2.8, 0.6

The two main alloys in this system are Monel 400 or alloy 400 and its age-hardenable version, alloy K-500. Alloy 400 was developed at the beginning of the twentieth century and, even after approximately 100 years, continues to be used in the modern-day chemical, petrochemical, marine, refineries, and many other industries. Alloy 400 containing about 30-33% copper in a nickel matrix has many similar characteristics of commercially pure nickel, while improving upon many others. Addition of some iron significantly improves the resistance to cavitation and erosion in condenser tube applications. The main uses of alloy 400 are under conditions of high flow velocity and erosion as in propeller shafts, propellers, pump-impeller blades, casings, condenser tubes, and heat exchanger tubes. Corrosion rate in moving seawater is generally less than 0.025 mm/year. The alloy can pit in stagnant seawater, however, the rate of attack is considerably less than in commercially pure alloy 200. Due to its high nickel content (approx. 65%) the alloy is generally immune to chloride stress corrosion cracking.

The general corrosion resistance of alloy 400 in nonoxidizing mineral acids is better compared to nickel. However, it suffers from the same weakness of exhibiting very poor corrosion resistance to oxidizing media such as nitric acid, ferric chloride, cupric chloride, wet chlorine, chromic acid, sulfur dioxide, or ammonia.

In unaerated dilute hydrochloric and sulfuric acid solution the alloy has useful resistance up to concentrations of 15% at room temperature and up to 2% at somewhat higher temperature, not exceeding 50°C. Due to this specific characteristic, alloy 400 is also used in processes where chlorinated solvents may form hydrochloric acid due to hydrolysis, which would cause failure in standard stainless steel.

Alloy 400 possesses good corrosion resistance at ambient temperatures to all HF concentration in the absence of air. Aerated solutions and higher temperature increase the corrosion rate. The alloy is susceptible to stress corrosion cracking in moist aerated hydrofluoric or hydrofluorosilic acid vapor. This can be minimized by deaeration of the environments or by stress relieving anneal of the component in question.

Neutral and alkaline salt solutions such as chloride, carbonates, sulfates and acetates have only minor effect even at high concentrations and temperatures up to boiling. Hence the alloy has found wide use in plants for crystallization of salts from saturated brine.

Alloy K-500, the age-hardenable alloy, which contains aluminum and titanium, combines the excellent corrosion resistance features of alloy 400 with the added benefits of increased strength, hardens, and maintaining its strength up to 600°C. The alloy has low magnetic permeability and is nonmagnetic to —134°C. Some of the typical applications of alloy K-500 are for pumpshafts, impellers, medical blades and scrapers, oil well drill collars, and other completion tools, electronic components, springs and valve trains. This alloy is primarily used in marine and oil and gas industrial applications. In contrast alloy 400 is more versatile, finding many uses in roofs, gutters, and architectural parts on a number of institutional buildings, tubes of boiler feedwater heaters, seawater applications

(sheathing, others), HF alkylation process, production and handling of HF acid, and in refining of uranium, distillation, condensation units, and overhead condenser pipes in refineries and petrochemical industries, and many others.

7.4.3 Ni-Fe Alloys






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