Intergranular Corrosion Intergranular corrosion occurs when corrosion attack develops along the grain boundaries. One important example is the sensitization of 18-8 austenitic stainless steel (SS 304). The steel contains 18 wt% Cr and 8wt% Ni with a low carbon content. But the weldability characteristics of these stainless steels are poor because the carbon content is enough to form deleterious carbides at the grain boundary areas of the heat-affected zone upon welding as observed in the quasi-binary phase diagram (Figure 5.63). When the material is fusion welded, the area near the weld zone (i. e., the heat affected zone or HAZ) experiences a high temperature, but does not melt the metal. As the welded steel is cooled down, chro­mium carbide (Cr23C6) formation takes place at the grain boundaries, as illustrated in Figure 5.64a. Because these particles contain a high amount of chromium, they make the regions adjacent to the grain boundary lean in chromium (<12 wt%), as illustrated in Figure 5.64b. In order for the stainless steel to retain its corrosion — resistant (stainless) property, it must have at least 12 wt% Cr. Even though the grain interiors contain higher Cr contents, the areas surrounding grain boundaries in the HAZ become lean in Cr. This condition is known as sensitization. This type of sensitized steel becomes susceptible to intergranular corrosion. This is a serious problem for this type of stainless steels.

Sensitization problem can be solved in three different ways: (i) After welding, the plate is cooled rapidly across the temperature range where chromium carbide forms. If the chromium carbide formation can be avoided, there will be no problem of sensitization. However, quenching the plate may have other undue conse­quences of residual stresses or distortion. (ii) Special stainless steel grades with very low carbon content (~0.01 wt% C) have been developed such as in 304L stain­less steel grades. If there is not much carbon present, there will be less amount of chromium carbide formation. (iii) Stabilized grades of austenitic stainless steels (like 316, 324, etc.) have been developed. Strong carbide-forming elements like molybdenum, titanium, niobium, and so on are added to the steel composition and during welding, corresponding carbides (TiC etc.) are preferentially formed throughout the bulk of the material (not just the grain boundaries) with not much carbon left for chromium carbide formation.

Stress Corrosion Cracking Stress corrosion cracking involves cracking of a material under static load by the combined action of stress and a chemical environment. SCC is generally found in alloys, not in pure metals. SCC occurs only in a specific environment for a given alloy. The presence of a tensile component of stress is nec­essary for SCC to take place. In majority of cases, the crack path is intergranular; transgranular cracking is rare. Cracking occurs in two stages: crack initiation and crack propagation. It has been noted that titanium alloys are immune to crack initi­ation in a chloride environment, but a precracked material shows susceptibility to crack propagation. Season cracking and caustic embrittlement are two examples of SCC. We will discuss this topic in detail in Section 6.4.4.

The crack growth mechanism that is involved is thought to be by anodic dissolu­tion at the crack tip. However, in materials that form passive films, the crack tip does get exposed to the corrosion medium as the plastic deformation at the crack tip exposes fresh metal surface. Thus, an active-passive cell gets created between the

crack tip and the crack faces. Since only a small area at the crack tip gets exposed, a very high current density is generated and corrosion occurs. In a laboratory environ­ment, slow strain rate tensile testing in the chosen chemical environment is used to determine the SCC properties of a material. Another way to study the time-depen­dent fracture in a corrosive environment is to precrack a specimen and surround it in the corrosion environment and keep the specimen under constant load. The measured time to fracture is plotted as a function of mode-I fracture toughness (Kj) in Figure 5.65a. If the applied stress corresponds to the critical fracture toughness (KIc), fracture occurs without any delay time. When Kj < KIc, fracture occurs after a delay period implying that crack grows slowly. However, when the stress intensity factor is below a critical level (Kjscc), no fracture occurs even after very long periods. Figure 5.65b shows the measured crack growth rates as a function of Kj. Below Kjscc, the crack growth rate is essentially zero. Above Kjscc in region-I, the crack propagates at an accelerating rate till it reaches region-II in which crack moves at a constant speed independent of the stress intensity factor. In region-II, the crack propagation is plausibly controlled by electrochemical factors. Finally, crack moves again at an accelerated rate resulting into fracture. The energy balance equation used to develop Griffith’s equation needs to be modified as electrochemical energy is released due to anodic dissolution at the crack tip. The final equation for KIscc is given by where cp is the plastic work term at the crack tip, E is the Young’s modulus, n! is the number of electrons taking part in the anodic dissolution process, F is Faraday’s con­stant, q is the density of the alloy, d is the opening at the crack tip, M is the atomic weight, and V is the electrode potential. Following Eq. (5.91), KIscc decreases with an increase in yield strength (through decreasing yp) and with an increase in propensity toward corrosion (via electrode potential V).