Method of inhibiting boiler corrosion and compositions for it

Abstract

 ''A method of inhibiting corrosion in boilers and boiler feedwater systems comprises adding to boiler feedwater containing dissolved oxygen an additive that is at least one trihydroxybenzene compound, hexahydroxybenzene, levodopa, and/or a salt, homologue or analogue of levodopa. There may also be added, as second corrosion inhibitor, one or more of hydroquinone, methoxypropylamine, cyclohexylamine, diethylethanolamine, morpholine, diethyl hydroxylamine, dimethyl amino-2-propanol, carbohydrazide, 2-amino-2-methylpropanol, erythorbic acid, and salts of erythorbic acid. Compositions comprising the additive and such a second corrosion inhibitor are novel.

Description

[0001] This invention relates to the inhibition of corrosion in boiler feedwater systems and boilers due to the presence of dissolved oxygen. It is also concerned with compositions for such use.

[0002] Protection of boiler feedwater systems is becoming an increasingly important aspect of plant operation. The presence of dissolved oxygen in boiler feedwater is a primary cause of corrosion of material in contact with the water. Moreover, an increase in the quality of boiler feedwater results in savings of energy and consequently of cost for the total boiler system.

[0003] Historically, the action of dissolved gases such as oxygen and carbon dioxide has been a principal factor that leads to corrosion of water feed systems and boilers. In order to understand the role of dissolved gases in corrosion, it is necessary to understand the electrochemical nature of corrosion. Under most conditions, there is a tendency for iron to dissolve in water, and two electrons are released for each iron atom that dissolves. These electrons are transferred to hydrogen ions present in the water, and the ions are reduced to elemental gaseous hydrogen. All action ceases at this point if the hydrogen remains on the surface of the metal, since a protective coating forms with the passage of electrons. However, any agent that increases the number of hydrogen ions present in the water, or that will cause the removal of the protective film, serves to increase the rate of corrosion.

[0004] The presence of oxygen in boiler feedwater causes a two-fold reaction to occur. Some molecules of oxygen combine with displaced hydrogen, thereby exposing the metal to fresh attack. Other oxygen molecules combine with iron ions to form insoluble iron oxides.

[0005] The first product of corrosion may be ferric oxide, which is only loosely adherent and blocks off areas to oxygen access. These areas become anionic and iron oxide couples are set up. The iron under the oxide deposit then dissolves, and pitting develops, which thus aggravates corrosion.

[0006] The severity of attack by oxygen will depend on the concentration of dissolved oxygen in the water, and the pH and temperature of the water. As the water temperature increases, corrosion in feed lines, heaters, boilers, steam and return lines made of iron and steel increases.

[0007] A major approach to reducing oxygen in boiler feedwater is mechanical deaeration. Efficient mechanical deaeration can reduce dissolved oxygen to as low as 5-10 ppb (parts per thousand million) in industrial plants and 2-3 ppb in utility operations. However, even with this trace amount of oxygen, some corrosion may occur in boilers. Removal of the last traces of oxygen from boiler feedwater is generally accomplished by the addition of chemicals that react with oxygen and that are hereinafter referred to as oxygen scavengers.

[0008] Several oxygen scavengers are known, e.g. sodium sulphite, hydrazine, diethylhydroxylamine, carbohydrazide and hydroquinone, as disclosed in numerous U.S. Patent Specifications.

[0009] Thus, US-A-3 551 349 discloses the use of quinones, particularly hydroquinone, as catalysts for the hydrazine-oxygen reaction; US-A-4 096 090 discloses the use of hydrazine compounds, a catalytic organometallic complex, and preferably a quinone compound for deoxygenating feedwater; US-A-3 808 138 discloses the use of cobalt maleic acid hydrazide with hydrazine for oxygen removal; and US-A-3 962 113 discloses the use of organic-substituted hydrazines such as monoalkyl hydrazines, dialkyl hydrazines and trialkyl hydrazines as oxygen scavengers. Disadvantages of hydrazine and related compounds include toxicity and suspected carcinogenicity. Hydrazine is toxic if inhaled and is also an irritant to the eyes and skin. Carbohydrazide, which is a derivative of hydrazine, decomposes to form hydrazine and carbon dioxide at temperatures above 3600F (180°C). US-A-4 269 717 discloses the use of carbohydrazide as an oxygen scavenger and metal passivator.

[0010] US-A-4 278 635 and US-A-4 282 111 disclose the use of hydroquinone, as well as other dihydroxy, diamino and amino hydroxy benzenes, as oxygen scavengers; US-A-4 279 767 and US-A-4 487 708 disclose the use of hydroquinone and "mu-amines", which are defined as amines compatible with hydroquinone, e.g. methoxypropylamine; US-A-4 363 734 discloses the use of catalysed 1,3-dihydroxyacetone as an oxygen scavenger; US-A-4 419 327 discloses the use of amine or ammonia neutralized erythorbates as oxygen scavengers. Additionally, diethylhydroxylamine (DEHA) has been used as an oxygen scavenger, and US-A-4 192 844 discloses the use of methoxypropylamine and hydrazine as a corrosion inhibiting composition. European Patent Specification EP-A-0 054 345 discloses the use of amino-phenol compounds or acid addition salts thereof as oxygen scavengers.

[0011] UK Patent Specification GB-A-2 138 796 discloses the use of trivalent phenols, preferably pyrogallol, to improve the activity of hydrazine/trivalent-cobalt compositions.

[0012] The present invention provides a method for controlling corrosion in boilers and boiler feedwater systems comprising adding to boiler feedwater containing dissolved oxygen an additive that is at least one trihydroxybenzene compound, hexahydroxybenzene, levodopa, and/or a salt, homologue or analogue of levodopa. Optionally there may also be added, as a second oxygen scavenger or neutralizing amine, one or more of hydroquinone, methoxypropylamine, cyclohexylamine, diethylethanolamine, morpholine, diethyl hydroxylamine, dimethyl amino-2-propanol, carbohydrazide, 2-amino-2-methylpropanol, erythorbic acid, and salts of erythorbic acid, and the present invention also provides compositions containing a trihydroxybenzene compound, hexahydroxybenzene, levodopa, and/or a salt, homologue or analogue of levodopa, together with a second oxygen scavenger or neutralizing amine from those just mentioned.

[0013] Though the compounds can be used with hydrazine, this is not preferred because of the toxicity of hydrazine.

[0014] Any effective amount of the compounds may be used in accordance with the present invention. The term "effective amount" means an amount that inhibits corrosion in the system being treated. The preferred dosage is from 0.1 to 1,000 parts per million in the feedwater being treated, particularly from 1 to 100 parts per million. The preferred mol ratio of the compound to dissolved 0₂ ranges from 0.01:1.0 to 100:1, particularly 0.1:1 to 20:1.

[0015] When used in combination with the second corrosion inhibitor, the weight ratio of the compound to the second corrosion inhibitor should be from 1:99 to 99:1, preferably 1:50 to 50:1 and particularly preferably 10:1 to 1:10. At least 0.1 ppm to 1,000 ppm, preferably 1 to 100 ppm, of the composition should be added.

[0016] Any trihydroxybenzene compound can be used, viz. (and in order of preference) 1,2,3-trihydroxybenzene (pyrogallol), 1,2,4-trihydroxybenzene (hydroxyhydroquinone), or 1,3,5-trihydroxybenzene (phloroglucinol). The trihydroxybenzene compounds can if desired be used in combination with each other or with other known corrosion inhibitors, e.g. filming amines and neutralizing amines.

[0017] Preferred compounds for use with trihydroxy benzene compounds are hydroquinone, carbohydrazide, diethylhydroxylamine, erythorbic acid, and salts of erythorbic acid, especially sodium erythorbate. Particularly preferred compounds are hydroquinone and diethylhydroxylamine.

[0018] About 0.66 mole of hexahydroxybenzene is required for every mole of oxygen in the water being treated. By contrast, about 2 moles of hydroquinone, a commonly used oxygen scavenger, is required per mole of oxygen. Additionally, the kinetic properties of hexahydroxybenzene are more favourable than those of hydroquinone, and hexahydroxybenzene has better passivating properties.

[0019] Levodopa, i.e. 2-amino-3-(3,4 dihydroxyphenyl) propanoic acid, may be represented as follows:

[0020] Any salt of levodopa, or homologue or analogue of levodopa, can be used, including dopamine (i.e. 3,4-dihydroxyphenethyl amine).

[0021] Other materials can be used in the compositions of the present invention if desired. Such compounds include catalysts such as cobalt, scale/deposit inhibitors such as chelants, dispersants, sequestrants, polyelectrolytes and organic or inorganic phosphates.

[0022] The compositions may be fed to the boiler feedwater by any known means. Thus, they may be pumped into boiler feedwater tanks or lines, or added by some other suitable means. Though for convenience it is recommended that the trihydroxybenzene, hexahydroxybenzene or levodopa and any second corrosion inhibitor be added as a composition, they may be added separately.

[0023] The following examples are illustrative and do not limit the invention.

EXAMPLES 1-11

[0024] Examples 1-11 show the oxygen scavenging capability of pyrogallol. Pyrogallol, at the concentration indicated in Table I below, was added to a simultated boiler feedwater at a pH of 9.0 and at the temperature shown. Percent oxygen removal values after 2, 4, 6, 8 and 10 minutes are shown in Table I.

EXAMPLES 12-14

[0025] Examples 12-14 show the oxygen scavenging capability of 1,2,4-trihydroxybenzene (phloroglucinol), which was added to simulated boiler feedwater at pH 9 and at the temperatures and dosages shown. Percent oxygen removal values after 2, 4, 6, 8 and 10 minutes are shown in Table II, below.

EXAMPLES 15-32

[0026] These examples compare the oxygen scavenging capabilities of several well-known oxygen scavengers with those of compositions comprising pyrogallol and a second conventional oxygen scavenger. Results are shown in Table III, below.

EXAMPLE 33

[0027] Although the traditional method of measuring the effectiveness of oxygen scavengers as boiler water corrosion inhibitors has been to measure the relative speed with which they react with dissolved oxygen, such results can be misleading. This is true because, in operating systems, oxygen is an intermediary in the corrosion reaction and the first product of corrosion is ferric oxide. Oxygen alone would not necessarily be detrimental were it not for this corrosion reaction. The primary function of an oxygen scavenger may therefore be to reduce ferric ions to their original state. Under such conditions, it is the iron itself that is the primary "oxygen scavenger"; the dosing agent functions primarily as a reducing agent for ferric ions.

[0028] Accordingly, a test procedure was used to measure the relative effectiveness of boiler corrosion inhibitors with respect to their ability to reduce ferric ions. This procedure compared the time required for equal molar concentrations of reducing agents to reduce a constant ferric concentration to a specified level. Thus, the reducing agents being tested were reacted with a ferric standard in a test cell. The sensing head of a Brinkman Colorimeter Model PC/800, set at 520 nm, was placed in the cells. The drop in ferric ion concentration was continuously recorded using a Brinkman Servogor Model 210 set at 12 cm/minute. Using the data obtained, curves showing time in minutes on the ordinate versus percent absorbance on the abscissa were developed. The negative slopes of these curves are indirectly proportional to the relative effectiveness of their respective reducing agents. The most effective inhibitor evaluated was pyrogallol, which had an inverse slope of 10.0. The least effective inhibitor was sodium sulphite, which had an inverse slope of 1.2. These results are shown in Table IV, below.

EXAMPLES 34-39

[0029] Examples 34-39 show the oxygen scavenging capability of hexahydroxybenzene. Hexahydroxybenzene, at the concentration indicated in Table V, was added to a simulated boiler feedwater at a pH of 9.0 and at the temperature shown. Percent oxygen removal values after 2, 4, 6, 8 and 10 minutes are shown in Table V below.

EXAMPLES 40-42

[0030] Examples 40-42 show the oxygen scavenging capability of levodopa. Levodopa, at the concentration indicated in Table VI, was added to a simulated boiler feedwater at a pH of 9.0 and at the temperature shown. Percent oxygen removal values after 2, 4, 6, 8 and 10 minutes are shown in Table VI, below.

Claims

1. A method of inhibiting corrosion in boilers and boiler feedwater systems comprising adding to boiler feedwater containing dissolved oxygen an additive that is at least one trihydroxybenzene compound, hexahydroxybenzene, levodopa, and/or a salt, homologue or analogue of levodopa.

2. A method as claimed in Claim 1, in which the amount of the additive is from 0.1 ppm to 1,000 ppm.

3. A method as claimed in Claim 2, in which the said amount is from 1 to 100 ppm.

4. A method as claimed in any one of Claims 1 to 3, in which the additive is pyrogallol or 1,2,4-trihydroxybenzene.

5. A method as claimed in any one of Claims 1 to 3, in which the said additive is dopamine.

6. A method as claimed in any preceding claim, in which there is added, as a second corrosion inhibitor, one or more of hydroquinone, methoxypropylamine, cyclohexylamine, diethylethanolamine, morpholine, diethyl hydroxylamine, dimethyl amino-2-propanol, carbohydrazide, 2-amino 2-methylpropanol, erythorbic acid, and salts of erythorbic acid.

7. A method as claimed in Claim 6, in which the second corrosion inhibitor is one or more of hydroquinone, carbohydrazide, diethylhydroxylamine, erythorbic acid, and salts of erythorbic acid.

8. A corrosion inhibiting composition comprising an additive as defined in Claim 1 and a second corrosion inhibitor as defined in Claim 6 in a weight ratio of from 1:99 to 99:1.

9. A composition as claimed in Claim 8, in which the second corrosion inhibitor is as defined in claim 7.

10. A composition as claimed in Claim 8 or 9, in which the compound defined in Claim i is pyrogallol, 1,2,4-trihydroxybenzene or dopamine.