Isothermal corrosion

Heat/Solvent Recovery. The primary appHcation of heat pipes in the chemical industry is for combustion air preheat on various types of process furnaces which simultaneously increases furnace efficiency and throughput and conserves fuel. Advantages include modular design, isothermal tube temperature eliminating cold corner corrosion, high thermal effectiveness, high reHabiHty and options for removable tubes, alternative materials and arrangements, and replacement or add-on sections for increased performance (see Furnaces, fuel-FIREd). 

Ferritic stainless steels depend on chromium for high temperature corrosion resistance. A Cr202 scale may form on an alloy above 600°C when the chromium content is ca 13 wt % (36,37). This scale has excellent protective properties and occurs iu the form of a very thin layer containing up to 2 wt % iron. At chromium contents above 19 wt % the metal loss owiag to oxidation at 950°C is quite small. Such alloys also are quite resistant to attack by water vapor at 600°C (38). Isothermal oxidation resistance for some ferritic stainless steels has been reported after 10,000 h at 815°C (39). Grades 410 and 430, with 11.5—13.5 wt % Cr and 14—18 wt % Cr, respectively, behaved significandy better than type 409 which has a chromium content of 11 wt %. 

During the materials selection procedure isothermal corrosion testing may indicate the suitability of a material for handling a corrosive process fluid. In many cases where heat transfer is involved the metal wall temperature experienced in service is higher than the bulk process fluid temperature. This, and the actual heat transfer through the material, must be taken into account since both factors can increase corrosion rates significantly. 

Hodges, R. J., Intergranular Corrosion in High Purity Ferritic Stainless Steel. Isothermal Time-Temp. Sensitisation Measurements , Corrosion, 27, 164 (1971)... 

The dissolution of passive films, and hence the corrosion rate, is controlled by a chemical activation step. In contrast to the enhancement of the rate of dissolution by OH ions under film-free conditions, the rate of dissolution of the passive film is increased by increasing the ion concentration, and the rate of corrosion in film-forming conditions such as near-neutral solutions follows the empirical Freundlich adsorption isotherm ... 

The discussion to date has assumed isothermal conditions, but in many practical situations corrosion reactions have to be considered when the electrode is acting as a cooler or a heater. 

Mass-transfer deposits can lead to blockages in non-isothermal circulating systems, cis in the case of liquid-metal corrosion. In fused salts, the effect can be reduced by keeping contamination of the melt by metal ions to a minimum e.g. by eliminating oxidising impurities or by maintaining reducing conditions over the melt . 

In addition to impurities, other factors such as fluid flow and heat transfer often exert an important influence in practice. Fluid flow accentuates the effects of impurities by increasing their rate of transport to the corroding surface and may in some cases hinder the formation of (or even remove) protective films, e.g. nickel in HF. In conditions of heat transfer the rate of corrosion is more likely to be governed by the effective temperature of the metal surface than by that of the solution. When the metal is hotter than the acidic solution corrosion is likely to be greater than that experienced by a similar combination under isothermal conditions. The increase in corrosion that may arise through the heat transfer effect can be particularly serious with any metal or alloy that owes its corrosion resistance to passivity, since it appears that passivity breaks down rather suddenly above a critical temperature, which, however, in turn depends on the composition and concentration of the acid. If the breakdown of passivity is only partial, pitting may develop or corrosion may become localised at hot spots if, however, passivity fails completely, more or less uniform corrosion is likely to occur. 

There are few systematic guidelines which can be used to predict the properties of AB2 metal hydride electrodes. Alloy formulation is primarily an empirical process where the composition is designed to provide a bulk hydride-forming phase (or phases) which form, in situ, a corrosion— resistance surface of semipassivating oxide (hydroxide) layers. Lattice expansion is usually reduced relative to the ABS hydrides because of a lower VH. Pressure-composition isotherms of complex AB2 electrode materials indicate nonideal behaviour. 

Electrochemical impedance spectroscopy was used to determine the effect of isomers of 2,5-bis( -pyridyl)-l,3,4-thiadiazole 36 (n 2 or 3) on the corrosion of mild steel in perchloric acid solution <2002MI197>. The inhibition efficiency was structure dependent and the 3-pyridyl gave better inhibition than the 2-pyridyl. X-ray photoelectron spectroscopy helped establish the 3-pyridyl thiadiazoles mode of action toward corrosion. Adsorption of the 3-pyridyl on the mild steel surface in 1M HCIO4 follows the Langmuir adsorption isotherm model and the surface analysis showed corrosion inhibition by the 3-pyridyl derivative is due to the formation of chemisorbed film on the steel surface. 

Mass Transport at Very Low Concentrations. Reactor Circuits. Early in the development of water-cooled reactors, it became apparent that at temperatures of 250-300 C with a non-isothermal circuit, corrosion of carbon steel could lead to significant mass transport of iron if the chemistry of the system were not properly controlled. The resulting buildup of large deposits of crud" on fuel surfaces caused fuel failure. However, the large cost differential between carbon steel and stainless steel provided an incentive to identify chemistry conditions for the successful use of carbon steel. 

The adsorption isotherms for metallic surfaces are reported in the literature however, an important part of the atmospheric corrosion process takes place under rust layers, which play a decisive role in the long-term course of corrosion because of its sorption capacity for water. The influence of the chloride and sulfate anions has a real effect only when the corrosion products layer is already formed. Thus, the adsorption isotherms of the steel corrosion products formed in different atmospheres were determined. 

Water adsorption should be caused by salts content but, perhaps, the presence of high levels of contaminants can determine the formation of a corrosion product having a more hygroscopic nature. To make clear this possibility, adsorption isotherm of corrosion products from Viriato corrosion station after elimination of salts were determined. The values of adsorption are lower after salt elimination, but higher than those corresponding to Quivican. It could be due to a more hygroscopic nature of corrosion products formed in coastal stations. 

The superficial characteristics of atmospheric corrosion products of steel depend on the type of atmosphere where the sample has been exposed. The way of adsorption of the corrosion products obtained in the coastal atmosphere is polymolecular due to a higher content of salts. This makes easier the presence of water in the metal-corrosion products interface and determines a high corrosion rate. The adsorption of water of a corrosion product formed in a rural zone obeys a Langmuir isotherm, i.e. a monomolecular adsorption takes place. It causes a lower corrosion rate. 

The determination of adsorption isotherms offers the possibility to study the atmospheric corrosion process when the metal is covered by a corrosion products layer. 

Fig. 4. EDS spectroscopy results of E-particles from a high bum-up LWR fuel superimposed on the isothermal section of ternary phase diagram from Kleykamp (1985) at 1700 CC. These analyses show that there is distinct heterogeneity in the composition of metallic particles in the fuels. Hence, spot analysis of an individual e-parlicle may not provide direct evidence of corrosion. The metallic system is dominated by the hexagonal close packing (e) that occupies most of the phase space. The tr-space and the body centered cubic

O Brien. 1235 Ohmic drop, 811, 1089, 1108 Ohmic resistance, 1175 Ohm s law, 1127. 1172 Open circuit cell, 1350 Open circuit decay method, 1412 Order of electrodic reaction, definition 1187. 1188 cathodic reaction, 1188 anodic reaction, 1188 Organic adsorption. 968. 978. 1339 additives, electrodeposition, 1339 aliphatic molecules, 978, 979 and the almost-null current test. 971 aromatic compounds, 979 charge transfer reaction, 969, 970 chemical potential, 975 as corrosion inhibitors, 968, 1192 electrode properties and, 979 electrolyte properties and, 979 forces involved in, 971, 972 977, 978 free energy, 971 functional groups in, 979 heterogeneity of the electrode, 983, 1195 hydrocarbon chains, 978, 979 hydrogen coadsorption and, 1340 hydrophilicity and, 982 importance, 968 and industrial processes, 968 irreversible. 969. 970 isotherms and, 982, 983... 

Following preliminary DSC studies, isothermal decompositions of small quantities (1-3 mg) of compound are performed at temperatures generally below the observed DSC exothermic maximum. Samples are usually thermolyzed in sealed Pyrex tubes. Use of Pyrex usually precludes reaction with the container that often occurs with metal reaction vessels. Sealed vessels also prevent corrosive decomposition products, e.g. NO2 or HF, from damaging laboratory instrumentation. Sealed reaction vessels confine the decomposition products where they can easily be identified and quantified. It is obvious that highly reactive decomposition products such as formaldehyde are not observed by this technique, but they would not be expected to survive over the time of these decomposition experiments (seconds to hours, depending on the temperature). Seal vessel thermoylses mimic real storage scenarios where the sample is self-confined. However, autocatalysis may occur in sealed vessels that would not be observed in open ones. On the other hand, in unsealed tubes sublimation of the sample may become competitive with decomposition. 

The Heat Recycle Urea Process (HRUP) was developed by Urea Technologies in the 1970 s and is now offered by Monsanto Enviro-Chem. This process is shown in Figure 11.5. In this process ammonia, recycled carbamate and about 60% of the make-up CO2 feed are charged to the top of an open-ended reactor coil at 3200 psig (220 bar). Ammonium carbamate is formed within the coil, exits the coil at the bottom and flows up and around it. The exothermic heat of carbamate formation in the coil drives the endothermic dehydration of carbamate to urea on the outside of the coil. This isothermal reactor design allows the conversion rate to reach 77%. The reactor has a uniform temperature profile that eliminates corrosion in the bottom of the reactor112. 

Improved versions of OLGA (versions II and III) enabled the applications of corrosive gases such as hydrogen chloride or hydrogen bromide, chlorine, thionyl chloride or boron tribromide vapor etc. This made it possible to synthesize volatile halides and measure their retention times in isothermal quartz columns. 

The results of studies of copper surfaces by low-temperature adsorption isotherms may be summarized as follows. True surface areas of metallic specimens as small as 10 sq. cm. can be derived with a precision of 6% from low-temperature adsorption isotherms using vacuum microbalance techniques. This method is of special value in determining the average thickness of corrosion films formed by the reaction of gases or liquids with solids. The effect of progressive oxidation of a rough polycrystalline metal surface is to decrease the surface area to a point where the roughness factor approaches unity. 

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