Solution crack velocity, effect

Fig. 8.11 Effect of beam deflection rate of cantilever beam specimens upon stress-corrosion crack velocity of carbon steel in carbonate-bicarbonate solution...

Temperature effects indicate an activation energy of 113 kJ/mol for Stage I and 16 kJ/mol for Stage II in 7079-T651 alloy. Crack velocity in Stage II is lowered as the solution viscosity is increased. 

Figure 7. Effect of solution concentration upon stress-corrosion-crack velocity in...

Alloy 400 exhibits excellent resistance to hydrofluoric acid solutions at all concentrations and temperatures, as shown in Figure 15.1. Again, aeration or the presence of oxidizing salts increases the corrosion rate. This alloy is widely used in HF alkylation, is comparatively insensitive to velocity effects, and is widely used for critical parts such as bubble caps or valves that are in contact with flowing acid. Monel 400 is subject to stress corrosion cracking in moist, aerated hydrofluoric or hydrofluorosilicic acid vapor. However, cracking is imlikely if the metal is completely immersed in the acid. 

Figure 3 Examples of crack velocity-stress intensity curves for SCC, showing the effects of alloy composition and cold work on SCC of austenitic stainless steels in a hot chloride solution. (From Ref 57. Courtesy of Pergamon Press.)...

SCC of austenitic or duplex stainless steels in CE/H2S environments usually occurs from pits, even at moderate temperatures such as 80°C, whereas crevice or undeideposit corrosion is normally needed to nucleate CE-SCC at such temperatures. This may be understood from Figure 26 pits in Cl solutions at 80°C are normally growing faster than cracks (so no SCC occurs), whereas in Cl H2S solutions llie crack velocity is increased and the pitting velocity can be lower without repassivation, owing to the stabilization of pit dissolution by adsorbed sulfur and possibly also the resistive effect of the black corrosion product that forms in the pits. 

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. 

Scully, J.C., Powell, D.T. The effect of strain-rate upon stress corrosion crack velocity in [alphaj-brass in ammoniacal solutions. Corros. Sci. 10, 719 (1970)... 

A mechanism such as that given above provides explanations for the known effects of many process variables ". The reductive dissolution and undermining processes require access of the acid to the metal surface, hence the benefits obtained by the deliberate introduction of cracks in the oxide by cold-working prior to pickling. Also the increase in pickling rate with agitation or strip velocity can be explained in terms of the avoidance of acid depletion at the oxide-solution interface. 

We consider plane contact and crack problems in this chapter, without neglecting inertial effects. Such problems are typically far more difficult than the non-inertial problems discussed in Chaps. 3 and 4, and require different techniques for their solution. This is an area still in the development stage so that it will not be possible to achieve the kind of synthesis or unification which is desirable. We confine our attention to steady-state motion at uniform velocity V in the negative x direction. We begin by deriving boundary relationships between displacement and stress. These are applied to moving contact problems in the small viscoelasticity approximation, and to Mode III crack problems without any approximation. 

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