General stress corrosion

This aspect and the general stress corrosion and hydrogen embrittlement behaviour are considered in detail in Reference 37. 

Corrosion control requires a change in either the metal or the environment. The first approach, changing the metal, is expensive. Also, highly alloyed materials, which are resistant to general corrosion, are more prone to failure by localized corrosion mechanisms such as stress corrosion cracking. 

Few, if any, failure mechanisms have received as much attention as stress-corrosion cracking (SCC). Yet despite an enormous research effort over many years, an acceptable, generalized theory that satisfactorily explains all elements of the phenomenon has not been produced. SCC is a complex failure mechanism. Nevertheless, its basic characteristics are well known, and a wealth of practical experience permits at least a moderately comfortable working knowledge of the phenomenon. 

General description. In incomplete fusion, complete melting and fusion between the base metal and the weld metal or between individual weld beads does not occur (Fig. 15.8). Incomplete fusion that produces crevices or notches at surfaces can combine with environmental factors to induce corrosion fatigue (Chap. 10), stress-corrosion cracking (Chap. 9), or crevice corrosion (Chap. 2). See Fig. 15.9. 

Concentrating on overall or general corrosion. Ignoring pitting, crevice corrosion, stress corrosion, cracking, etc. 

The condition of the test metal is important. Clean metal samples with uniform finishes are preferred. The accelerating effects of surface defects lead to deceptive results in samples. The ratio of the area of a defect to the total surface area of the metal is much higlier in a sample than in any metal in service. This is an indication of the inaccuracy of tests made on metals with improper finishes. The sample metal should have the same type of heat treatment as the metal to be used in service. Different heat treatments have different effects on corrosion. Heat treatment may improve or reduce the corrosion resistance of a metal in an unpredictable manner. For the purpose of selectivity, a metal stress corrosion test may be performed. General trends of the performance of a material can be obtained from such tests however, it is difficult to reproduce the stress that actually will occur during service. 

Process severity The indication of the degree of aggressiveness of the process medium on the hardware aggressiveness would include erosion, stress, corrosion, temperature, blockage, etc. Four categories of severity are used in this book Clean, General Industry, Moderately Severe, Severe. (See Chapter 2 for further explanation of these categories.)... 

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. 

The corrosive environments which cause SCC in any material are fairly specific, and the more common combinations are listed in Table 53.2. In the case of chloride stress corrosion cracking of the 530 series austenitic stainless steels it is generally considered that the risk is... 

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