High austenitic stainless steels

This section deals with the corrosion behavior of duplex stainless steel, some high austenitic stainless steels, and a nickel-based alloy in 28% HCl acidizing solutions, either in inhibited or uninhibited conditions, at 130 °C. Weight loss, crevice corrosion, and stress corrosion cracking tests were carried out for 6 and 24 hours, with amine-based commercial corrosion inhibitors, originally formulated for carbon steel. 

One laboratory investigation has studied four high austenitic stainless steels and a duplex stainless steel. The low-alloy steel was tested since the corrosion inhibitors used were formulated for carbon and low-alloy steels. Chemical compositions (% by weight) and relevant mechanical properties in as-received conditions are respectively reported in Tables 8.6 and 8.7. 

High austenitic stainless steels, in the presence of corrosion inhibitors, suffer erevice corrosion attacks occurring in all locations. Table 8.11 shows the results of the erevice corrosion tests in 28% HCl at 130°C. All results of SCC tests are reported in Table 8.12. 

Commercial corrosion inhibitors, formulated for low-aUoy steels, showed good effectiveness when used with high austenitic stainless steels and nickel-based alloys. 

Crevice corrosion tests revealed susceptibility to corrosion attack in the presence of corrosion inhibitors, while in their absence no preferential attack in crevice sites was observed, see, in the presence of commercial inhibitors, did not occur for any tested materials in the absence of corrosion inhibitors, transgranular microcracks were present at the bottom of pits on high austenitic stainless steel. 

Formic acid is commonly shipped in road or raH tankers or dmms. For storage of the 85% acid at lower temperatures, containers of stainless steel (ASTM grades 304, 316, or 321), high density polyethylene, polypropylene, or mbber-lined carbon steels can be used (34). For higher concentrations. Austenitic stainless steels (ASTM 316) are recommended. 

The highly aHoyed austenitic stainless steels are proprietary modifications of the standard AISI 316 stainless steel. These have higher creep—mpture strengths than the standard steels, yet retain the good corrosion resistance and forming characteristics of the standard austenitic stainless steels. Nickel-Base Superalloys. 

Corrosion. Copper-base alloys are seriously corroded by sodium thiosulfate (22) and ammonium thiosulfate [7783-18-8] (23). Corrosion rates exceed 10 kg/(m yr) at 100°C. High siUcon cast iron has reasonable corrosion resistance to thiosulfates, with a corrosion rate <4.4 kg/(m yr)) at 100°C. The preferred material of constmction for pumps, piping, reactors, and storage tanks is austenitic stainless steels such as 304, 316, or Alloy 20. The corrosion rate for stainless steels is <440 g/(m yr) at 100°C (see also Corrosion and corrosion control). 

Depth-of-Gut Notching. Depth-of-cut notching (DOCN) is a localized wear process common when machining materials such as austenitic stainless steels or high temperature alloys. Notching is attributed to the chemical reaction of the tool material and the atmosphere, or to abrasion by the hard, sawtooth outer edge of the chip. DOCN may lead to tool fracture. 

Hydrogen Atmospheres Austenitic stainless steels, by virtue of their high chromium contents, are usually resistant to hydrogen atmospheres. 

A specific corrodent. One of the unusual and interesting features of SCC is the specificity of the corrodent. A particular alloy system is susceptible to SCC only when exposed to certain corrodents, some or all of which may be unique to that particular alloy system. For example, austenitic stainless steels (300 series) are susceptible to cracking in chloride solutions but are unaffected by ammonia. Brasses, on the other hand, will crack in ammonia but remain unaffected by chlorides. The corrodent need not be present at high concentrations. Cracking has occurred at corrodent levels measured in parts per million (ppm). 

Microstructural examinations revealed that the cracks originated on the external surface (Fig. 9.15). The cracks were highly branched and transgranular. The branched, transgranular character of these cracks is typical of stress-corrosion cracking of austenitic stainless steels. The thick-walled fracture faces are also typical of cracking by this mode. 

Microstructural examinations disclosed highly branched, predominantly transgranular cracks originating on the internal surface. Cracks of this form are typical of SCC in austenitic stainless steels. 

Use fully killed or fine grain steel, controlled rolling temperatures high Mn/C ratios eliminate sharp corners in design, remove defects from steel heat treat steel. For cryogenic operations use high nickel alloy steels or austenitic stainless steels, depending on temperature. 

Poor Weldability a. Underbead cracking, high hardness in heat-affected zone. b. Sensitization of nonstabilized austenitic stainless steels. a. Any welded structure. b. Same a. Steel with high carbon equivalents (3), sufficiently high alloy contents. b. Nonstabilized austenitic steels are subject to sensitization. a. High carbon equivalents (3), alloy contents, segregations of carbon and alloys. b. Precipitation of chromium carbides in grain boundaries and depletion of Cr in adjacent areas. a. Use steels with acceptable carbon equivalents (3) preheat and postheat when necessary stress relieve the unit b. Use stabilized austenitic or ELC stainless steels. 

Above temperatures of 900°F, the austenitic stainless steel and other high alloy materials demonstrate inereas-ingly superior creep and stress-rupture properties over the chromium-molybdenum steels. For furnace hangers, tube supports, and other hardware exposed to firebox temperatures, cast alloys of 25 Cr-20 Ni and 25 Cr-12 Ni are frequently used. These materials are also generally needed because of their resistanee to oxidation and other high temperature corrodents. 

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. 

Residual stresses occur from welding and other fabrication techniques even at very low stress values. Unfortunately, stress relief of equipment is not usually a reliable or practical solution. Careful design of equipment can eliminate crevices or splash zones in which chlorides can concentrate. The use of high-nickel stainless steel alloy 825 (40% nickel, 21% chromium, 3% molybdenum and 2% copper) or the ferritic/austenitic steels would solve this problem. 

Austenitic stainless steels Chloride (and other halide) ion Polythionic acids High-temperature water Hydrogen sulfide (aqueous)... 

Steel is the most common constructional material, and is used wherever corrosion rates are acceptable and product contamination by iron pick-up is not important. For processes at low or high pH, where iron pick-up must be avoided or where corrosive species such as dissolved gases are present, stainless steels are often employed. Stainless steels suffer various forms of corrosion, as described in Section 53.5.2. As the corrosivity of the environment increases, the more alloyed grades of stainless steel can be selected. At temperatures in excess of 60°C, in the presence of chloride ions, stress corrosion cracking presents the most serious threat to austenitic stainless steels. Duplex stainless steels, ferritic stainless steels and nickel alloys are very resistant to this form of attack. For more corrosive environments, titanium and ultimately nickel-molybdenum alloys are used. 

The shape of a vessel determines how well it drains (Figure 53.7). If the outlet is not at the very lowest point process liquid may be left inside. This will concentrate by evaporation unless cleaned out, and it will probably become more corrosive. This also applies to horizontal pipe runs and steam or cooling coils attached to vessels. Steam heating coils that do not drain adequately collect condensate. This is very often contaminated by chloride ions, which are soon concentrated to high enough levels (10-100 ppm) to pose serious pitting and stress corrosion cracking risks for 300-series austenitic stainless steel vessels and steam coils. 

Nickel-chromium alloys can be used in place of austenitic stainless steels where additional corrosion resistance is required. These alloys are still austenitic but are highly resistant to chloride-induced stress corrosion cracking when their nickel content exceeds 40 per cent. 

A more detailed treatment of sensitisation of austenitic stainless steels, of intergranular corrosion of austenitic stainless steels without sensitisation, and of sensitisation and intergranular corrosion of ferritic stainless steels and high-nickel alloys, is given by Cowan and Tedmon . 

In practice, by far the most common case of stress corrosion is that occurring when austenitic stainless steels are simultaneously exposed to tensile stresses and hot, aqueous, aerated, chloride-containing environments. In this case the major variable is alloy composition and structure virtually all austenitic stainless steels are more or less susceptible to stress-corrosion cracking in these environments, while ferritic and ferritic/austenitic stainless steels are highly resistant or immune. 

Stainless steels are particularly prone to crevice corrosion, and even the Fe-18Cr-8Ni-3Mo type of austenitic stainless steel, which is highly resistant to pitting when the surface is free from crevices, is susceptible although initiation of attack may take 1-2 years... 

Carbon The solubility of carbon in sodium has been measured it is considered lower than the corresponding value for oxygen (2 p.p.m. of carbon at 520°C) but is sufficiently high to give rise to undesirable effects. Carburisation of refractory metals and of austenitic stainless steels has been observed in sodium contaminated with carbon e.g. oil, grease or a low alloy ferritic steel the source of which can be either decomposed organic material, e.g. oil, or a ferritic steel of low or zero alloy content. The latter is an example of... 

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. 

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