Nickel-base alloys, intergranular corrosion

Henthorne, M., Intergranular Corrosion of Iron and Nickel Base Alloys, Localised Corrosion-Cause of Metal Failure, ASTM STP 516, ASTM, 66 (1972)... 

Many instances of intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel-based alloys have occurred in the reactor water systems of BWRs. IGSCC, first observed in the recirculation piping systems (21) and later in reactor vessel internal components, has been observed primarily in the weld heat-affected zone of Type 304 stainless steel. 

Nickel-based alloys-Determination of resis-ance to intergranular corrosion Corrosion tests in artificial amospheres at very low concentrations of polluting gas(es) Test /. .-sulphur dioxide test for contacts and connectors... 

Fig. 7.57 Effect of carbon and nickel content on intergranular corrosion penetration rate of 1 8 wt% Cr-base stainless steels. Alloys sensitized for 100 h at 550 °C. Immersion in boiling 65% nitric acid. Pds., periods (48 h) of exposure. Redrawn from Ref 84...

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... 

Crain Boundary Segregation and Intergranular Corrosion in Ferrous and Nickel-base Alloys... 

The most common intergranular corrosion tests are outlined in ASTM A 262 and G 28. Most of these test solutions are strong oxidizing acids or acids with added oxidizers that maintain the specimen at a stable electrochemical potential where accelerated intergranular corrosion will occur in a susceptible material. The test duration is typically 24-120 h, depending on the alloy. Recommended exposure periods for various nickel-base alloys are listed in ASTM G 28 but not in ASTM A 262. Though only austenitic stairrless steels are listed in ASTM A 262, some of the test methods are used for nickel-base alloys. It is important to note that ASTM A 262, Practice A, the cBialic add etch test, is recommended for stainless steels. It is not normally used for nickel-base alloys. 

Corrosion of metals and alloys—Corrosivity of atmospheres—Guiding values for the corrosivity categories Corrosion of metals and alloys—Corrosivity of atmospheres—Measurement of pollution Corrosion of metals and alloys—Corrosivity of atmospheres—Determination of corrosion rate of standard specimens for the evaluation of corrosivity Corrosion tests in artificial atmospheres—Salt spray tests Nickel-based alloys—Determination of resistance to intergranular corrosion Corrosion of aluminium alloys—Determination of resistance to stress corrosion cracking Corrosion tests in artificial atmosphere at very low concentrations of polluting gas(es)... 

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

Intergranular corrosion, usually of stainless steels or certain nickel-base alloys, that occurs as the result of sensitization in the heat-affected zone during the welding operation. 

Another type of nickel alloy with which problems of intergranular corrosion may be encountered is that based on Ni-Cr-Mo containing about 15% Cr and 15% Mo. In this type of alloy the nature of the grain boundary precipitation responsible for the phenomenon is more complex than in Ni-Cr-Fe alloys, and the precipitates that may form during unfavourable heat treatment are not confined to carbides but include at least one inter-metallic phase in addition. The phenomenon has been extensively studied in recent years . The grain boundary precipitates responsible are molybdenum-rich M C carbide and non-stoichiometric intermetallic ix... 

Austenitic stainless steels are the most significant class of corrosion-resistant alloys for which intergranular corrosion can be a major problem in their satisfactory use. The problem is most often encountered as a result of welding but also may result from stress-relief annealing or incorrect heat treatments. Intergranular corrosion also can occur in ferritic stainless steels and in nickel- and aluminum-base alloys. 

Intergranular corrosion occurs in stainless steels and alloys based on nickel, aluminium, magnesium, copper and cast zinc. In the following sections we shall look at the three former groups in some detail. 

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