Reducing nickel-based alloys

Increasingly, titanium alloys are competing with nickel-base alloys on the basis of cost, strength, and corrosion resistance. The alloy Ti-3Al-2.5 V, for example, is finding expanded use in die process industries because of its resistance to mildly reducing chloride en ironments. 

The major alloying element contributing to resistance to pitting corrosion in iron- and nickel-base alloys is chromium. The effect of chromium in reducing both the critical current density and the passivating potential of iron in 1 N H2S04 is shown by the polarization curves of... 

Nickel based alloys, such as Hastelloy, have outstanding corrosion resistance in reducing acids, mixed acids and acids at high temperatures. The principal restriction in their use is the high cost. 

Hodgekiess and Vassilion (2003) once stndied the corrosion behavior of copper-10% nickel-based alloy in NaCl solntion. They condncted the tests at 19°C and exposure time of 4 h. It was concluded that the corrosion behavior of this particnlar alloy is a function of the impingement velocity. Craddock et al. (2006) stndied the inhibitor performance of some formulated and unidentified inhibitors in sonr conditions. The inhibitors were evaluated at a dose of 50 ppm generally and at typical worst-case temperatnres in 5-h test dnration. They observed in their investigation that more rapid redaction in corrosion rate after 5 h is possible when 50-ppm formulation A was injected. This shows that the chemistry is effective in the presence of dissolved H2S, reducing the corrosion rate of brine on mild steel and maintaining it at a low level. 

Recently, incorporation of nitrogen into the surface of especially nickel-based alloys by thermal nitration was shown feasible. In this way, the contact resistances are reduced and the corrosion resistance of the materials is increased, thus meeting the targets set by the DoE [77]. Nitration of cheaper Fe-based stainless steels is being considered. 

Besides heat-treatment, the nickel content of the alloy also affects SCC behavior. Fig. 1-18 (eopson, 1959). erNi steels containing 10% nickel have the greatest susceptibility to see in boiling Mgei2 solution. With increasing nickel content See susceptibility is reduced and nickel-based alloys are resistant. Ferritic nickel-free stainless chromium steels are also highly resistant. 

The corrosion behavior of non-ferrous alloys such as those based on nickel, cobalt, copper, zirconium, and titanium has been reviewed in detail in this chapter. Besides exotic materials such as tantalum and platinum, nickel-based alloys are the most resistant to corrosion by mineral acids, and they are especially resistant to localized corrosion in chloride-containing environments, which troubles stainless steels. Nickel-based alloys can broadly be divided into alloys, e.g. Ni-Mo (B-2, B-3) and Ni-Cu (alloy 400), that do not contain chromium, and are not, therefore, passivated under oxidizing conditions, and alloys, e.g. Ni-Cr-Mo (C-22, C-2000,59,686, etc.) and Ni-Cr-Fe (G-30, 825, etc.), that form a chromium oxide passive film under oxidizing conditions. Ni-Mo alloys such as B-3 have excellent corrosion resistance in hot reducing acids such as hydrochloric and sulfuric. Ni-Mo alloys cannot withstand oxidizing conditions such as nitric acid and hydrochloric acid contaminated with ferric ions. Ni-Cr-Mo alloys such as C-2000 alloy are multipurpose alloys that can be used both in reducing and oxidizing conditions. 

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