Corrosion resistance of copper nickel alloy used in chemical plant

Stainless steel and nickel alloys are widely used in chemical plant equipment, which are usually in contact with highly corrosive solutions such as sulfuric acid and hydrochloric acid). Although Mo and Ni alloys are expensive and increase the cost of raw materials, they are beneficial to improve the corrosion resistance of materials. Therefore, nickel alloys with high Mo content, such as UNS N10276 or N06022, have been successfully applied. The researchers found that relatively cheap copper can also improve the corrosion resistance of the material.

According to the different corrosivity of using environment, improper selection of nickel alloy will lead to poor cost-effectiveness. Some research reports suggest that copper, which is cheaper than molybdenum and nickel, can also improve the corrosion resistance of stainless steel and other alloys. For example, the corrosion performance of stainless steel in sulfuric acid solution is improved by forming a copper protective layer on the surface of the material.
This paper focuses on the study of Cu as an alternative element of Mo in nickel alloy, and evaluates the effect of Cu and Mo content on the corrosion resistance of 46ni-23cr-4w-fe alloy in reducing acid.

According to the results of laboratory corrosion test, a new type of nickel alloy UNS N06845 with 3% Cu and 6% Mo, which has the same corrosion resistance as N10276 and N06022, was designed and developed by optimizing the content of Cu and mo. The corrosion resistance of N06845 was compared with that of N10276 and N06022 under different corrosion environment by laboratory test and field test in industrial sulfuric acid plant.

Laboratory corrosion test

In order to evaluate the effect of copper on corrosion resistance, corrosion tests were carried out on 46ni-23cr-4w-fe alloy with different copper content. In order to ensure that the alloy has high corrosion resistance to oxidizing acid and strong local corrosion resistance, Cr and W are added to the alloy. For comparison with conventional nickel alloys, the chemical composition of the test material is given in Table 1.
Table.1 Chemical composition of alloy for laboratory test
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The ingot used in this study is smelted in a 180 kg vacuum induction furnace, and then hot forged and hot rolled into a 20 mm thick plate. The hot-rolled plate was annealed at 1150 ℃ for 10 minutes, then quenched and cooled. The hot-rolled plate was cold rolled to obtain 12 mm thick plate, and then solution treatment was carried out at 1150 ℃. Cut the sample (3mm thick × 10 mm wide × 40 mm long) from the thin plate, grind the surface of the sample mechanically with No. 400 sandpaper, then carry out ultrasonic cleaning in acetone solution, and weigh the sample after cleaning. The corrosion test was carried out in 3% HCl solution at 60 ℃ for 6 hours, or in 20% H2SO4 solution at 80 ℃ for 24 hours. After the immersion test, the sample is cleaned by ultrasonic in acetone solution to remove the corrosion products on the surface of the sample, and then the corrosion rate is calculated by weighing the sample.
Based on the above corrosion test results, a new nickel alloy with Cu 3% and Mo 6% registered as no6845 was developed. In order to compare with the corrosion resistance of existing nickel alloy, the corrosion resistance of no6845 alloy was tested under different corrosion test conditions. The corrosion tests carried out include: 3% HCl solution at 60 ℃, soaking for 6 hours; 20% H2SO4 solution at 80 ℃, soaking for 24 hours; 40% HNO3 boiling solution, soaking for 24 hours; three different types of mixed acid solutions (0.1% HCl + 0.5% H2SO4; 2% HCl + 10%) H2SO4;2%HCl+10%HNO3)。

Alloy design based on laboratory test

Figure 1A shows the average corrosion rate of 46ni-23cr-0mo-4w-fe alloy at different Cu contents. When the content of Cu increases to 3%, the corrosion resistance of the alloy to hydrochloric acid and sulfuric acid solution is improved. When the content of Cu is more than 3%, the effect is saturated. According to these experimental results, the content of Cu is determined to be 3%.
Figure 1b shows the average corrosion rate of 46ni-23cr-3cu-4w-fe alloy at different Mo contents. The corrosion resistance of the alloy to sulfuric acid increases with the increase of Mo content, while when 3% Mo is added to the alloy, the corrosion rate is the highest, and when 6% Mo is added to the alloy, the corrosion resistance is the best, and the corresponding corrosion rate is 0.04mm/year. The reason for the high corrosion rate of the alloy with 3% Mo in hydrochloric acid will be discussed in the future.
Fig.1 A and 1b effect of chemical composition on corrosion resistance
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According to the above results, the chemical composition of the new nickel alloy UNS N06845 is determined to be 46ni-23cr-3cu-6mo-4w-fe. As shown in Table 2, N06845 has been listed in ASTM B163, B423, B424, B425 and ASME code case 2794.
Table.2 Chemical composition requirements of developed alloy N06845
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Figure 2 shows the average corrosion rate of the alloy in H2SO4, HCl and HNO3 solutions. N06845 has the same corrosion resistance as N10276 and n06022 in oxidizing acid solution (HNO3) and reducing acid solution (HCl, H2SO4).

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Fig.2 Corrosion test results of different acid solutions in laboratory

Fig.3 shows the corrosion resistance evaluation results of N06845 in mixed acid solution. The corrosion resistance of n06845 in mixed acid solution is due to other nickel alloys.

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Fig.3 Test results of mixed acid corrosion in laboratory

Surface analysis

In order to study the characteristics of passivation film on alloy surface in H2SO4 solution, X-ray photoelectron spectroscopy (XPS) was used to analyze the alloy surface. In XPS analysis, the surface of the sample was detected by using the monochromatic alka x-ray (h =1486.6ev) as the excitation source. The diameter of the sampling area was 0.2mm, and the sampling depth was 30nm below the surface of the sample. The high-resolution spectrum used in statement analysis is recorded with the energy of 23.50ev. The surface of the sample is analyzed at a sweep angle of 45 degrees. The depth distribution of Cu, Ni, Mo, Cr and Fe was studied by spraying the sample surface with Ar ion gun. Considering that the injection rate of SiO2 conversion is set at 0.8nm/min.

Effect of Cu on improving corrosion resistance

Figure 4 shows the concentration distribution of the alloy with copper content of 0.1% and 3% respectively after the corrosion test in H2SO4 solution.
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Fig.4 XPS surface analysis results after soaking in 20% H2SO4 solution at 80 ℃ for 24 hours

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Figure.5  It is a partial enlarged figure of Figure.4

The concentration of Cr oxide reached the maximum value in the 4 nm area below the surface of the two alloy samples, while Cu was concentrated under the Cr oxide. It is believed that this Cr oxide was formed by air oxidation after the corrosion test, because H2SO4 is a reducing acid. That is to say, in H2SO4 solution, Cu is enriched below the surface of the sample. This shows that the Cu dissolved in the base metal gathers around the surface in the initial reaction, while the deposition of Cu on the surface of the material inhibits the corrosion reaction.
These experimental results show that as an alternative element of Mo, adding Cu is the best way to improve the acid corrosion resistance of the alloy.

Field test of chemical plant

Under the laboratory corrosion test conditions, the sulfuric acid corrosion resistance of n06845 alloy is better than that of existing nickel alloy (N06022 and N10276), which proves that the alloy is suitable for industrial manufacturers. N06845 alloy has been tested for one year in industrial sulfuric acid plant. Under the same test conditions, the corrosion properties of N06022 and N10276 alloys were also tested.
Table 3 shows the chemical composition of the tested alloy. The 3.5t N06845 alloy steel ingot is smelted in vacuum induction furnace. After the ingot is hot forged, the outer diameter of the hot extrusion is 204mm, and the wall thickness is 9.5mm seamless pipe. The seamless tube was annealed at 1100 ℃, then cold rolled to 168.5mm outer diameter and 6.95mm wall thickness seamless tube, and then solution treated at 1100 ℃. Two specimens were cut from the tube. The specimen size was 6mm thick × 15mm wide × 50mm long. Then, the specimens were butt welded by TIG welding method. The filler metal used for welding was AWS ernicrmo-10.
Table.3 Nominal chemical composition (mass%) of alloy used in field test

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The samples of alloy N06022 and N10276 are taken from the existing industrial plates. The samples are butt welded by tungsten inert gas welding method. The filler metals used are AWS ernicrmo-10 and AWS ernicrmo-4, respectively. All specimens were mechanically ground with 600 sandpaper and then cleaned with ultrasonic in acetone solution.
The sample is placed at the outlet of H2SO4 tank containing a small amount of NH4SO4 for one year. After the exposure test, optical microscope was used to analyze the cross section of the sample surface.
Figure 6 shows the appearance and cross-sectional morphology of the sample after the exposure test. The corrosion depth of N06022 weld metal area is 50 mm, the total corrosion depth of N10276 heat affected area is 70 mm, while the intergranular corrosion depth of N06845 heat affected area is only 15 mm.
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Figure.6 Appearance of weld after field test in sulfuric acid tank (left) microstructure (right)

It is believed that N06022 and N10276 are corroded by oxides such as NH4SO4 under test conditions, because the Cr content of the two alloys is lower than that of N06845. N06845 has the same or better corrosion resistance as N06022 and N10276 in industrial application, which shows that N06845 with low content of Cu and Cr is an economic and practical alloy material in this application case.

Conclusion

  • 1. The effect of Cu and Mo contents on the corrosion resistance of 46ni-23cr-4w-fe alloy in reducing acid environment was studied. The results show that both Cu and Mo can improve the corrosion resistance of the alloy.
  • 2. As Cu is concentrated below the surface of the corrosion sample, it can be considered that the deposition of Cu on the alloy surface inhibits the corrosion reaction.
  • 3. Based on the optimized design of Cu and Mo contents, a new type of nickel alloy containing copper was put forward.
  • 4. Under the laboratory test conditions, n06845 has the same corrosion resistance to reducing acid as n06022 and N10276, and the corrosion resistance to oxidizing acid is better than the latter two.
  • 5. N06845 shows excellent corrosion resistance in all kinds of strong corrosion environments even if the content of Ni and Mo is reduced. N06845 is expected to become the most economical alloy material in the field of chemical plant application.
  • 6. N06845 has been proved to have excellent corrosion resistance by various industrial manufacturers and laboratory tests.

Source: China Alloy Steel Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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From: the 80th issue of stainless steel branch of China Special Steel Enterprise Association

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corrosion resistance of copper nickel alloy used in chemical plant - Corrosion resistance of copper nickel alloy used in chemical plant
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Corrosion resistance of copper nickel alloy used in chemical plant
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In order to evaluate the effect of copper on corrosion resistance, corrosion tests were carried out on 46ni-23cr-4w-fe alloy with different copper content.
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