Stress corrosion cracking SCC mechanisms

It has been suggested [114] that the threshold stress, Tscc is related to the yield stress. In single crystals, it has been reported [169] that the yield strength must be exceeded to produce plastic deformation and this is necessary for SCC. However, Table 8.1 [5] indicates that ctscc is significantly less than the yield stress for many alloy-environment combinations. 

Logan [161,170] proposed an alternative electrochemical mechanism. He postulated that, when the surface film is ruptured by an applied strain 

Logan s hypothesis [161,170] that the SCC was entirely electrochemical was based on the observation that cathodic polarisation could prevent SCC. However, HE models generally require the exposure of film-free surfaces, which may be prevented by cathodic polarisation. Furthermore, Logan calculated from Faraday s law that the observed crack propagation rates (10 X 10 m/s) required an effective current density of 14 A/cm. Similarly Pugh et al. [171] observed crack velocities between 6 x 10 and 40 X 10 m/s for Mg-7.6A1 in a chloride-chromate solution, which correspond to current densities between 8 and 60A/cm. Such current densities were considered by Pugh et al. to be prohibitively high. 

The most commonly proposed mode of TGSCC of Mg alloys is discontinuous cleavage. Most Mg alloys have hexagonal close packed (HCP) crystal 

Chakrapani and Pugh [115] determined from measurements of the distance between acoustic emission spikes that the average crack velocities in Mg-7.6A1 were between 5 and 30 x 10 m/s. Similarly, using a travelling microscope Pugh et al. [171] determined that, for the same alloy, average crack velocities were between 6 and 40 x 10 m/s. Such velocities could not 

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Equally, boiler surface failure may result solely from poor operational practices or other indirect problems, although more usually it is due to a combination of waterside and operational problems such as corrosion fatigue and other stress corrosion cracking (SCC) mechanisms. 

Where caustic deposits occur, the resultant corrosion of steel by caustic gouging or stress corrosion cracking (SCC) mechanisms produces particulate iron oxides of hematite and magnetite. It is common to see white rings of deposited sodium hydroxide around the area of iron oxide formation. 

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