Corrosion by Ammonia Free

ANHYDROUS AMMONIA (NH₃) is a major commercial chemical that is used in the manufacture of fertilizers, nitric acid (HNO₃), acrylonitrile, and other products. The world total production of NH₃ in 1996 was 93,500 thousand metric tons of nitrogen. In 2002, according to the International Fertilizer Association, the world production was 108,320 thousand metric tons (89,025 thousand metric tons of nitrogen). Approximately 80% of the U.S. apparent domestic NH₃ consumption was for fertilizer use, including anhydrous NH₃ for direct application, urea, ammonium nitrate (NH₄NO₃), ammonium phosphates, and other nitrogen compounds.

Ammonia is made by a number of processes; the two principal ones are:

• Steam reforming of natural gas or other light hydrocarbons (natural gas liquids, liquefied petroleum gas, naphtha). Modified steam reforming using excess air in the secondary reformer and heat-exchange autothermal reforming are also being used to some extent.

• Partial oxidation of heavy fuel oil, coal, or vacuum residue is used.

The hydrogen-containing gas from the primary reformer is mixed with nitrogen in the secondary reformer to produce a gas containing CO₂, H₂, and N₂. The CO₂ is stripped off, and the remaining synthesis gas (syngas) is reacted over an iron catalyst to produce NH₃ that is condensed and separated from the gas stream. Ammonia that leaks into cooling towers can be converted to HNO₃ by nitrifying bacteria and pose a corrosion threat throughout the plant. Since much of the NH₃ production process operates at elevated temperatures, materials resistant to high-temperature corrosion and with sufficient mechanical strength are needed. The anhydrous NH₃ is purified and shipped or stored, usually in steel tanks.

Liquid anhydrous NH₃ is stored in one of three ways, selected on economic grounds (Ref 1, 2):

• Flat-bottomed steel storage tanks are used for storing very large volumes of NH₃, with typical capacity of 10,000 to 30,000 tons (up to 50,000). These tanks are designed for low pressure so the anhydrous NH₃ must be fully refrigerated at atmospheric pressure and at the atmospheric boiling point of −33 °C (−28 °F).

• Semirefrigerated tanks or spheres are used where larger volumes of NH₃ must be stored. These are held at some intermediate temperature, for example, −12 °C (10 °F), between ambient and fully refrigerated −33 °C (−28 °F) conditions. Cylindrical storage vessels (bullets) are also commonly employed.

• Pressurized tanks (around 2 MPa, or 300 psig) at ambient temperature in spheres or horizontal cylinders up to about 1700 tons are also used.

Ammonia and ammonium hydroxide (NH₄OH) are not particularly corrosive in themselves, but corrosion problems can arise with specific materials, particularly when contaminants are present. Air or oxygen contamination is a factor in many instances, causing general corrosion of some materials and localized corrosion, specifically stress-corrosion cracking (SCC), in others. Contamination with carbon dioxide can lead to corrosion due to carbamates, which are sometimes encountered in NH₃ recovery systems. High-temperature corrosion will occur in hot dissociated NH₃.

Aluminum and its copper-free alloys show good resistance to dry, gaseous NH₃ at ambient or elevated temperatures. Corrosion rates of<0.025 mm/yr (<1 mil/yr) at 21 °C (70 °F) and<0.05 mm/yr (<2 mils/yr) at 100 °C (212 °F) are typical (Ref 3).

Aluminum may be used for cargo tanks for the anhydrous product. If moisture is present there is some attack on aluminum, but a protective film soon forms and corrosion stops. Aluminum tubing is used in NH₃ refrigeration units operating in liquid NH₃ containing 5% water. In moist NH₃ vapor, corrosion is low (<0.1 mm/yr, or 3.9 mils/yr) below about 50 °C (120 °F). Under condensing conditions of steam and NH₃, aluminum can be attacked and the rate does not decrease with time. Attack is prevented if the CO₂-to-NH₃ ratio is at least 2.5 to 1. This high level of carbon dioxide can be achieved, for example, in NH₃ recovery plants by the decomposition of alcohols or other organic chemicals. Hydrogen sulfide also inhibits corrosion under condensing conditions. Aluminum is used for compressors, heat exchangers, evaporators, condensers, and piping in the production of NH₃. Aluminum pressure vessels are used in the storage and transport of NH₃ (Ref 4).

There is mild action on aluminum in NH₄OH solutions at temperatures below about 50 °C (120 °F). The greatest attack occurs in concentrated solutions (around 25%) and at about 5% concentration. For aluminum to perform well in NH₄OH the solution must be free from heavy metals and halogen ions. Attack is limited if the solution is saturated with aluminum ions before exposure takes place. As with anhydrous NH₃, aluminum is attacked at higher temperature, but this attack stops as a protective film forms (Ref 5, 6). Even at ambient temperature, corrosion rates decrease with time of exposure (Ref 7).

The initial attack of aluminum by dilute NH₃ solutions (up to ∼10%) is controlled by the diffusion of OH⁻ ions to the surface and is a function of pH. The surface is passivated once sufficient corrosion product has been produced to form a protective film. Under some exposure conditions, the corrosion product may continue to dissolve rather than form a protective film (Ref 4).

Ferrous alloys are generally not corroded by NH₃ or NH₄OH at ambient or elevated temperatures. Corrosion can, however, occur in the presence of contaminants and ferrous alloys can be subject to SCC.

Cast iron has been widely used in NH₃ production and handling. Since it is generally thick walled, somewhat higher corrosion rates can be tolerated as long as the iron in solution is acceptable. Cast iron strippers have been used to concentrate crude gas liquors with about 1 to 2% NH₃ up to 25% NH₄OH. A corrosion rate of 1 mm/yr (40 mils/yr) has been found in the vapor space of a ductile cast iron NH₃ pressure distillation plant operating at about 88 °C (190 °F). White cast iron with >18% Cr content (ASTM A532 grade IID) has been used in some applications.

High-nickel cast irons have been used in valves, pumps, and so forth, in NH₄OH solutions. Austenitic nickel cast iron NiResist cooling coils corroded at about 0.005 mm/yr (0.1 mil/yr) at 25 °C (75 °F) and 0.15 mm/yr (5.9 mils/yr) at 70 °C (160 °F) in flowing NH₃ gas (Ref 3). Laboratory testing of NiResist (International Nickel Company) cast irons and unalloyed cast irons gave the results shown in Table 1 (Ref 8). This table also shows data from NH₃-containing environments in industrial applications. In most applications unalloyed cast iron is at least as good as the NiResist irons, but in the concentrated and contaminated solutions the alloyed irons are superior. In most cases in this table, the recommended NiResist irons are types 1 (F41000) and 2 (F41002) with type 3 (F41004) also appropriate for the dilute solutions.

Ammonia is essentially noncorrosive to steels at ambient temperatures, which accounts for their widespread use. Carbon steels (commonly referred to as “steels” in many standards and regulations pertaining to NH₃ storage and transport) include both carbon steels and carbon-manganese steels and are commonly specified for process equipment. These alloys are by far the most commonly used materials for the storage and handling of NH₃. The standard used to specify the steel is selected based on the mechanical properties required by the application, with consideration of environmental factors.

Even at elevated temperatures, corrosion rates in anhydrous NH₃ are low, less than 0.05 mm/yr (1.9 mils/yr) in the range from 297 to 589 K (24 to 316 °C, or 75 to 600 °F) (Ref 9). One very important exception relative to corrosion resistance of steels is the tendency of anhydrous NH₃ to cause SCC (see below).

Ordinary carbon and alloy steels are satisfactory in NH₄OH service, although a superficial rusting will occur in the vapor space.

Ammonia SCC in carbon steel vessels was first reported in the mid-1950s in agricultural service tanks. Cracking occurred in areas of high residual stress, such as welds and cold-formed dished heads. Hot forming or stress relieving the heads considerably reduced the occurrence of cracking, as did the addition of a small amount of water to the NH₃.

Throughout the 1960s and early 1970s, cracking problems appeared to be mainly associated with high-strength quenched-and-tempered steels. Later, there were reports of cracking occurring in spheres containing anhydrous NH₃ with water additions and also in spheres that had been stress relieved after finding and repairing cracks.

The cause of this cracking is now accepted to be high local stresses and the presence of air contamination, although nitrogen and carbon dioxide are also thought to play a role. Cracking is accelerated by the use of high-strength steels, the presence of hard welds and air contamination (Ref 4, 10). The highest susceptibility to SCC has been found to be in liquid NH₃ with 3 to 10 ppm oxygen and a water content <100 ppm. Stress-corrosion cracking can occur, however, in NH₃ with an oxygen content down to 0.5 ppm when the water content is very low (Ref 11).

Possible ways to control or reduce liquid NH₃ SCC in carbon steels include (Ref 12):

• Eliminate oxygen

• Add around 0.2% water to the NH₃

• Use steel with an actual yield strength less than 300 MPa (44 ksi)

• Reduce residual stress by stress relieving

• Inspect often enough to detect cracks before they grow to dangerous proportions

• Use a sacrificial anode, such as a thermal sprayed zinc coating

• Cathodically protect the steel

• Use a different material such as stainless steel

• Paint the steel with a suitable protective coating

Some of these options are impractical at the present time (2005) since:

• There is no commercially available paint suitable for NH₃ submersion duty.

• Using stainless steel would increase the capital cost of vessel construction by at least 100% and increase steel mass.

• Cathodic protection has not yet been shown to be reliable in refrigerated installations and would not protect in vapor spaces, for example, in road tankers.

• Zinc thermal spraying of NH₃ tank internals has been used, largely on a trial basis. More experience is required.

The remaining options that are currently used for storage vessels or road tankers are evaluated in Table 2 (Ref 12).

Stress-corrosion cracking of carbon steel occurred in NH₃ receiver tanks used for recycling NH₃ in a urea plant. These tanks (SA 516 grade 70) suffered extensive cracking in welds and HAZ in spite of the addition of 0.2% water to the system. It was concluded that the water was not uniformly distributed throughout these vessels and was, probably inadequate to protect the vapor space in the condensing NH₃ (Ref 13).

At high temperatures, NH₃ may dissociate into hydrogen and nascent nitrogen. The nascent nitrogen has a high affinity for iron and reacts to form a very hard, brittle metal nitride. This is sometimes a desirable reaction, and wear-resistant surfaces are commonly produced on steel parts by nitriding in an NH₃ atmosphere.

Although commercial nitriding is performed at temperatures above the normal service temperature for steels (495 to 565 °C, or 925 to 1050 °F), significant nitriding can occur at lower temperatures, resulting in loss of ductility. Ammonia dissociation is catalyzed by iron, which also contributes to the damage potential. For these reasons, steels are restricted to use at temperatures below 300 °C (600 °F) in NH₃ service. In NH₃ converters, nitriding layers can develop over time to a depth of several millimeters and these hard layers can cause brittle, surface cracks to form. Austenitic steels, in converter baskets for example, develop thin, hard nitrided layers that tend to flake off (Ref 1). Nitriding of pipes, grid supports, and so forth in the NH₃ converter is limited by using stainless steels with a higher nickel content, for example, type 347.

At high temperatures and pressures, hydrogen that is present in NH₃ synthesis can dissociate; the atomic hydrogen entering the steel lattice reacts with carbon to form methane (CH₄). This weakens the steel in two ways, by removing carbon from the steel (decarburization) and by forming blisters or fissures in so doing. This can eventually cause the vessel or pipe to rupture, often without any obvious prior deformation. Areas around weld seams are particularly prone to this phenomenon.

The risk of attack may exist at temperatures as low as 200 °C (390 °F) and hydrogen partial pressure as low as 7 bar (700 kPa). Selection of appropriately resistant materials can largely eliminate this problem. The classic Nelson curves described in the American Petroleum Institute (API) document 941 (Ref 14) give guidance on the stability limits of various alloys in terms of temperature and hydrogen partial pressure. These original guidelines have been modified in recent years on the basis of ongoing experience. For example, the low-alloy steels with 0.25 and 0.5% Mo are now classed as unalloyed steels from the viewpoint of resistance to hydrogen attack (Ref 1).

Steels with low levels of molybdenum and no chromium failed in a number of cases in catalytic reforming service, so care was recommended in the use of these alloys for that application. However, failures caused by hydrogen also occurred in hydrodesulfurization, NH₃ synthesis, and other parts of NH₃ production plants, leading to their removal from these safe operating charts (Ref 15). Curves (modified from Nelson curves) that incorporate findings from the extensive operating experience are available that provide safe operating conditions for carbon and low-alloy steels (Ref 14, 16). An example of a modified Nelson curve is shown in Fig. 1.

The standard chromium-molybdenum steels are being modified and improved so that they can be used at higher temperatures. A modified 2$14$Cr-1Mo has been developed that is usable at temperatures up to 484 °C (903 °F) instead of being limited to 454 °C (849 °F) as are conventional steels of this type. This higher-temperature operation is permitted under ASME II rules. The modification included the addition of $14$% V. The API 941 standard has placed this modified steel at the same limits in terms of temperature and hydrogen as the 3Cr-1Mo steel. The first vessel made from this steel was an NH₃ converter fabricated in 1995. The vanadium-modified steel also has a better resistance to temper embrittlement than does the standard, unmodified version (Ref 16).

Hydrogen damage can also occur from the dissociation of molecular hydrogen into atomic hydrogen at elevated temperatures. Atomic hydrogen can diffuse into metal structures and recombine into molecular hydrogen at defects or discontinuities within the metal. This is also known as hydrogen embrittlement and is usually associated with welds that have not received correct postweld heat treatment (PWHT). In contrast to the phenomenon described above, this effect of hydrogen is reversible and hydrogen can diffuse back out of the structure if held at atmospheric pressure at around 300 °C (570 °F). Slow cooling from elevated-temperature and pressure operation is often recommended to permit this diffusion process to occur.

If heat-resistant steels are held at tempering temperatures, that is, above about 400 °C (750 °F), for long periods their impact properties can decline. The transition temperature between ductile and brittle behavior can be elevated to 60 °C (140 °F) from the normal 0 °C (32 °F) or below. This tendency to embrittle can be reduced by controlling the level of trace elements (Si, P, Mn, Sn) in the steel. In this respect, modern steels are generally much cleaner than some of the older steels and are less prone to this phenomenon. Vessels or pipes in which temper embrittlement is anticipated should not be pressurized at low temperatures.

Most high-temperature steels are attacked by hydrogen sulfide (H₂S) in the gas stream in partial oxidation plants. The use of austenitic stainless steels eliminates this problem, but stress relief of welds is advised in these plants to avoid SCC by chlorides sometimes present in the feed oil (Ref 1).

Corrosion can also occur from the condensation of sulfur acids at the colder ends of flue gas ducts. Type 304 stainless steel is used at the cold ends. Carbon steel air preheater tubes in this area can corrode. Alternative materials include cast iron, glass, or coated steel.

Steels alloyed with small amounts of chromium, molybdenum, nickel, or other elements are referred to as low-alloy steels, and they can provide an economical alternative to high-alloy steels at both high and low temperatures. Low-alloy steels have been used effectively in NH₃ storage vessels. These steels are also subject to SCC with high stresses and air contamination being the main factors causing cracking. The addition of 0.1 to 0.2% water inhibits this attack (Ref 4). Chromium-molybdenum steels are the most common steels of this type used in NH₃ applications.

Chromium-molybdenum steels are commonly used at elevated temperatures where the alloying additions provide resistance to hydrogen attack and increase the strength of the alloy. Typical alloys are 1.25Cr-0.5Mo (K11597), 2.25Cr-1Mo (K21950), 5Cr-0.5Mo (K41545), 7Cr-0.5Mo (S50300), and 9Cr-0.5Mo (S50400). Their use in NH₃ service is chiefly in NH₃ synthesis, the alloy selection being based on API Publication 941 (Ref 14). Old editions of this standard have curves showing the resistance of low-alloy steels, with 0.5% Mo without chromium. As mentioned previously in the section “Hydrogen Attack,” service experience has shown that this alloy has little more resistance to hydrogen attack than carbon steel so the API publication was updated in 1991 to reflect this experience. Other uses for these chromium-molybdenum steels are in high-strength parts such as fasteners.

Addition of nickel to steel greatly enhances the low-temperature toughness (impact properties). For this reason, these materials are sometimes specified for low-temperature NH₃ service, especially as weld filler metals. Typically, the nickel-alloy steels contain 3.5% (K32025), 6%, or 9% Ni (K81340).

From the point of view of corrosion, stainless steels are not normally required in NH₃, but they do find many applications, particularly for low temperatures and to avoid iron contamination. All grades of stainless steel are resistant to NH₄OH solutions at up to the atmospheric boiling point.

The standard ferritic grades are rarely used in NH₃ service, but the modern superferritic steels find occasional applications. The first superferritic steels were based on 26% Cr, 1% Mo (S44627), and the niobium-stabilized XM-27 (S44627). These were developed to provide better resistance to chloride SCC than the austenitic 300 series. Superferritic steels include AL 29-4C (Allegheny Ludlum Corporation) (S44735), AL 29-4-2 (Allegheny Ludlum Corporation) (S44800), Sea-Cure (Plymouth Tube Co.) (S44660), and Monit (Uddeholm, originally; now, Outokumpu Stainless) (S44635). The superferritic AL 29-4C (S44735) has been used successfully in an NH₃ stripper reboiler handling water, NH₃, H₂S, and steam (Ref 17).

The PH stainless steels see limited service in NH₃ environments, but play an important role nonetheless. These alloys exhibit very high strengths combined with good notch toughness and corrosion–resistance properties. For this reason, they are often the preferred material for such parts as valve stems and certain critical fasteners. Some of these alloys are susceptible to hydrogen embrittlement in corrosive or hydrogen-rich environments.

Duplex stainless steels are not used in NH₃ service except where their resistance to chloride SCC is useful from the water side of heat exchangers. The modern grades, for example, alloy 2205 (S31803), typically also contain molybdenum to achieve a composition of about 22% Cr, 5.5% Ni, 3% Mo, and 0.03% C max. They are strengthened and stabilized by nitrogen. The mixed austenite-ferrite structure imparts strength and resistance (but not immunity) to chloride pitting and SCC. The duplex 3RE60 (S31500) has been used to replace standard austenitic stainless steels in this type of situation. The duplex stainless steels have higher strength than the lower austenitic grades such as type 304 (S30400), but are subject to temper embrittlement at about 475 °C (885 °F).

Welding tends to lead to variations (20 to 80%) in the austenite/ferrite ratio in the as-cast weld bead and fusion zone. To minimize this effect, they are welded with a nickel-rich, overmatching rod.

The standard austenitic grades, for example, types 304 (S30400) and 316 (S31600), find widespread use in NH₃ service since they are resistant to general corrosion and NH₃ SCC. Stabilized and low-carbon grades are not generally needed in NH₃ applications. Type 304 (S30400) stainless steel was found to be resistant to intergranular attack (IGA) in 28% NH₃ solution at room temperature even after sensitizing for 1 h at 677 °C (1250 °F) (Ref 18). Stainless steels can, however, fail by SCC that is initiated by sensitization formed during heating, for example when welding. Such a failure in a reformer tube occurred at the welded junction between the type 321 flange and HK-40 catalyst tube. The weld and HAZ were sensitized when joined using a high-carbon welding rod. The high-nickel alloy weld that was intended to protect the cast steel (HK-40) from corrosion had inadvertently been machined off, thus exacerbating the corrosion (Ref 19). Fuel gas lines that contain H₂S (stripped from feedstock naphtha) are subject to corrosion. The use of steam-traced type 304 limits this problem.

The high cost of stainless steels relative to carbon and low-alloy steels often precludes their use for large equipment, but their many advantages lead to their selection for smaller parts and components. They are used in low-temperature service since they have an extremely low nil-ductility transition temperature (NDTT) and exhibit excellent notch toughness at temperatures far below the atmospheric boiling temperature of NH₃. In elevated-temperature services in NH₃ synthesis, they are used because of their resistance to hydrogen attack and nitriding. Start-up heater coils in the NH₃ synthesis section are vulnerable to high-temperature corrosion and fatigue. Usually, type 321 performs well here.

Some shallow nitriding does occur after years of service in aggressive environments, but due to the excellent notch toughness of the base material, this effect does not reduce the structural integrity. Another common area of application of the basic grades is in heat-exchanger tubes, where the material selection is influenced by the fluid used to heat or cool the NH₃.

These standard grades can suffer from chloride SCC, particularly in partial oxidation plants in which the feed oil contains chlorides. Chloride SCC failures can also occur under upset condition. An example of such a failure occurred in type 321 and 310S tubes in a waste heat boiler in an ammonium synthesis converter. The direct cause of this SCC was the failure of a feedwater boiler pump that was not repaired or replaced and permitted a buildup of deposits in the system. General and intergranular corrosion was also observed on these tubes (Ref 20).

There are many more specialty grades of stainless steels that see service where some factor limits the use of other materials. A very common example is the use of special ferritic grades or duplex grades of stainless steels as heat-exchanger tubes where austenitic stainless steels are subject to pitting or chloride SCC from cooling waters. The martensitic grades of stainless steel are used where high strength is required, especially at elevated temperatures, as in compressor rotors.

The equivalent cast version of types 304 (S30400) and 304L (S30403) are CF-8 (J92600) and CF-3 (J92500), respectively, and they exhibit approximately the same corrosion response as the wrought alloys. However, castings can have surface layers containing more than the maximum allowable carbon content of 0.08%, which can significantly reduce corrosion resistance of the surface.

In the cast form, the difference between CF-8 or CF-3 and the corresponding molybdenum-bearing grades CF-8M (J92900) and CF-3M (J92800) is insignificant. The CF-3 or CF-8 cast steels are not particularly common, and manufacturers of cast pumps and valves tend to standardize on CF-8M, which has a broader range of applications. The molybdenum grade castings are often more available and less expensive than the 304 or 304L wrought grade equivalents. Since the cast version of these alloys is unlikely to be welded, there is rarely a justification for specifying the low-carbon (L) grades in this case. This assumes that the valves or pumps, if weld repaired by the manufacturer, are properly reheat treated (solution annealed) to restore optimum corrosion resistance. Availability and price are likely to favor the non-L grade, and properly heat treated castings in CF-8 or CF-8M are likely to be as corrosion resistant as their low-carbon cast or wrought equivalents.

Many of the operations in NH₃ production take place at elevated temperatures. Some of the materials used for these applications are conventional iron-base alloys that have been discussed previously or nickel-base alloys. However, specialized elevated-temperature duties in NH₃ production, petroleum refining, and so forth, have generated a demand for ever better materials specific to these duties. Many of the materials used here are proprietary and have been developed to have high strength and good creep-resistant properties in aggressive, gaseous environments.

The standard stainless steels—types 304, 310, and 347—were commonly used but often failed by cracking at elevated temperatures. A better alloy, HK-40 (J94204, 25Cr-20Ni), was developed, and this became the industry standard (Ref 21).

A later material development produced the HP (N08705, 26Cr-35Ni) alloy with 26% Cr and 35% Ni, which was modified by the addition of alloy elements such as molybdenum, niobium, or tungsten. These modified alloys have improved creep resistance, but still possess good ductility and weldability. Various HP-modified alloys are used in NH₃ applications, one of the most common being HP-45Nb (Ref 22). A more recent development is the production of microalloyed HP alloys in which trace quantities of titanium, zirconium, and rare earths are added during casting. These alloys have better resistance to carburization and better high-temperature creep-rupture properties (Ref 21).

There are new alloys, many of them proprietary such as 25Cr-35Ni-15Co-5W, 28Cr-48Ni-5W, and 35Cr-45Ni plus additional elements, and these offer higher strength and good high-temperature properties (Ref 22).

There has also been some development of coatings to resist high-temperature attack by NH₃. Uncoated type 304 (S30400) and 316 (S31600) and the same steels coated with silica applied by a sol-gel procedure were tested in anhydrous NH₃ at high temperature. The uncoated samples were attacked and formed a nitride scale that embrittled the metal. After 115 h of testing at 500 °C (932 °F) uncoated samples were completely degraded, while the coated samples were only lightly attacked. Multilayer coatings were most effective, and the stainless steel substrates were not sensitized to intergranular attack by the high-temperature coating process (Ref 23).

The phenomenon of metal dusting occurs during high-temperature operation, for example, in steam superheaters downstream of the secondary reformer. It is related to the process of carburization in which carbon migrates into the structure forming hard carbides. Carburization occurs above about 800 °C (1470 °F) in the presence of hydrocarbons that crack to provide the carbon.

Metal dusting occurs at 500 to 800 °C (930 to 1470 °F) on iron-nickel or iron-cobalt alloys in gases containing carbon monoxide. Carbon formation is catalyzed by iron, nickel, or cobalt, and the effect is to produce a surface dust layer consisting of a mixture of metal, oxides, and carbon. This dusting is usually observed as pitting or general corrosion attack. Theoretically, alloys that form protective films of the oxides of chromium, silicon, or aluminum should be more resistant. Virtually all high-temperature alloys can be prone to this attack, but higher steam/CO levels help as does coating with aluminum. In nickel-base alloys such as 601, 601H, 625, and so forth, the attack is tolerable in normal operation (Ref 1).

Hydrogen sulfide in the gas offers some protection from metal dusting since the adsorbed sulfur blocks the surface for the adsorption of CO or CH₄ and other hydrocarbons, and the molecules cannot adsorb and dissociate if their adsorption sites are occupied by sulfur. Similarly, dense oxide scales can prevent ingress of carbon into the structure. If sulfur cannot be tolerated in the process, nickel alloys with high chromium and aluminum or silicon additions are the best materials to resist metal dusting (Ref 24).

Values of Cr equivalent for alloys (some of which are proprietary) commonly used in high-temperature applications in ammonia plants are shown in Table 3 (Ref 22).

There are also nickel alloys that are said to have exceptional resistance to metal dusting and elevated-temperature corrosion. The standard alloys such as alloy 600 and alloy 601 have been used in these environments, but other alloys have been developed with superior high-temperature behavior. Alloy 693 (N06693) was tested for a year in CO and 20% hydrogen at 621 °C (1150 °F). Pit depths measured on this alloy were only 0.031 mm (1.2 mils). Pit depths for other alloys tested under the same conditions were (Ref 25):

An example of the failure of alloy 601 occurred in a heat exchanger on an NH₃ plant after only 2$12$ years service. This failure was found to be caused by contamination of the process side by steam. This caused mineral deposition that destroyed the protective film within about 6 months after the steam ingress into the process side (Ref 26).

Another nickel alloy developed for resistance to metal dusting and carburizing is alloy 602CA (N06025). The metal wastage rate of this alloy is compared with other nickel alloys in a strongly carburizing CO-H₂-H₂O gas in Fig. 2 (Ref 27). This alloy has been tested in an NH₃ plant in Europe and showed no metal dusting at temperatures of 450 to 850 °C (840 to 1560 °F). Alloy 601 had some attack, and alloy 800H was severely attacked.

There are also coatings being developed to resist metal dusting. Diffusion coatings based on oxide formers such as silicon, titanium, chromium, and aluminum have been tested and found to show good potential to extend the lifetime of iron-base and nickel-base alloys under metal dusting conditions (Ref 28).

Nickel alloys are seldom used in NH₃ service except at elevated temperatures. They are very resistant to dry NH₃, but can be attacked by gaseous NH₃ if more than about 1% water is present. They are resistant to anhydrous NH₃ and exhibit good resistance to nitriding (Ref 29).

Data in Table 4 show the effect of temperature on nitrogen absorption and nitriding depth for nickel alloys compared with a type 310 stainless steel (Ref 30). Samples were exposed to pure NH₃ for 168 h.

Nickel alloy 200 (N02200) will resist NH₄OH only up to about 1% concentration. Dissolved oxygen (DO) may maintain passivation up to about 10% concentration. Higher concentrations are highly corrosive to nickel even in the presence of air (Ref 31).

The corrosion rate of alloy 200 in agitated solutions of various strengths of NH₄OH at room temperature is shown in Table 5 (Ref 31). These data show that even with agitation nickel is attacked by moderate concentrations of NH₄OH at room temperature.

Alloy 400, with about 30% Cu content, is more resistant than alloy 200 as shown in Table 6 (Ref 31). In solutions of >3% NH₄OH the corrosion rate is increased considerably by aeration and agitation.

The corrosion rate of alloy 800 in 5% and 10% NH₄OH at 80 °C (176 °F) was <0.003 mm/yr (0.12 mil/yr) in 7 day laboratory tests (Ref 32).

The Ni-Cr-Mo alloys such as alloy 625 (N06625) and alloy C-276 (N10276) are resistant but find no application because of the adequate resistance of lesser alloys.

Copper alloys are generally to be avoided in NH₃ service. Although resistant to pure, dry NH₃, contamination by water and oxygen will cause SCC and general corrosion. Corrosion of various alloys in deaerated NH₃ are shown in Fig. 3(a) and are compared with the behavior of the same alloys in aerated ammonia in Fig. 3(b) (Ref 33). These data clearly show the corrosive effect of the presence of air in the solution. Carbon steel (A-285) is included in these data for comparison, and its corrosion behavior is also adversely affected by the presence of oxygen at lower oxygen levels.

Ammonia and copper typically react to form an intensely blue copper/ammonium complex. All copper-base alloys can be made to crack in NH₃ vapor, NH₃ solutions, ammonium ion solutions, and environments in which NH₃ is formed. It is generally true that any metal with a small grain size is more resistant to SCC, irrespective of whether the cracking is transgranular or intergranular. The effect of grain size on the time to cracking of yellow brass (C26800) in NH₃ is shown in Fig. 4 (Ref 34).

While some copper-base alloys are far superior to others in their resistance, and dry anaerobic NH₃ does not cause corrosion, it is general industry practice to avoid all use of copper-base alloys in NH₃ and related services. It can be noted in passing that copper in solid solution in ferrous metals (generally less than 3%) added to attain certain physical, mechanical, or corrosion properties does not pose a problem in NH₃ applications.

Copper alloys C11200 and C26000 were penetrated at a rate of 5 μm/yr (0.2 mil/yr) in anhydrous NH₃ at atmospheric temperature and pressure. Corrosion rates were also low if small amounts of water were present, but oxygen was also probably excluded (Ref 4).

Copper alloy tubes in utility condensers have often been attacked by NH₃ on the steam side. The NH₃ in this case comes from decomposition of amines or hydrazine added to the boiler feed to control oxygen and passivate the boiler surface. Admiralty brass is commonly used to tube such condensers and has been subject to this attack by NH₃. Copper alloy condensers form a surface layer of cuprous oxide when placed in service. If excess oxygen is present, this oxide layer is converted to cupric oxide, which is readily complexed by NH₃. Attack by NH₃ in this type of utility condenser is accelerated by air in-leakage. In cases where boiler chemistry cannot be altered or controlled to avoid NH₃ formation, a more resistant alloy, such as copper-nickel, is used (Ref 35, 36).

All copper-base alloys are attacked by NH₄OH unless air is rigorously excluded, which is not feasible in plant practice. The deep royal blue of the copper/ammonium complex is immediately obvious.

Dealloying can occur also in some copper alloys in NH₃ solutions. It was found that dezincification occurred in 70Cu-30Zn brass in 10 N NH₄OH solution at 32 °C (90 °F). Corrosion and dezincification were increased by increasing stress applied to the alloy. The mechanism of this increased attack was shown to be due to an increase in open circuit potential and a shift in the polarization curve under the influence of applied stress (Ref 37).

Titanium is not attacked by atmospheres containing NH₃, but can be corroded at elevated temperatures. The protective oxide film is effective in NH₃ up to at least 300 °C (570 °F) (Ref 4). At higher temperatures, NH₃ will decompose into nitrogen and hydrogen that may cause hydrogen embrittlement of titanium. Titanium corroded at 11.2 mm/yr (440 mils/yr) in an NH₃-steam mixture at 221 °C (430 °F). This high corrosion rate was thought to be associated with hydriding. Titanium shows excellent resistance to corrosion in up to 70% NH₄OH up to the boiling point (Ref 38). The corrosion rate of titanium in 100% anhydrous NH₃ is <0.13 mm/yr (5 mils/yr) at 40 °C (105 °F). In 28% NH₄OH solution at room temperature, the rate is 0.0025 mm/yr (∼0.1 mil/yr) (Ref 39).

Zirconium is resistant to NH₃ even at elevated temperatures. The corrosion rate of zirconium Zr702 (R60702) in wet NH₃ at 38 °C (100 °F) is less than <0.127 mm/yr (<5 mils/yr) and in 28% NH₄OH at up to 100 °C (212 °F) is <0.025 mm/yr (<1 mil/yr) (Ref 40). Zirconium is stable in NH₃ up to about 1000 °C (1830 °F) (Ref 4).

Niobium is not attacked by 13% and 25% NH₄OH solutions at 20 to 100 °C (68 to 212 °F) (Ref 3).

Tantalum is resistant to anhydrous liquid NH₃, but should not be exposed to the gaseous mixtures encountered in NH₃ synthesis at elevated temperature. Above about 250 °C (480 °F), it reacts rapidly with hydrogen to form brittle hydrides. Tantalum is not corroded by 10% aqueous NH₄OH solutions up to 100 °C (212 °F), but is attacked by hot, concentrated NH₄OH solutions (Ref 3).

Lead is resistant in NH₃ at temperatures up to 60 °C (140 °F) and hard lead (alloyed with antimony) is satisfactory up to 100 °C (212 °F) in dry NH₃. Lead is also resistant to NH₄OH at room temperature (Ref 41). The corrosion rate is very sensitive to the presence of air and agitation. In 27% NH₃ solution at 20 °C (68 °F), lead had a corrosion rate of 110 g/m²/d (3.3 mm/yr) with rapid agitation and only 21 g/m²/d (0.63 mm/yr) without agitation (Ref 7).

Tin is resistant to dry NH₃ and saturated NH₃ solutions, but dilute NH₃ solutions corrode tin.

Zinc is not resistant to NH₄OH.

Magnesium is not attacked by wet or dry NH₃ at ordinary temperatures, but attack may occur if water vapor is present.

The precious metals are also not used. In fact, there is a potential hazard if silver is exposed under some conditions because explosive azides may be formed. Precious metals are not employed, and silver or silver-rich alloys are not to be employed in NH₃ or NH₄OH.

Although ferrous and nonferrous alloys are the most commonly used materials for the manufacture, handling, and storage of NH₃ and NH₄OH, nonmetallic materials are also used for some applications. These include:

• Elastomers

• Plastics, both thermoplastics and thermosetting resins

• Carbon and graphite

• Glass

Some elastomeric materials are attacked by NH₃, for example, butyl, Viton A (E.I. Du Pont Co.), hard rubber, isoprene, and natural rubber. Satisfactory materials include Chemraz (Greene, Tweed), Kalrez (E.I. Du Pont Co.), chloroprene rubber, acrylonitrile rubber (Buna N), and butadiene-styrene rubber (Buna S). Ethylene-propylene(-diene) rubber (EPDM) is resistant to NH₃, but may be attacked by oils present in compressed gas systems. Buna N elastomer used in pumps is rated as suitable for cold NH₃ gas but fair to poor in hot gas. Neoprene is suitable in cold NH₃ while polytetrafluoroethylene (PTFE) is suitable in cold or hot NH₃ (Ref 42).

Satisfactory materials in NH₄OH include Chemraz, Kalrez, and chloroprene rubber. Some elastomers that are attacked and are unsatisfactory are butyl, Viton A, hard rubber, isoprene, and natural rubber.

Temperature limits for various elastomers are given in Table 7. These data are a compilation from various sources.

Most plastics are chemically resistant to NH₃ and NH₄OH at ambient temperatures. However, due to the hazardous nature of NH₃, the use of plastics is generally not recommended except in seals and gaskets, where fluoropolymers are typically used.

Resistant grades of fluoropolymers are ethylene chlorotrifluoroethylene (ECTFE), for example, Halar (Allied Corp.), ethylene trifluoroethylene (ETFE), for example, Tefzel (E.I. Du Pont Co.), polyvinylidene fluoride (PVDF), for example, Kynar (Arkema), fluorinated ethylene propylene (FEP), perfluoro alkoxy (PFA), and polytetrafluoroethylene (PTFE). Some plastics that are generally unsatisfactory in NH₃ are acrylonitrile-butadiene-styrene (ABS), epoxy, certain polyesters, polyisobutylene, and polystyrene. Data on temperature limits for various common thermoplastics are shown in Table 8. These data are a compilation from various sources. Various manufacturers list elevated-temperature limits for plastics in liquid NH₃, and these are included in this table. They are not, however, of very practical interest since it is highly unlikely that plastics would be used in elevated-temperature (therefore, also high-pressure) liquid NH₃. In the unlikely event that an application for a plastic was being considered in liquid NH₃, the subzero properties would be more relevant and should be investigated.

One area of interest is the difference in behavior of polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC). In many environments CPVC is more resistant than PVC and can withstand higher temperatures. This is not the case, however, in NH₃ and amines. Polyvinyl chloride has generally good resistance to NH₃ and some amines, even at somewhat elevated temperatures, while CPVC has extremely poor resistance to NH₃ or NH₄OH, and limited resistance to most amines, even at ambient temperatures. This is due to the extremely high reactivity of amines and chlorine, the higher availability of chlorine in CPVC, and the lower bond strength of CPVC versus PVC. Even at fairly low concentrations and temperatures, NH₃ and many amines are capable of rapid dehydrochlorination of CPVC. One CPVC pipe handling 28% NH₄OH at ambient temperature failed after only 1 year in service (Ref 43).

If polyethylene is used to store concentrated aqueous ammonium solutions, there is a weight loss due to outward diffusion through the plastic. A solution of 27% NH₄OH solution kept in a 500 mL bottle (1 mm (0.04 in.) wall thickness) at 20 °C (68 °F) for 54 days lost 3.6% of its weight (Ref 3).

Use of many of these thermoplastics is confined to linings for pipe and vessels, rather than as solid construction items, because of the hazardous nature of NH₃. Recommended temperature limits for plastic-lined steel pipes are given in Table 9 (Ref 44).

Thermoplastics are also used as the resistant liner in dual-laminate construction in which fiber-reinforced plastic (FRP) is used as the reinforcing, structural element. The corrosion resistance depends on the resistance of the thermoplastic liner, although resistant resins are often used in the FRP reinforcement in case of permeation and leaks in the thermoplastic liner. Most common thermoplastics are used in this type of construction, and many of them are suitable for use in NH₃ applications. Data on temperature limits for various thermoplastics used for dual-laminate construction for NH₃ service are given in Table 10 (Ref 45).

The fiberglass-reinforced thermosetting composites, commonly called FRPs, have a resistance determined by the polymer used. Some suggested concentration and temperature limitations of FRP are shown in Table 11. Data are a compilation from various sources.

Carbon and graphite are resistant to NH₄OH. However, with impervious graphite heat exchangers, an epoxy resin should be specified rather than phenolic, which is attacked by alkaline chemicals. Carbon and graphite are resistant to all solutions of NH₃ up to their limiting temperature, which depends on the individual grade and formulation. Graphite is resistant to anhydrous NH₃ over the full range of concentration up to the temperature limit of the graphite. Fluorocarbon-bonded graphite, Diabon F100 (SEL Carbon Group), is resistant in 20% NH₃/caustic NH₃ at up to 40 °C (105 °F) (Ref 46).

Glass is resistant to about 60 °C (140 °F) in very dilute solutions of NH₄OH (perhaps 1%), but will withstand solutions to pH 14 at room temperature in lined steel (Ref 47).