Corrosion of Nickel-Bases Alloys

Nickel-base alloys under certain conditions of composition, thermal history, and environment are susceptible to intergranular corrosion. 

Because of the broad variation in composition and response to thermal treatment of the nickel-base alloys, it is not possible to generalize mechanisms responsible for developing susceptibility to intergranular corrosion. Therefore, the following discussion of the behavior of a Ni-Mo-Cr alloy is used to illustrate the complexity of an interrelationship between alloy composition, heat treatment, corrosion environment and corrosion rate. The alloy has the nominal composition in weight percent of 14.5 to 16.5 Cr, 15 to 17 Mo, 3 to 4.5 W, and 4 to 7 Fe with maximum limits on carbon and silicon. The alloys for which the corrosion data are shown in Fig. 7.59 contained 0.045 to 0.06 wt% carbon and 0.53 to 0.80 wt% silicon and were initially quenched from 1225 15 °C (2235 25 °F), which produced a dispersion of M6C type carbides (M = Mo, W, Si) in austenite (Ref 94). These carbides were not involved in the subsequent corrosion behavior or heat treatments. Heat 

60 Comparison of the time-temperature-transformation curves of Hastelloy alloys C and C-276. The latter contains less carbon and silicon. Redrawn from Ref 95 

The more significant result of the lower carbon and silicon content alloy is the reduced susceptibility to intergranular corrosion. This has been correlated with the much slower rate of precipitation in this alloy when compared with the higher carbon and silicon concentrations. The difference is evident in Fig. 7.60, where the time of appearance of precipitates is some 30 times longer for alloys with the lower concentration of these elements. Consistent with this difference in precipitation rate is the ability to satisfactorily weld the latter alloys without introducing susceptibility to intergranular corrosion. 

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While the few examples quoted provide some general guidance as to the behaviour of nickel-rich materials in contact with molten metals and salts, it cannot be over-emphasised that such behaviour can be very considerably modified by the presence of very small amounts of contaminants in the liquid media (see Sections 2.9 and 2.10). The effect of very small contents of sodium chloride on the corrosion of nickel-base alloys by sodium sulphate has been referred to previously and other reported examples involving trace amounts, particularly of gaseous impurities, underline the need for great care in interpretation of experimental results. 

C. M. Chun, G. Bhargava and T. A. Ramanarayanan, Metal Dusting Corrosion of Nickel Based Alloys,/. Electrochem. Soc., 154 C231-C240(2007). 

Scarberry RC, editor. Corrosion of Nickel-base Alloys. Proceedings of Conference Oct. 1984, Cincinnati, Ohio ASM, 1985. 

Kolts, J. and Sridhar, N., Temperature Effects in Localized Corrosion, Corrosion of Nickel-Base Alloys, R. C. Scarberry, Ed., ASM International, Metals Park, OH, 1985, pp. 191-198. 

Wensley, D. A., Reid, R. C., and Dykstra, H., Corrosion of Nickel-Based Alloys in Chlorine Dioxide Washer Service, Corrosion/90, Paper 537, Houston, TX NACE International, 1990. 

Factors affecting localized corrosion of nickel-base alloys are chloride concentration, pH, temperature, crevice geometry (depth and tightness), and crevice former material (non-metallic or similar or dissimilar metal, metal surface condition, area ratio of exposed to shielded metal). A wide range of results can be obtained as these factors are varied. 

Mechanism of Stress Corrosion Cracking of Alloy X-750 in High-Piuity Water Conference Corrosion of Nickel-Base Alloys Cincinnati, Ohio, USA, 23-25 Oct. 1984... 

Sufficient evidence is not available to present a more acceptable mechanism of intergranular corrosion of nickel base alloys. 

U. Feld, A. Rahmel, M. Schmidt, Investigation on the interactions between creep and corrosion of nickel-based alloys in sulfate melts, in Corrosion and Mechanical Stress at High Temperatures, Applied Sci. Publishers Ltd, Barking, 1981, pp. 171-194, discuss. 195-196. 

Asphahani, A. I., Corrosion of Nickel-Base Alloys, in Metals Handbook Corrosion. Metals Park, Ohio, ASM International, 1987, pp. 641-657. 

MOLTEN-SALT CORROSION OF NICKEL-BASE ALLOYS... 

Chun C M, Bhargava G, Ramanarayanan T A, Metal dusting corrosion of nickel based alloys , J. Electrochem. Soc., 2007 154 C231-C240... 

Bimetallic corrosion of nickel-iron alloys may be of significance in welding operations. Ni-45 Fe alloys are used as filler materials in the welding of cast irons but the favourable area relationship of weld metal to base plate... 

Mitton DB, Yoon JH, Cline JA, Kim HS, Eliaz N, Latanision RM. The corrosion behavior of Nickel base alloys in SCWO systems. Ind Eng Chem Res 2000 39 4689. 

Corrosion Resistance of Nickel-based Alloys. Nickel-based alloys are solid solutions based on nickel. Nickel-based alloys used for low-temperature aqueous or condensed systems are generally known as corrosion-resistant alloys (CRA), and nickel alloys used for high-temperature applications are known as heat-resistant alloys (HRA), high-temperature alloys (HTA), or superalloys. The corrosion performance could change due to the presence of second phase or a weld seam. (Rebak)5... 

Based on the chemical composition, corrosion-resistant nickel-based alloys consist of commercially pure nickel. Ni-Cu alloys, Ni-Mo alloys, Ni-Cr-Mo alloys, and Ni-Cr-Fe alloys.63 The cast versions of the nickel-based alloys do not have the same corrosion resistance as the corresponding wrought products, mainly due to the higher carbon and silicon contents and the anisotropic microstructure of the cast products. (Rebak)5... 

Corrosion Resistance of Nickel-Based Alloys There are three types of nickel-based alloys (i) CRA (ii) heat-resistant alloys and (hi) high-temperature alloys or super alloys. The corrosion performance can change because of the presence of a weld seam as a second phase. 

D. E, Jordan, Stress-Corrosion Cracking of Nickel-Base Alloy Weldments, International Institute of Welding Annual Assembly, Montreal (1990). 

Llewelyn, G., Protection of Nickel-Base Alloys Against Sulfur Corrosion by Pack Aluminizing, Hot Corrosion Problems Associated with Gas Turbines, ASTM STP 421, ASTM International, West Conshohocken, PA, 1967, p. 3. 

Kane, R. D., Greer, J. B., Hanson, J. R., et tJ., Stress Corrosion Cracking of Nickel Base Alloys in Chloride Containing Environment, Paper 174, CORROSION/79, NACE, Atlanta, GA, April 1979. 

Removal of the corrosion product or oxide layer by excessive flow velocities leads to increased corrosion rates of the metallic material. Corrosion rates 2ire often dependent on fluid flow and the availability of appropriate species required to drive electrochemical reactions. Surface shear stress is a measure of the force applied by fluid flow to the corrosion product film. For seawater, this takes into account changes in seawater density and kinematic viscosity with temperature and salinity [33]. Accelerated corrosion of copper-based alloys under velocity conditions occurs when the shear surface stress exceeds the binding force of the corrosion product film. Alloying elements such as chromium improve the adherence of the corrosion product film on copper alloys in seawater based on measurements of the surface shear stress. The critical shear stress for C72200 (297 N/m, 6.2 Ibf/ft ) far exceeds the critical shear stresses of both C70600 (43 N/m, 0.9 Ibf/ft ) and C71500 (48 N/m, 1.0 Ibf/ft ) copper-nickel alloys [33]. 

The BWR also uses some carbon steel and low-alloy steel for steam and other piping (to avoid IGSCC, when corrosion resistance is adequate) and a limited amount of nickel-base alloys (e.g., for reactor vessel/piping safe ends). 

An ability to build coded heat exchangers in a variety of nickel-based alloys, oxide dispersion strengthened alloys (ODS) and ceramic materials to address the temperature, life and corrosion issues associated with these demanding duties. 

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