Which Alloys prevent environmental degradation?

Nickel based alloys containing high nickel content have a wide range of applications. The electrochemical properties of nickel and its crystallographic characteristics enable it to accommodate large magnitudes of alloying elements. The alloys utilized for corrosion resistant purposes are primarily solid solution strengthened alloys supplied in the fully annealed form.

Nickel is nobler than iron however more active than copper in electromotive series. In the reducing conditions for example dilute sulfuric acid, nickel offers superior resistance than iron however is less resistant than copper or NiCu alloys. The nickel-molybdenum alloys offer superior resistance in reducing acid media as compare to nickel or nickel-copper alloys.

Nickel can develop a security passive layer in some conditions. Although this passive layer is not specifically stable and hence nickel cannot be usually in oxidizing conditions like nitric acid. When alloyed with chromium it develops a more stable layer and corrosion resistance is attained to a variety of oxidizing conditions. Although these alloys can corrode in environments comprising of significant magnitudes of chlorides or other halides, particularly if oxidizing species are present. Combination with molybdenum or tungsten is essential to enhance its corrosion resistance by oxidizing chloride solutions.

Monel alloy 400

Alloys containing 33% copper with nickel produces Monel 400, offers an alloy with several characteristics of pure nickel with enhancements. A major region of application is in water handling, including brackish water and marine. As similar to nickel 200, alloy 400 pits in stagnant seawater however the corrosion rate is significantly decreased. The unavailability of chloride stress corrosion cracking is also a factor in choosing this alloy.

The general corrosion resistance offeredby Monel 400 wire in nonoxidizing acids like sulfuric, hydrochloric and phosphoric is better than pure nickel. This alloy cannot withstand oxidizing media for example nitric acid, ferric chloride, chromic acid, wet chlorine, sulfur dioxide or ammonia.

Alloy 400 attains outstanding resistance to hydrofluoric acid solutions at different concentrations and temperatures. It is exposed to SCC in moist, aerated hydrofluoric or hydrofluorosilicic acids and vapor. Monel corrosion is nominal in all kinds of environments. Indoor exposure develops an extremely light tarnish that is easily eradicated by occasional wiping. Outer surfaces subjected to rain develop a thin gray green patina. In sulfurous conditions, a smooth brown adherent film develops.

As high nickel concentration of Monel 400 is resistant as nickel 200 to caustic soda throughout the concentration range. It also resists anhydrous ammonia and to ammonium hydroxide solutions of about 3% concentration. It is subjected to stress corrosion cracking at high temperatures, in concentrated caustic and in mercury.

Incoloy alloy 800

Alloy 800 contains about 20% chromium, 32% nickel and 46% iron as balance. It is primarily used for its oxidation resistance at high temperatures. At average temperatures the general corrosion resistance of Incoloy 800 is similar to that of other austenitic nickel-iron-chromium alloys. Although with increase in temperature, alloy 800 continues to attain supreme corrosion resistance whereas other austenitic alloys are unsatisfactory for this service. It prevents corrosion in nitric acid at concentrations up to 70%. It prevents corrosion in variety of oxidizing salts, however not halide salts.