Temperature effects corrosion products

The overall effect is that corrosion is usually more rapid at higher temperatures, the corrosion products being often more objectionable in nature. There are, however, exceptions to this generalisation and the increased rate... 

By substituting the appropriate values for viscosity and diffusion at various temperatures, they found that corrosion rates could be calculated which were confirmed by experiment. The corrosion rates represent maxima, and in real systems, corrosion products, scale and fouling would reduce these values often by 50%. The equation was useful in predicting the worst effects of changing the flow and temperature. The method assumes that the corrosion rate is the same as the limiting diffusion of oxygen at least initially this seems correct. 

Variation in the pressure of the reacting gas can affect corrosion processes in two ways. In the cases more usually met with in practice, in which the corrosion rate is controlled by diffusion processes in the surface film of corrosion product, the influence of gas pressure on corrosion rate is slight. If, however, the dissociation pressure of the oxide or of a constituent of the scale lies within the range involved, the stability of the corrosion product will be critically dependent on the pressure. The effect of temperature is, however, far more critical and thus, in practical cases, pressure variations rarely decide the stability of corrosion products. 

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. 

In situ Raman spectroscopy is being used to investigate corrosion products from zinc in a humid atmosphere and sodium chloride70 and from Type 304L stainless steel in aerated water at elevated temperatures and pressures.71 The changes in detected species over time helped identify possible corrosion mechanisms and the effect of different variables on corrosion rates and mechanisms. 

Thermal insulation effects by limiting the substrate and membrane temperature to prevent thermal damage and (3) Reduce permeation of corrosive fluid to the substrate, thus minimizing its corrosion rate. CRM linings, such as acid brick and monolithic cements, also prevent "wash", which is the removal of the membrane or substrate corrosion products by the circulating medium. Even when the fluid eventually reaches the membrane or substrate surface, the amount is relatively small, thus limiting chemical attack, and any corrosion products are trapped beneath the masonry shield. 

K. Makela, T. Buddas, M. Zmitko, J. Kysela, The effect of hydrazine on high temperature water chemistry and corrosion product transport in primary circuit of a VVER 440 unit, Water Chemistry Conference, Nice, France, 1994. 

Activity transport effects can be minimized by selecting materials with a low cobalt content and by rigid adherence to chemical specifications for the coolant. Because of the important role of corrosion product particles in this transport, filtration has been studied extensively as a means of reducing the rate of growth of radiation fields. High flows are needed to be effective and therefore the filters must operate at full coolant temperature. Two types of filter which have proved successful in pilot tests at the NPD reactor are a deep bed of graphite particles and a bed of steel balls in an electromagnetic field (61). 

Since wear and friction are system-based properties, it is also important to measure other factors such as bearing temperature, although this generally only carried out in critical applications. Service wear can comprise a complex mixture of wear processes and the effect of machine variables such as vibration, ingress of dirt and corrosion products needs to be taken into account for each application, although a quantitative assessment of the effect of each is virtually impossible. 

Paralinear corrosion (related to dissolution of corrosion product) does not occur for all aluminum alloys in water at all high temperatures. In Figure 13 are plotted data for an alloy (Al, 1% Ni, 0.1% Ti) corroded in water at 350°C (10). The corrosion rate was low and constant, as shown better in other figures in the same publication. For some specimens in the figure 1/3 or 2/3 of the corrosion product was removed mechanically after the first exposure period. There was no discernible effect on subsequent corrosion, indicating that control of corrosion probably resided close to the metal-oxide interface. Similar experiments for the alloys and temperatures where paralinear behavior occurs showed that removing some of the product caused an increase in subsequent corrosion rate. 

The resulting corrosion processes not only lead to leakage but also may induce failure of system components owing to clogging with corrosion products (e.g. scales and precipitates). Corrosion inhibitors for water treatment should be effective at variable water composition, temperature, and flow conditions for a wide range of inhibitor concentrations. They should protect all exposed metals and should not be aggressive to other materials in the system (e.g. solder, rubber, and plastics). Furthermore, they should not stimulate the buildup of scales, thermally isolating... 

Atmospheric corrosion is electrochemical corrosion in a system that consists of a metallic material, corrosion products and possibly other deposits, a surface layer of water (often more or less polluted), and the atmosphere. The general cathodic reaction is reduction of oxygen, which diffuses through the surface layer of water and deposits. As shown in Section 6.2.5, the thickness of the water film may have a large effect, but it is more familiar to relate atmospheric corrosion to other parameters. The main factors usually determining the accumulated corrosion effect are time of wetness, composition of surface electrolyte, and temperature. Figure 8.1 shows the result of corrosion under conditions implying frequent condensation of moisture in a relatively clean environment (humid, warm air in contact with cold metal). 

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