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NACE SP0391-20011.6 Concentrated sulfuric acid is an oxidizing agent, and because of its affinity for water, it is also a desiccant. T
he major problems in its handling and storage relate to its hygroscopic nature (absorbing atmospheric humidity), the exothermic reactivity with water on dilution, and
velocity effects that tend to accelerate corrosion of iron- and leadbased alloys....
2.1 Carbon Steel
2.1.1 Carbon steel is satisfactorily resistant to concentrated sulfuric acid at ambient temperatures under static or low-velocity conditions (less than 0.9 m/s [3 ft/s]); for some restrictions, see Paragraphs 2.1.4 and 3.3.6. The initially high corrosion rate is quickly reduced by the formation of an insoluble sulfate film that is highly protective unless physically disturbed. However, even at nominal flow velocities within the prescribed limits, rapid, localized attack may occur. For example, short-radius elbows or excessive internal protrusion of circumferential welds may cause downstream turbulence resulting in high corrosion rates.
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2.2 Cast Iron
2.2.1 Gray cast iron and ductile cast iron are both more resistant to corrosion by concentrated sulfuric acid than is carbon steel. Velocities up to 1.5 m/s (4.9 ft/s) can be tolerated. Frequently, velocities up to 2 m/s (7 ft/s) are accepted in large-diameter pipe (approximately 350 mm [14 in.] in diameter and larger), but accelerated corrosion may occur at joints and bends as a result of flow disturbance.
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2.3 Stainless Steel
2.3.1 The conventional austenitic stainless steels of the Cr-Ni and Cr-Ni-Mo type are generally resistant to ambient-temperature, concentrated acid because they have a naturally formed passive film. UNS S31600 is suitably resistant within the 90 to 100% concentration range. UNS S30400 should only be used at concentrations of 93% or greater. Conventional austenitic stainless steel grades find application in piping in which moderately high velocity is a factor or in areas where iron contamination of the acid must be minimized. Although intergranular corrosion of sensitized stainless steel is not a problem in concentrated acid, the low-carbon grades (UNS S30403 and UNS S31603) are used for welded piping systems to prevent intergranular attack if dilution or contamination can occur.
2.3.2 The molybdenum-bearing grades (e.g., UNS S31600/S31603 or UNS J92900/J92800) are recommended for improved resistance to velocityenhanced corrosion or impingement in valves in the entire concentration range.
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3.3 Piping
3.3.1 Materials selection for concentrated sulfuric acid piping depends on a number of factors including size, velocity, pumping schedule, contaminants, desired life, and potential dilution.
3.3.2 Carbon steel piping may be used at ambient temperature and low flow velocity (i.e., maximum 0.9 m/s [3 ft/s]). Flow velocities of up to 1.5 m/s (4.9 ft/s) may be allowed if the pumping schedule is brief (e.g., a
few hours per day). Good-quality welds are essential, and multipass welds should be used. See Paragraph 2.1.6 for information on potential problems caused by lack of internal smoothness.
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3.3.4 Ductile cast iron pipe has an increased tolerance for higher velocities when compared with steel. Ductile cast iron is preferred to gray cast iron, because it offers better ductility in addition to comparable corrosion and erosion resistance in the acid strengths covered by this standard (see Paragraph 2.2).
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3.3.6 For small-diameter piping (e.g., less than 75 to 100 mm [3 to 4 in.]), the use of austenitic stainless steel is recommended at ambient temperatures and velocities up to 1.8 m/s (5.9 ft/s). UNS S30400/S30403
may be used for 93 to 100% acid and UNS S31600/S31603 may be used for 90 to 100% acid (see also Paragraph 2.3.1).
3.3.7 UNS N08020 has been successfully used for small-diameter piping, especially when flow velocities exceed 1.8 m/s (5.9 ft/s).
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