Corrosion-resistance Critical shear rate

Table 7.5 does not give any clear information about critical velocities, but it indicates that such thresholds exist for the copper alloys in the velocity range represented in the table (1.2-8.2 m/s). More specifically, both Figure 7.46 and Table 7.6 show examples of critical velocities for erosion corrosion. The values are not absolute they depend on the composition of the environment, the temperature, geometrical conditions, the exposure history, the exact composition and treatment of the material etc. In connection with Figure 7.46 it can be mentioned that austenitic stainless steels show excellent resistance to erosion corrosion in pure liquid flow at high velocities, while some ferritic [7.42] and ferritic-austenitic steels are attacked less than the austenitic ones if the liquid carries solid particles. The data in Table 7.6 originate from work by Efird [7.43], who interpret his results as follows for each alloy in a certain environment, there exists a critical shear stress between the liquid and the material surface. When this shear stress is exceeded, surface films are removed and the corrosion rate increases markedly. 

If the critical shear stress that acts on the layer of corrosion products present at a surface is exceeded, mechanical film damage leads to a strong increase in the rate of corrosion. According to this view, stainless steel and titanium are not susceptible to flow accelerated corrosion because the thin passive oxide films formed on these metals are more resistant to shear stresses than the corrosion product layers found on copper and its alloys [16]. 

Erosion is one of several wear modes involved in tribocorrosion. Solid particle erosion is a process by which discrete small solid particles, with inertia, strike the surface of a material, causing damage or material loss to its surface. This is often accompanied by corrosion due to the environment. A major environmental factor with significant influence on erosion-corrosion rates is that of flow velocity, but this should be set in the context of the overall flow field as other parameters such as wall shear stress, wall surface roughness, turbulent flow intensity and mass transport coefficient (this determines the rate of movement of reactant species to reaction sites and thus can relate to corrosion wall wastage rates). For example, a single value of flow velocity, referred to as the critical velocity, is often quoted to represent a transition from flow-induced corrosion to enhanced mechanical-corrosion interactive erosion-corrosion processes. It is also used to indicate the resistance of the passive and protective films to mechanical breakdown [5]. 

The resistance of aluminum alloys to flow induced corrosion depends on the stability of the protective oxide films on the surface. Dissolution of these films leads to accelerated corrosion. The protective films of bayerite and boehmite could be eroded by shear forces resulting from flow beyond a critical velocity. Aluminum alloys of series 5xxx are not adversely affected by velocities up to 3 m/s in the absence of abrasives in water. The removal of a film adjacent to a film surface sets up local corrosion cell which accelerates the corrosion process. AUoys of 5xxx series (such as 5454) show a good resistance to corrosion at velocities up to 3ms at temperatures up to 140°C. The corrosion rate increases with increased velocities in the presence of abrasive particles, which need to be controlled. The water chemistry, water velocity and pH needs to be controlled to minimize the effect of flow on localized corrosion. Maintaining pH below 9 would not allow aluminum to dissolve as AlO. The preventive measures include the minimizing of turbulent flow or changing water chemistry. 

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