Erosion-corrosion, resulting from

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. 

Impingement Attack localised corrosion resulting from the action of corrosion and/or erosion (separately or conjoint) when liquids impinge on a surface. 

The combination of wear or abrasion and corrosion results in more severe attack than with either mechanical or chemical corrosive action alone. Metal is removed from the surface as dissolved ions, as particles of solid corrosion products, or as elemental metal. The spectrum of erosion corrosion ranges from primarily erosive attack, such as sandblasting, filing, or grinding of a metal surface, to primarily corrosion failures, devoid of mechanical action. 

Erosion-corrosion can be defined as the accelerated degradation of a material resulting from the joint action of erosion and corrosion when the material is exposed to a rapidly moving fluid. Metal can be removed as solid particles of corrosion product or, in the case of severe erosion-corrosion, as dissolved ions. 

Impediments to water flow resulting from inadequate equipment design or lodgement of foreign objects in the tubes can exercise a dramatic effect on the erosion-corrosion process. Much of this influence is linked to the creation of turbulence and the simple increase in fluid velocity past obstructions. The importance of these factors is quickly recognized when the phenomenon of threshold velocity is considered. 

Erosion-corrosion of these components was caused by high-velocity turbulent flow resulting from incomplete opening of the valve. In this case, erosion is the dominant factor in the metal loss, corrosion being a minor contributing factor. 

Galvanic corrosion may also occur by transport of relatively noble metals, either as particulate or as ions, to the surface of an active metal. For example, ions of copper, perhaps resulting from corrosion or erosion-corrosion at an upstream site, may be carried by cooling water to the surfaces of aluminum, steel, or even stainless steel components. If the ions are reduced and deposit on the component surfaces, localized galvanic corrosion may result. 

Turbulence and high fluid velocities resulting from normal pump operation accelerated metal loss by abrading the soft, graphitically corroded surface (erosion-corrosion). The relatively rapid failure of this impeller is due to the erosive effects of the high-velocity, turbulent water coupled with the aggressiveness of the water. Erosion was aided in this case by solids suspended in the water. 

The use of Ni-base superalloys as turbine blades in an actual end-use atmosphere produces deterioration of material properties. This deterioration can result from erosion or corrosion. Erosion results from hard particles impinging on the turbine blade and removing material from the blade surface. The particles may enter through the turbine inlet or can be loosened scale deposits from within the combustor. 

High water velocities can result in erosion or corrosion due to the abrasive action of particles in the water and the breakdown of the protective film which normally forms on the inside surface of the pipe. Erosion can also result from the formation of flash steam and from cavitation caused by turbulence. Publishing data on limiting water velocities are in conclusive. Table 27.9 summarizes the available information. 

Impingement attack (sometimes termed erosion corrosion) is a result of the combined effect of flow and corrosion on a metal surface and it occurs when metal is removed from the surface under conditions where passivation is insufficiently rapid. It is a function of flow, corrosion and passivation. 

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. 

Copper ions, resulting from erosion or corrosion processes elsewhere in the system that have been transported and deposited, can cause serious galvanic corrosion. 

One report stated thickness measurement a short distance away from the rupture showed the line was a nearly full design thickness. Investigators concluded the line failure was the result of the thinning of the Schedule 120 carbon steel 90-degree elbow due to long-term erosion/corrosion. [21] Another story stated the piping was originally or nominally 0.625 inches thick, but had worn down to 0.085 inches. [27] That represents an 86-percent wall loss. 

If there is any doubt concerning suitable materials for construction of equipment, reference should be made to the literature, or laboratory tests should be carried out under conditions similar to the final operating conditions. The results from the laboratory tests indicate the corrosion resistance of the material and also the effects on the product caused by contact with the particular material. Further tests on a pilot-plant scale may be desirable in order to determine the amount of erosion resistance or the effects of other operational factors. 

The toxicity of hypochlorite arises from its corrosive activity on skin and mucous membranes. Corrosive burns may occur immediately upon exposure to concentrated bleach products. Most of this corrosiveness stems from the oxidizing potency of the hypochlorite itself, a capacity that is measured in terms of available chlorine . The alkalinity of some preparations may contribute substantially to the tissue injury and mucosal erosion. Sodium hypochlorite when combined with an acid or ammonia may produce chlorine or chloramine gas, respectively. An inhalation exposure to these gases may result in irritation to mucous membranes and the respiratory tract, which may manifest itself as a chemically induced pneumonitis. 

Reactor cooling gases should be treated both at inlets, to decrease particulate matter exposed to neutron fluxes, and at outlets to remove particulates resulting from corrosion, erosion, or fuel element rupture. Operating temperatures sometimes prove a limitation for this application. The treatment of dissolver exit-gases imposes acid fume resistance as a condition in the treatment equipment. 

Oxidation-corrosion data obtained from the pilot plants generally compare well with laboratory data in ranking of high-temperature alloys. Pilot plant results, however, indicate more severe corrosion than laboratory oxidation-corrosion data. This should be expected because of cyclic operation of pilot plants and additional variables comprising the pilot plant environments. The contribution of erosion and erosion-corrosion by coal ash, char, and sulfur sorbents to the corrosion process in the pilot plants has not been defined. 

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