the Cu-containing alloy is much better than the Cu alloy, and its corrosion resistance is much better.
9.2.3.7 Nitric acid
In nitric acid, these nickel-chromium-molybdenum-copper alloys have a high resistance to HN03 corrosion due to their high chromium content. For example, 0Cr22Ni56Mo6. 5Cu6. 5 alloy can withstand the corrosion of all concentrations of HN03 below 70 °C; when the temperature of HNO3 reaches the boiling point, the alloy can withstand the corrosion of 25% HNO3. The test results of the 0Cr22Ni56Mo6.5Cu6.5 alloy are listed in Table 9-11. Due to the increase in chromium content and the resistance to HNO3, the HN03 resistance of the nickel-chromium-molybdenum-copper alloy containing a mass fraction of about 28% is better than that of 0Cr22Ni56Mo6. 5Cu6.

Table 9-11 Corrosion resistance of 0Cr22Ni56Mo6. 5Cu6.5 alloy in HN03

Since the performance of stainless steel for nitric acid resistance can well meet the needs of engineering, in order to solve the corrosion problem of HNO3, it is not necessary to use expensive nickel-chromium-molybdenum-copper alloy. However, for environments that are resistant to HNO3 and H2SO4 or H3PO4, for conditions such as resistance to HNO3, H2SO4, H3PO4, etc., for environments resistant to HNO3 media containing F- and Cr, nickel-chromium-molybdenum-copper corrosion-resistant alloys are used. Can be considered.
9.2.3.8 Organic acids
Among organic acids and organic compounds, nickel-chromium-molybdenum-copper alloys are very resistant to most organic acids and organic compounds such as anhydrides, wakes, ketones, alcohols, esters and other organic solvents. Corrosive. 0050mm/ao。 Corrosion rate of 0. 0050mm / a 0. 0050mm / a
9.2.3.9 Halogen gas
Among the halogen elements and their hydride gases, the nickel-chromium-molybdenum-copper alloy generally has good corrosion resistance in a high-temperature dry halogen gas. However, once water is condensed in chlorine, bromine, and iodine, these alloys are rapidly corroded, even at room temperature. At this time, pitting often occurs.

The treatment of the nuclear fuel zirconium shell is often carried out with HC1 gas at 400-600 ° C to react with the Zr cladding to make Zr a volatile tetrazed zirconium oxide. The manufacture of such a reactor requires the use of materials that are resistant to high temperature gases such as HC1 and ZrCl4. The test results show that the corrosion rate of 0Cr21Ni68Mo5Cu3 alloy is <0.03~0. 06mm/ao
The etch rate of the 0Cr22Ni56Mo6.5Cu6.5 alloy is 0. 25 ~ 0. 37mm / a0 in the 700% medium containing HF, nitrogen oxide and steam.
Due to the high volatility of the fluorides of chromium and molybdenum, the nickel-chromium-molybdenum-copper alloy is not resistant to corrosion in high-temperature fluorine gas. Table 9-12 lists the test results of 0Cr21Ni68Mo5Cu3 alloy in flowing fluorine gas. Obviously, this alloy can only be used in the fluorine gas of S370T.

Table 9-12 Corrosion resistance of 0Cr21Ni68Mo5Cu3 alloy in flowing fluorine

9.2.4 Thermal processing, cold working, heat treatment and welding performance
0Cr21Ni68Mo5Cu3 alloy is a deformed alloy that can be processed for forging, milking, drawing, stamping, spinning, etc. without special difficulties. The alloy has a suitable heat distortion temperature of 1065 to 1230 ° C, but allows slight thermal processing between 950 and 1065 ° C. The suitable heat treatment process of 0Cr21Ni68Mo5Cu3 alloy is 1120~1177 °C and then water-cooled. The maximum allowable cold deformation of this alloy after each heat treatment is 25%. For 0Cr21Ni68Mo5Cu3 alloy, it is most suitable to use TIG and MIG welding.
0Cr22Ni56Mo6.5Cu6.5, 0Cr28Ni56Mo8. 5Cu5. 5Si4B and 0Cr28Ni55Mo8. 5Cu5 All three alloys are cast alloys and can only produce castings. Their heat treatment process is water cooled by llSOTM heat. 0Cr28Ni50Mo8. 5Cu5. 5Si4B alloy has poor welding performance and generally needs to be preheated to about 960 °C before welding.
9.2.5 Application
0Cr21Ni68Mo5Cu3 alloy is a deformed alloy. It can supply metallurgical products such as sheet metal, pipe, bar, strip, wire and forging. It can also produce castings. 0022Ni56Mo6. 5Cu6. 5, 0Cr28Ni50Mo8. 5Cu5. 5Si4B, 0Cr28Ni55Mo8. 5Cu5. 5 three alloys can only be supplied in the form of cast products. Since these nickel-based alloys have high contents of chromium, molybdenum and copper, they are particularly resistant to corrosion by sulfuric acid and phosphoric acid (including sulfuric acid containing Cr and F-), and are suitable for the production of cast products such as pumps and valves.