Flow-induced Corrosion

High velocities of aqueous solutions impinging on copper and brass tubes produce impingement on the metal or alloy. The aluminum brass and cupronickel alloys have great resistance to flow-induced attack up to a well-defined maximum for the flow rate, beyond which the film on the metal surface will be disrupted. Admiralty brass and aluminum brass have lower values for maximum velocity of flow than cupronickels. Admiralty brass and aluminum brass are preferred to cupronickels for use in media containing sulfide species. Coatings have been developed for cupronickel and aluminum brass condenser tubes for land-based and marine systems. 

Addition of a small quantity of iron to the alloy or to the solution improves the erosion-corrosion resistance of the alloy because of the tenacity of the iron film on the metal surface. This protection function of iron has led to the successful use of iron sacrificial pieces in preference to the familiar zinc sacrificial anodes in water boxes of condensers and heat exchangers. 

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Bends and tee-pieces in pipework often create locally turbulent flow. This enhances the corrosivity of the process liquid. These effects should be minimized by the use of flow straighteners, swept tees and gentle bends. Flow-induced corrosion downstream of control valves, orifice plates, etc. is sometimes so serious that pipework requires lining with resistant material for some twelve pipe diameters beyond the valve. 

Silverman has defined a number of useful expressions that allow one to utilize the rotating cylinder method with a variety of practical geometries (12,15). Both shear stresses and mass transfer coefficients are included in the derivations described (12). Table 1 in NACE standard TM-0270-72 summarized the various features of experimental systems for studying flow induced corrosion (22). 

Fig. 4 Flow-induced corrosion of a pump impeller. (View this art in color at www. dekker.com.)...

Lotz U. Velocity effects in flow induced corrosion. Corrosion/90. Paper No 27, National Association of Corrosion Engineers, NACE, Houston, Texas, 1990. 

On the other hand, diffusion-controlled corrosion inhibition may be nnllified in flowing fluids as a result of saturation of the boundary layer. The resulting pnrely chemical ranoval of material therefore no longer corresponds to the usual term of corrosion-erosion and is called flow-induced corrosion. 

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]. 

NACE, Flow Induced Corrosion Fundamental Studies and Industry Experience, K. J. Kennelly, R. H. Hausler, and D. C. Silverman, Eds., An Official NACE Publication, 1991. 

Weber, J. (1992), Flow induced corrosion - 25 years of industrial research, Br. Corros. J., Vol. 27,193-199. 

With the exception of cavitation, flow-induced corrosion problems are generally termed erosion-corrosion, encompassing flow enhanced dissolution and impingement attack. The fluid can be aqueous or gaseous, single or multiphase [23]. There are several mechanisms described by the conjoint action of flow and corrosion that result in flow-accelerated corrosion (FAC) [24 25] ... 

Heitz E. Chemo-mechanical effects of flow on corrosion. In KermeUey KJ, Hausler RH, Silverman DC, eds. Flow-Induced Corrosion Fundamental Studies and Industry Experience. Houston, Tex. NACE International, 1991 1-29. 

The best way to minimize corrosion in flowing water system is at the fabrication stage as discussed above. Other methods to minimize flow-induced corrosion are summarized below. 

Z. Ahmad and B. J. Abdul Aleem, Effect of Suspended Solids in the Flow Induced Corrosion of Modified Al-2.5 Mg Alloy in Arabian Gulf water, Jr. Mat. Sc. and Eng., 1, p.61,1992. The technique of studying the effect of velocity on the corrosion resistance of materials is described in this paper. 

Alloying elements of Mn, Al, Si, P, minimize flow induced corrosion of steels (annealed) in 3.0% NaCl, whereas C promotes corrosion. The effect of alloying elements on flow induced corrosion are shown in Fig. 9.14 [26]. 

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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