Heat-flux corrosion rates

Deposit control is important because porous deposits, under the influence of heat flux, can induce the development of high concentrations of boiler water solutes far above their normally beneficial bulk values with correspondingly increased corrosion rates. This becomes an increasingly important feature with increase in boiler saturation temperature. In addition, deposits can cause overheating owing to loss of heat transfer. Finally, carryover of boiler water solutes, which can be either mechanical or chemical, can lead to consequential corrosion in the circuit, either on-load or off-load. Material so transported can result in corrosion reactions far from its point of origin, with costly penalties. It is therefore preferably dealt with by a policy of prevention rather than cure. 

It has been concluded from data reported in these studies that the skin temperature is the major controlling factor in corrosion, not the rate of heat flow through the metal . It has also been concluded, however, that corrosion rates at a given mid-specimen temperature do depend on the presence or absence of thermal flux . The difference between temperatures at skin and mid-specimen positions may account for this discrepancy. 

Heat-flux corrosion rates can also be determined in plant tests using steam-heated tubular specimens which are weighed or callipered. 

Economizer corrosion rates are enhanced by higher heat-transfer rates excessive heat flux may create localized nucleate boiling zones where gouging, as a result of chemical concentration effects, can occur. Air heaters are also located in the exit gas system. They do a job similar to that of economizers except that they preheat combustion air. 

Caustic gouging usually occurs only in areas of high heat flux but may also result when heat transfer rates are low, as in horizontal or inclined WT boiler tubes under circumstances in which the steam-water velocity is particularly low. Here, the relatively small volume of BW surrounding the steam bubbles concentrates very quickly, the alkalinity soars, and caustic corrosion develops. 

Where general corrosion occurs, the rate of hydrogen production is relatively independent of heat flux. Thus, the concentration of hydrogen is inversely proportional to steam flow. 

Where localized corrosion occurs, the rate of hydrogen production grows with increase in heat flux, and the dissolved hydrogen concentration rises with increase in steam flow. 

The linear dependence of the deposition rate R on the concentration of corrosion products in the reactor water is largely undisputed. On the other hand, there are differences in opinion as to the impact of the heat flux, whether R is directly proportional to the heat flux Q or whether it is proportional to the square of the heat flux. According to Alder et al. (1992), the deposition of corrosion products onto the fuel rod surfaces preferentially occurs in the zone where nucleate boiling begins (about 1 m from the bottom end of the fuel rod) and is expressed in the following equation ... 

The cone calorimeter can also be used to obtain other data in support of fire safety engineering analysis. For example, the instrument can be used to determine ignition characteristics of a material by measuring the time to ignition at different heat flux levels. A laser smoke photometer is moimted on the duct to determine the smoke production rate. A continuous gas sample can be taken from the exhaust duct and analyzed to determine the concentration of different toxic and corrosive products of combustion in the effluents. Fourier transform infrared spectroscopy is now a common method to measure the concentration of toxic and irritant gases in the exhaust duct of the cone calorimeter. 

Heat flux (W/m ), which is the heat transfer rate (W) divided by the exposed metal tube surface area (m ), is constant as long as no deposits accumulate on the specimen. However, deposits coming from hardness salts, corrosion by-products, silt, process contamination, biomass, etc., can form on the heated surface, raising the surface temperature. If the surface temperature becomes excessive, there is temperature cutoff switch to protect the device. 

Heat flux, a major influence on scale/deposition rate. Corrosion rates will also be influenced but to a lower degree. Heat flux of 0-6300 W/m (0-2000 BTU/tf-h) are considered low. Heat flux of 6300-15 800 W/m (2000-5000 BTU/ft -h) are average. Heat flux of 15 800-31 500 W/m (5000-10000 BTU/ft -h) are considered high. 

Konig created a heat balance by assuming that the heat flux into the refractory surface must equal the heat flux through the furnace shell. If the heat flux into the refractory surface exceeds the heat flux out of the shell, the interface temperature (Tr) must increase, causing increased corrosion rates until the heat balance is maintained once again. Terms used in Konig s analysis are given in Table 1. 

The corrosion test is designed to measure the rate of corrosion of a very thin copper wire. It is critical that the copper wire be thin because the test depends upon the difference in resistance of the wire over time. Therefore, no solder is used in preparing the test coupons—only flux, solder paste flux, or cored-wire flux should be applied to the copper anode and heated to soldering temperatures. This test provides complementary information to the SIR test that measures the insulating characteristics of the laminate. SIR readings, however, reflect the combined effect of (a) residues that are corrosive to the conductor tracks and (b) residues that interact with the laminate. 

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