Transgranular SCC

Ammonia SCC of Cu-base alloys 02 and C02 accelerate SCC failure in a wide range of NH3/air ratios of 20-80% NH3 Alloys of 60-65% Cu resulted in both intergranular and transgranular SCC predominantly transgranular SCC with alloys of >70% Cu... 

Moist HF Monel (Ni-Cu) susceptible to SCC. Alloys with 15-30% Cu resulted in intergranular and transgranular SCC Yellow brass susceptible to SCC in HF (petroleum industry)... 

In addition to the potential for SCC from the inside as a result of the process fluids, austenitic SS s can experience SCC from the outside, if the austenitic SS is neither low carbon nor stabilized, intergranular SCC can occur as low as at room temperature as a result of either chlorides or sulfur dioxide in the atmosphere. In the 140 to 220°F (60 to 104°C) range, transgranular SCC of stabilized, low carbon, and regular grades of austenitic SS s has occurred as a result of chlorides from seacoast atmospheres. 

An example of the behavior is shown by stainless steel in 1.0 M sulfuric acid solution. Transgranular SCC can occur in two ranges of potentials. Intergranular SCC can occur in a wider potential range. The potential zones 1 and 2 correspond to the active-passive and passive-active state transitions. The crack tip corresponds to the crack tip and the passive state, or film formation corresponds to zone 2. Zone 2 is frequently above the pitting potential, indicating the possible pit initiation and propagation. 

Figures 14.16 and 14.17 are examples of intergranular corrosion and transgranular corrosion of stainless steels, respectively. Transgranular corrosion refers to the corrosion of the metallic grains, while intergranular corrosion refers to the corrosion in the grain boundary region. Although carbon steel is considered to be unaffected by transgranular corrosion, transgranular SCC was observed with mild steels and low alloy steels in a H2O-CO-CO2 environment [18,19].

SCC in aluminum alloys is typically intergranular, although transgranular SCC has been observed for a few alloys under highly specific environmental conditions [16,19] and may be part of the propagation mechanism in some 7xxx alloys. SCC requires the interaction of a metalliugically susceptible material, a corrosive environment (water vapor may be sufficient) and sustained (but not necessarily continuous) tensile stress. 

Figure 1-12. Transgranular SCC of a stainless steel X2CrNiMol8-14-3 heat exchanger tube (water, 40 mg L Cr, 150 °C, 25 bar).

Transgranular SCC progresses by localized subsurface attack in which a narrow path is corroded randomly across grains without any apparent effect of the grain boundary on the crack direction. Transgranular SCC may occur during the SCC of austenitic stainless steels and less commonly during the SCC of low-alloy steels. It can also occur in the SCC of copper alloys in certain media (e.g., ammonia). It seldom occurs in aluminum alloys. 

MeUianol/ Like other titanium alloys, transgranular SCC... 

Figure 4.40 Alloy 800 in 10% NaOH solution at 288°C showing regions of intergranular and transgranular SCC. (From Jones, R.H. and Ricker, R.E. (1987). Metals Handbook, 13, Corrosion. Reproduced by kind permission of ASM, Metals Park, Ohio, USA)...

L Fairman, HJ Bray, Transgranular SCC in magnesium alloys. Corrosion Science, 1971, II, 533-541. 

In May 1985, the heavy ceiling in a swimming pool located in Uster, Switzerland, collapsed with fatal consequences after 13 years of service. The failure mechanism was established to be transgranular SCC, as illustrated in Fig. 5.16. The presence of a tensile stress was clearly created in the stainless rods by the weight of the ceiling. Chloride species dispersed into the atmosphere, together with thin moisture films, in all likelihood represented the corrosive environment. A characteristic macroscopic feature of the failed stainless steel rods was the brittle nature of the SCC fractures, with essentially no ductility displayed by the material in this failure mode. 

Figure 5.16 Transgranular SCC on stainless steel supporting rods.

Solid solution composition classically controls the SCC of brasses [71,72], austenitic stainless steels in hot chloride solutions [73,74], and noble-metal alloys [75]. In all these systems there is evidence that dealloying dominates the SCC mechanism, although this remains controversial for stainless steels. Transgranular SCC ceases above a critical content (parting limit or dealloying threshold) of the most noble alloying element, either 80-85% in Cu-Zn or Cu-Al or 40% in Ni-Cr-Fe or Au-X alloys (Figures 11.9 and 11.10). These values... 

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