Austenitic stainless steels, corrosion carbonate

Table 9.1 summarizes environmental alloy combinations that have been shown to produce see. The test temperature accelerates the See for most of the systems listed in Table 9.1. Electrochemical methods and stress corrosion tests should be performed to evaluate possible corrosion environments for a given alloy. More information on these and additional systems may be found in the ASM Metals Handbook [30]. Other significant alloys include nickel alloys [31], austenitic stainless steel [30], carbon steels [32], copper alloys [33], ferritic, martensitic, duplex [31,32], titanium alloys [33], and aluminum alloys [34].

Although Hitec is nonflammable, it is a strong oxidizer and supports the combustion of other materials. Consequendy, combustible materials must be excluded from contact with the molten salt. Hitec is compatible with carbon steel at temperatures up to 450°C. At higher temperatures, low alloy or austenitic stainless steel is recommended. Adding water to Hitec does not appreciably alter its corrosion behavior. 

Addition of niobium to austenitic stainless steels inhibits intergranular corrosion by forming niobium carbide with the carbon that is present in the steel. Without the niobium addition, chromium precipitates as a chromium carbide film at the grain boundaries and thus depletes the adjacent areas of chromium and reduces the corrosion resistance. An amount of niobium equal to 10 times the carbon content is necessary to prevent precipitation of the chromium carbide. 

Austenitic stainless steels are the most corrosion-resistant of the three groups. These steels contain 16 to 26 percent chromium and 6 to 22 percent nickel. Carbon is kept low (0.08 percent maximum) to minimize carbide precipitation. These alloys can be work-hardened, but heat treatment will not cause hardening. Tensile strength in the annealed condition is about 585 MPa (85,000 Ibf/in"), but workhardening can increase this to 2,000 MPa (300,000 Ibf/in"). Austenitic stainless steels are tough and ducdile. 

Furnace tubes, piping, and exchanger tubing with metal temperatures above 800°F now tend to be an austenitic stainless steel, e.g., Type 304, 321, and 347, although the chromium-molybdenum steels are still used extensively. The stainless steels are favored beeause not only are their creep and stress-rupture properties superior at temperatures over 900°F, but more importantly because of their vastly superior resistance to high-temperature sulfide corrosion and oxidation. Where corrosion is not a significant factor, e.g., steam generation, the low alloys, and in some applications, carbon steel may be used. 

Carbon dioxide produces a solution of carbonic acid (as in boiler condensate, see Section 53.3.2). Carbon steel is often employed but corrosion rates of up to 1 mm/yr can be encountered. Coatings and non-metallic materials may be employed up to their temperature limits (Section 53.5.6). Basic austenitic stainless steels (type 534) are suitable up to their scaling temperatures. 

Intercrystalline corrosion was a serious problem with the austenitic stainless steels early in their development since carbon contents then were relatively high, e.g. En58J contained up to 0.12type stainless steel contained up to 0.08 Vo C. The problem in relation to surgical implants has been reported by Scales eta/. and as a result of this and several other reports the British, American and International Standards specified the use of a 316S12 type austenitic stainless steel which contains 0.03 Vo C max. The use of the lower carbon content stainless steels as specified in the various standards has now eliminated the problem of sensitisation of implants. If manufacturers do use the 0.08% C versions they have to be very careful with the forging temperatures or anneal the prostheses afterwards. 

A form of boiler waterside, caustic stress-corrosion cracking corrosion affecting carbon steels and austenitic stainless steels (300 series). Particularly associated with high localized concentrations of deposited sodium hydroxide (caustic soda). 

The austenitic stainless steels that are not stabilized or that are not of the extra-low-carbon types, when heated in the temperature range of 450 to 843°C (850 to 1,550°F), have chromium-rich compounds (chromium carbides) precipitated in the grain boundaries. This causes grain-boundary impoverishment of chromium and makes the affected metal susceptible to intergranular corrosion in many environments. Hot nitric acid is one environment which causes severe... 

The martensitic alloys contain 12 to 20 percent chromium with controlled amounts of carbon and other additives. Type 410 is a typical member of this group. These alloys can be hardened by heat treatment, which can increase tensile strength from 550 to 1380 MPa (80,000 to 200,000 Ibf/in ). Corrosion resistance is inferior to that of austenitic stainless steels, and martensitic steels are generally used in mildly corrosive environments (atmospheric, freshwater, and organic exposures). In the hardened condition, these materials are very susceptible to hydrogen embrittlement. 

If austenitic stainless steel parts exposed to conditions that promote intergranular corrosion are to be fabricated, hard faced, overlaid, or repaired by welding, these parts shall be made of low-carbon or stabilized grades. 

NOTE Overlays or hard surfaces that contain more than 0,10% carbon can sensitise both low-carbon and stabilised grades of austenitic stainless steel unless a buffer layer that is not sensitive to intergranular corrosion is applied. 

High-carbon austenitic structures can be preserved at ambient temperatures if the iron is alloyed with sufficient nickel or manganese, since these metals form solid solutions with 7-Fe but not with a-Fe. If over 11% chromium is also present, we have a typical austenitic stainless steel. Such steels are corrosion resistant, nonmagnetic, and of satisfactory hardness, but, because the a-Fe 7-Fe transition is no longer possible, they cannot be hardened further by heat treatment. Figure 5.9 summarizes these observations. 

Nitric acid is normally stored in flat-bottomed, roofed tanks that are made from low-carbon, austenitic stainless steel. Most concentrations of nitric acid are available in tank cars and by truck. Stainless steel is necessary for concentrations up to 80% to 85%. Stronger solutions are less corrosive and may be stored in aluminum. Approximately 90% of nitric acid production is consumed on site to make ammonium nitrate (AN) fertilizers and some industrial explosives. Thus, the merchant market for nitric acid accounts for only 10% of the total. The production of AN fertilizers and most industrial explosives do not require acid concentrations in excess of the azeotropic composition of 68.8%. ... 

Chloride stress cracking corrosion was also a problem when austenitic stainless steels were covered with thermal insulation. The NACE Paper also discusses the protective coatings for stainless steels and provides sandblasting and coating application guides for stainless and carbon steels. 

Storage and Distribution. Nitric acid is normally stored in flat-bottomed, roofed tanks that are made from low-carbon, austenitic stainless steel. Most concentrations of nitric acid are transported in tank cars and by truck. Stainless steel is necessary for concentrations up to 80 to 85 percent. Stronger solutions are less corrosive and may be stored in aluminum. 

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