Stress-corrosion cracking, discussion

The most important mechanism involved in the corrosion of metal is electrochemical dissolution. This is the basis of general metal loss, pitting corrosion, microbiologically induced corrosion and some aspects of stress corrosion cracking. Corrosion in aqueous systems and other circumstances where an electrolyte is present is generally electrochemical in nature. Other mechanisms operate in the absence of electrolyte, and some are discussed in Section 53.1.4. 

Resistance to stress-corrosion cracking Commercially pure titanium is very resistant to stress-corrosion cracking in those aqueous environments that usually constitute a hazard for this form of failure, and with one or two exceptions, detailed below, the hazard only becomes significant when titanium is alloyed, for example, with aluminium. This latter aspect is discussed in Section 8.5 under titanium alloys. 

The above simple concepts need to be modified and expanded to allow discussion of stress-corrosion cracking of stainless steels in general, especially as the role of hydrogen in assisting crack growth must be accounted for. 

For a general discussion of the mechanism of stress corrosion cracking see Fontana (1986). 

Stress corrosion cracking in stainless steels is discussed by Turner (1989). 

Season cracking of high zinc brasses is a severe form of embrittlement resulting in cracking or disintegration. Somewhat similar forms of stress-corrosion cracking occur in many other metals and alloys. Embrittlement of boiler plate, discussed below, may be considered a special case. 

Reactors are normally made of low-alloy steel (selected per API Publication 941) and clad or weld overlaid with type 347 (UNS S34700) SS. The stabilized grades (i.e., type 947 or 321 [UNS S32100] SS) are normally used for all stainless equipment in these units to avoid intergranular stress corrosion cracking (SCC) during downtime. Downtime corrosion is discussed in the next section of this chapter, Feed-Effluent Exchangers."... 

Stress corrosion cracking (see also in Section 8.6 in pressurized ammonia vessels and tanks is a problem which has been discussed in many papers [1252]-[1265], [1271]-[1276], The mechanism of this phenomenon, the influence of water, and the role of oxygen are not yet completely understood, in spite of extensive research. A review is given in [1277]. As it is generally accepted that addition of water may inhibit stress corrosion [1263], [1264] it has become a widely used practice to maintain a water content of 0.2% in transport vessels [1264]. Protection may also be achieved by aluminum or zinc metal spray coating [1275], [1277], More recent research [1273], [1277], however, has shown that water may not give complete protection. 

In this chapter, we have not discussed materials corrosion under mechanical stresses, such as environmental cracking, stress corrosion cracking, erosion-corrosion, and cavitation-corrosion. 

As with all standardized tests (e g., the ASTM A 262 procedures previously discussed), correlations must be established between the EPR Pa values and service performance. For example, a criterion of Pa < 2 C/cm2 has been proposed for adequate resistance to intergranular corrosion leading to intergranular stress-corrosion cracking (IGSCC) of type 304 and 304L pipe and welds. Other limits would be set depending on the material, application, and environment (Ref 105, 106). 

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