Carbon steels environment-sensitive

Corrosion Film Chemistry. A linear relationship exists between the mass of corrosion product formed on carbon steel, Cor-Ten A, zinc, galvanized steel, and copper and the mass of metal in the corrosion film. This relationship is independent of site and the wide variation in environmental parameters between the sites in short-term exposures of 1 and 3 months. The ratio of the two masses is relatively sensitive to the composition of the corrosion film. The independence of this ratio from substantial variations in air quality, meteorology, and rain chemistry is interpreted as indicating, at least for the major constituents, that the composition of the corrosion film is independent of the environment in short-term exposures. 

Strain-rate dependence of ductility of the form shown in Fig. 7.81 is presented in Fig. 7.82 for a carbon steel in a carbonate-bicarbonate environment (Ref 119). The ductility is represented as the ratio of the reduction in area (RA) in the environment relative to the value in inert oil. The tests were conducted at the indicated constant potentials and illustrate that the strain-rate dependence can be sensitive to the potential, particularly the minimum ductility and the strain rate at which the minimum occurs. It follows, as an illustration, that if small changes in the environment, such as dissolved oxygen, shift the potential from -720 to -680 mV (SHE), significant changes in susceptibility to SCC would be predicted. 

Representative environments for which SCC has been reported in carbon steels are included in Table 7.7. The sensitivity of these steels to changes in composition and environment are illustrated by the effects of potential in Fig. 7.78 to 7.80 and by the slow strain-rate data of Fig. 7.82 and 7.83. These data support the conclusion that environment cracking is related to the susceptibility of the passive films to crack under stress, to the subsequent crack growth due to anodic dissolution and/or hydrogen embrittlement during the period of exposure of the alloy substrate, and to rates of repassivation of the exposed areas. Actual crack-front growth mechanisms are discussed in some detail in a later section. 

Observations on Stress Corrosion Cracking Initiation Sites in Carbon Steel Conference Corrosion 94, Paper No. 234 NACE, Houston, TX, USA, 1994, 13 S. [4 Van der Sluys, W. A. DeMiglio, D. S. Use of a Constant Delta Test Method in the Investigation of Fatigue Crack Growth in 288 deg C Water Environments. Conference Environment-Sensitive Fracture Evaluation and Comparison Test Methods, Gaithersburg, Maryland, USA, 26-28 Apr. 1982... 

The addition of chromium forms a family of Ni-Cr-Mo alloys such as Hastelloy alloys C-276, C-22, and C-2000. These alloys contain 16 to 22 percent chromium and 13 to 16 percent molybdenum and are very resistant to a wide variety of chemical environments. They are considered resistant to stress-corrosion cracking and very resistant to localized corrosion in chloride-containing environments. These alloys are resistant to strong oxidizing solutions, such as wet chlorine and hypochlorite solutions. They are among only a few alloys that are completely resistant to seawater. The carbon contents are low enough that weld sensitization is not a problem during fabrication. These alloys are also more difficult to machine than stainless steel, but fabrication is essentially the same. 

Polythionic acid SCC occurs only in anstenitic stainless steels and nickel-chromium-iron alloys that have become sensitized through thermal exposure. Sensitization occurs when the carbon present in the alloy reacts with chromium to produce chromium carbides at the grain boundaries. As a result, the areas adjacent to the grain boundaries become depleted in chromium and are no longer fuUy resistant to certain corrosive environments. 

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