corrosion by organic acid

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CORROSION BY ORGANIC ACIDS L. A. Scribner, Jr. Union Carbide Technical Center P.O. Box 8361 South Charleston, WV 25314 ABSTRACT The corrosion of materials by organic acids is complicated by the virtually unlimited number of possible compounds. The corrosion of metals by organic acid is often confounded by trace impurities such as oxygen and metallic salts. This paper concentrates on corrosion by acetic, formic and propionic acids and gives some information on longer chain organic acids. Keywords: Organic acids, acetic, formic, propionic, corrosion, impurities, manufacture INTRODUCTION A key group of industrial chemicals is organic acids. They are often used in their pure form and they are used as an intermediate in a wide variety chemical reactions to make products ranging from polyester clothing to amino acids used in vitamins. Acetic acid is synthetically produced in the largest volume of all of the carboxylic or organic acids and is best known by the general public as the weak aqueous solution, 'Vinegar'. The simple, straight chain aliphatic acids are discussed in this paper. They are often called "fatty acids" because those containing an even number of carbon atoms (four or greater) exist in a combined form with glycerol as fats and oils. l Since there are almost an unlimited number of organic acids possible, the subject is complicated. Often the acids are not handled as pure products but as mixtures with inorganic acids, salts, a wide variety of organic solvents, and in mixtures with other organic acids. They have odors that vary from sharp like formic and acetic acid, to rancid like butyric acid (butter) or even worse, smell like sweaty goats - caproic acid. Tingyue Gu - Invoice INV-475575-QJVHQQ, downloaded on 8/24/2011 4:13:18 PM - Single-user license only, copying and networking prohibited.

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Page 1: Corrosion by Organic Acid

CORROSION BY ORGANIC ACIDS

L. A. Scribner, Jr. Union Carbide Technical Center

P.O. Box 8361 South Charleston, WV 25314

ABSTRACT

The corrosion of materials by organic acids is complicated by the virtually unlimited number of possible compounds. The corrosion of metals by organic acid is often confounded by trace impurities such as oxygen and metallic salts. This paper concentrates on corrosion by acetic, formic and propionic acids and gives some information on longer chain organic acids.

Keywords: Organic acids, acetic, formic, propionic, corrosion, impurities, manufacture

INTRODUCTION

A key group of industrial chemicals is organic acids. They are often used in their pure form and they are used as an intermediate in a wide variety chemical reactions to make products ranging from polyester clothing to amino acids used in vitamins. Acetic acid is synthetically produced in the largest volume of all of the carboxylic or organic acids and is best known by the general public as the weak aqueous solution, 'Vinegar'. The simple, straight chain aliphatic acids are discussed in this paper. They are often called "fatty acids" because those containing an even number of carbon atoms (four or greater) exist in a combined form with glycerol as fats and oils. l

Since there are almost an unlimited number of organic acids possible, the subject is complicated. Often the acids are not handled as pure products but as mixtures with inorganic acids, salts, a wide variety of organic solvents, and in mixtures with other organic acids. They have odors that vary from sharp like formic and acetic acid, to rancid like butyric acid (butter) or even worse, smell like sweaty goats - caproic acid.

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Page 2: Corrosion by Organic Acid

CORROSION CHARACTERISTICS

Organic acids are weak acids when compared to the common inorganic acids like HC1 or H2SO4 but still hydrolyze well enough to act as true acids toward most metals. Aliphatic organic acids are usually considered to be slightly reducing. They are often handled in copper which does not directly displace hydrogen from acids. The 400 series stainless steels exhibit borderline passivity and are thus are seldom selected whereas the 300 series stainless steels are today's work-horse. The 300 grade of stainless steels require oxidizing conditions to maintain their passivity, especially at high temperatures. The reversal of corrosion resistance as the environment changes from oxidizing to reducing characteristics make contaminants extremely important because they tend to shift the oxidizing capacity of the acid mixture. Aeration (i.e. dissolved oxygen or DO), ferric ions, peracids or peroxides will cause rapid attack of copper and copper alloys while the presence of chlorides, which are reducing, can have disastrous effects on stainless steels. 2

Corrosion testing in organic acid media can be either difficult or misleading, mainly because of the effect of impurities. Electrochemical measurements are used almost exclusively in the dilute aqueous solutions of organic acids, wherein hydrolysis effects improve conductivity. Electrical conductivity is very low in high concentrations of organic acids and in solutions in non-aqueous solvents like benzene. The addition of sodium or chloride salts is reported to allow electrochemical measurements in these types of solutions. 3 Electrochemical data obtained in strong acetic acid, acetic acid-anhydride and formic acid solutions showed active-passive behavior for stainless steels and nickel containing alloys. This is consistent with field experience. 4

Laboratory data from by immersion tests often show erroneous results unless the atmosphere is carefully controlled. Short tests of metals, especially those that exhibit active-passive behavior, can be misleading because the metal may remain passive during the initial exposure only to corrode in an active state at a rapid rate after longer exposure. Without atmospheric control, the solution will be saturated with air at the beginning of the test but will lose dissolved oxygen as the temperature is increased until at boiling conditions almost all of the oxygen will be removed. Testing with other metals that are corroding will contaminate the testing solution and also give erroneous results. These situations can lead to results that vary widely, depending on the duration of the test.

Truly anhydrous organic acids are usually much more corrosive to stainless steels than acid containing even traces of water. Corrosion rates reported for "glacial" acids can vary because the acids may be truly anhydrous or may contain trace amounts of water.

CORROSION PREDICTION

The electrochemical corrosion of non-passive metals in simple organic acids (those that do not complex or form chelates like citric acid) can generally be predicted from a basic understanding of the oxidation-reduction or emf series. Rule: In order for an electrochemical corrosion reaction to proceed, the potential (measured in volts) of the anodic or oxidation reaction must be more negative than the corresponding cathodic or reduction reaction 5. Oxidation reactions generated electrons whereas the reduction reactions consume electrons. Both reactions are required before electrochemical corrosion can occur.

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Page 3: Corrosion by Organic Acid

T A B L E 1

Standard Oxidat ion - R e d u c t i o n

Potentials, 25°C Potential at Standard State o f ions, volts.

Potential at 1 x 1 0 -4

molar concentra t ion o f ions, volts. 2,3

Potent ial at 1 x 10 6

molar concentra t ion o f ions, volts. 2.3

4H + + 02 + 4e- <---) 2 H 2 0 +1.228 +1.17 +1.14

+0.799 +0.56 +0.45 Ag <---> Ag + + e-

Fe +++ + e - +-~ Fe ++ +0.771

Cu ~ Cu ÷+ + 2e- +0.337 +0.22 +0.16

2H + + 2e- ~ H2 0.000 +0.12 +0.18

Fe ~ Fe +÷ + 2e- -0.440 -0.56 -0.62

Notes: 1) The standard state is 1 molar concentration of dissolved ions and a partial pressure of one atmosphere. 2) Potential calculated at metal ion concentration of 1 x 10 .4 and 1 x 10 -6 molar. This corresponds to a concentration of 1 to

10 PPM and 10 to 100 PPM of dissolved metal, respectively. Concentration of H ÷ ions always held at one molar. 3) When molar concentrations of ions are at 1 x 10 .4 and 1 x 10 -6 molar, the corresponding H2 and 02 partial pressure is 1 x

10 -4 and 1 x l 0 -6 atmospheres.

Whi le referring to Table 1, not ice that the e lec t rochemica l potentials o f copper are a lways more posi t ive or above those o f hydrogen reduct ion:

Cu <--+ Cu ++ + 2e- +0 .160 to +0.337 volts No corros ion (1)

2H + + 2e- ~ H2 - 0.18 to 0 .000 (2)

In this case, corros ion cannot p roceed since the chosen oxidat ion or corros ion react ion is more e lec t rochemica l ly posi t ive than the cor responding cathodic react ion and breaks the rule given above.

B y the same logic, the corros ion o f iron b y hydrogen reduct ion to form hydrogen gas will result in corros ion because , in this case, the corros ion react ion is more negat ive than its cor responding reduct ion reaction.

2H ÷ + 2e- <--+ H2 0 .000 (3)

Fe ~+ Fe ++ + 2e- - 0 .440 Corros ion poss ib le (4)

B y using the same logic, the effect o f impuri t ies like metal ions in solut ion and d i sso lved oxygen can also be unders tood. For example , i f d i s so lved oxygen is not p rec luded f rom the acidic solution, copper can be cor roded because a o x y g e n / h y d r o n i u m ion reduct ion react ion exis ts that is more e lec t rochemica l ly posi t ive than the copper corros ion reaction.

4H + + 02 + 4 e <--+ 2 H 2 0 + l . 2 2 8 v o l t s (5) Cu ~ Cu ++ + 2e- +0.337 Corros ion poss ib le (6)

The ef fec t o f impuri t ies like oxidiz ing metal ions can also be demonst ra ted .

Fe ÷++ + e- ~ Fe +÷ +0.771 volts

Cu +--r Cu ++ + 2e- +0.337 Corros ion poss ib le (7) (8)

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Page 4: Corrosion by Organic Acid

The above discussion is of course not at all rigorously valid since the effect of the specific acid, its concentration, activity, hydrolysis [which for the organic acids is quite low and 1,000 to 100,000 times less than a corresponding inorganic acid like hydrochloric] have been conveniently ignored. Nevertheless, the practical application to organic acid streams is often valid. It is also important to point out that just the tendency to corrode is forecast, the rate of corrosion being determined by other important reaction variables such as the concentration and diffusion of ions.

MANUFACTURE OF ORGANIC ACIDS

Acetic acid is made by a variety of processes, the best known being Butane Oxidation and the Monsanto Low-Pressure processes. Historically, acetic acid was produced by fermentation of grain and then by oxidation of acetaldehyde. The fermentation process was usually carried out in copper equipment or wooden tanks, and combinations of copper and stainless steel equipment were used for acetaldehyde oxidation.

The Monsanto process is based upon the carbonylation of methanol (with carbon monoxide) using a rhodium catalyst and an alkyl halide such as methyl iodide. This process has all but replaced the now antiquated Wacker Process. 6 The Monsanto process (and its various improvements thereof) is the most popular method for the production of acetic acid. As can be surmised, the addition of the halogen reaction promoter to the reaction cycle requires the use of very corrosion resistant alloys and spreads trace halides throughout the rest of the plant. Various patents have been filed to overcome the corrosion caused by the halogens that are in the reaction cycle. These have varied from the use of silver compounds to form insoluble silver salts to ingenious distillation schemes to minimize carrying bromides into other parts of the plant. The bromide addition, if carried down-stream in sufficient quantities, certainly has the potential to pit, crack and corrode the stainless steel used in the downstream equipment. Data derived directly from the field exposure of alloys is not available, but the problem facing the corrosion engineer who selects materials for these processes has been outlined by Togano and others. 7 The reaction system contains the halogen promoter and Togano suggests that Alloy B should be tested along with titanium, zirconium and tantalum for this high-pressure, high temperature system. He considers nickel-molybdenum and nickel-copper alloys attractive for the recovery system that still contains the halide, using the more conventional materials only to handle the acid after the halide has been removed. Yau, in the Outlook Journal, reports excellent corrosion resistance of commercially pure zirconium in the reaction system 8

The Butane Oxidation process, in which butane is partially combusted to acetic acid, has the notorious habit of generating a variable but small quantity of peroxide compounds in the reaction cycle and these are forever changing the corrosion in the downstream equipment from reducing to oxidizing conditions. Secondly, the Butane oxidation process produces a variety of other organic acids, such a formic and propionic, along with a wide variety or organic solvents like ketones. This further complicates the recovery and purification of the reaction products.

Formic acid is produced as a by-product of butane oxidation and by the hydrolysis of methyl formate in the Leonard/Kemira process. The methyl formate is made by the reaction of steam with carbon monoxide (probably in methanol) and the formic acid is recovered from a dilute process stream and concentrated. Since no oxygen is present in the first reaction steps, the formic acid produced is very reducing often requiring the use of zirconium in the distillation train.

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Page 5: Corrosion by Organic Acid

The dominant processes for the production C3 acids, C4 acids, C5 acids and C8 acids (2- ethylhexanoic) is the Low Pressure OXO process of Union Carbide/Kvaerner. It is based upon the hydroformylation of olefins with syngas (H2/CO) in the presence of rhodium/phosphorous catalysts or cobalt catalysts to produce the required aldehyde. The aldehyde is then simply oxidized to form the corresponding acid - i.e. propionaldehyde to propionic acid. The Low Pressure process is so mild that ordinary type 316L stainless steel has been found to be satisfactory. The higher aldehydes, used to produce to produce linear C7 and C9 aldehydes are also made by a similar low pressure OXO process using Celanese technology.

ALLOYS USED TO HANDLE ACETIC ACID

Steel. Steel is attacked quite rapidly, with hydrogen evolution, at concentrations stronger than 1 x 10 -3 molar, even at room temperature. Glacial acetic acid at room temperature is less aggressive than weaker acid but still corrodes at a rate of 0.75 to 1.25 mm/y. Steel is therefore normally unacceptable for use in acetic acid service.

Aluminum. Aluminum shows good resistance to nearly all concentrations of acetic acid at temperatures up to 50 ° C. Grades 1100 (A91100), 3003 (A93003) and the 5000 series like 5052 (A95052) are used for storage and shipment of the acid. It is rapidly attacked below about 95% acetic acid at the boiling point and is again attacked very rapidly in concentrations near 100 percent or in those mixtures containing excess acetic anhydride. Aluminum again becomes resistant to pure acetic anhydride, although it causes contamination of the anhydride due to formation of a white crystalline solid, aluminum triacetate, which precipitates in the liquid. The copper bearing "2000" series, e.g. A92000, alloys of aluminum are not suitable for acetic acid service because of high corrosion caused by the copper rich precipitates.

McKee and Binger have written an excellent summary of the use of aluminum in acetic acid and anhydride. 9 Figures 1 and 2 show the resistance of aluminum in acetic acid and acetic anhydride. The data is for aluminum 1100 but similar rates would be expected for alloys such as 3003,6063 (A96063), or 5086. The corrosion resistance of aluminum is strongly affected by contaminants in acetic acid, and aluminum can corrode in almost any concentration of acetic acid at any temperature if the acid is contaminated with the proper species.

Copper and Copper Alloys. With the exception of the alloys that contain more than 15% zinc, copper (C12000) and all of its alloys show good resistance to all concentrations of acetic acid up to and even above the atmospheric boiling temperature in the absence of oxygen or other oxidants. This good performance is predicted by the elementary discussion in Section 3 on the electrochemical stability of copper in organic acids. Copper was used almost exclusively to handle acetic acid until the advent of the stainless steels; today Type 316L ($31603) stainless steel and higher alloys are often used.

The absence of oxidizing agents is a requirement for copper to be useable acetic acid solutions, as well as other organic acids. Slight contamination of acetic acid with air through storage under an air atmosphere or by ingress of air through a vacuum leak can increase the corrosion to rates that are unsuitable whereas copper is nearly immune to attack by pure, uncontaminated acetic acid.

Laboratory tests that introduced oxygen into room temperature 50% acetic acid gave corrosion rates of 1.9 mm/y compared to only 0.08 mm/y in nitrogen blanketed tests. Similar data in 6% acetic acid showed a corrosion rate of only 0.03 mm/y when passing hydrogen over the solution whereas when oxygen is passed over the solution, the rate rose to 0.58 mm/y.

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Page 6: Corrosion by Organic Acid

FIGURE 1 - EFFECT OF CONCENTRATION A N D TEMPERATURE ON CORROSION OF 1100- H14 (A91100) A L U M I N U M ALLOY IN ACETIC ACID 9

25.4

20.3

15.2

~ 10.2

E E 5.1

~2 .5" z ~ 2.0 0

~ 1.5 0

1.0

0.5

%

0 1 2

• BOILING TEMP. A 50 DEG. C. o ROOM TEMP.

_ , - , 1 . 4 -

3 10 30 50 70 90 97 98 99 100 PERCENT, ACETIC ACID

FIGURE 2 - METAL LOSS FOR 1100-H14 (A91100) A L U M I N U M ALLOY IN ACETIC ACID- ACETIC A N H Y D R I D E SOLUTIONS AT ATMOSPHERIC BOILING TEMPERATURE 9

is o >,

o.

E E I,u k-

n- z 0 (/) 0 E

0 o

63.5 • I I I I

. ~ • B O I L I N G T E M P . 50.8 - - ~ A 5 0 D E G . C .

38.1

25.4

1 2.7 ~ A , " ~--

0 . 5 1 - - ~ , ~ ~ - -

o ~ '~ -~ ~ ~ *'-' - 0 20 40 60 80 100

P E R C E N T , A C E T I C A N H Y D R I D E

I I I I I I 100 80 60 40 20 0

P E R C E N T , A C E T I C A C I D

Copper alloyed with increasing quantities of nickel showed increased corrosion resistance to acetic acid. The nickel addition also increased the resistance to the effect of oxidants. Tests in air sparged 50 percent aqueous acetic acid at the boiling point gave rates of 7.75 mm/y for copper, 4.7 mm/y for copper containing 30 percent nickel and 2.1 mm/y for copper containing 70% nickel. Similar reductions in corrosion rates with increasing nickel content were noted when ferric ion was added to the

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Page 7: Corrosion by Organic Acid

solution. However, the rates still are still too high for economical use even though the nickel addition was beneficial - again pointing out the harmful effects of oxidizing impurities.

Stainless Steel. Low corrosion rates for the straight chromium 400 series stainless steels in dilute acetic acid solutions can often be shown in laboratory tests. However, very tenuous passivity is exhibited by these alloys and field experience with these materials indicates a susceptibility to high corrosion rates and pitting attack. An exception is high purity chromium-molybdenum stainless steel, e.g. Alloy 26-1 ($44627) which shows good resistance.

Type 304L ($30403) stainless steel is the lowest grade commonly used. Type 304 ($30408) stainless steel finds wide use in dilute acetic acid solutions and in the shipment and storage of concentrated acetic acid. Previously published data ~ show that glacial acetic acid can be handled in Type 304 to a temperature of about 80°C (175°F), and it has been satisfactory for lower concentrations to the boiling point of the acid. Intergranular corrosion of sensitized Type 304 stainless steel will occur in 60°C and hotter acetic acid. To prevent this intergranular attack, the use of the low carbon grade 304L is recommended for welded construction. The effect of oxidizing impurities cannot be overlooked when using type 304 stainless steel. The hotter the solution, the more critical for the presence of adequate quantities oxygen or other oxidizing impurities. If oxidizing conditions are lost, rapid corrosion can ensue.

Type 316L ($31603) stainless steel is the alloy most commonly used in equipment processing acetic acid. Even glacial acid at temperatures above the atmospheric boiling point can be handled if the impurities are held within proper levels. The low-carbon grade 316L is required for the higher temperature applications where welding is required. Type 316L, rather than type 304L is most often the required alloy for tankers shipping C. P. acetic acid because of the lower metal pick-up from corrosion.

The old acetaldehyde oxidation process for manufacturing acetic acid also produced acetic anhydride as a co-product in and is often found in acetic acid streams. If acetic acid only contains small quantities of acetic anhydride or if the acid is truly anhydrous, the rate of attack on Type 316L is very high. The introduction of just a few tenths percent of water will reduce the corrosion back to normal rates. Figure 3 illustrates the reduction in corrosion rate as more acetic anhydride is added.

Leakage of chloride-bearing water to acetic acid process streams (e.g., from leaking condensers) results in contamination of the acid with sodium chloride, with subsequent formation of hydrochloric acid by the following reaction:

NaC1 + CH3COOH ---) CH3COONa + HC1 ~ (9)

The hydrochloric acid moves through the system causing its own problems. Corrosion of stainless steel equipment by the hydrochloric acid, for example, produces ferric chloride (along with chromium and nickel chlorides) which, because of its volatility and strong oxidizing nature, is particularly pernicious. The volatility of ferric chloride allows it to move freely through a processing system and, because it is a strong oxidant, it promotes stress-corrosion cracking (SCC). Besides SCC, a condition of accelerated corrosion and pitting results from chloride contamination. Reportedly, slightly less than 20 PPM of chloride in the organic acid stream can be tolerated, but higher concentrations are likely to cause rapid equipment failure. 12

Heat transfer, as in heat exchangers, can alter the corrosion mechanism and rate of corrosion. A method to test metals under heat-transfer conditions has been described by Groves, et. al. and he has

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Page 8: Corrosion by Organic Acid

developed data on stainless steels and some nickel alloys under heat transfer conditions. 13 Higher alloys such as Alloys 20Cb3, 825 (N08825) and 904L(N08904) show better resistance to acetic acid

FIGURE 3 - LABORATORY CORROSION OF TYPE 316 STAINLESS STEEL ($31600) IN ACETIC ACID - ACETIC ANHYDRIDE MIXTURES 11

1 0 0

8O

6 0

40

20

1 0 0 - ,--

8 0 "

60 " q

40 -

20 "~

0 0 . 0 3

i l.IJ

n -

> - . r

z < [

I -

n,. W

_a

w

Z

I11 n

I I I I 0 . 1 3 0 . 2 5 0 . 3 8 0 . 5 1

C O R R O S I O N R A T E , r n m p e r y e a r

than does Type 316 stainless steel - especially when being used to heat the acid, e.g. in evaporators. Alloy 20Cb3 weld overlay has been used successfully to combat crevice corrosion of Type 316 stainless steel in areas such as flange faces.

Among the duplex grades, Alloy 2205 offers an advantage over Type 316L only in the presence of chlorides. In 80% acid containing 2000 PPM chlorides at 90°C, it showed a rate of less than 0.05 mm/y vs. greater than 1 mm/y for Type 316L. However, in boiling 99.5% acid with 200 PPM chlorides, Alloy 2205 exceeded 1 mm/y whereas Alloy 2507 ($39275) corroded at less than 0.02 mm/y. The older duplex stainless steels like type 329 ($32900) also exhibited this improvement in corrosion resistance to acetic acid mixtures as shown in Table 2.14 The anomalous behavior of the type 304 in the 40% acetic acid / 55% water / 5% glycol diacetate is probably the result of too short an exposure time, which preserved the passivity temporarily. The expected rate would be much higher and would undoubtedly have been encountered in a longer test exposure or under operating plant conditions.

Titanium. Titanium is resistant to all concentrations of acetic acid up to the atmospheric boiling point. Electrochemical corrosion studies in acetic acid solutions suggest that it is possible to attack titanium in anhydrous acetic acid although titanium has been successfully used in commercial practice. However, some failures of titanium in hot strong acetic acid have been reported with most failures associated with hydrogen embrittlement. 15 Laboratory tests confirmed hydrogen absorption in 95% acid at 210 ° C with 1000 ppm hydrobromic acid. The high-strength titanium alloys should not be used because of their propensity for stress corrosion cracking.

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Page 9: Corrosion by Organic Acid

TABLE 214

Type of Test

Plant

Lab

Lab

Lab

Corrosive Medium

99.9% acetic acid <0.1% water <0.01% formic acid O.02%acetaldehyde 87.5% acetic acid 6% water 0.5% formic acid 4% acetaldehyde 40% acetic acid 55% water 5% glycol diacetate 25% acetic acid 4% formic

Temperature, °C

102

126

115

110

Stainless Alloy

Type 329 Type 316

Type 329 Type 347

Type 329 Type 304

Type 329 Type 316

Corrosion Rate, mm/yr. 0.03 0.01

0.01 1.09

<0.01 0.08

<0.02 0.08

Nickel Alloys. Alloys C-276 (N10276) and B-2 (N10665) are resistant to acetic acid solutions at all normal concentrations and temperatures. These materials are sometimes used where the acetic acid is mixed with inorganic acids and salts, which limits the use of stainless steel or copper alloys. Alloy B-2 is used under reducing conditions, such as with combinations of acetic acid and sulfuric acid, while Alloy C-276 is commonly used in acetic acid solutions which are highly oxidizing in nature. Alloy C- 276 has been used for glacial acetic acid evaporators.

Zirconium. Zirconium (R60702) has found good use in the Monsanto Process for the production of acetic acid. It is able to withstand the very corrosive conditions that are encountered in the reaction cycle where both acetic acid and very corrosive concentrations of iodide ions are present. Alloy B-2 has been used in similar environments with good corrosion resistance but with occasional cracking, perhaps caused by ordering reactions within improperly heat treated plate. 16 Zirconium withstands all concentrations of acetic acid with or without oxidizing conditions or halogen impurities but does not perform well in the presence of copper ions if the acetic acid also contains acetic anhydride.

Other Alloys. Silver has been used frequently in European practice to handle acetic acid, and it is quite resistant to all concentrations at normal temperatures. Because of the high electrochemical potential (see Section 3, Table 1) silver will be much more resistant to acids contaminated with metals ions like iron and copper since it is more electrochemically positive than these metals in the reduced form. Because of cost considerations, silver has found very little use in the United States.

Lead has been used to store glacial acetic acid where temperature, degree of aeration, and velocity was low, but is generally not considered to be a suitable metal. Room temperature stagnate dilute acetic acid corrodes lead at rate at rates exceeding 1 mm/y.

Non-Metallic Materials. In the food industry, wooden stave storage tanks have been used for the storage of dilute acetic acid and many years of service are common with wood tanks in 3 to 4% acid. However, stainless steel tanks are now widely used with the exception of pickle production, wherein copious amounts of salt are dissolved in the weak acid.

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Page 10: Corrosion by Organic Acid

Successful use of both hard and soft rubber linings has been experienced in the storage of acetic acid, provided that discolorization of the product is not objectionable. Soft rubbers tend to swell. Butyl rubber will withstand glacial acetic acid to about 80°C. EPDM is satisfactory above 5% concentration at room temperature.

Of the plastic materials, polyethylene drums are used to handle C.P. glacial acetic acid. The fluorinated plastics are completely resistant to their normal temperature limits. Many plastics tend to be susceptible to solvent action by the acid. Of the thermosetting resins, chlorinated polyethers and furanes have been used to about 100°C. Reinforced phenolics are suitable to about 120°C. FRP construction using a bisphenol polyester and vinyl esters has been successfully employed.

Carbon and graphite are resistant to the boiling point and impervious graphite heat exchangers have been used in many demanding acetic acid services.

Glass linings have been used to handle acetic acid. No attack on borosilicate glass is reported below about 149 to 177°C. When the glass is attacked, it becomes porous and fades from a dark blue to a pale blue via ion exchange. This is easily detected by simple wetting of the surface to detect a color change.

ALLOYS USED TO HANDLE FORMIC ACID

Formic acid is the most highly ionized of the common organic acids and thus the most corrosive, its dissociation constant being about ten times greater than that of acetic acid. It is very reducing. Its corrosion reactions with copper are predictable in solutions that contain substantial concentrations of water, in conformance with Section 3. It reacts readily with many oxidizing and reducing compounds and is somewhat unstable as the concentration approaches 100 percent, decomposing to carbon monoxide and water. Formic acid is soluble in water, alcohol and ether. Its specific gravity is 1.220, it melts at 8.4°C, and it boils at 100.8°C at one atmosphere.

Steel. Steel is not normally not considered for formic acid service because it is attacked quite rapidly at all concentrations and temperatures.

If alloy steels are heat treated to high strengths and placed under stress, they can fail by hydrogen-assisted cracking (HAC) when exposed to corrosion by strongly ionized acids. Stressed 18% nickel maraging steel cracked when exposed to 10 percent aqueous formic acid but did not crack in 91 percent formic or in either glacial or 10 percent acetic acid. 17

Aluminum. As long as there is no contamination of the formic acid, aluminum exhibits fair resistance at any concentration while at ambient temperatures (Figure 4).~8 However, contamination with a wide variety of materials, e.g., heavy metal salts, can cause severe corrosion of aluminum, most often in the form of very aggressive pitting. Therefore, except for concentrations exceeding 90%, aluminum is seldom used for formic acid containment. The shipping of 95 to 99 percent formic acid can be done in aluminum tanks but it is important to prevent the loading of hot acid.

Typical rates of corrosion on grade 5086 aluminum in formic acid are shown in Figure 5. The maximum temperature normally encountered during transportation is 45°C. At this temperature, as the concentration of water goes up in the acid, the rate of attack increases rapidly. Additionally, acid stored in aluminum becomes turbid - the haze of aluminum salts making the acid unmarketable.

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Page 11: Corrosion by Organic Acid

Copper and Copper Alloys. Copper and all of its alloys, except yellow brasses which can dezincify, respond in approximately the same manner to exposure to formic acid. This is in good agreement with the Section 3 electrochemical prediction information. The successful use of copper and its alloys is contingent on the absence of air, oxygen, or other oxidizing species. Corrosion of these alloys is autocatalytic because of the buildup in the solution of cupric ions (Cu ++ ), which are themselves an oxidizing species. Thus, the exposure of copper to formic acid is dependent on having a short residence time to prevent the build-up of sufficient cupric ions which increase the corrosion rate over and above that which would normally occur. Table 3 shows typical corrosion rates for copper (C10300) and 90-10 copper nickel (C70600) in various concentrations of formic acid. In laboratory tests, a proprietary aluminum bronze, Ampco 8 (C61900), showed rates of less than 0.05 mm/y at 90C.

FIGURE 4 - RATE OF CORROSION FOR 1100-H1 (A91100) ALUMINUM IN AQUEOUS REAGENT GRADE FORMIC ACID 18

63.50

38.10

12.70 O

E E 2.54

t O ROOM TEMP. El 50°C. ~'- BOILING

2.03

1.02

0

A

,x ' -c----vo------v~ , o - T o -

20 40 60 80 PERCENT, FORMIC ACID

100

Copper and copper-based alloys (other than high-zinc brasses) can be successfully employed in formic acid, providing oxidizing species are absent and no accumulation of copper corrosion products can occur. Resistance is maintained in all concentrations of formic to the atmospheric boiling point and above. Copper and its alloys used to be the most widely used materials for handling formic acid but have been supplanted by the many grades of molybdenum-bearing containing stainless steels and high-nickel alloys. The anomalies in the data shown in Table 1, such as the higher rate of attack in 50 and 70 percent formic acid, are probably caused by incomplete deaeration during laboratory tests, although some increase in the rate of corrosion in intermediate acid strengths is to be expected because of maximum ionization at these concentrations.

Stainless Steels. The 400 series and precipitation hardening stainless steels are usually not resistant to formic acid and they are seldom used. However, if a PH grade were needed, type 15-7Mo (S 15700) would be a candidate based upon its similarity to type 316L.

Type 304L stainless steel exhibits nil corrosion rates at all concentrations of formic at ambient temperatures and is therefore the preferred material of construction for the storage of the acid. However,

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Page 12: Corrosion by Organic Acid

at the atmospheric boiling point, Type 304L stainless steel is resistant to only 1 or 2 percent formic acid where it will corrode at rates of about 0.2 m m / y . For formic acid concentrations above 5% and below about 95%, Type 304L will not be acceptable unless oxidizing contaminants stronger than oxygen are present. For process streams, actual corrosion tests are advisable anytime that Type 304L stainless steel is considered for handling formic acid at elevated temperatures. It is doubtful that Type 304L stainless will prove to be suitable in any process and it should not be considered. Table 4 shows typical rates of attack on various stainless steels in several concentrations of formic acid at the atmospheric boiling temperature.

FIGURE 5 - CORROSION OF 5086 (A95086) A L U M I N U M IN FORMIC ACID AT 45°C ~8

>,

@ D.

E E

0.76

0.64

0.51

0.38

0.25

0.13

0 90

O

- - O O

- o - - . o .

95 1 O0

P E R C E N T F O R M I C ACID

TABLE 3 - CORROSION OF COPPER AND 90-10 COPPER NICKEL BY FORMIC ACID (LABORATORY TESTS, ATMOSPHERIC BOILING TEMPERATURE, 96 HOURS EXPOSURE-

DEAERATED) l ]

Acid Concentration. %

1.0

Copper mm/yr.

0.02

90-10 Copper Nickel mm/yr .

0.02 5.0 <0.02 0.02 10.0 <0.02 0.02 20.0 0.20 0.39 40.0 0.14 0.33 50.0 0.25 0.54 60.0 0.05 0.03 70.0 0.75 0.75 80.0 0.19 0.13 90.0 0.22 0.19

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Page 13: Corrosion by Organic Acid

Type 316L stainless steel is the preferred material, showing excellent resistance to formic acid in all concentrations at ambient temperatures and being resistant to at least 5 percent formic acid at the atmospheric boiling temperature. In the corrosion data reported in Table 4, the corrosion rate for Type 304 was about 8 times more than for Type 316 under the same conditions. Since intergranular corrosion is common in acetic acid service, the low carbon grades are generally selected if any elevated temperatures are expected. However, Type 316L stainless steel can be seriously attacked, especially by intermediate strengths of formic acid at higher temperatures, and corrosion tests are advisable in any given situation.

TABLE 4 - CORROSION OF STAINLESS STEEL BY FORMIC ACID (LABORATORY TESTS, ATMOSPHERIC BOILING TEMPERATURE, 96 HOUR EXPOSURE) II

Acid Concentration

1.0

304 1 mm/yr.

0.17

316 1 mm/yr.

0.09

316 z mm/yr.

Alloy 20-Cb3 3 mm/yr.

26 Cr- lMo 1 mm/yr.

<0.02 5.0 0.77 0.04 10.0 1.33 0.26 20.0 1.89 0.27 40.0 3.40 0.20 50.0 4.20 0.50 0.46 0.03 60.0 3.40 0.46 70.0 3.97 0.48 0.64 <0.02 80.0 4.20 0.47 90.0 3.23 0.41 0.61 0.10 100.0 0.25

Note: 1. Oxygen not controlled 2. Deaerated, coupons exposed to the refluxed condensate. 3. 48 Hour exposure

Alloy 20Cb3, is more resistant to formic acid than Type 316 stainless steel, and its use should be considered in higher concentrations at higher temperatures. Other alloys with chromium, nickel, and molybdenum contents higher than Type 316 stainless steel, such as Alloys 28 (N08028) and 904L, also show superior resistance to mixtures of formic and acetic acid and would be expected to be better in formic acid itself. Table 5 shows that the newer duplex alloys such as Alloy 2205 and Alloy 255 ($32550) are superior to Type 316 stainless steel and even Type 317L ($31703) stainless steel. ~9 Even then, the super duplex alloys are subject to higher rates when the acid concentration goes above 90% and can corrode at higher rates in the acid reflux than in the liquid in the kettle. Alloy 2507 reportedly resist all concentrations at or slightly below the atmospheric boiling point.

TABLE 5 CORROSION OF 317L AND DUPLEX STAINLESS STEEL IN BOILING FORMIC ACID, MM/Y 19

20% Formic Acid 40% Formic Acid Type 317L 0.23 0.43

<0.025 -- I Alloy 2205 ($31803) Alloy 255 ($32550) 0.025 <0.025

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Page 14: Corrosion by Organic Acid

Under gaskets and in other occluded areas (e.g., threaded connections), Type 316L stainless steel sometimes undergoes crevice corrosion. Weld overlays of Alloy 20Cb3 have proven partially successful for solving this problem. The superaustenitic alloys containing 6% molybdenum (e.g. Alloy 6XN; N08367) should be even better than the Alloy 20Cb3 which contains only 2-3% Mo. A 6% Mo version of the 20-type alloy is now available, Alloy 20Mo-6 (N08026).

The super ferritics, like Alloy 26-1, appear to have exceptional corrosion resistance to formic acid in preliminary laboratory studies and plant usage, and it should definitely be considered for tubing for heat exchangers handling formic acid. In laboratory tests, Alloy 29-4-2 ($44800) which contains 4% Mo did not corrode in boiling 45% formic acid.

High-Nickel Alloys. Several of the high nickel alloys such as Alloys B-2, C-276 and C-4 (N06455) have shown outstanding resistance to formic acid in process equipment and are reported to have very good resistance even at temperatures above the atmospheric boiling point.

Corrosion data for Alloy C (N10002) are shown in Figure 52°. Notice that Figure 5 also shows a zone of higher corrosion for Alloy B-2. Alloy B (N 10001) sometimes exhibits lesser corrosion resistance in intermediate strengths than Alloy C but, because Alloy B usually costs more, is not often selected. However, in those services that are very reducing, such as sulfuric acid catalyzed esterification reactions, Alloy B is often the better choice. In services where contamination from sodium chloride or other halogens is possible, the Alloy B should be superior to the Alloy C . As is to be expected, the Alloy B is not better than the Alloy C types when strongly oxidizing conditions prevail.

The chromium-molybdenum-nickel alloys are all similar to the Alloy C and B alloys. Alloy 625 (N 10625) contains only 9% Mo and should be expected to be somewhat less corrosion resistant than Alloy C-276 but is so similar in resistance as to be useable in all but the most severe of services.

Titanium. While titanium will resist acetic acid under almost all conditions to the boiling point, it will resist formic acid only under strongly oxidizing conditions. Titanium exhibits borderline passivity in formic acid and under normal conditions, pure formic will corrode titanium at the boiling point in all concentrations above 10%. 21 A titanium heat exchanger disappeared in less than one day when strong conditions, approaching the anhydrous state, had been encountered in a distillation system. When the titanium does corrode, even slightly, hydriding of the metal occurs and loss of ductility results.

Zirconium. Commercially pure zirconium exhibits very good resistance to boiling formic acid with rates below 0.02 mm/y, even when the acid is contaminated with metal salts or iodine. This suggests that zirconium is a good candidate material for the distillation portions of formic acid plants, in which the acid concentration and the temperatures are both high. 22

Non Metallics. Formic acid is an excellent solvent, which makes it very destructive to most organic materials and coatings. Therefore, plastics are not normally considered for this service. An exception is polyethylene, which is good with all concentrations to about 35 ° C The fluorocarbon plastics are resistant to their normal temperature limitations, except as coatings, which have a temperature limitation of about 93°C.

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Page 15: Corrosion by Organic Acid

Hard rubber, neoprene and butyl rubbers are resistant at ambient temperatures and fluoroelastomers to about 50°C. Rubber linings can be used for storage if discoloration of the acid is not a matter of concern.

FIGURE 5 - ALLOY C-276 (N10276) AND ALLOY B-2 (N10665) IN FORMIC ACID 2°

d t/)

144

uJ

u~

uJ D..

kU I--

100

60

20

0.20 mm per year BOIUNG POINT CURVE / /-

I I I I 0 20 40 60 80 100

PERCENT, FORMIC AC ID

ALLOYS USED TO HANDLE PROPIONIC ACID

Propionic acid or methyl acetic acid boils at 141.4°C and is water-soluble. It is a weaker acid than acetic but otherwise very similar to acetic acid.

Steel. Steel is attacked at rates of about 0.6 mm/y in pure propionic acid at room temperature and at much higher rates in aqueous solutions of the acid. Therefore, steel has very limited usefulness in this service.

Aluminum. Aluminum will resist 100% propionic acid at room temperature but becomes unacceptable at about 80°C. The corrosion diagram displayed in Figure 6 23 shows that the attack is very similar acetic acid. Like acetic acid, if the propionic acid is contaminated with anhydride or heavy metal ions, high rates are possible.

Copper and Copper Alloys. Copper and copper alloys containing not more than 15% zinc will handle propionic acid in all concentrations. The data shown in Figure 7 indicates attack on copper in boiling 100 percent propionic acid, but this is believed to be an anomaly caused by incomplete deaeration of the solution since propionic acid been routinely handled in copper lined equipment. As with formic and acetic acid, copper and its alloys are satisfactory only if the solutions are completely deaerated and do not contain other oxidizing agents.

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Page 16: Corrosion by Organic Acid

Stainless Steel. The 400 series stainless steels will pit in propionic acid solutions, probably because of borderline passivity, and are therefore unreliable. They should not be used unless they are tested in the exact environment.

Type 304 stainless steel shows good resistance to propionic acid at room temperature and to aqueous solutions up to about 50 percent concentration at the atmospheric boiling point. Figure 7 shows the resistance of various materials, including type 316 stainless, to boiling propionic acid solutions.

These were short term tests in which the gaseous atmosphere above the boiling solutions was not

FIGURE 6 - RESISTANCE TO CORROSION OF 1100-H14 (A91100) ALUMINUM ALLOY IN PROPIONIC ACID SOLUTIONS AT VARIOUS TEMPERATURES 23

25.4

20.3 -

15.2 >,

~10.2 E E 5.1 ui -

¢z: z ----- 2.0 o~ 0 m 1.5 O O

1.0

0.5

• •,, 50 ROOM BOILING DEG.TEMp.C.TEMP" I 114"0ram @ 99"8% A-_

0 ~ 0 1 2 3 10 30 50 70 90 97 98 99 100

PERCENT, PROPIONIC ACID

controlled. It is likely that this resulted in two erroneous results. Above 65 percent acid, the rate of corrosion of copper is shown to increase rapidly. This would be true if air were present but not in its absence. In process equipment, copper has shown to have low rates of corrosion in all concentrations of acid, providing air or other oxidants are absent. Although Type 304L stainless steel has shown decreasing corrosion rates in laboratory tests between 80 and 100% propionic acid, field experience has been that the alloy demonstrates borderline passivity above 80% and in not suitable for such service.

Type 316L stainless steel is the preferred material for handling hot concentrated solutions of propionic acid. The low -carbon grade should be utilized to avoid possible intergranular attack. It is quite suitable for all ranges of concentration noting that oxidizing conditions are beneficial and that the rate can be somewhat high around 65% concentration as show in Figure 7.

High-Nickel Alloys. Alloys B-2 and C-276 show excellent resistance to propionic acid solutions under reducing and oxidizing conditions, respectively. Other nickel alloys of similar

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Page 17: Corrosion by Organic Acid

composition show similar good resistance to propionic acid but are not normally used since type 3 16L is suitable for most concentrations at high temperatures.

OTHER ORGANIC ACIDS

The solubility in water decreases with increasing molecular weight of the aliphatic monobasic acids (e.g. butyric, pentanoic, etc.). Such acids are usually non-corrosive to Type 304L, for example, until the temperature becomes high enough to promote dissociation. This critical temperature is usually close to the atmospheric boiling point, at which temperature Type 316L is usually required.

It is impossible to cover here the corrosion characteristics of the many different organic acids. However, a sampling of corrosion rates of various metals in several of the longer chain aliphatic acids, aromatic acids and some dicarboxylic acids is shown in Table 6 and 7 24. Table 7 shows data on the corrosion of some less familiar alloys in various organic acids. Good summaries of the corrosion characteristics of di- and tricarboxylic acids (oxalic, maleic, etc.), naphthenic acids and multifunctional acids have been published. 4

FIGURE 7 - CORROSION OF METALS IN BOILING PROPIONIC ACID 23

G}

L_

t~

E E Z 0 I - <

I.- U.I Z I.U

U.I

< n- U.I

1 . 2 7

1 . 0 2

0 . 7 6

0 . 5 0

0 . 2 5

. i= . °~=~ "

0 10 20

J /

.,31~6304 . . . . " . . . . "" - ~ " - t soo

,__f.~.~ ___... I " ~,.~-

• ,.___._. - ' - - - - - - - - - - - - - ~ A L LO y C.~.._

30 4 0 5 0 6 0 7 0 80 9 0 1 0 0

ACID , P E R C E N T IN W A T E R

It is possible to make some generalized statements concerning the higher molecular weight acids.

Steel. Steel is usable at ambient temperatures in the higher molecular weight acids, and it is used occasionally to store many of the much heavier acids and their corresponding anhydrides, providing iron contamination is not a matter of concern.

Aluminum. Aluminum shows good resistance to such acids at room temperature and is widely used for their shipment and storage. Some of the higher molecular weight acids cause severe attack of

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Page 18: Corrosion by Organic Acid

aluminum at highly elevated temperatures, so the use of aluminum must be considered for the specific acid and temperature desired.

TABLE 6 - CORROSION OF METALS IN REFINED ORGANIC ACIDS II

Acid

Acrylic, 50% in an ether, 88°C

Benzoic, 90% 138°C Butyric

26°C, Room Temp. 115°C

163°C (Boiling) 2-Ethyl butyric

26°C, Room Temp. 150°C

2-Ethyl hexoic 26°C, Room Temp.

190°C Heptanedienoic (Pimelic) 225°C

Iso-Octanoic 26°C, Room Temp.

190°C Iso-decanoic

26°C, Room Temp 190°C

2-Methyl pentanoic 26°C, Room Temp.

150°C Pentanedioic

(Glutaric) 210°C Pentanoic (Valeric) 26°C, Room Temp.

114°C

Steel mm/y

0.15

0.18 0.85

0.02 1.25

<0.02 0.88

<0.02 0.83

0.02 0.53

0.05 1.35

Copper mm/y

0.05

0.02 0.40

<0.02 <0.02

<0.02 <0.02

<0.02 <0.02

0.07 0.30

0.05 0.68

Silicon Bronze mm/),

0.05

0.02 0.23

<0.02 <0.02

<0.02 <0.02

<0.02 <0.02

0.10 0.07

0.05 0.13

304SS mm/y

($30408)

<0.02 0.38

<0.02 0.07 1.40

<0.02 0.53

<0.02 0.20

0.93

<0.02 0.20

<1 0.20

<0.02 <0.02

0.68

<0.02 <0.02

316SS mm/y

($31608)

<0.02 0.13

<0.02 0.07 0/13

<0.02 <0.02

<0.02 <0.02

0.18

<0.02 <0.02

<0.02 <0.02

<0.02 <0.02

<0.02

<0.02 <0.02

Copper and Copper Alloys. Copper and copper alloys show good resistance to all of the higher molecular weight acids and can be used quite widely to handle the acids, even at elevated temperatures, in the absence of oxidants. The problem with copper is that the long chain fatty acids can contain dissolved oxygen over and above that of the simple acids like acetic. Even worse, the dissolved oxygen may remain in solution and not be boiled out at atmospheric pressure. 2-Ethylbutyric acid will exhibit this property and cannot be handled in coppery

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Page 19: Corrosion by Organic Acid

TABLE 7 - CORROSION OF MISCELLANEOUS ALLOYS BY ORGANIC ACIDS (EXPOSURE: 48 HOURS, TEMPERATURE: ATMOSPHERIC BOILING, ATMOSPHERE: NOT CONTROLLED) 11

[ Corrosion Rate mm/y ] Test Medium

Acetic Acid, Glacial

99% Acetic Acid: 1% Acetic Anhydride

90% Acetic Acid: 10% Acetic Anhydride

50% Acetic Acid: 50% Acetic Anhydride

90% Acetic Acid: 10% Formic Acid

70% Formic Acid, Aqueous

20% Formic Acid, Aqueous

2-Ethylbutyric Acid

10% Oxalic Acid, Aqueous

Type 329 SS

<0.02

0.58

0.70

1.25

0.35

Tantalum

Nil

Nil

Nil

Nil

Nil

<0.02

<0.02

Nil

Nil

Titanium

<0.02

<0.02

<0.02

0.18

<0.02

<0.02

Zirconium

Nil

<0.02

<0.02

<0.02

<0.02

<0.02

<0.02

<0.02

<0.02

Crucible 223

7

22.50

0.13

4.68

0.02

0.58

Alloy 26-1

<1

0.20

<0.02

<0.02

<0.02

0.35

MP35N (R30035)

<0.02

0.13

0.10

Stainless Steel. Type 304L stainless steel has excellent resistance to the higher molecular weight organic acids at room temperature and at lower concentrations at high temperatures. With the concentrated acids, Type 304L stainless steel is sometimes severely corroded. Type 316L stainless steel is then required and is usable in almost all of the acids, even at elevated temperatures.

Nickel Alloys. The nickel-molybdenum and nickel-ruolybdenum-chromium alloys show excellent resistance to the higher molecular weight acids, but the expense of these alloys is rarely justified unless other contaminants, such as inorganic acids, are also present.

The nickel-based alloys, particularly nickel-copper Alloy 400 (N04400), have found usage for processing the higher molecular weight acids at elevated temperatures. They can be particularly useful when contamination of such acid prohibits the use of 316L stainless steel.

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Page 20: Corrosion by Organic Acid

CONCLUSIONS

The corrosion of common materials in the more common, simple organic acids has been presented. The effect of process impurities and how to predict their roles in the corrosion of some of the metals was proposed. Although a lot of data has been presented, laboratory tests of candidate materials is always suggested because of the deleterious effect of impurities and the ability to predict their presence.

ACKNOWLEDGEMENT

The author extends his appreciation and thanks for the review and comments of C. P. Dillon - pre-eminent corrosion expert, mentor and friend.

REFERENCES

i Linstromberg, W. W., Organic Chemistry, (Lexington, MA: D. C. Heath, 1970) p. 259. 2 Dillon, C. P. , Materials Performance 21, 9 (1965): p. 4 3 Tsinman, A. E. et al, Electrokhimya II, 1 (1975): p. 127. 4 "Corrosion Resistance of Nickel Containing Alloys in Organic Acids and Related Compounds,"

Toronto, Ontario, Canada: Nickel Development Institute,, Corrosion Engineering Bulletin CEB-6. 5 Fontana, M. G. et al, Corrosion Engineering, (New York, NY: McGraw-Hill, 1967) p. 304. 6 "The Monsanto Acetic Acid Process," The Organometallic HyperTextBook,

www.ilpi.com/organomet/monsanto.html. 7 Togano, H. et al, "Corrosion Tests on Materials Used in the Synthesis of Acetic Acid from Methanol

and Carbon Dioxide. I. Examination in Acetic Acid solutions at Increased Temperature," Tokyo Kogyo Shikensho Hokoku, 57 (1962) p. 342-50. Part II in 60, 6 (1965) p. 221-231.

8 Yau, Te-Lin, Outlook, 16, 1 (1995) p.3. 9 McKee A. B. et al, Corrosion, 10, 1 (1954) p. 786t. 10 Tomashov, N. D., Theory of Corrosion and Protection of Metals, (New York, NY: Macmillan

Company 1966) p. 598 ~1 Elder, G. B., Process Industries Corrosion, (Houston, TX: National Association Engineers 1975) p.

251. 12 Dillon, C. P. et al, MS 2: Formic, Acetic and Other Organic Acids, Materials Selector for Hazardous

Chemicals, (St. Louis, MO: Materials Technology Institute 1997) p.77. 13 Groves, N. D. et al, Corrosion 17, 4 (1961) p.173t. 14 Debold, T. A. et al, "Duplex Stainless Offers Strength and Corrosion Resistance," Duplex Stainless

Steels, (Metals Park, OH: American Society for Metals 1983) p. 177. 15 Yau, Te-Lin, ibid. 16 Yau, Te-Lin, ibid. 17 Elder, G. B., Metals Handbook Ninth Edition, Volume 13, (Metals Park: American Society for

Metals, 1987)p. 1157. ~8 T-5A-7c Work Group Report, Materials Performance 13,7 (1974), p 13. 19 Dillon, C. P. ibid., p. 24. 20 T-5A-7c Work Group Report, ibid. p. 15. 21 Godard, H. P. et al, The Corrosion of Light Metals (New York, NY: John Wiley and Sons 1967)

p. 338 22 Yau, Te-Lin, Outlook, Vol. 16, No. 1 (1995) p. 4. 23 Elder, G. B., ibid., p. 252. 24 Elder, G. B., ibid., p. 253. 25 Dillon, C. P. ibid., p. 127.

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