Select alloys that perform well in sulfuric acid

A variety of stainless steels and high-nickel alloys, as well as carbon steel, meet a wide range of temperatures and concentrations.

Although it is highly corrosive, concentrated sulfuric acid at ambient temperatures is customarily handled in carbon steel equipment. In general, the aggressiveness of the acid varies with its concentration and temperature, its velocity relative to exposed surfaces, and the nature of possible contaminants.

This article examines the behavior of several stainless steel and nickel-bearing alloys (Table 1), taking account of the above variables. Discussion begins with a look at alloys suitable for equipment used in the manufacture of sulfuric acid and in the storage and handling of cold, concentrated commercially-pure acid; then, we present guidelines for the selection of corrosion-resistant materials in processes using sulfuric acid. The following concentration ranges are defined:

This article does not cover concentrations in excess of 98%, e.g., anhydrous, fuming sulfuric acid, or oleum.

The various alloys interact with sulfuric acid in a complex manner. A single process may involve not only a wide range of sulfuric acid concentrations, but also the presence of such contaminants as ferric and cupric ions, nitrates, chlorides, ammonia, sulfur dioxide, and metallic oxides. While the hydrogen ion activity of sulfuric acid attains its maximum at about 5N (25% concentration), its activity diminishes above and below this level. The foregoing variables exert significant influence not only on the oxidizing capacity of sulfuric acid solutions, but also on the performance of stainless steels and nickelbearing alloys in contact with them.

Sulfuric acid manufacture

The past two decades have witnessed major changes in sulfuric acid manufacture. Plants are growing steadily larger and more energy-efficient; they are designed to minimize process energy consumption, while maximizing energy recovery and using steam to generate electricity. The Claus process is rapidly being supplanted by three newer technologies:

1. Combustion of sulfur;

2. Recovery of SO^sub 2^ from metallurgical processes, such as roasting of pyrites and other sulfide ores; and

3. Regeneration from spent acid.

For all three routes, the last two processing steps are catalytic conversion and absorption. The catalytic converter is the most important part of the plant. In it, SO^sub 2^ at about 425 deg C (800 deg F) reacts with excess air in the presence of a catalyst in a multipass operation. Since the reaction is highly exothermic, the gases traverse a cooling chamber after each pass through the catalyst bed. The SO^sub 3^ then enters the absorbing towers where it reacts to form sulfuric acid. About 35% of the heat generated during the process serves to raise the temperature of the sulfuric acid produced. It has been found that in the temperature range 155-190 deg C (310-370 deg F) relatively low corrosion rates prevail for stainless alloys such as American Iron and Steel Institute (AISI) Type 304L - Unified Numbering System (UNS) S30403 and Type 310L (S31003), and for duplex stainless compositions such as Type 2205 (S31803), and for high-silicon modified stainless steel, nominal composition 18% Cr, 18% Ni, 5.5% Si. Figure 1 shows the relevant isocorrosion curves for selected alloys used in the heat-recovery step.

Stainless steel has replaced carbon steel and cast iron as the preferred material of construction for converters and acid coolers. Newer alloys and anodic passivation surface treatments have increased resistance to corrosion by hot acid and oleum. In general, Type 304L is specified for the shell and internals of the converter. Drying towers, acid pump tanks and piping, as well as the acid coolers, are made from silicon-modified stainless steel. As an alternative, anodically protected AISI Type 316L (S31603) stainless steel has become the alloy of choice for acid coolers. Silicon stainless steel works well at acid temperatures higher than those for which anodically protected 316L is recommended. Table 2 provides typical corrosion rates for these and other alloys in 99% sulfuric acid at a temperature of 105-115 deg C (220-240 deg F) in an absorption tower.

Cold, concentrated sulfuric acid

The term "concentrated sulfuric acid" generally refers to the concentration range 90-100%. At concentrations of 93-98%, commercial sulfuric acid can be stored in carbon steel tanks, provided that iron contamination does not present a problem.

Carbon steel provides satisfactory corrosion resistance in the concentration range between 80 and 100% at ambient temperatures in static conditions, and at low velocity. In this service, carbon steel depends for its corrosion resistance on a ferrous sulfate film that is easily removed by erosion or turbulence. Rapid corrosive attack takes place in the absence of that film. Therefore, the maximum permissible design velocity in carbon steel is set at 2 ft/s.

Turbulent flow occurs in elbows, tees, valves, and rough or uneven internal pipe surfaces, e.g., steps or ledges created by improper pipe alignment during welding. Turbulence so created may extend over a distance of several pipe diameters downstream. Moreover, the solubility of the ferrous sulfate layer is directly proportional to temperature. For carbon steel, 50 deg C (120 deg F) is considered the upper limit in acid of 92-98% concentration; between 78 and 90% concentration, the corresponding temperature is only 25 deg C (80 deg F).

The remainder of this section suggests materials that are suitable for equipment and components in contact with concentrated sulfuric acid:

Storage tanks - These may be carbon steel, provided that a corrosion allowance of 1/8-1/4 in. (3-6 mm) above the design wall thickness can be accommodated. Discharge nozzles should be Type 316L.

Piping - Use ductile iron for large-diameter piping, and 304L or 316L for piping having an internal diameter of 3 in. (7.6 cm) or less.

Valves and pumps - Types CF3M (J92800) (19% Cr, 11% Ni, 2.5% Mo) and CN-7M (J95150) (20% Cr, 29% Ni, 2.5% Mo, 3.5% Cu) are usually selected to cope with velocity effects. CN-7M and CD4-MCu (J93370) (26% Cr, 5% Ni, 2% Mo, 3% Cu) can be specified for valves and pumps at little additional cost. These casting alloys also offer resistance to dilute acids up to specific limiting temperatures.

Tank cars and road tankers - An iron sulfate sludge tends to accumulate in these tanks and must be removed periodically. Because handling and brushing the internal surfaces is unavoidable during sludge removal, the tanks should be epoxy coated internally or preferably specified in 304L. Lightweight road tanker construction favors stainless steel, either in 304L or 316L. Tanks made from these alloys can carry a wide range of products, are easy to clean, and reduce the hazards of product contamination.

When concentrated sulfuric acid is fed into an aqueous stream, the temperature of the diluted acid rises. To prevent attack by the hot acid, the mixing section is usually a spool of 14.5% silicon iron. A check valve is provided to prevent backup of the intermediate-strength acid. Corrosion rates inside a dilution device usually exceed the capabilities of the various stainless steels and Alloy 20 (N08020). Cast or wrought control valves can be made from Alloy C-276 (N10276).

In summary, the major problems associated with handling and storage of concentrated sulfuric acid relate to:

1. Hygroscopic effects;

2. Exothermic temperature rise on dilution; and

3. Velocity effects that erode or otherwise damage protective surface layers.

Selecting corrosion-resistant materials

Corrosion is a complex and baffling phenomenon. Seemingly unimportant variables, such as minute amounts of impurities, can materially change the performance of alloys. Nevertheless, concisely summarized information is useful in that it permits a bird's eye view of a situation, a preliminary screening that helps to minimize the number of materials to he considered and evaluated. Figure 2, an isocorrosion chart based on pure sulfuric acid, presents such a screening tool. It summarizes the limits of usefulness for some of the alloys shown in Table 1. Figure 2 also indicates the inhibiting effect which metal sulfate corrosion products exert on the performance of stainless steels (the dotted lines in the figure).

Effect of contaminants

Oxidizing contaminants include sulfur dioxide, dissolved oxygen, nitrite or nitrate ion, chromates or vanadates, and oxidizing metallic cations -- notably ferric and cupric ions. If these are present in sufficient quantity, they can passivate stainless steel. Figure 3 demonstrates the relevant effect for 304L and 316L. The effect of ferric ion is not significant in highly-concentrated acid, 96% and beyond. Great care must he exercised if oxidizing agents are to be relied on as corrosion inhibitors to passivate alloys. This practice can be very dangerous, especially in the presence of chlorides or in equipment that contains undercuts or crevices wherein the inhibitors may be depleted locally.

Reducing contaminants include the halides, various compounds of arsenic and antimony, as well as hydrogen sulfide. These adversely affect the performance of stainless steel.

Specialty alloys

Iron- and nickel-base alloys have been developed to cope with intermediate concentrations of sulfuric acid in conditions beyond the capability of Type 316L. These alloys can be specified across the full range of acid concentrations up to 65 deg C (150 deg F). Comparable performance is obtained from the 6% molybdenum alloy 904hMo (N08925) and Alloy 28 (N08028) stainless steels, as well as the highnickel alloys 625 (N06625), G-3 (N06985), and C-276. The performance of some representative compositions in reagent sulfuric acid is compared in Figure 4 in which isocorrosion lines are for 20 mpy. The differences between their effectiveness are governed by the nature of the impurities that are present. In some cases, these impurities can extend the passivity of the alloys up to a temperature of 80 deg C (175 deg F), or even to the boiling point in the concentration range up to 40%.

The literature often describes corrosion resistance in terms of isocorrosion lines for rates of 5, 20, or 50 mpy (mils/yr -1 mil = 0.025 mm). Corrosion rates below 3 mpy are meaningful for passive film forming alloys; alloys with corrosion rates above 20 mpy should not be used.

In nonpassivating nickel-base alloys such as Alloy 400 (N04400) and Alloy B-2 (N10665), corrosion resistance usually depends on the insolubility of the protective surface film of metallic sulfate corrosion product. In the absence of oxidizing species, these alloys perform better than the chromium-bearing grades in acids of less than 25% concentration and temperatures up to the boiling point. Under these conditions, Alloy B-2 may be specified in concentrations up to 50%, while Alloy 400 may be used at temperatures up to 65 deg C (150 deg F) in the range of 25-50%, provided that no accumulation of cupric corrosion products can occur. The resistance of Alloy B-2 diminishes at acid concentrations of 70%, and Hastelloy Alloy D can be used for acid concentrations of 70-100% at temperatures over 65 deg C (150 deg F). Alloy B-2 and Alloy 400 will also resist chloride ion contamination in the acid.

Casting alloys

Comparable casting alloy specifications are available for many of the alloys mentioned above. Although their composition usually deviates somewhat from the corresponding wrought specifications, the casting alloys offer substantially similar corrosion resistance. Traditional high-nickel casting alloys, such as Illium and Chlorimet, have been largely replaced by compositions typically containing 32% Cr, 35% Ni, 16% Fe, 6% Co, 4% Mo, and 3.5% Si. These offer exceptionally good resistance to acids at temperatures up to 125 deg C (260 deg F), even under conditions in which erosion or abrasion may occur.

Miscellaneous materials

At acid concentrations below 80%, lead has been a traditional material of construction. However, its use has decreased because of its toxicity and because other more cost-effective materials have become available. In weak sulfuric acid, titanium is of interest only if powerful oxidants are present, as, for example, in the high-pressure leaching of copper ores at 15% acid concentration. Zirconium can be useful in acid concentrations up to about 60%, although care is required at approximately 60% and 80 deg C (180 deg F) because, in these conditions, the corrosion products are pyrophoric and pose an ignition hazard. Tantalum may be used at all acid concentrations up to the atmospheric boiling point; however, in concentration ranges between 77 and 100%, it should not be specified at temperatures above 190 deg C (375 deg F). Similar performance may be expected of glass, although neither it nor tantalum should be used in the presence of fluorides where glass suffers direct chemical attack while tantalum undergoes rapid deterioration as a result of hydriding phenomena. The 14.5% silicon-iron alloy can handle the full range of sulfuric acid concentrations up to the boiling point, but is also attacked by fluoride contamination that removes the protective siliceous film. Now, we will look at alloys for common sulfuric acid services.

Hydrometallurgy

AISI Type 316L is satisfactory for the tanks, piping, and other equipment in a copper refinery. Acid concentrations vary between 13 and 15%; the temperature in the electrowinning bath varies from 50-65 deg C (120-150 deg F). Cupric sulfates inhibit corrosion at the surface of the alloy. However, 316L is not suitable for the 100-psi heating coils that cause the metal to attain temperatures beyond its permissible limit. Alloy 20 or 904L (N08904) would be more appropriate. In zinc refineries, extensive applications for stainless steel include 304L for cathode starting sheets, and 316L for evaporators and crystallizers that handle byproducts such as copper sulfate. The finished crystalline product is conveyed and stored in equipment made from Type 304L. In Figure 2, dotted lines show the inhibiting effect of metal sulfate salts on the performance of Types 3i6L and 904L. Similar benefits are obtained for Type 304L.

Organic sulfation and sulfonation

Sulfuric acid at 93% concentration and at temperatures between room and 60 deg C (140 deg F) is suitable for sulfation of oils to make wetting agents and penetrants. Usually processed in batches, the sulfation product is salted out, washed, and neutralized with caustic soda. Frequently, all of these steps take place in the same vessel. Alloy 400 is specified not only for the vessel, but also for the heating coils, pipe, agitators, and pumps. A wide variety of detergents and wetting agents is now made by sulfation of fatty alcohols or fatty esters. Steel and cast iron reaction equipment may be used, provided that the sulfuric acid concentration is not permitted to fall below 80% during the reaction. If the concentration of the acid is likely to fall below that limit, Alloy 400 or Alloy 20 should be specified.

Petroleum refining

In sulfuric acid alkylation, the hydrocarbons are emulsified in 98% acid and reacted at temperatures in the range of -1 deg C to +10 deg C (30 deg F to 50F). The acid remains fairly concentrated, diluting to about 88%. Mild steel offers satisfactory corrosion resistance except in areas where high velocity may be encountered, such as in pumps, valves, or return bends, for which 304L or 316L should be specified.

Acid treatment of lubricating oils and other distillates is generally carried out in 83% acid at 65 deg C (150 deg F), although some processors may operate at temperatures as high as 105 deg C (220 deg F). The acid is diluted with water to facilitate separation of the sludge which arises after treatment. Batch operations commonly use Alloy 400, which can also be specified for the high-speed centrifugal equipment that accomplishes the separation of oil from sludge. Because acid and oil remain mixed in this equipment, corrosion is usually not severe at moderate temperatures.

Ammonium sulfate

Ammonia-rich gases pass upward through a tower countercurrently to a stream of ammonium sulfate solution that contains sulfuric acid at a temperature of 50 deg C (120 deg F) and concentrations between 4 and 10%. The exiting solution is pumped to a crystallizer and then to centrifuges or filters. In this process sequence, both Alloy 400 and Type 316L have performed well for the scrubber, crystallizer, motherliquor mixing tank, and the settling tanks. In an alternative process sequence, ammonium sulfate is generated by reaction of synthetic ammonia gas with concentrated sulfuric acid in a mother liquor of concentrated ammonium sulfate. Operating temperatures range between 80 and 105 deg C (180-220 deg F). While Alloy 400 is unsuitable in this temperature range, 316L may be used for reactor and crystallizer equipment.

Phosphate fertilizers

Phosphate rock feed material enters the system as a slurry that reacts with a mixture of concentrated sulfuric acid and partially spent acid that has been recycled from the filtering operation. The agitator paddles and shafting are commonly made from Alloy 20, Alloy 825, or Alloy 904L. The same alloy selection applies to a slight variant of the foregoing process sequence where the sulfuric acid is prediluted with filter acid prior to entering the reactor. However, the service life of the foregoing alloys has proved to be significantly shorter in newly designed isothermal reactors that are characterized by higher velocities and more severe agitation and abrasion by the gypsum solids. Here, Alloy G-3 has proved to be successful for the draft tube, shafting, and impeller. Depending upon the type of feedstock, corrosive attack in these units can become exceptionally severe. Some phosphate rocks contain high concentrations of fluorides and chlorides in addition to oxidizing metal salts, chlorates, and manganese peroxides. Occasionally, only alloys such as Alloy C-276 or Alloy 625 will suffice in this environment.

Flue-gas scrubbers

After the effluent gases from power-generating plants have been scrubbed free of sulfur dioxide, these wet gases are routinely mixed with some 10% by volume of nonscrubbed hot gases. This is to raise the temperature of the scrubbed gases from 60 to 70-80 deg C (135-160 deg F) to increase stack efficiency. This practice has brought about extremely severe corrosion in the outlet ducting, the stack breeching section, and the stack liner. The attack is particularly virulent during startup and abnormal operating conditions when a higher-than-normal percentage of sulfur dioxide-bearing flue gas may bypass the scrubbers. Since the mixed gases will be at a temperature below their dew points, they will throw off condensate. The adiabatic saturation curve for sulfuric acid (Figure 5) helps to determine the approximate concentrations of the acid to be expected in the condensing droplets. Under normal operating conditions, these concentrations range from 25-55% sulfuric, but they can rise to as high as 80% under full bypass conditions. Several plants have successfully applied Alloy 625 and Alloy C-276 for the exhaust ducting, breeching section, and stack liners. High-alloy cladding and thin-gage alloy liners have been employed as an added economy measure.

Acid pickling

Hot-rolled, forged, or heat-treated steel parts are pickled in hot sulfuric acid solutions to remove oxide scale. Acid concentrations range from 5 to 15% at temperatures of 60-95 deg C (140-200 deg F). Alloy 400, containing 66.5% nickel and 31.5% copper, is the accepted material of construction for crates, racks, baskets, hooks, chains, and other components that support the steel during the pickling sequence. The pickling reactions appear to consume any oxygen which may have been dissolved in the acid. Hydrogen evolved by reaction of the acid with the steel helps to keep the metallic ion contaminants in a reduced state. Moreover, Alloy 400 may become galvanically protected by the steel parts with which it comes in contact. Alloy 400 racks are also chosen for pickling brass and copper, and for the racks that hold the product to be pickled prior to enameling or carburizing.

Alloy 825 (N08825) may be specified for most pickling applications. In one case, it replaced Alloy 400 for a hook that conveyed wire bundles through an acid bath. The first stage was a dip into H2SO4 at a concentration of 12% and a temperature of 70C (160oF) in the presence of sodium dibromate. This treatment was followed by a dip into a mixture consisting of 60% H^sub 2^SO^sub 4^, 25% HNO^sub 3^, and 0.2% HCl, by weight. Alloy 825 achieved excellent results not only for the hook, but also for the heating coils in both pickling baths.

To sum up

Nickel, chromium, molybdenum, copper, and silicon are the most important of the elements that enhance the corrosion resistance of alloys in sulfuric acid service. Relatively newer alloys, such as Alloy 904L and Alloy 28, have proved to be highly corrosion resistant as well as cost-effective. Please refer to Table 3 for relative cost indications of the various alloys. Many of these alloys resist sulfuric acid both under reducing and oxidizing conditions. Established alloys that have performed satisfactorily over several decades include Alloy 20 and Alloy 825. Under the most severe conditions, Alloy 625 and Alloy C-276 receive consideration. Bronzes and nickel-base alloys containing 28% molybdenum or 14.5% silicon, respectively, have lost ground to the stainless steels and nickel-bearing alloys that rely for their corrosion resistance on a passivated surface layer. Newer sulfuric acid manufacturing flowsheets have been developed and promoted by major chemical plant constructors seeking maximum operating economy through energy savings and low maintenance costs. These installations make effective use of 304L, 316L, and Alloy 904L.

In general, three strategies may be employed to tackle corrosion problems that arise with equipment in sulfuric acid service:

1. Addition of an oxidizing agent;

2. Application of anodic protection; and

3. Selection of a more-corrosionresistant alloy.

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Copyright (C) Nickel Development Institute, Toronto, ON, Canada. Used with permission.

[Author Affiliation]

C. M. SCHILLMOLLER is head of Schillmoller Associates, Belleair, FL (Tel. and Fax: 813/4435114). He has over 36 years of experience in the market development of stainless steels and nickel-containing alloys for corrosion control and high-temperature service. Schillmoller has held senior marketing positions with INCO and VDM in the U.S., Australia, and Europe. He is a chemical engineering graduate from the University of Sydney, Australia, a PE in California, and a NACE International Accredited Corrosion Specialist. He is the author of over 275 technical papers and articles dealing with alloy applications in the petroleum, petrochemical, and chemical industries, has lectured at Stanford and the University of California (Berkeley), and has taught numerous corrosion short courses. He is a member of NACE and AIChE.