2.08.2.3.1 Chemical compositions, physical properties, and mechanical properties

The chemical compositions of typical nickel — molybdenum-iron, nickel-molybdenum-chromium — iron, and nickel-chromium-molybdenum-iron alloys are shown in Table 3, along with those of other nickel — based alloys.

corrosion rates.29 It is seen that the corrosion rate in 10% hydrochloric acid dramatically decreases with increasing molybdenum content. Commercial nickel-molybdenum alloys include about 30% molybdenum.

Alloy B (UNS N10001) (nickel-based 28% molybdenum-5% iron) is one of those rare materials which is resistant to corrosion in hydrochloric acid up to its boiling point. The alloy shows excellent corro­sion resistance in reducing and oxidizing chloride solutions. However, because of its lack of chromium

content, care must be taken to avoid using this alloy in oxidizing environments.

Alloy B-2 (UNS N10665) is an advanced version of Alloy B. It has superior corrosion resistance in weld-heat-affected zones compared to Alloy B, due to reduced carbon and silicon contents and a restricted range of iron content.

Alloy B-3 (UNS N10675) was developed to minimize problems associated with the fabrication of B-2 alloy components. Alloy B-3 has excellent resistance to hydrochloric acid at all concentrations and temperatures.30 It also withstands sulfuric, acetic, formic, and phosphoric acids, as well as other nonoxidizing media. Alloy B-3 has a special chemistry designed to achieve a level of thermal stability superior to that of Alloy B-2. It has been applied to similar components as Alloy B-2, but cannot be used in environments containing ferric or cupric salts because these salts may cause rapid corrosion failure.

Alloy C (UNS N10002) (nickel-based 18% chromium-16% molybdenum-5% iron-4% tungsten) is also an advanced version of Alloy B. It has superior corrosion resistance to oxidizing environments com­pared to Alloy B due to the added chromium. How­ever, Alloy C is degraded after heating in the temperature range 650-1090 °C due to the precipita­tion of M6C carbides and of m phase along grain boundaries. Solution heat treatment is therefore nec­essary after welding in the case of this alloy.

Alloy C-276 (UNS N10276) improves upon this weakness by using reduced carbon (<0.01%) and sili­con (<0.08%) contents compared to Alloy C. The alloy can be used in most cases in the as-welded state (with­out solution heat treatment after welding).31

Alloy C-4 (UNS N06455) improves upon the long-range aging characteristics of Alloy C-276 by the addition of titanium and a reduction in the iron

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content.

Alloy C-22 (same as Alloy 22) (UNS N06022) shows improved corrosion resistance in oxidizing environments due to increased chromium content (about 22%) compared to Alloy C-276 and maintains its corrosion resistance in reducing environments.33

Alloy 625 (UNS N06625) was originally devel­oped as a gas-turbine material. It is a typical nickel- chromium-molybdenum-iron alloy as well as a solid-solution-hardenable alloy. It has high creep — rupture strength at high temperatures, due to the added molybdenum and niobium, and high resistance to corrosion and pitting in oxidizing environments such as nitric acid due to its higher chromium (about 22%) and lower molybdenum (about 9%) content compared to Alloy C-276.34 However, the corrosion resistance of the alloy in reducing environments such as hydrochloric acid and sulfuric acid is inferior to that of Alloy C-276. Alloy 625 is used where welding is required, based on the stabilization of carbon by niobium addition (about 3.5%) for preventing sensi­tization. Also, the alloy shows excellent SCC resis­tance to chloride solutions and seawater, due to its high nickel content.

Alloy 625 LCF (UNS N06626), a modified Alloy 625, shows improved low-cycle fatigue properties and cold formability for bellows applications.

Alloy 686 is very similar in composition to Alloy C-276 but where the chromium level has been increased from 16 to 21% while maintaining molyb­denum and tungsten at similar levels. Alloy 686 is used for resistance to aggressive media in chemical processing, pollution control, pulp and paper manu­facture, and waste management applications. This alloy contains chromium, molybdenum, and a tung­sten content of around 41%. To maintain its single­phase austenitic structure, this alloy has to be solution-annealed at a high temperature of around 1220 °C followed by rapid cooling to prevent precipi­tation of intermetallic phases.35

Alloy 59 has high chromium and molybdenum con­tent with low iron content. This alloy has excellent resistance to general corrosion, SCC, pitting, and crev­ice corrosion in aggressive corrosive environment. The alloy is a nickel-chromium-molybdenum alloy with­out the addition of any other alloying element. This purity and balance ofnickel-chromium — molybdenum is mainly responsible for its thermal stability.36

Alloy 825 (UNS N08825) was developed from alloy 800 with the addition of molybdenum (about 3%), copper (about 2%), and titanium (about 0.9%) for providing improved aqueous corrosion resistance in a wide variety of corrosive media. In this alloy, the nickel content confers resistance to chloride-ion SCC. Nickel in conjunction with molybdenum and copper gives outstanding resistance to reducing envir­onments such as those containing sulfuric and phos­phoric acids. Molybdenum also aids resistance to pitting and crevice corrosion. In both reducing and oxidizing environments, the alloy resists general cor­rosion, pitting, crevice corrosion, IG corrosion, and SCC. Some typical applications include various com­ponents used in sulfuric acid pickling of steel and copper, components in petroleum refineries and pet­rochemical plant (tanks, valves, pumps, agitators), equipment used in the production of ammonium sulfate, pollution control equipment, oil and gas recovery, and acid production.37

The mechanical and physical properties of typical nickel—molybdenum—iron, nickel-molybdenum— chromium-iron, and nickel—chromium—molybdenum— iron alloys are shown in Tables 4 and 5 respectively, along with those of other nickel-based alloys.

2.08.2.3.2 Applications to nuclear power industrial fields

Alloy 625, as a typical nickel-chromium— molybdenum—iron alloy, has been investigated for its SCC resistance in high-temperature water as an alternative material to austenitic stainless steels, from the view point of preventing sensitization. The alloy has also been studied for corrosion resistance in highly caustic solutions as a candidate material for components of supercritical light water-cooled reac­tors. Alloy 625 is one of the candidates for reactor — core and control-rod components in water-cooled reactors and a candidate component material for supercritical water-cooled reactors, due to its high strength, excellent general corrosion resistance, SCC resistance, and pitting resistance in high-temperature water. The alloy is also being considered in advanced high-temperature reactors because of its high allow­able design stress at elevated temperatures, especially between 650 and 760 °C.

Alloy C-22 has been investigated for corrosion resistance in highly caustic solutions and concentrated chloride solutions as a candidate material for high — level radioactive waste-disposal storage containers, due to its excellent corrosion resistance in oxidizing and reducing environments.