Types of Intergranular Corrosion

One of the most important forms of stress corrosion that concerns the nuclear industry is chloride stress corrosion. Chloride stress corrosion is a type of intergranular corrosion and occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions, and high temperature. 

The same metal can be sensitive to both types of intergranular corrosion depending on the prevailing potential. The susceptibility to intergranular corrosion of a given alloy such as austenitic stainless steel, therefore not only depends on the thermal treatment but also on the conditions to which it is exposed potential, temperature, anions. 

Intergranular corrosion is an especially severe problem in the welding of stainless steels, when it is often termed weld decay. Figure 17.19 shows this type of intergranular corrosion. 

I Various types of intergranular corrosion, (a) In- terdendritic corrosion in a cast stmcture. b) In-terfragmentary corrosion in a wrought, unrecrystallized... 

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... 

Cihal and Prazak determined the resistance of 18/8 stainless steel to this type of corrosion. They claimed that the technique could be used on steels which are difficult to test by other methods, including steels of low carbon content, and steels in which stabilising elements are present. By means of potentiostatic curves and light etching at constant potential they confirmed that the extent of intergranular corrosion depended upon the amount of precipitated chromium carbide. 

Streicher s work indicates how useful the potentiostat has been in studying intergranular corrosion. Ideally, future data would be expanded to provide Pourbaix-type diagrams that also contain kinetic information showing various rates of attack within the general domain of intergranular corrosion. (Similar data for cases other than intergranular attack would be equally valuable.)... 

Effect of Thermal History of Austenitic Stainless Steels on Susceptibility to Intergranular Corrosion. The time dependence for the local depletion of chromium sufficient to cause susceptibility to intergranular corrosion as functions of temperature and carbon content is of the form represented in Fig. 7.54 (Ref 83). The curves are typical of type 3xx alloys with nominal chromium concentrations of 17 to 25 wt% and, since they represent times for initiation of intergranular corrosion,... 

Retardation of some types of local corrosion (intergranular, selective, stress),... 

The general term covers pitting corrosion, crevice corrosion, and selection of intergranular corrosion, spongiosis, dezinciflcation, and line and layer type corrosion. 

Precipitation processes of this kind are always caused by heat treatments, snch as sensitizing annealing, that are inappropriate for the alloy in question. For the austenitic chronuum-nickel-molybdenum steels used for the fabrication of chemical plant equipment, the critical tanperature range is 400-800°C. Chromium depletion through formation of chromium-rich carbides, mostly of the type (M23Cg), is the main cause of intergranular corrosion in these steels. The precipitation of chromium nitrides of importance only that the chromium-rich nitride (CrjN) can initiate intergranular corrosion, especially in ferritic steels. Since the intermetalUc phases in stainless steels contain appreciably less chromium than carbides and nitrides and their deposition is far slower, the chromium depletion related to these phases is minimal. 

In addition to carbides, grain boundary precipitates in this material also consist of an intermetal-lic chromium- and molybdenum-rich intermetallic phase, which leads to depletion of chromium and molybdenum in the grain boundary zones. The critical temperature range for precipitation is very wide (500-1150°C). More recent types of this family of alloys, such as NiMol6Crl6Ti (material no. 2.4610) and NiCr21Mol4W (material no. 2.4602), exhibit a considerably reduced susceptibility due to their lower carbon and silicon contents (both <0.1%) thus, there is no further risk of intergranular corrosion in practical applications (Shell et al. 1964). 

Figure 7.40 Mechanisms of intergranular corrosion. Type I zone depleted of passivating metal. Type II anodic precipitate.

In the first mechanism type I), the precipitate is inert or even cathodic with respect to a zone immediately adjacent, which is depleted in elements that promote passivation. The depleted zone being anodic with respect to the rest of the surface it selectively corrodes. Type-I intergranular corrosion is particularly obnoxious under conditions where the zone adjacent to the grain boundary becomes active, while the remainder of the surface remains passive. Intergranular corrosion of austenitic stainless steel due to the precipitation of chromium carbide is the best-known example of type I intergranular corrosion. 

In the selection of materials, the possibility of intergranular corrosion must also be taken into account. This type of corrosion is selective and is based, for example, on the precipitation of chromium-rich carbides on the grain boundaries (e.g. by welding). The chromium depletion can lower the corrosion resistance so that the grains disintegrate. This modification of the material is known as sensitisation [30]. Thus, for example, steel no. 1.4301 (Table 4) can be sensitised and, according to [32], the use of this material is not recommended. 

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