Corrosion-erosion, control

The most important method for controlling corrosion-erosion is the use of materials that are resistant to this form of attack, and further information can be obtained by consulting the sections that are devoted to metals and alloys see particularly Section 4.2). 

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. 

The transmission of stresses from piping to equipment can be eliminated by installation of manufactured expansion joints. Corrosion, erosion, and cyclic stresses, however, limit the use of mechanical expansion joints. Piping stresses on equipment is usually controlled by proper piping arrangement and use of supports and anchors. 

Corrosion, erosion, mechanical fatigue, operation at design limit and/or beyond it, calibration failure, design fault (such as wrong material selection), obsolescence, etc. are major contributing factors for degradation of equipment/system, and/or electrical control and instmmentation (EC I) items. 

Technology for quality control of the liquid metal coolant and methods for the protection of structural materials against corrosion/erosion, as well as structural materials themselves, developed for marine nuclear reactors [XXIII-5]. 

Corrosion due to mechanical and metallurgical problems also exist. Metal tools used in drilling wells are often softer than the formation being penetrated. The abrasiveness of formation solids can easily erode protective films from drilling equipment, leaving metal exposed to corrosion-erosion attack. Mechanical and chemical separation of abrasive solids helps control this attack. It is difficult, however, to control stress concentrations in a string of driU pipe that may reach many kilometers into the earth. 

The studies reported in this section indicate that corrosion-erosion problems of high-temperature (250 to 300°C) aqueous oxide slurry systems may be satisfactorily controlled. However, under nnfavorable conditions very aggressive attack has been noted. 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

Pinch The industrial equivalent of controlling flow bv pinching a soda straw is the pinch valve, Mves of this tvpe use fabric-reinforced elastomer sleeves that completely isolate the process fluid from the metal parts in the valve. The valve is actuated bv applying air pressure directly to the outside of the sleeve, causing it to contract or pinch. Another method is to pinch the sleeve with a linear actuator with a specially attached foot. Pinch valves are used extensively for corrosive material service and erosive sliirrv senice. This type of valve is used in applications with pressure drops up to 10 bar (145 psi),... 

This chapter deals with the essential factors in the selection of systems and equipment during the design stage principles of controls, noise-reduction systems, and problems such as erosion, corrosion, maintenance and equipment cleaning, etc. 

The geometry of the system is a further factor that enables control to be effected, and in general, reducing the velocity of the solution and ensuring that flow is laminar and not turbulent will reduce the tendency for attack by erosion-corrosion. Thus the pipe diameter should be as large as possible, consistent with other considerations bends should have a large radius and inlet and outlets should be streamlined so that there is not a sudden change in section. 

Flow thins protective film to equilibrium thickness which is a function of both mass transfer rate and growth kinetics. Erosion corrosion rate is controlled by the dissolution rate of the protective film. 

A number of workers have suggested that there are situations in which two processes in series control the erosion corrosion rate, for example diflfusion plus partial activation control, leading to a lower dependency on mass transfer than expected. 

Most mass transfer data is for smooth surfaces . It is well established that as a surface suffers erosion corrosion it will roughen and the mass transfer rate will increase. It has been suggested that when this occurs the roughness becomes more important than the original shape in controlling the mass transfer. 

Environmentally the most important variables are pH, oxygen content and temperature of the water (Figure 1.96). In single phase conditions both high pH and additions of low levels of oxygen have been used to prevent erosion corrosion . However, because of partitioning effects between water and steam this is more difficult to achieve in two-phase flow. Although additions of morpholine or AMP (2-amino-2-methyl-propan-l-ol) have been successfully used to control pH. 

Fig. 1.96 Summary of factors controlling the erosion corrosion of steels in water...

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