Stress-corrosion cracking elimination

Remedial measures include reduction or elimination of chlorides or replacement of the 304 stainless steel with a metal that is resistant to chloride stress-corrosion cracking. 

Austenitic stainless steels (the 300 series) are particularly su-sceptible to stress-corrosion cracking. Frequently, chlorides in the process stream are the cause of this type of attack. Remove the chlorides and you will probably eliminate stress-corrosion cracking where it has been a problem. 

Eliminate unfavorable environments. The presence of oxygen and other oxidizers is a critical factor in stress corrosion cracking. For example, the cracking of austenitic stainless steel in chloride solutions can be reduced or completely eliminated if oxygen is removed. 

Al-Mg-Si alloys are strengthened by precipitation hardening in which MgjSi is formed. They are not very susceptible to stress-corrosion cracking which only occurs in specimens subjected to a high solution-treatment temperature followed by a slow quench Ageing such material eliminates susceptibility . 

Heat treatment may also affect the extent and distribution of internal stresses. These may be eliminated by appropriate annealing treatments which can remove susceptibility to stress-corrosion cracking. This must be explored in any studies of the performance of materials in environments where stress-corrosion cracking is a hazard. In particular cases, stress-relief annealing treatments may result in the appearance of new phases which, while eliminating the stress-corrosion effects, will induce another type of path of attack. This possibility must be kept in mind in assessing the overall benefits of heat treatments applied primarily for stress relief. 

It has been reported that hydrogen embrittlement is a form of stress corrosion cracking (SCC). Three basic elements are needed to induce SCC the first element is a susceptible material, the second element is environment, and the third element is stress (applied or residual). For hydrogen embrittlement to occur, the susceptible material is normally higher strength carbon or low-alloy steels, the environment must contain atomic hydrogen, and the stress can be either service stress and/or residual stress from fabrication. If any of the three elements are eliminated, HE cracking is prevented. 

The net result of the two reactions is the decomposition of water into hydrogen and oxygen. Since the chemistry involves only sulfur, oxygen and hydrogen compounds, many of the development issues associated with more complex thermochemical processes, such as cross-contamination and halide-induced stress corrosion cracking, are eliminated. 

Air-cooled condensers are especially attractive at locations where water is scarce or expensive to treat. Even when water is plentiful, air coolers are frequently the more economical alternative. Elimination of the problems associated with the water side of water-cooled equipment, such as fouling, stress-corrosion cracking, and water leaks into the process, is an important advantage of air-cooled equipment. In many cases, carbon steel tubes can be employed in air-cooled condensers when more expensive alloy tubes would otherwise have been necessary. The use of air-cooled heat exchangers may eliminate the need for additional investment in plant cooling water facilities. 

Careful design, good fabrication techniques and annealing processes will do much to reduce or eliminate, the problem of stress corrosion cracking. 

A typical rack employed for installation of specimens in pilot plants is shown in Fig. 6. Both corrosion coupons, 2 X 1 X 0.35 in. thick, and bend specimens intended to determine stress-corrosion cracking susceptibility, are included in the installation for aqueous corrosion testing. Specimens are separated by high density alumina spacers to eliminate electrochemical effects. During exposure, the racks are welded to existing components in the pilot plant equipment. 

In alloyed air-hardened steels (13 CrMo44), thorough tempering at 750°C is needed after welding to eliminate the hardness structure. Only when the hardness is below 220V are these steels resistant to stress corrosion cracking. 

Stress-corrosion cracking of steel was first encountered in a practical way in riveted steam boilers. Stresses at rivets always exceed the elastic Unfit, and boiler waters are normally treated with alkalies to minimize corrosion. Crevices between rivets and boiler plate allow boiler water to concentrate, until the concentration of alkali suffices to induce S.C.C., sometimes accompanied by explosion of the boiler. Because alkalies were recognized as one of the causes, failures of this kind were first called caustic embrittlement. With the advent of welded boilers and with improved boiler-water treatment, S.C.C. of boilers has become less common. Its occurrence has not been eliminated entirely, however, because significant stresses, for example, may be established at welded sections of boilers or in tanks used for storing concentrated alkalies. 

Eliminate stress risers in equipment components to reduce stress corrosion cracking. 

Improved reactor coolant system materials to reduce the chance of reactor coolant system cracking, which can lead to reactor coolant system leakage and associated safety challenges and cleanup/repair operation radiation exposures. A specific example of this improvement is the elimination of Inconel 600, to prevent stress corrosion cracking affecting the reactor coolant system pressure boundary. 

---