Systems exhibiting stress-corrosion cracking

Table I Alloy/Environment Systems Exhibiting Stress Corrosion Cracking...

Mechanisms of SCC. Crack initiation of EAC is complex and not well understood till now. Most of the SCC systems exhibit short initiation times ranging from minutes to weeks and cracking often occurs due to the change in the environment rather than to a very long initiation time. Stress-corrosion crack growth rates are usually 10 11 and 10-6 m s In systems such as stainless steels in chloride solutions, localized corrosion may create the local conditions prone to crack development, but it is still difficult to explain the initiation of the crack in the absence of localized corrosion in environmental conditions different from that of the crack propagation.95 It should be mentioned that dealloyed surface layers such as certain copper alloys in ammonia-containing solutions are believed to cause SCC.54... 

In many of the alloy systems shovm in Table 1/ the stable configuration of the alloy surface is that it is filmed. Many of the alloys, e.g., stainless steels, Al, Ti, Zr and Mg alloys are only usable in such a condition. Such a consideration applies not only to these alloys covered with a thin passive film but also to those on which relatively thick films are formed. The possible mechcUiisms by which stress corrosion cracking occurs are concerned with reactions between unfilmed metal and the environment. Before consideration of these it is necessary to consider how these various types of film break down initially. While many of the alloys exhibit pitting, it is not necessary for pitting as such to precede crack propagation. Pitting is associated with static unstressed metals whereas cracking is associated with a metal whose surface is stressed. 

Stress corrosion crack growth rates are usually between 10" and 10 m s . The fastest growth rates are met in systems such as stainless steels in acidic chloride, or copper alloys in concentrated ammonia solutions. Alloy 600 exhibits the slowest crack growth rate, in high temperature water. 

This behavior is characterized by a plateau region, which prevails above a definite threshold K. It is often referred to as stress-corrosion fatigue because SCC systems usually exhibit this behavior, and the most common theory assumes that the crack growth rate results from the addition of SCC, and pure fatigue crack advance. This is a type of... 

Those alloy/environment systems that form relatively thick corrosion-product layers (e g., brass in ammonia environments) frequently exhibit preferential penetration of the corrosion products along grain boundaries. These are generally brittle products such that cracking will occur on reaching a critical depth in the presence of tensile stresses across the grain boundary. The environment again has access to the... 

CF results in fine-to-broad cracks with little or no branching, unlike SCC that exhibits branching cracks. The cracks appear as families or parallel cracks and are filled with dense corrosion product. The sample may have pits, grooves, or some other forms of stress concentrator. Transgranular fracture paths, often ramified or branched are more common than intergranular fracture with the exception of lead and zinc (Fig. 1.18). Some systems show a combination of transgranular and intergranular forms of fracture (8, 9). 

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