Stainless steels critical temperature

Fluorine can be handled using a variety of materials (100—103). Table 4 shows the corrosion rates of some of these as a function of temperature. System cleanliness and passivation ate critical to success. Materials such as nickel, Monel, aluminum, magnesium, copper, brass, stainless steel, and carbon steel ate commonly used. Mote information is available in the Hterature (20,104). 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

Fatty acids are corrosive at high temperatures and selection of materials of constmction for distillation systems is critical. Stainless steels with various contents of molybdenum have proved satisfactory. For example, 316 L has 2% Mo and is satisfactory for service up to 260°C 317 L has 3% Mo and can be used satisfactorily up to 285°C, whereas 904 L can be used up to 310°C (31). 

It is known that the common austenitic stainless steels have sufficient corrosion resistance in sulfuric acid of lower concentrations (<20%) and higher concentrations (>70%) below a critical temperature. If with higher concentrations of sulfuric acid (>90%) a temperature of 70°C is exceeded, depending on their composition, austenitic stainless steels can exhibit more or less pronounced corrosion phenomena in which the steels can fluctuate between the active and passive state [19]. 

In critical applications, if stainless steel is to be used near its limit (in terms of corrosion), and for cases such as welds, where a good finish cannot be otherwise achieved, additional passivation is required. Nitric acid (10-15 per cent by volume) is the best passivator. It also dissolves iron contamination. In circumstances where the use of nitric acid is not possible for safety or physical reasons (such as the underside of vessel roofs) passivation paste is appropriate. Both materials are used at ambient temperature and require a contact time of approximately 30 minutes. They must be removed by thorough rinsing with low chloride-content water. 

Compared with ferritic carbon and low-alloy steels, relatively little information is available in the literature concerning stainless steels or nickel-base alloys. From the preceding section concerning low-alloy steels in high temperature aqueous environments, where environmental effects depend critically on water chemistry and dissolution and repassivation kinetics when protective oxide films are ruptured, it can be anticipated that this factor would be of even more importance for more highly alloyed corrosion-resistant materials. 

The critical pitting temperature (CPT) is widely used as a measure of the resistance of stainless steel against pitting attack. Various methods for determination of the CPT are described here, special attention being given to the choice of test potential for the control of stainless steel quality. 

Using this method, it takes many days to determine a single reliable CPT value. Salinas-Bravo and Newman published in 1994 what they called An alternative Method to Determine Critical Pitting Temperature of Stainless Steels in Ferric Chloride solution. ... 

Validation of the database. This is the final part in producing an assessed database and must be undertaken systematically. There are certain critical features such as melting points which are well documented for complex industrial alloys. In steels, volume fractions of austenite and ferrite in duplex stainless steels are also well documented, as are 7 solvus temperatures (7 ) in Ni-based superalloys. These must be well matched and preferably some form of statistics for the accuracy of calculated results should be given. 

SFC chromatographs represent hybrids between GC and HPLC instruments (Fig. 6.4). In order to deliver the supercritical fluid, syringe pumps or reciprocal pumps are used and maintained above the critical temperature using a cryostat regulated at around 0 "C. In instances where an organic modifier is used, a tandem pump is employed which has two chambers, one for the critical fluid and one for the modifier. The liquid then passes through a coil maintained above the critical temperature so that it is converted into a supercritical fluid. Stainless steel packed columns like those used in HPLC (1 to 4 mm in diameter) or fused silica capillary columns like those used in capillary GC (2 to 20 m in length, internal diameters as low as 50 pm and stationary phase film thickness of at least 1 pm) are used in SFC. 

The thermal time constant of a reactor characterizes the dynamics of the evolution of the reactor temperature. In fact, since it contains the ratio of the mass proportional to volume with the dimension L3 to the heat exchange area with the dimension L2, it varies non-linearly with the reactor scale, as is explained in Section 2.4. Some values of the time constant obtained with normalized stainless steel reactors [1] are summarized in Table 9.3. The variation by a factor of about 7, over the range considered here, is critical during scale-up. The heating or cooling times are often expressed as the half-life, the time required for the temperature difference to be divided by two ... 

Design of the experiment and scale-up. It is at this stage of the product s life cycle that many control parameters are evaluated (e.g., glass container in lieu of stainless steel, temperature controls). Information should be collected at critical processing stages and included in the master production record. This information will be used to assist in scale-up activities. 

The choice of possible mobile phases is more limited. The critical properties (critical pressure pc and temperature Tc) should be within practical reach. Moreover, stable compounds are required, which do not show disintegration at elevated temperatures and pressures. Also, the mobile phase must not be too agressive towards the materials used in the column (usually silica-based phases) and the instrumentation (mainly stainless steel). Therefore, mobile phases that are extremely interesting from a chemical point of view, such as supercritical ammonia and, especially, supercritical water, have found little use so far. Table 3.9 lists some possible mobile phases for SFC together with their chemical properties. 

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