Stress-corrosion cracking locations

Materials of Construction. GeneraHy, carbon steel is satisfactory as a material of construction when handling propylene, chlorine, HCl, and chlorinated hydrocarbons at low temperatures (below 100°C) in the absence of water. Nickel-based aHoys are chiefly used in the reaction area where resistance to chlorine and HCl at elevated temperatures is required (39). Elastomer-lined equipment, usuaHy PTFE or Kynar, is typicaHy used when water and HCl or chlorine are present together, such as adsorption of HCl in water, since corrosion of most metals is excessive. Stainless steels are to be avoided in locations exposed to inorganic chlorides, as stainless steels can be subject to chloride stress-corrosion cracking. Contact with aluminum should be avoided under aH circumstances because of potential undesirable reactivity problems. 

General description. Incomplete penetration describes the condition in which the weld fails to reach the bottom of the weld joint, resulting in a notch located at the root of the weld (Fig. 15.12). This critical defect can substantially reduce the intrinsic mechanical strength of the joint and can combine with environmental factors to produce corrosion fatigue (Chap. 10), stress-corrosion cracking (Chap. 9), or crevice corrosion (Chap. 2). 

Figure 15.19 shows various crack orientations that can occur in connection and attachment welds. Applied stresses from external loading of these components can add to the residual weld stresses, producing still higher stress loads. This can increase the susceptibility to stress-corrosion cracking and can affect orientation and location of crack paths. 

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. 

Stress Corrosion Cracking One of the serious forms of pipeline corrosion is see. This form of corrosion consists of brittle fracture of a normally ductile metal by the conjoint action of a specific corrosive environment and a tensile stress. In the case of underground pipelines, see affects the external surface of the pipe, which is exposed to soil/ground water at locations where the coating is disbonded. 

Static tests carried out with non-precracked specimens allow us to characterize, for a given environment, the sensitivity of an alloy to stress corrosion cracking and hydrogen embrittlement. TyfpicaUy, the test specimen subjected to a constant load or a constant strain is exposed to a corrosive environment and one measures the time to failure. The test specimens sometimes contain a notch—not to be confused with precracking—that fixes the location where failure will occur. In constant strain tests, the time to failure corresponds to the appearance of the first cracks. This kind of experiment thus indicates the crack initiation time. In constant load tests, on the other hand, the time to failure is the time leading to fracture of the specimen. It corresponds to the sum of the crack initiation and the crack propagation times. 

Specific tests frequently used are (a) neutral 5 % Sodium Chloride salt spray (ASTM B 117, Test Method of Salt Spray (Fog) Testing), (b) 3.5 % Sodium Chloride by alternate immersion (ASTM G 44, Practice for Evaluating Stress Corrosion Cracking Resistance of Metals in 3.5 % Sodium Chloride Solution), and (c) exposure to various outdoor atmospheres. Guidelines for outdoor exposure are contained in ASTM G 50, Practice for Conducting Atmospheric Corrosion Tests on Metals. Generic types of atmospheres used are seacoast, industrial, urban, and rural. Sometimes specific geographical locations or local chemical conditions are important because they can produce unique results [2i],... 

The main degradation mechanisms identified are stress corrosion cracking (SCC) at different locations of tubes, intergranular attack (IGA), "denting" and "fretting. ... 

The low water temperature, the water quality control, and the removal of 10 mils of surface after the final heat treating will each reduce the probability of stress corrosion cracking. The location of parts made of 17-4 H900 have been Identified so that any failures... 

The literature contains many references to the use of notched, as opposed to pre-cracked or plain, specimens in laboratory studies of stress corrosion, for reasons of improved reproducibility, inability to crack plain specimens under otherwise identical conditions or ease of measuring some parameter such as crack growth rate when the crack location is predetermined. However, the developments in fracture mechanics (see Section 8.9), have resulted in a whole new field of stress-corrosion testing involving the use of specimens... 

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