Sulfided stainless steel, carbon

Corrosivity. Anhydrous hydrogen sulfide has a low general corrosivity toward carbon steel, aluminum. Inconel, Stehite, and 300-series stainless steels at moderate temperatures. Temperatures greater than ca 260°C can produce severe sulfidation of carbon steel. Alternative candidates for hydrogen... 

Shipment nd Stora.ge, Sulfur monochloride is minimally corrosive to carbon steel and iron when dry. If it is necessary to avoid discoloration caused by iron sulfide formation or chloride stress cracking, 310 stainless steel should be used. Sulfur monochloride is shipped in tank cars, tank tmcks, and steel dmms. When wet, it behaves like hydrochloric acid and attacks steel, cast iron, aluminum, stainless steels, copper and copper alloys, and many nickel-based materials. Alloys of 62 Ni—28 Mo and 54 Ni—15 Cr—16 Mo are useful under these conditions. Under DOT HM-181 sulfur monochloride is classified as a Poison Inhalation Hazard (PIH) Zone B, as well as a Corrosive Material (DOT Hazard Class B). Shipment information is available (140). 

Stainless steels attacked by sulfate reducers show well-defined pits containing relatively little deposit and corrosion product. On freshly corroded surfaces, however, black metal sulfides are present within pits. Rust stains may surround pits or form streaks running in the direction of gravity or flow from attack sites. Carbon steel pits are usually capped with voluminous, brown friable rust mounds, sometimes containing black iron sulfide plugs fFig. 6.10). 

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. 

The vessel is generally constructed of carbon steel that has been heat treated. Control of metal hardness is required because of the potential of sulfide-stress cracking. The iron-sponge vessel is either internally coated or clad with stainless steel. 

The petroleum feedstocks that contain sulfur as an impurity are handled under ambient conditions in carbon-steel tanks and pipelines where the corrosion attack by sulfur is less severe. In the desulfurization step, vaporized feedstock is processed at 400°C in the presence of hydrogen sulfide (H2S) and carbonyl sulfide (COS) - both of which are highly corrosive. Stainless Steels (SS) 304, 316 or 321 (for the fired heater) are used as the material of construction for various pieces of process equipment in this section of the plant. Equipment failures occur because of external corrosion and thinning of fired-heater coils and interior deposition of carbon from coking which leads to overheating. Fuel-gas lines, that contain hydrocarbon vapors and H2S, should be constructed of SS 304 and heat traced to avoid condensation88. 

The inner surface of the stainless steel reactor was deactivated by the following procedure. The tube was cleared by passing air at the rate of 1-2 L/min. for 30 min while maintaining a temperature of 575°C. Hydrogen sulfide was then passed for 30 min at the rate of 250 mL/min. Finally, hydrogen was passed for 30 min at the rate of 3 L/min. To equilibrate the surface a small amount of the liquid hydrocarbon was pumped before starting the experiment. This treatment apparently eliminates any major catalytic effects of the reactor surface, as indicated by the lack of carbon deposits in all runs. 

Carbon monoxide, trace metals, and sulfur compounds, such as HjS, COS, mercaptans, and thiophenes, exist in hydrogen produced from coal gasification and used in molten carbonate Hj/Oj fuel cells. In addition, nitrogen compounds from coal, such as HCN and HCNS can be present or they might oxidize to corrosive NO. While carbon monoxide is reactive in these cells, the rest impurities can either poison the Ni anode or they can attack chemically cell and electrodes 249), for example, HjS sulfidizes nickel and stainless steel. HjS could also undergo oxidation to deposit sulfur 250) ... 

Redox-type waste contains considerable mercury, which must be removed. Advantage is taken of the presence of mercury to use it as a carrier for the ruthenium and technetium when this group is precipitated as the sulfides. This involves fairly corrosive chemical solutions, but they can be handled in equipment fabricated with special grades of stainless steel. The filtrate contains only the alkaline and rare earths which are then precipitated as carbonates, the same as the Purex-type procedure. The waste from this step is treated separately. 

External polythionic acid attack may occur in plants having atmospheric sulfide pollution including gas applications with water condensation. However, the problem does not usually occur on external surfaces in fired equipment because excess air causes sulfates to form, instead of sulfides. Wet sour systems containing carbon steel heat transfer surfaces may generate substantial iron sulfide. Such systems should not be flushed or drained into stainless steel piping or equipment prior to a shut-down unless appropriate precautions are taken. Several metallurgical methods address intergranular corrosion caused by sensitization. 

Bubble-cap trays in the exdiange towers are made of stainless steel, preferably type 304 18-8. Carbon steel is unsuitable because the impingement of spray to which the trays are sulqected prevents an iron sulfide layer from forming, and under these conditions carbon steel conodes rapidly. The same ccmsiderations apply to other parts exposed to erosion by spray... 

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