Erosion corrosion critical velocities

Velocity Accelerated Corrosion This phenomenon is sometimes (incorrecdy) referred to as erosion-corrosion or velocity corrosion. It occurs when damage is accelerated by the fluid exceeding its critical flow velocity at that temperature, in fliat metal. For that system, fliis is an undesirable removal of corrosion products (such as oxides) which would otherwise tend to stifle the corrosion reaction. 

The critical velocity, which when exceeded may result in erosion corrosion, can be calculated by the equation presented in API RP 14E, which is [199]... 

Craig, B., Critical Velocity examined for effects of erosion-corrosion, Oil and Gas Journal, May 27, 1985. 

Whenever corrosion resistance results from the formation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be to prevent their normal formation, to remove them after they have been formed, and/or to preclude their reformation. All metals that are protected by a film are sensitive to what is referred to as its critical velocity i.e., the velocity at which those conditions occur is referred to as the critical velocity of that chemistry/temperature/veloc-ity environmental corrosion mechanism. When the critical velocity of that specific system is exceeded, that effect allows corrosion to proceed unhindered. This occurs frequently in small-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities (e.g., condenser and evaporator tubes), in the vicinity of bends in pipelines, and on propellers, agitators, and centrifugal pumps. Similar effects are associated with cavitation and mechanical erosion. 

The corrosion rate of carbon steel increases with increase in velocity until a critical velocity is reached. This behavior is different from that of the carbon steel in fresh water where the corrosion rate decreases beyond a critical velocity due to the formation of a passive him. In seawater passive films are not formed because of the presence of high concentrations of chloride. The erosion corrosion occurs after critical velocity 20 m/s is reached. The maximum corrosion rate of 1,0/mm/yr is reached at velocities up to 4 m/s. 

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. 

Figure 7.46 Critical velocities for erosion corrosion of different materials in seawater. (From Bemhardson et al. [7.41].)...

Using equation (10.50) we can relate the critical flow velocity for erosion corrosion. Vent lo a critical shear stress... 

The critical shear stress that causes erosion corrosion of copper in a pipe containing sea water under turbulent flow conditions is taken to be 9.6 N/m. What would be the critical flow velocity in pipes of 5 cm and 20 cm respectively We assume that the friction coefficient is given by the Blasius relation ... 

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 velocity profiles and transverse momentum transfer close to the solid/liquid interface dictate wall shear stress levels and mass transport efficiencies, both of which are important drivers for erosion-corrosion. Therefore, critical velocity values are very geometry-specific and cannot be readily applied to predict component service life in generic flow systems. 

Erosion corrosion is associated with a flow-induced mechanical removal of the protective surface film that results in subsequent corrosion rate increases via either electrochemical or chemical processes. It is often accepted that a critical fluid velocity must be exceeded for a given material. The mechanical damage by the impacting fluid imposes disruptive shear stresses or pressure variations on the material surface and/or the protective surface film. Erosion corrosion may be enhanced by particles (solids or gas bubbles) and impacted by multi-phase flows [29]. Increased flow stream velocities and increases of particle size, sharpness, density, and concentration increase the erosion corrosion rate. Increases in fluid viscosity, density, target material hardness, and/or pipe diameter tend to decrease the corrosion rate. The morphology of surfaces affected by erosion corrosion may be in the form of shallow pits or horseshoes or other local phenomena related to the flow direction. 

Water velocity and turbulence can damage protective films and deposits on metal surfaces causing increased corrosion. Soft metals, such as copper, are particularly susceptible to erosion corrosion, but steel and other metals are also susceptible if the water velocity is sufficiently high. The critical velocity for erosion-corrosion of copper in freshwater is about 5 s (1.52 m/s), but this velocity can drop sharply as the chemical corrosivity of the water increases. Suspended solids in the water can increase the erosion characteristics of the water [1]. Deposits can result in accelerated corrosion from the formation of oxygen differential concentration corrosion cells. 

Erosion corrosion is mainly observed in hydraulic installations (pumps, turbines, or tubes). This form of corrosion appears at a point where the flow velocity of the bulk solution exceeds a critical limit, or where this limit is exceeded by local turbulence. Erosion corrosion results from an interaction between mechanical and chemical influences. Fig. 1-26. One model describing the mechanism of erosion corrosion assumes that local shear forces acting on the metal surface as a result of the high flow velocity forms pores or unprotected areas. Accelerated mass transfer then occurs in these areas and aggravates corrosion damage. 

The protective oxide film of most metals is subject to being swept away above a critical water velocity. After this takes place, accelerated corrosion attack occurs. This is known as erosion-corrosion. For some metals, this can occur at velocities as low as 2-3 ft/s. The critical velocity for titanium in seawater is in excess of 90 ft/s. Numerous corrosion-erosion tests have been conducted and all have shown that titanium has outstanding resistance to this form of corrosion. 

Line sizes based on velocity limitations are calculated only in special cases where corrosion, erosion or deposits on the pipe wall have to be accounted for or where critical flow conditions exist. 

Material degradation may be caused by sand erosion or corrosion. As shown in Figure 1, two potential threats leading to sand erosion or corrosion are identified. The first threat focuses on excessive sand production rate. This does not only mean that there is an increase in sand production, but also that the velocity of the flow in which sand is carried exceeds a critical threshold. In order to prevent this threat to develop into pipeline material degradation, two safety barriers are in place ... 

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