Austenitic nickel cast iron, corrosion

Second, better inherent corrosion resistant can also be used to increase the erosion-corrosion resistance of cast irons. Austenitic nickel cast irons can have hardness similar to unalloyed cast irons but may exhibit better erosion resistance because of the improved inherent resistance of nickel alloyed irons compared to unalloyed irons. 

Nickel—Iron. A large amount of nickel is used in alloy and stainless steels and in cast irons. Nickel is added to ferritic alloy steels to increase the hardenabihty and to modify ferrite and cementite properties and morphologies, and thus to improve the strength, toughness, and ductihty of the steel. In austenitic stainless steels, the nickel content is 7—35 wt %. Its primary roles are to stabilize the ductile austenite stmcture and to provide, in conjunction with chromium, good corrosion resistance. Nickel is added to cast irons to improve strength and toughness. 

Austenitic cast irons (either flake graphite irons or nodular graphite irons) are produced by mixing in nickel from 13-30%, chromium from 1-5% and copper from 0.5-7.5 (to lower nickel-containing grades to augment the corrosion resistance at lower cost). 

The addition of about 20% nickel to cast iron produces materials with a stable austenitic structure these materials are sometimes known as austenitic cast irons but are more often referred to commercially as Ni-Resist cast irons. The austenitic matrix of these irons gives rise to very different mechanical and physical properties to those obtained with the nickel-free grey cast irons. The austenitic matrix is more noble than the matrix of unalloyed grey irons and it was shown in the early work of Vanick and Merica that the corrosion resistance of cast iron increases with increasing nickel content up to about 20% (Fig. 3.42). 

The austenitic cast irons show better corrosion resistance than the ferritic irons primarily due to the nickel content of the austenitic matrix. 

Potential-current density (E-i) curves, which have been determined for a number of the austenitic cast irons and also for the nickel-free ferritic irons, indicate that in general the austenitic cast irons show more favourable corrosion characteristics than the ferritic irons in both the active and passive states. 

Austenitic cast irons show particularly good corrosion resistance in alkaline environments, even better than that shown by low alloy cast irons. The resistance to corrosion improves with increasing nickel content (Fig. 3.51),... 

ASTM A 744/A 744M-00(2004) Standard Specification for Castings, Iron-Chromium-Nickel, Corrosion Resistant, for Severe Service (contains some of the austenitic corrosion grades)... 

The austenitic cast iron alloys with 13-35% nickel and 1-5% chromium, also known under the commercial name Ni-Resist [93], show, due to their austenitic structure, much better corrosion behaviour in stagnant and flowing seawater than unalloyed castings. The corrosion resistance increases markedly with increasing chromium content. The carbon may be present either as graphite in lamellar form... 

Some austenitic cast iron types may show a level of sensitivity to stress corrosion cracking in seawater and salt solutions at higher temperatures [96]. At high nickel contents of around 35%, pitting corrosion increases [97]. 

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