Editors Note:

This is a unique issue of Outlook. It is one of the lengthiest issues in the publication's 15-year history and contains the longest article (11 pages) ever to appear in the newsletter. Outlook usually presents shorter articles, but this particular study on metals in organics by well-known corrosion expert Te-Lin Yau was too good to condense. For those who are not interested in this topic, the back cover contains other metals information. The next issue of Outlook will return to covering a broader range of metals and chemicals news.

Zircadyne improves organics production

By Te-Lin Yau, Corrosion Engineer

Zirconium is well known for its corrosion resistance in a wide range of inorganic and organic media.^1-3 It has played a key role in advancing the production technologies of urea and formic, acetic, hydroxyacetic, lactic and methacrylic acids, methyl methacrylate, rayon, alcohols, phenolic resins and other organic acids and compounds. Modern plants are using various types of zirconium equipment, such as reactors, columns, heat exchangers, reboilers, evaporators, heating devices, chemical mixing tanks, pumps, valves, piping systems, trays, and packings.

Zirconium's excellent corrosion resistance in a wide range of organic media has allowed chemical plants such as the one shown above to improve production by stretching pressure and temperature operating parameters.

Compared to other engineering alloys, zirconium allows producers to make products of high quality by operating processes at higher temperatures/pressures for improved efficiency and yield. Unlike most elements, zirconium produces colorless ions. Most transition metals produce ions of different colors depending on their valence state. Another important benefit of using zirconium is that it's considered nontoxic and biocompatible.

No materials would be free of corrosion problems under all conditions. Zirconium may be vulnerable in certain organics, for example, acetyl chloride, that are incompatible with water. In water-soluble organics, such as methanol, some amounts of water would be needed in order to inhibit localized corrosion. There are impurities, such as copper ions in acetic acid, which are undesirable for zirconium. Control measures, such as water addition and stress relieving, can be applied to zirconium equipment in certain extreme conditions.

The corrosivity of anhydrous organic media is often underestimated. The corrosivity of many organic media increases when some water is present. However, in the absence of water and oxygen, certain organic compounds, such as organic halides and unsaturated organic compounds, can react with metals to form organometallic compounds. Sometimes, this underestimation can happen simply because organics are just carbon-based compounds and are not extreme in pH values.

Indeed, there are many highly corrosive organic media. There is an increasing emphasis on product quality in order to meet the requirements of purity and color stability. Process equipment that corrodes at low rates may no longer be acceptable. For example, at a corrosion rate of 50 µma/y (2 mpy), there is over 1 kg of metal dissolving from a 1,000m² area each day. This can create great concern if toxic compounds are produced from the corrosion processes. Compounds of chromium, nickel and lead are regarded as toxic materials. Consequently, highly corrosion-resistant materials should always be considered as structural materials for process equipment.

Urea

In 1928, Wohler used ammonium cyanate to prepare urea. Urea became the first organic compound to be synthesized from an inorganic material. This didn't remove the boundary separating inorganic chemistry from organic chemistry, but did change the concept that organic compounds had to be associated only with living organisms.

Today, urea is produced commercially in vast amounts from ammonia and carbon dioxide. These two materials are combined at high temperatures/pressures to form ammonium carbamate, which then decomposes to yield urea and water. Modem urea plants are both energy efficient and make maximum use of feedstocks. Highly corrosive conditions are created. A carbamate solution is particularly difficult for common alloys, including stainless alloys, to handle. Many processes are based on recycling carbamate to the urea reactor. Certain processes, e.g., the CPI-Allied process⁴, were developed to avoid the carbamate recycle. These processes are based on recovery and recycle of unconverted NH₃ and CO₂. To achieve this goal, the reactor temperature has to increase from <190°C to 193°C-232°C. The higher temperatures result in high conversion rates, 80 to 85% versus 65 to 70% for carbamate recycle processes. These features allow chemical companies to reduce the size of urea plants. The key requirement in this type of process is the use of zirconium-lined reactors to solve the serious corrosion problems. Certain zirconium-lined reactors and heat exchangers in urea service for over 20 years have shown no signs of corrosion.⁵

Zirconium clearly has been established as the most corrosion-resistant metal for urea-synthesis service.^6,7 Stainless alloys are actively corroding in urea-synthesis conditions at rates exceeding 2 mm/y. Even silver has a high corrosion rate, such as 0.76 mm/y. Titanium is known as a corrosion-resistant metal for urea service. However, it has experienced several problems, such as erosion⁷, and problems resulting from surface contamination and embrittlement.⁸

Nevertheless, urea producers have not fully taken advantage of zirconium's strength in corrosion resistance. One of the reasons is that zirconium is perceived as being an exotic, expensive metal. In fact, zirconium equipment is very competitive with equipment made of stainless alloys because of the following factors.

1.Zirconium's price has been stable for decades.
2.Compact equipment can be designed considering zirconium's excellent corrosion resistance and thermal conductivity.
3.Advancements in cladding techniques allow zirconium to be economically used at elevated temperatures.

Another reason is that the high corrosion rates of stainless steels can be greatly reduced, provided that oxygen is present for passivation. Oxygen injection is a popular corrosion control measure in urea plants. This measure has certain drawbacks: lower plant efficiency and greater safety concerns. Explosion of urea plants (like the one in Reference 9) has occurred in different countries.

Recently, zirconium is returning to urea plants. The driving force for the current interest in zirconium is the concern for the presence of heavy metal ions in fertilizers. Stainless steel equipment still has a meaningful corrosion rate in urea production conditions with the oxygen injection measure: To protect the environment, allowables for the presence of heavy metal salts in fertilizers are being tightened. It can be costly not to take advantage of zirconium's corrosion resistance and nontoxicity.

Acetic Acid

The importance of acetic acid in the organic chemistry industry is comparable to that of sulfuric acid in the inorganic chemistry industry. Acetic acid is widely used in a variety of organic syntheses. It is used in the production of acetate esters (such as vinyl, methyl and cellulose acetates), acetic anhydride, terephthalic acids, and pharmaceuticals (such as aspirin). It is also a common ingredient in many organic process streams. To meet these demands, millions of tons of acetic acid are being produced annually. Production capacity is on the rise at a fast pace. Materials selection for acetic acid service needs to be thorough.

Acetic acid Can effectively acidify aqueous solutions with increased corrosivity. It is not highly corrosive at low temperatures. Many materials, such as wood, rubber, aluminum, copper, stainless alloys, titanium, and silver have been used in acetic acid service with varying degrees of success. While each one has merit, corrosion problems can arise due to variations in acid concentration, temperature, solution impurities or catalysts, and heat transfer.¹⁰

For over 30 years, zirconium has been recognized as one of the most versatile corrosion-resistant materials in acetic acid media.^10-13 Zirconium shows nil corrosion rates (<25 gin/y) in most acetic acid media at temperatures up to 300°C. In fact, acetate ions have a mild inhibitive effect on the localized corrosion of zirconium in halide solutions. Conditions that lead to the corrosion of zirconium are few and will be discussed later. Zirconium's excellent corrosion resistance allows it to answer corrosion problems occurring in severe acetic acid media. Some of these severe conditions exist in the production of acetic acid by the synthesis of methanol and carbon monoxide. This synthesis process offers economical attractiveness over the older routes, such as the oxidation of acetaldehyde or straight chain hydrocarbons. It produces fine product which can be used in food and pharmaceutical applications. It has become the technology of choice in the world market. However, production conditions involve corrosive, hazardous chemicals. The process equipment must be made of the most corrosion-resistant material available.

The methanol-carbon monoxide processes have been studied since the 1920s.¹⁴ The reaction systems used a catalyst at 300°C to 400°C under high CO pressures (>20 MPa). Phosphoric acid, copper phosphate, hydrated tungstic oxide, iodides, and other materials were tried as catalysts. Nickel iodide proved to be particularly effective. These processes faced many difficulties, e.g., loss of catalysts, corrosive conditions, and dangerously high pressures. In order to commercialize these processes, noble metals needed to be used to construct process equipment. A breakthrough for this technology was achieved in the 1960s.¹⁵ In this process, rhodium promoted with iodide is used as a catalyst. The catalyst possesses remarkable activity and selectivity for conversion to acetic acid. The reaction system operates at reduced temperatures (150°C to 200°C) and pressures (3.3 to 6.6 MPa) from the previous routes. A plant based on this technology was built in 1970. Zirconium's advantage over stainless alloys can be seen in Table 1. It was confirmed by engineers and resulted in the use of zirconium at critical areas in the aforementioned plant. In this process, all unfavorable factors for stainless alloys are encountered. These factors include:

1.An intermediate acid concentration
2.An elevated temperature
3.The presence of highly corrosive methanol and iodides

Table 1. Results of 48-hour tests in 50% acetic acid with cobalt acetate and potassium iodide as catalysts with pressurized carbon monoxide at 260 °C (Ref. 11)

Because of its strength advantage, Alloy B or B-2 was also chosen for use in the production equipment. This alloy has been experiencing cracking problems due to aging and stress-corrosion cracking. Zirconium has been replacing this alloy and other stainless alloys after their failures for some years now.^16-18 Zirconium is the most cost-effective structural material when all issues, such as process efficiency, product yield, quality, safety, maintenance and replacement costs, and toxicity are considered.

Titanium seemingly should be as suitable as zirconium for handling corrosive acetic acid media, as indicated in Table 1. However, most acetic acid media are too reducing for titanium to form a high-quality oxide film on its surfaces. Therefore, titanium is susceptible to hydrogen embrittlement. There have been cases of hydrogen embrittlement of titanium equipment in acetic acid service.¹⁹ Results of recent autoclave tests, as shown in Table 2, confirm this.²⁰ In general, zirconium is more suitable than titanium for handling reducing environments.

Table 2 indicates that zirconium would also be suitable for the production of terephthalic acid. However, in high in temperature areas (~300°C), zirconium alloys (Zr-l.5 Sn or Zr 704; Zr-2.5Nb or Zr 705) are more appropriate than unalloyed zirconium as in-plant tests have proven.

Table 2. Hydrogen absorption in 95% acetic acid plus 1000 ppm hydrobromic acid at 210°C.

Formic Acid

Formic acid exhibits unique properties. Compared to acetic acid, the acid is higher in dielectric constant (56.1 versus 6.2 at 25°C) and, consequently, in ionization. Formic acid is more corrosive than acetic acid.

Indeed, formic acid attacks many metals. Steel is attacked rapidly by this acid at all concentrations, even at ambient temperatures. Aluminum, copper, and their alloys show fair resistance to the acid only at ambient temperatures. If the acid is free of oxygen and other oxidants, copper's resistance improves to warm temperatures. Lead is usually poor in organic acids, including formic acid. Stainless alloys are better than the mentioned metals but have some serious limitations. Type 304 stainless steel resists only 1 to 2% formic acid at boiling. Type 316 stainless steel can be seriously attacked by intermediate strengths of hot formic acid. Nickel-based alloys may corrode at high rates in the presence of certain impurities, such as halides, and under heat transfer conditions. Although titanium and its alloys have a greater usefulness than these metals in formic acid service, they are not consistent due to factors such as aeration and water content.

Zirconium is versatile and corrosion resistant in formic acid solutions?²¹ As indicated in Table 3, zirconium clearly outperforms stainless alloys in formic acid, particularly, with impurities present.

Zirconium has played a key role in the commercialization of the Leonard/Kemira process.^22,23 Formic acid is produced in this process by the hydrolysis of methyl formate. It is economical since only carbon monoxide and water are consumed. Common materials, such as glass linings, resin and plastic coatings, stainless alloys and specialty metals proved to be inadequate as structural materials for this process.²³ Zirconium is extensively specified for process equipment used in formic acid plants based on this process.

Previously, formic acid was produced by two main routes: (1) acidolysis of formate salts; or (2) as a by-product in the production of acetic acid by the oxidation of hydrocarbons. These routes are no longer adequate in meeting the increasing demand for this acid because of its environmentally friendly nature. Formic acid is replacing mineral acids in many industrial applications since it is also strongly acidic and reducing.

One of the most important uses for formic acid is in fodder preservation developed by Finnish chemist A. Virtanen. Applying formic acid to freshly cut grass prior to ensilation, the nutritional value of the ensuing silage is enhanced. Specifically, lactic fermentation is promoted, and the undesirable formation of butyric acid is avoided.

Formic acid finds a range of diverse applications as an intermediate in the production of drugs, dyes, flavors, and perfume components. It also has various uses in the textile, leather, and rubber industries.

To address pollution problems in the pulp and paper industry, several sulfur- and chlorine-free chemicals are being proposed for the pulping and bleaching processes. Mixtures of formic acid and hydrogen peroxide have demonstrated their capability in making fully bleachable pulp.^24,25 The pulp can be bleached by alkaline hydrogen peroxide. Zirconium is not just corrosion resistant in formic acid and hydrogen peroxide but is also non-catalytical to peroxide decomposition. Zirconium has been rated as the best material for handling hydrogen peroxide solutions.²⁶ A zirconium vessel has been used for developmental work of this organosolv pulping process?²⁷

Sulfuric Acid-containing Organic Media

Sulfuric acid (H₂SO₄) is one of he most important of all chemicals. It is a corrosive of complicated characters, changing from the reducing nature of dilute solutions to the oxidizing nature of concentrated solutions. Under the reducing condition, it is difficult for common metals and alloys to handle the acid. For example, hot 10% to 40% H₂SO₄ can be used to pickle steel and stainless steel.

In various concentrations, sulfuric acid is used in manufacturing many organic and inorganic chemicals.

Table 3. Corrosion of metals in boiling formic acid solutions for eight days

Formic Acid %	Impurity	Zirconium NW	Zirconium W	Alloy B-2 NW	Alloy B-2 W	Alloy C-276 NW	Alloy C-276 W
50	1% Cu2+	<2.5	<2.5	>5000^1	>5000^1	178	128
70	1% Cu2+	<2.5	<2.5	W.G.^1	W.G.^1	155	100
96	1% Cu2+	<2.5	<2.5	3175^1	3860	53	36

NW: Nonwelded W: Welded W.G.: Weight gain

1: Coupons were plated with copper

Note: for more information, refer to Reference 21.

Zirconium finds many applications in H₂SO₄ -containing media since it has excellent corrosion resistance in the acid at all concentrations up to 70% and at temperatures to boiling and above. Examples for zirconium's organic applications are in the production of methacrylic acid (MAA), methyl methacrylate (MMA), alcohols, rayon, and hydroxyacetic acid (HAA).

MAA and MMA

MAA and its chief derivative, MMA, are important materials for the manufacture of specialty polymers. MMA polymers exhibit excellent mechanical, optical, and weathering properties. They are used in acrylic sheets as glazing, sign and building materials, molding resins for automobile parts, paints, oil additives, and other applications.

The acetone cyanohydrin (ACN) process is the major route for making MAA and MMA. It uses plentiful raw materials such as acetone, hydrogen cyanide, methanol and sulfuric acid, which create a process stream of high corrosivity and toxicity.

In the ACN process, methacryla-mide sulfate is produced in a series of reactions at 80°C to 100°C. The reactor effluent is heated in the range of 125°C to 145°C to complete the reaction, and it is then transferred to the esterification section for the making of MMA. The spent acid is re-converted to sulfuric acid for recycle or is treated with ammonia to give fertilizer-grade ammonium sulfate.

There are highly corrosive conditions in the production and recycling areas of the ACN process. The acid strength ranges from 25% to 35% H₂SO₄ which is very difficult for stainless alloys to handle. Other corrosion-resistant materials, such as glass-lined materials and graphite, can't meet the mechanical requirements when process efficiency and safety are emphasized.

In order to get the most out of the ACN process, Rohm and Haas, a major MAA/MMA producer, has extensively used zirconium equipment for over 20 years.²⁸ Zirconium equipment includes pressure vessels, columns, heat exchangers, piping systems, pumps, and valves. With zirconium, Rohm and Haas is able to push the envelope of the ACN process by using stronger acid and higher temperature/pressures in its operations. Consequently, zirconium technology has advanced as well. For example, zirconium welds may be susceptible to intergranular corrosion when the acid concentration is too strong and/or too hot. Heat treatment at 774°C ± 14°/hr/25mm has been developed to improve the corrosion resistance of zirconium welds.^29,30 Also, a one-ton zirconium pump was built to meet the demand of processing huge stream volume. A large one-piece zirconium casting was made for this pump.³¹

Alcohols

Alcohols are a series of organic compounds characterized by one or more hydroxyl (OH) groups attached to a carbon atom that is attached to three other atoms. Common ones are methyl, ethyl, isopropyl, and butyl alcohols. They are neither acid or alkaline. They are active, nevertheless.

Alcohols are used as foods, fuels, solvents, raw materials for making coatings, resins, pharmaceuticals, and other products. Sulfuric acid, as a reactant or a hydrating agent, is needed in certain alcohol production processes. It becomes necessary to use zirconium although most alcohols are not corrosive.

In the 1950s, the Tennessee Valley Authority (TVA) experimented with a process to convert wood to sugars via single-stage dilute sulfuric acid hydrolysis. Ethyl alcohol (Ethanol) can be made from sugar through fermentation. Further development of this technology was revived after the energy crises of the 1970s. A modified process is based on two-stage dilute sulfuric acid hydrolysis and fermentation of pentoses and hexoses.³² Hydrolysis equipment has to withstand temperatures up to 200°C, and concentrations of up to 5% sulfuric acid. Suitable structural material needs to be high in corrosion resistance and strength. Zr 705 uniquely meets these requirements, while stainless alloys rapidly corrode in dilute sulfuric acid at elevated temperatures.³³ Because of the abrasive conditions, Zr 705 equipment was air oxidized at 566°C for four to six hours to develop a thick layer of hard oxide film on its surface. Residual stresses in the welds and worked areas were relieved as well, so that the delayed hydride-cracking tendency of Zr 705 was removed. All Zr 705 equipment has been working well for over eight years.³⁴ Practically, this technology can convert any wood fiber-containing material (e.g., wood chips, waste paper, and agricultural residues) into ethanol.

In the presence of sulfuric acid, isopropyl and isobutyl alcohols can be produced from propylene and isobutylene respectively through a hydration process. The operating conditions are up to 65% H₂SO₄ at about boiling temperatures. Zirconium has been used for column internals, piping systems, vessels and reboilers.

Zirconium is well suited for service in alcohol plants where structural materials need to be highly corrosion resistant as well as strong.

Because of the strong acid condition, it is standard to perform post-weld heat treatment at 774°C on zirconium equipment.

The successful uses of zirconium in alcohol plants led to the addition of a zirconium-clad (zirconium explosion bonded to carbon steel) vessel at an Exxon plant.³⁵ Because of the great difference in the thermal expansion coefficients of zirconium and steel, the vessel was not heat treated and experienced cracking in a nominal 65% H₂SO₄ process. Research efforts of Exxon and Teledyne Wah Chang (TWC) uncovered zirconium's susceptibility to stress corrosion cracking (SCC) in 64% to 69% H₂SO_4. This susceptibility had been previously suppressed since zirconium equipment was made of solid materials and was heat-treated. Subsequently, stress relieving at much lower temperatures (425°C to 480°C) has been developed to apply on zirconium-clad equipment. The vessel was repaired and stress relieved. As an extra measure of safety, a hydrocarbon was added to the medium as an inhibitor and the vessel was shot peened. The vessel has been performing well without any cracking problem.³⁶

Rayon

Rayon is a man-made textile fiber that comes from the plant substance called cellulose. In addition to apparel applications, rayon is used in liners for solid rocket boosters in the space shuttle and in the nose cones of missiles.

Most of today's rayon is made by the viscose process. This versatile process has many steps. Each stage of processing and final spinning requires close attention to yield the desired product properties. The viscose process is very demanding. It requires continuous, yearlong operation to prevent gelling and ultimate product loss. Reliability is a difficult task since sulfuric acid, hydrogen sulfide, and caustic soda are involved in the process.

Equipment made of graphite is common in handling sulfuric acid environments. It was popular in ray-on-making plants. It is vulnerable to breakdowns. Avtex Fibers Inc., a leading rayon producer, began experimenting with zirconium equipment in 1970. Zirconium's excellent performance prompted Avtex to convert more pieces of equipment to zirconium, which included 10 acid evaporators, 14 shell-and-tube heat exchangers, and 12 bayonet heat exchangers.^37,38 In addition to dramatically reducing maintenance costs and downtime, the zirconium equipment improved operating efficiency and lowered overall energy costs.

HAA

Hydroxyacetic (glycolic) acid is a hydroxy carboxylic acid used in textile and leather processing and in the manufacture of detergents, in metal cleaning and plating, in medicine, and in dairy sanitation.

HAA is produced in a synthetic process. Under high pressure, 30-90 MPa, and temperature, 160°C to 200°C, formaldehyde reacts with carbon monoxide and water in the presence of an acidic catalyst, such as sulfuric acid, to form HAA.

DuPont, a major HAA producer, could not even rely on silver lining for reliable service?³⁹ Silver showed poor erosion resistance in the piping sections. There were several cases of blowouts in the piping due to failure of the silver lining.

By the mid-1980s, zirconium lining was considered when many other materials were found unsuitable. Zirconium is well known for its corrosion resistance in weak sulfuric acid at temperatures up to and greater than 260°C. The excellent corrosion resistance in HAA at 205°C was confirmed at TWC. An eight-month field test at DuPont indicated that a zirconium tube would not corrode in the most severe service section of the process. Consequently, DuPont replaced its silver lining with zirconium lining in piping sections over five years ago. The company estimated that the zirconium would last three times as long as the silver.

Hydrochloric Acid-containing Organic Media

Hydrochloric acid (HCl) is one of the most corrosive chemicals. In strong HCI solutions, most metals corrode rapidly. In weak HCI solutions, most metals are susceptible to localized corrosion (e.g., pitting, SCC and crevice corrosion). Hydrochloric acid is a common reactant, catalyst or solvent in organic processes.

Zirconium is one of the few metals that have good corrosion resistance in the whole range of HCl solutions at temperatures above the boiling curve. It has some susceptibility to localized corrosion under oxidizing conditions. Various measures, such as surface conditioning and electrochemical protection techniques, can be applied to control the localized corrosion of zirconium in oxidizing HCl solutions.⁴⁰

Lactic Acid

Lactic acid occurs naturally in some milk, fruits, blood, and muscle tissues. It can be produced synthetically for the manufacture of calcium and sodium stearoyl-2-1actylates in baking uses. It is completely nontoxic and is suitable as a general food additive. Structural materials for the production equipment ought to be corrosion resistant and nontoxic.

Commercially, lactic acid is produced either by fermentation or by synthesis. The synthetic process was developed by Wislicenus in 1863. It is based on lactonitrile, which is prepared by reacting acetaldehyde with hydrogen cyanide at up to 200°C. Lactonitrile is then hydrolyzed in the presence of HCl to yield lactic acid. In the HCI-affected areas, suitable structural materials are limited. Glass-lined materials are prone to breakdowns. Stainless alloys corrode and introduce toxic materials to the process stream. Titanium and the Ti-Pd alloy are susceptible to crevice corrosion in hot chloride solutions.

Zirconium is ideal for handling the lactic acid production media. It is one of the best materials in HCl solutions. Since lactic acid is produced as a fine chemical, contamination has to be prevented in all areas. Oxidizing HCl conditions resulting from the presence of Fe3+ or Cu2+ are avoided. Moreover, zirconium is highly resistant to crevice corrosion in chloride solutions. Since the 1970s, zirconium equipment has provided excellent service in lactic acid production.

Methyl Isobutyl Ketone (MIBK)

MIBK is commonly used as a solvent for gums, resins, nitrocellulose, as well as other materials. It played a critical role in the extraction of zirconium for nuclear applications.

Zirconium and its alloys are standard materials for cladding nuclear fuels, since they are corrosion resistant and transparent to thermal neutrons. However, hafnium, zirconium's sister metal, is always present in zirconium ores. Most of zirconium's and hafnium's properties are similar. One major difference is that hafnium is opaque to thermal neutrons. For nuclear applications, hafnium has to be removed from zirconium, which is difficult. One commercial process for the Zr/Hf separation involves the use of MIBK as the solvent. Enough of a difference exists between the solubilities of zirconium and hafnium in an MIBK-based solution to separate them. One of the other ingredients in the separation stream is HCl.

Welded and nonwelded coupons and U-bends of Types 304, 310, and 316 stainless steels, titanium, and zirconium were tested in an MIBK still pot. Stainless samples showed some local attack. U-bends of titanium showed some fine cracks in the weld metal. Only zirconium samples did not show any meaningful corrosion. Zirconium has been successfully used in MIBK still pots at critical areas, such as heating coils, for years.

Phenolic Resins

Major building blocks for phenolic resins are phenol and formaldehyde. First, these two reactants with an acid catalyst (oxalic, sulfuric, or hydrochloric acid at pH 0.5 to 1.5) are prepolymerized, then the mixture is heated to 120°C to 1800C. The product is recovered by flash devolatilization. The residue is then separated into an aqueous and a resin phase.

Depending on applications, phenolic resins of a wide range of quality are produced. Carbon steel can be used as the material of construction of the equipment when iron contamination and some color are permissible. Stainless steel is used when a better quality is needed.

Furthermore, there is a continuous effort to improve the quality of phenolic resins. Metallic impurities in the resins have to be controlled to much lower than 1 ppm. Therefore, zirconium is the material of choice to construct the equipment when the ultimate quality is required. Practically, zirconium is totally unaffected by phenol and formaldehyde, regardless of which acid (oxalic, sulfuric, or hydrochloric) is involved in production.

Adipic Acid

Adipic acid is the most important of all the aliphatic dicarboxylic acids. It is primary used in manufacturing nylon-6,6. The major commercial route to adipic acid involves the oxidative cleavage of cyclohexane, followed by oxidation with nitric acid at elevated temperatures.

Most passive metals and alloys are useful in nitric acid due to the acid's oxidizing power to yield oxide films; however, they develop some serious limitations when the temperature exceeds the boiling curve of the acid.

Table 4. Corrosion of zirconium in certain organic halides

The corrosion resistance of nickel-based alloys is too poor to be considered for most nitric acid applications. Corrosion rates of stainless steels increase quickly with increasing temperatures and corrosion products, such as salts of iron and chromium. On the other hand, titanium and its alloys require the presence of an inhibitor, such as Ti 4+ and V 5+, to become adequately corrosion resistant in nitric acid.¹⁹

Zirconium is one of the most corrosion resistant materials for handling nitric acid solutions with very few limitations?⁴¹ In most HNO₃ solutions, the corrosion rate of zirconium is typically below 25 µm/y. It doesn't require any inhibitor when the acid is pure. The passivating power of the acid allows zirconium to tolerate some amounts of harmful impurities, such as FeCl₃.

For over 15 years now, nitric acid producers increasingly use zirconium in demanding areas, such as cooler condensers, reboilers, and piping areas. They often regard zirconium as the ultimate solution to their corrosion problems in nitric acid environments. Now, zirconium is starting to find its way into adipic acid plants.

Limitations and Control Measures

Zirconium is highly corrosion resistant in most organic media. Actually, most organic compounds are not regarded as corrosive since they are basically carbon-based compounds. The corrosive conditions of organic media are often created due to the presence of inorganic acids. Zirconium is excellent in inorganic acids too.

However, the number of carbon (organic) compounds (currently known) is larger than the number of compounds of all other elements put together. Highly corrosive organics do exist even without the presence of inorganic corrosives.

This section discusses a few factors that affect zirconium's corrosion resistance in organic media.

Water

Water is a critical component in corrosion processes. It promotes the corrosivity of organic media by lowering pH and providing a matrix for oxidation and reduction reactions. It also has available oxygen to oxidize passive metals. Zirconium's corrosion resistance is insensitive to pH changes. Zirconium doesn't have the kind of difficulty experienced by most passive metals in taking oxygen from water to form protective oxide films. The presence of water in organic streams is often undesirable for most passive metals. It seems to be always beneficial to zirconium. One of the exceptions for zirconium is when chlorine gas is involved. Zirconium resists dry chlorine but not wet chlorine.

The importance of water in organics was first recognized for zirconium in mixtures of methanol and halogens or halides.^42,43 In water-deficient conditions, zirconium is susceptible to SCC or stress-induced intergranular corrosion in methanol, particularly in the presence of halogens or halides. An addition of 2% or more water to the medium removes this susceptibility. It seems that the presence of high stresses is also required for the occurrence of these types of corrosion.

As indicated in Table 4, zirconium coupons (low in residual stresses) have nil corrosion rate in CH₃OH + 1% KI at 200°C. In fact, zirconium equipment has been successfully used in processing methanol media, e.g., mixing tanks for the preparation of methyl iodide. Water addition and/or stress relieving can be used to control the corrosion of zirconium in mixtures of methanol and halogens or halides.

Zirconium behaves similarly in other alcohols. The susceptibility to SCC decreases quickly with increasing molecular weight. It is much more difficult for SCC to occur on zirconium in higher alcohols. This implies that less water (<2%) is needed to inhibit SCC. There is no report of SCC cases for zirconium equipment in higher alcohols services. In fact, isopropyl alcohol can be used to inhibit the SCC of zirconium in 64-69% H2SO4.³⁶ The SCC possibility can be realized in labs.

Zirconium was thought to be immune to SCC in acetic acid. This possibility has been evaluated at TWC. The U-bend test method is not practical in finding this possibility since it is difficult to maintain water-content constant. The slow strain-rate test (SSRT) method is a much more useful way to observe the SCC of zirconium in acetic acid. Table 5 gives test results.

Organic Halides

It has been speculated that zirconium's corrosion problems in dry methanol are caused by halide impurities rather than by methanol.⁴⁴ Truly, organic halides, like inorganic halides, can be quite corrosive.

According to their solubilities in water, organic halides can be classified into three groups: soluble, insoluble, and incompatible.

Water-soluble halides, such as aniline hydrochloride, chloroacetic acid, and tetrachlorethane, are not corrosive to zirconium. They may become more corrosive when water content is low and/or zirconium is highly stressed. More active halides, such as dichloroacetic and trichloroacetic acids, are more corrosive to zirconium. It is expected that water addition and stress relieving are effective measures to control corrosion of zirconium in water-soluble halides.

Water-insoluble halides, such as trichloroethylene and dichlorobenzene, are not corrosive to zirconium, probably because of the stability. They won't dissolve in water, and they won't exclude water. They and water can be physically mixed.

Water-incompatible halides, such as acetyl chloride, are highly corrosive to zirconium. They are not stable. They react violently with water.

(1) Test was done in oil to serve as the base line
(2) The acid was from a newly opened bottle
(3) The acid was from a previously opened bottle
(4) Water was added to the acid from a newly opened bottle

Table 5. Results of slow strain-rate tests at 2.5 x 10 -6 sec -1 in glacial acetic acid at 100 °C

There is no chance for water to be present in this type of halide, which is the most undesirable organic environment for zirconium, and maybe other metals, to handle.

Copper Ions

When various metals were evaluated in numerous acetic acid environments, zirconium was identified as the most corrosion resistant.^11,12 Nevertheless, in certain tests, zirconium exhibited general corrosion and pitting in mixtures of acetic acid and anhydride when copper ions were also present. Copper ions came from corroded copper and copper-containing alloys. Zirconium is quite resistant to mixtures of acetic acid and anhydride. Copper ions seem to play a catalytic role in corrosion.

In conducting corrosion tests, it is expedient not to test different alloys in the same vessel. Corrosion products from one alloy may greatly alter the corrosion behavior of the other alloys.

Somehow, copper ions appear to be effective in promoting pitting on

zirconium in acetic acid (refer to Table 6 below) but not in formic acid (see Table 3 above). There isn't any known effect from ions of iron, nickel, chromium, or molybdenum on zirconium in acetic acid and in formic acid. Copper ions are harmful to zirconium in certain organics but not in others. The role of copper ions in the corrosion of zirconium in organics is not clear. However, copper is known to serve as a catalyst in certain organic reactions, e.g.:

Cu
2CH₃Cl + Si à (CH₃)₂SiCl₂

It should be noted that copper is the low-end noble metal. Copper ions can be reduced to copper on the surface of passive metals to support the catalytical function in certain environments. On the safe side, zirconium equipment in organic and also in inorganic services should not see the effect of copper ions. Copper ions can come from the corrosion of copper and its alloys as well as copper-containing alloys.

^a 1 as the least pitting and 10 as severe pitting
^b Coupons had some salt deposits

Table 6. Corrosion of zirconium in boiling 1000ppm Cu-containing acetic acid solutions for ten days.

Design and Operation

Many organic compounds are volatile. They can be vaporized easily. The composition of a process stream can change in a dramatic manner. When equipment is improperly designed and/or operation is under an upset condition, a persistent boiling or evaporating condition can occur at a constant area such as a hot spot or a ferrule. Consequently, impurities may become concentrated as a concentrating mechanism exists at this constant area. To the extreme, certain impurities can precipitate out of the solution or a dry-out condition is created. At elevated temperatures, some of these precipitates may decompose to yield new species.

It is important to keep the equipment full all the time. Over designed equipment and heat exchangers with plugged tubes are the examples of undesirable situations.

Corrosion Mechanisms

Zirconium has excellent corrosion resistance properties in most organic compounds. It has some vulnerabilities when some halides are present and water is absent. Zirconium is very resistant to most inorganic halides, such as HCl and HI. It is also adequate in dry gases of halogens, such as Cl₂ and I₂. It is not reasonable for these inorganic halides and halogens to become corrosive to zirconium in organic compounds under dry conditions even at ambient temperatures. Maybe, organic halides are formed when they are added to organic compounds. Certain organic halides are corrosive to zirconium, particularly under dry conditions.

The reactivity of organic halides toward metals has been known for over 100 years. In 1849, Frankland prepared diethylzinc directly from zinc and ethyl iodide.⁴⁵ These reactions were investigated extensively by Grignard during the early 1900s and earned him a Nobel Prize in 1912. Today, organometallic halides are termed "Grignard" reagents. One key requirement in preparing these reagents is that water and oxygen should be excluded.

Alkyl and aryl halides are the common ones to react with metals for the preparation of organometallic compounds. Vinyl halides and unsaturated organic compounds can also be used but with greater difficulty.

Highly reactive metals, such as Li, Na, Mg, and K, are often used in these reactions. Reaction rates are fast allowing them to be utilized by the chemical process industry. In fact, certain organic halides, such as methyl halides, are corrosive to corrosion-resistant metals like Si and Pt as well.⁴⁵

Therefore, it is likely that organic halides may attack zirconium or intermetallic compounds at grain boundaries to form organometallic compounds. These reactions should not happen when a sufficient amount of water is present. Zirconium has a higher affinity for oxygen in H₂0 than for organics. Damages caused by high stresses or mechanical means can be repaired by the formation of ZrO₂. Otherwise, the reactions to form organometallics may continue.

Summary

Zirconium is one of the most corrosion-resistant metals in the production of organic compounds, such as acetic acid, acetic anhydride, formic acid, urea, methacrylic acid, methyl methacrylate, alcohols, rayon, hydroxyacetic acid, lactic acid and phenolic resins. It becomes the preferred structural material because of the advantages in improving process efficiency, yield, product quality, plant safety, and environmental protection. Increasingly important, zirconium is currently considered nontoxic and biocompatible.

Zirconium has some vulnerabilities in organic halides to form organometallic compounds under dry conditions but not under wet conditions. Corrosion-control measures include water addition and stress relieving. Zirconium is not compatible with organic halides, such as acetyl chloride, which react violently with water.

References

1. Hamner, N. E., Corrosion Data Survey, NACE, Houston, '74.

2. Cox, B., "Oxidation of Zirconium and Its Alloys," Advances in Corrosion Science and Technology, V. 5, M. G. Fontanan and R. W. Staehle, Eds., Plenum Press, New York, '76, p. 173.

3. Yau, T. L. and Webster, R. T., "Corrosion of Zirconium and Hafnium," Metals Handbook, 9th Ed., V. 13, Corrosion, '87, p. 707.

4. Guccione, E., Chem Eng. (CE), 9/26/66, p. 96.

5. "Zirconium Outlives Urea Synthesis Technology for Which It Was Designed," Outlook, V. 7, N. 1, W '86, p. 1.

6. McDowell, D. W., CE., 5/13/74, p. 118.

7. Miola, C. and Richter, H., Werkst. u. Korros., V. 43., '92, p. 396.

8. Krystow, P. E., Chem. Eng. Progress, V. 67, N. 4, '71, p. 59.

9. "Explosion Rips Louisiana Urea Plant," Chem. & Eng. News, 8/3/92, p. 14.

10. "Corrosion Resistance of Nickel-containing Alloys in Organic Acids and Related Compounds," CEB-6, INCO, '79.

11. Togano, H. and Osato, K., Boshoku Gijutsu, V. 10, N. 13, '61, p. 529.

12. Shimose, T., Takamura, A. and Segawa, S., ibid, V. 15, N. 2, '66, p. 49.

13. Yau, T. L. "Zirconium resists corrosion in a wide range of acetic acid and anhydride environments", Outlook, V. 8, N. 3, Su '87, p. 2.

14. Weymouth, F. J. and Millidge, A. F., Chem Ind. (London), 5/28/66, p. 887.

15. Paulik, P. E. and Roth, J. F., Chem. Commun., '68, p. 1578.

16. "Zirconium Heat Exchangers Resist Corrosion in the Production of Acetic Acid and Anhydride," Outlook, V. 4, N. 2, Sp '83, p. 1.

17. "In Acetic Acid - ICI Australia Increases Equipment Life by Converting to Zircadyne® 702," ibid., V. 11, N. 1, W/Sp '90, p. 6.

18. Bird, K., ibid, V. 13, N. 4, F '94, p. 1.

19. Yau, T. L., Werkst u. Korros., V. 43, '92, p. 358.

20. Yau, T. L. and Bird, K. "A Comparison of Zirconium and Titanium in Acetic Acid," The First NACE Asian Conference, 9/92, Singapore.

21. Yau, T. L., Outlook, V. 9, N. 3., Su '88, p. 6.

22. Leonard, J. D., European Patent No. 5,998, 12/12/79.

23. "Kemira Specifies Zircadyne® 702 for Use in a Formic Acid Application," Outlook, V. 11, N. 1, W/Sp '90, p. 1.

24. Sundquist, J., ibid, V. 11, N. 1, W/Sp '90, p. 4.

25. Poppius K., Hortling, B. and Sundquist, J., "Chlorine-free Bleaching of Chemical Pulps --The Potential of Organic Peroxyacids," International Symposium on Wood and Pulping Chemistry, 5/22-25/89, Raleigh, N.C.

26. Bloom, R., Jr., Weeks, L. E. and Raleigh, C. W., Corrosion, V. 16, '60, p. 164t.

27. Yau, T. L., Tappi Journal, V. 74, N. 3, '91, p. 149.

28. "Rohm and Haas Chooses Equipment Made from Zircadyne® Zirconium," Outlook, V. 11, N. 2, Su '90, p. 1.

29. Frechem, B. S., Morrison, J. G. and Webster, R. T., ASTM STP 728, '81, p. 85

30. Yau, T. L. and Webster, R. T., Corrosion, V. 39, '83, p. 218.

31. "Oregon Metallurgical Corporation Casts World's Largest One-piece Zirconium Pump from Zircadyne," Outlook, V. 12, N. 1, Su'91, p. 1.

32. "Zircadyne 705 Chosen for Use in TVA's Ethanol-from-Wood-Process," ibid., V. 7, N. 3, Su '86, p. 1.

33. Webster, R. T. and Yau, T. L., Materials Performance, 2/86, p. 15.

34. "Zircadyne® Reactor Recycles Waste," Outlook, V. 15, N. l, 1Q '94, p.1.

35. Fitzgerald, B. J., Yau, T. L. and Webster, R. T., "Stress Corrosion Cracking of Zirconium and Its Control in Sulfuric Acid," Corrosion 92, Paper N. 154, NACE, Houston.

36. Fitzgerald, B. J. and Yau, T. L. "The Mechanism and Control of Stress Corrosion Cracking of Zirconium in Sulfuric Acid," 12th International Corrosion Congress, Paper N. 92, 9/19-24/93, Houston.

37. Bowen, L. B., ASTM STP 728, '81, p. ll9.

38. "Rayon Producer Converts 26 Heat Exchangers to Zircadyne® Zirconium," Outlook, V. 10, N. 3, F '89, p. 1.

39. "DuPont Acid Dissolves Problems: Zircadyne-lined Tube Meets Belle Plant Production Challenge," ibid., V. 14, N. 3, Su '93, p. 1.

40. Yau, T. L. and Maguire, M., "Control of Localized Corrosion of Zirconium in Oxidizing Chloride Media," Advances in Localized Corrosion, NACE, Houston, '90, p. 311.

41. Yau, T. L., ASTM STP 917, '86, p. 57.

42. Mori, K., Takamura, A. and Shimose, T., Corrosion, V. 22, '66, p. 29.

43. De, P. K., Elayaperumal, K. and Balachandra, J., Corrosion Science, V. 11, '71, p. 579.

44. Cox, B., Reports AECL-3551, 3612 and 3799, Atomic Energy of Canada Ltd.

45. Thayer, J. S., Chemtech, V. 20, N. 3, '80, p. 188.

New wire yields surprising results

In response to customer requests for superconducting material with better high field current density, Teledyne Wah Chang (TWC) has developed a new type of modified jelly roll wire, a niobium-copper-tin product that performs better than anticipated.

According to Jim McKinnell, TWC' s Manager of Superconductor Development, customers have shown particular interest in the overall current density of the wire, especially in the high field region (>10 tesla).

TWC used its 84-element modified jelly roll (MJR) wire as a starting point for the new design. "The first thing we did was to take out as much copper as we could from our present billet design," McKinnell said. "This enabled us to add six additional nio-bium-tin hexs (elements or very thin strands of wire); so, now we had a billet with 90 niobium-tin elements and just one element of copper."

McKinnell calculated that wire drawn from the new billet would yield an increase of at least 7% in current density (I_c) over the present MJR design.

Before testing wire at the National High Magnetic Field Laboratory at Florida State University, TWC made a few slight but, as it turned out, significant modifications to the new design.

The results that came back from FSU were surprising. Instead of the anticipated 7% increase in current density, the wire's I_c jumped more than 20% at 10 tesla and more than 40% at 20 tesla (see graph below),

"The properties for this wire look comparable to the best results we've seen published," says McKinnell. "I think we have a racehorse here."

TWC makes its superconducting wires in a variety of diameters. For more information, call Teledyne Wah Chang at 503-967-6977.

Shape memory alloy a perfect fit for USAF

The US Air Force's next generation cargo aircraft, the C-17 Globe-master III, is using Nitinol Shape Memory Alloy fluid fittings to join components of its hydraulic system. The fittings are manufactured by Menlo Park, California-based AMCI (Advanced Metal Components, Inc.) from Nitinol barstock supplied by Teledyne Wah Chang (TWC).

The Nitinol barstock is machined into CryoFit® permanent couplings and Cryolive® dematable end fittings, according to AMCI's Joe Chappron. Both fittings are based on the same principles: (1) Nitinol barstock is first machined at room temperature to an ID (inside diameter) that is smaller than the OD (outside diameter) of the tube to be joined; (2) the fitting is then cooled in liquid nitrogen below its martensitic transformation temperature; (3) next it is expanded in the cold condition to an ID larger than the OD of the tube to be joined; and (4) the finished fitting is placed over the tube end, where it warms and recovers onto the substrate.

The resulting tube joints are highly reliable and qualified to rigorous 4000 psi performance standards. Each C-17 has hundreds of CryoFit® and Cryolive® fittings on board. To date, in the Air Force's flight test program, involving ten aircraft, more than 3500 flying hours, and more than 8500 sorties, no Nitinol fittings have leaked or failed. Similar fittings are currently found on the B-1 Bomber, the B-2 Stealth Bomber, the F-14 Tomcat, and will soon be used in manufacturing the Gulfstream V corporate jet.

For more information on CryoFit® couplings or Cryolive® fittings, contact AMCI at 415-617-8900. For more information on TWC's metals, call 503-967-6977.