SCC mechanisms

Equally, boiler surface failure may result solely from poor operational practices or other indirect problems, although more usually it is due to a combination of waterside and operational problems such as corrosion fatigue and other stress corrosion cracking (SCC) mechanisms. 

Where caustic deposits occur, the resultant corrosion of steel by caustic gouging or stress corrosion cracking (SCC) mechanisms produces particulate iron oxides of hematite and magnetite. It is common to see white rings of deposited sodium hydroxide around the area of iron oxide formation. 

This type of stress-related corrosion process may result in boiler failure through a sudden and violent rupturing of the boiler tube metal. Austenitic stainless steels also are corroded by SCC mechanisms in the presence of concentrated chlorides (chloride-induced SCC). 

Numerous SCC mechanisms have been proposed specific to certain alloys in specific environments. An excellent review written by Newman [35] is recommended for additional information on different stress corrosion mechanism. There is no universal model that explains various mechanisms proposed for SCC because of the inconsistency in SCC... 

Logan [42] suggested a film rupture or sHp-step dissolution model for SCC mechanism where the crack is initiated by locahzed anodic dissolution. This mechanism postulates that plastic strain in the metal at the crack tip causes a fracture of the oxide barrier film. The film rupture model (FRM) assumes a repetitive, cycling process of film rupture, dissolution of the underlying metal, and repassivation. 

Zirconium and zircaloy-4 in 1 M NaCl, 1 M KBr, and 1 M aqueous KI solutions were found susceptible to SCC only above the pitting potential (zone 1) [168]. Zirconium alloy SCC in aqueous halide solutions occurs as a result of electrochemical passive film breakdown followed by intergranular attack due to anodic dissolution (dealloying assisted by stress). The final step was a fast transgranular propagation. A surface-mobility SCC mechanism was suggested to explain experimental results. Figure 9.47 shows... 

In addition, crack advance occurs in environmental conditions that may be very different from those that prevail on free surfaces, and this is to be considered when evaluating the relevance of a SCC mechanism. 

The SCC mechanisms presented in the following sections are those that are presently considered to have the greatest interest from a mechanistic point of view, or, in the case of the slip dissolution model, to give some quantitative prediction, at least in specific cases. 

The most extensive mechanistic investigations have been carried out on passive systems showing cracking of type A or t5q)e B. All SCC mechanisms rely on the exposure of bare metal to the environment, and if this is too brief (owing to... 

The chapter builds on our critical reviews on Mg corrosion [1-4] and Mg SCC [5,6]. SCC [5-8] involves (1) a stress, (2) a susceptible alloy and (3) an environment. SCC is related to hydrogen embrittlement (HE). HE is SCC that is caused by hydrogen (H), which can be gaseous, can come from corrosion, or can be internal from prior processing. HE is often postulated as the SCC mechanism. SCC can be extremely dangerous. Under safe loading conditions, SCC causes slow crack growth. Fast fracture occurs when the crack reaches a critical size. SCC, for any alloy + environment combination, can be characterised by [7,8] the threshold stress, ctscc> threshold stress intensity factor, iscc> the stress corrosion crack velocity. 

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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