STABILIZED SODIUM ERYTHORBATE AND ITS USE AS A CORROSION INHIBITOR

Abstract: A method for inhibiting corrosion due to dissolved oxy­ gen comprises adding chelant-stabilized sodium erythorbate, alone or in combination with conventional corrosion in­ hibitors, to boiler water to prevent corrosion by reducing dis­ solved oxygen levels in boiler feedwater. Novel compositions contain sodium erythorbate and, as chelant, nitrilo­ triacetic acid, ethylenediamine tetraacetic acid, N-­ hydroxyethyl) ethylenediamine triacetic acid or citric acid, or a salt of such an acid.

Description: This invention relates to the inhibition of corrosion in boiler feedwater systems and boilers due to the presence of dissolved oxygen. 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. 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. 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. 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. 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. A major approach to reducing oxygen in boiler feedwater is mechanical dearation Efficient mechanical deaeration can reduce dissolved oxygen to as to 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. Several oxygen scavengers are known, e.g. sodium sulphite, hydrazine, diethylhydroxylamine, carbohydrazide and hydroquinone, as disclosed in numerous U.S. Patent Specifications. 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 carcinogeneicity. Hydrazine is toxic is 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 360°F (180°C). US-A-4 269 717 discloses the use of carbohydrazide as an oxygen scavenger and metal passivator. US-A-4 278 635 and US-A-4 282111 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 disclose 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 EPA-0 054 345 discloses the use of amino-phenol compounds or acid addition salts thereof as oxygen scavengers. 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. The present invention results from the discovery that chelants stabilize salts of erythorbic acid and erythorbic acid itself. This means that an improved chemical method for control of corrosion in boiler feedwater systems and boilers is now possible. The present invention provides a method of inhibiting corresion in boilers and boiler feedwater systems comprising adding to boiler feedwater containing dissolved oxygen from 0.1 ppm to 1000 ppm of a chelant-stabilized sodium erythorbate in which the chelant is nitrilotriacetic acid, ethylenediamine tetraacetic acid, N-(hydroxyethyl) ethylenediamine triacetic acid or citric acid, or a salt of such an acid. Optionally, hydroquinone, methoxpropylamine, cyclohexylaminp, diethylethanolamine, morpholine, diethyl hydroxylamine, dimethyl amino-2-propanol, 2-amino 2-methylpropanol, or carbohydrazide may be present, as a second oxygen scavenger or neutralizing amine. The invention also provides a method of stabilizing sodium erythorbate comprising adding to the sodium erythorbate an effective amount of a chelant as defined above, and the stabilized chelant-containing sodium erythorbate compositions thus obtained, the chelant:sodium erythorbate weight ratio being from 0.001:1 to 1:1, based on active sodium erythorbate. The chelant-stabilized sodium erythorbate may be used in conjunction with scale/deposit inhibitors such as chelants, dispersants, sesquestrants, polyelectrolytes, and organic and inorganic phosphates, or conventional boiler corrosion inhibitors such as those already mentioned under (b) above. The chelant unexpectedly retards oxidative degradation of the sodium erythorbate. Erythorbate degradation shortens shelf life, causing loss of erythorbate activity and making erythorbate inconvenient to use. Preferably, the chelant dosage is 0.1 to 100, especially 1 to 50, weight percent of the sodium erythorbate being stabilized, based on active sodium erythorbate. Thus, the preferred chelant:sodium erythorbate weight ratio ranges from 0.0001:1 to 1:1, particularly 0.01:1 to 0.5:1. The stabilized sodium erythorbate compositions may be used at any effective dosage. Dosages of from 0.1 to 1,000 parts per million by weight (ppm) in the feedwater being treated are preferred, with dosages from 1 to 100 ppm being particularly preferred. The preferred mole ratio of sodium erythorbate:O₂ ranges from 0.01:10 to 100:1, particularly 0.1:1 to 20:1. Optionally, stabilized sodium erythorbate compositions may be used in combination with other known corrosion inhibitors. When used in combination with a second corrosion inhibitor, the ratio of the stabilized erythorbate to the second corrosion inhibitor is preferably from 10:1 to 1:10 0.1 ppm to 1,000 ppm of such a composition should be added. The preferred dosage is 1 to 100 ppm of such a composition. 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. Although for convenience it is recommended that stabilized sodium erythorbate and the second corrosion inhibitor, if used, be added together, they may be added separately. The examples below, which are illustrative, compare the effects of ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA), which are the preferred chelants for use in accordance with the present invention, on the oxidative degradation of sodium erythorbate. The results are shown in Table I below. In these examples, stabilized or unstabilized aqueous solutions of sodium erythorbate were placed in a capped flask and allowed to stand for 3 days or 26 days. After standing, the percent active sodium erythorbate was measured by titration and compared with the starting sodium erythorbate concentration of approximately 10%. The chelant values shown in Table I are weight percentages, based on the total weight of the aqueous solution being tested.

Claims: A method of inhibiting corrosion in boilers and boiler feedwater systems comprising adding to boiler feedwater containing dissolved oxygen from 0.1 ppm to 1,000 ppm of a chelant-stabilized sodium erythorbate in which the chelant is nitrilotriacetic acid, ethylenediamine tetraacetic acid, N-(hydroxyethyl) ethylenediamine triacetic acid or citric acid, or a salt of such an acid. A method as claimed in Claim 1 in which the chelant is nitrilotriacetic acid or ethylenediamine tetraacetic acid. A method as claimed in Claim 1 or 2 in which the chelant is present at a chelant:sodium erythorbate weight ratio of from 0.001:1 to 1:1, based on active sodium erythorbate. A method as claimed in any preceding claim, in which the dosage is from 1 to 100 ppm. A method of stabilizing an aqueous solution of sodium erythorbate against oxidative degradation comprising adding to the sodium erythorbate aqueous solution a chelant as defined in Claim 1 or 2, the chelant:sodium erythorbate weight ratio being from 0.001:1 to 1:1, based on active sodium erythorbate.

Inventors: WALKER, JERRY L., ZUPANOVICH, JOHN D.
Applicants: CALGON CORPORATION
Application year: 1986
Main classification: C23F11
IPC codes: C09K3, C23F11