BACKGROUND OF INVENTION General surface corrosion of metals by chemicals dissolved in an electrolyte is a common problem in the operation of industrial processes which utilize amines, alkanol amines, hot carbonate solutions, or diglyme as reactants, products, solvents, etc. For example, it is known that acid gases, such as carbon dioxide and H2S, can be removed from gaseous feed streams, such as natural gas and synthesis gas, using dilute aqueous solutions of alkanol amines, such as monoethanolamine, diethanolamine, and methyl diethanolamine, and other weak bases. The presence of acid gases and the resulting necessity for its stripping by gas treatment services inherently results in a corrosion problem. This problem is particularly acute and/or critical when carbon steels are used in the equipment to save on capital costs over the use of more exotic and expensive metal alloys such as stainless steel. It is,

of course, common to use stainless steel and nickel alloys in sensitive areas such as in heat exchangers. There has been much activity devoted to solving the problem of metallic corrosion in the equipment used in the above-described pro- cesses. For example, U.S. Pat. No. 3,808,140 provides for inhibition of an alkanolamine solution by using minor amounts of vanadium and antimony com- pounds.

U.S. Pat. No. 3,896,044 provides for inhibition of alkanolamine solutions by using minor amounts of nitro substituted aromatic acids or salts thereof.

U.S. Pat. No. 3,959,170 discloses a corrosion inhibitor composition for steel equipment used in alkanol amine gas treating systems having antimony and vanadi- um, compounds or benzotriazole. The disclosure teaches that corrosion can be inhibited by combinations of antimony and vanadium compounds, stannous salts, organo-tin compounds, nitroaromatic acids and their salts or benzotriazole.

U.S. Pat. No. 4,096,085 provides for inhibition of alkanolamine solutions using minor amounts of a polyamine, with or without copper, and sulfur.

U.S. Pat. No. 4,116,629 provides for the corrosion inhibition of stainless steels (types 410 and 430) when in contact with carbonate solutions by using nickel salts.

U.S. Pat. No. 4,440,731 discloses a corrosion inhibitor composition for use in aqueous absorbent gas-liquid contacting processes for recovering carbon dioxide from industrial gas employing nickel and bismuth salts in combination with dihydroxyethylglycine.

U.S. Pat. No. 4,431,563 discloses an inhibitor solution comprising thionitrogen compounds, such as thiocyanates or thioamides, and alkanolamines in combination with synergistic metals such as cobalt, nickel, calcium, copper, chromi- um, zinc, tin, aluminum, and magnesium.

U.S. Pat. No. 4,595,723 discloses a corrosion inhibitor composition for treating gaseous streams containing carbon dioxide. Specifically, the above patent discloses a thiourea-amine-formaldehyde polymer preferably in combination with a nickel (11) ion producing material.

U.S. Pat: No. 4,959,177 discloses the reduction of stress corrosion cracking of metal weldments in the presence of an aqueous alkanol amine solution using a sulfiding agent selected from elemental sulfur or a sulfide ion yielding compound.

While the above compositions are effective, they each have various defects which detract from their universal use. For example, compounds of arsenic, antimony, vanadium and other heavy metals are known to be toxic and their use presents waste disposal problems for the plant operators.

SUMMARY OF INVENTION The invention, according to one embodiment, is directed to a method of protecting against corrosion in alkanolamine, hot carbonate solution, diglyme or other solvent gas treating systems. Specifically, the invention, according to one embodiment is directed to a method for inhibiting corrosion in environments in contact with amine based solvents and acid gases, such as CO2, H2S, or mixtures

thereof. The method comprises injecting into said environment of an aqueous amine resin.

In addition, the present invention, according to another embodiment, is directed to a new non-toxic corrosion inhibitor which provides protection against corrosion in alkanolamine, hot carbonate solution, diglyme or other solvent gas treating plants used for the removal of acid gases, such as CO2, H2S, or mixtures thereof.

According to a still further embodiment, the present invention is directed to an aqueous amine resin corrosion inhibitor for use, for example, according to a preferred embodiment in alkanolamine gas treating systems.

According to a particularly preferred embodiment, the present invention is directed to a non-toxic corrosion inhibitor for inhibiting corrosion in environments in contact with alkanolamine, hot carbonate solution, diglyme or other solvents and acid gases (CO2, H2S, and mixtures thereof). The compositions comprise amine resin solutions made by reacting sterically hindered amines, such as selected diamines, triamines, amino alcohols, and mixtures thereof, with aldehydes, aldehyde donors, the reaction products of lower alkanolamines and lower aldehydes, or mixtures thereof. The subject compositions are preferably made by reacting the sterically hindered amines and aldehyde components in about a 1:1 molar ratio at a temperature ranging from about 110 degree(s) to about 120 degree(s) F, (about 43 degree(s) to about 49 degree(s) C.), using a sufficient amount of the sterically

hindered amine to produce a reaction product having a pH ranging from about 10.5 to about 12.

BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a table showing test results.

Fig. 2 is a table showing test results.

Fig. 3 is a graph showing the effect of the inhibitor concentration on LPR corrosion rate of carbon steel.

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a method of inhibiting corrosion of steel, steel alloys, and ferrous containing steel alloys. The invention is specifically directed, according to one embodiment, to methods of inhibiting corrosion of steel, steel alloys, and ferrous containing steel alloys used in equipment in contact with alkanolamine, hot carbonate solutions, diglyme or other solvents and acid gases (CO2, H2S, and mixtures thereof); for example, equipment used in the process of stripping carbon dioxide out of a gaseous stream utilizing monoethanolamine.

The corrosion inhibitor of the present invention is preferably an amine resin solution made by reacting sterically hindered amines such as selected diamines, triamines, amino alcohols, and mixtures thereof, with aldehydes, aldehyde donors, the reaction products of lower alkanolamines and lower aldehydes, and mixtures thereof. The subject compositions are preferably made by reacting the sterically

hindered amines and aldehyde components in about a 1:1 molar ratio at a tempera- ture ranging from about 110° F to 1200 F, using enough or a sufficient amount of the sterically hindered amine to produce a reaction product having a pH ranging from about 10.5 to about 12.

Generally speaking, the compositions of the invention are preferably made by reacting a solution comprising free aldehyde and the reaction product of a lower aldehyde and a lower alkanolamine either with a solution comprising a sterically hindered amine, preferably amine heads, or with an activator comprising the reaction product of amine heads and formaldehyde. A particularly preferred lower aldehyde for use in making the subject compositions is formaldehyde. A particularly preferred lower alkanolamine for use in making the subject composi- tions is monoethanolamine.

As used herein, the term "amine heads" refers to an unrefined mixture of alkyl diamines that comprise from 4 to 6 carbon atoms. Examples of alkyl diamines typically found in amine heads include aminomethylcyclopentylamine; 1,2- cyclohexanediamine (1 ,2-diaminocyclohexane); 1 ,5-pentanediamine, 2-methyl; 1,6- hexanediamine; 1 H-azepine, hexahydro; and 1,4-butanediamine. Amine heads are commercially available from Monsanto Company and DuPont as a byproduct in the manufacture of hexamethylenediamine.

Although amine heads are a convenient and useful source of aliphatic diamines suitable for use in making the compositions of the invention, it should be understood that other diamines or triamines not present in amine heads can likewise

be used within the scope of the invention. Examples of other aliphatic diamines and triamines that can be satisfactorily used in making the subject compositions include 1 ,4-diaminocyclohexane and bis-hexamethylenetriamine.

One particularly preferred composition of the invention is made by reacting amine heads with formaldehyde.

Another preferred composition of the invention is made by reacting amine heads with a solution of free formaldehyde and the reaction product of monoethanolamine and formaldehyde. Another preferred composition of the inven- tion is made by reacting the reaction product of monoethanolamine and formalde- hyde with an activator comprising the reaction product of amine heads and formal- dehyde. An amine material that can be effectively substituted for monoethanolamine in making the foregoing compositions of the invention is available commercially from The Dow Chemical Company under the tradename Organic Amine 70.

Organic Amine 70 is a black, viscous liquid comprising about 5 weight percent aminoethylethanolamine, about 55 weight percent Sym- dihydroxyethylethylenediamine, about 37 weight percent Unsym- dihydroxyethylethylenediamine, about 3 weight percent trihydroxyethylethylenediamine, and a trace of tetrahydroxyethylethylenediamine.

Another composition of the invention comprises the reaction product of amine heads with a formaldehyde donor such as HMTA or hydantoin.

Other materials believed to be satisfactory for use in place of amine heads in making the compositions of the invention and for use in the methods of the invention include, for example, methyl-diethanolamine; 2 [(hydroxymethyl)amino] ethanol, 2-amino-2-methyl- 1 -propanol; methyl-ethanol amine; 2-methyl-l-amino ethanol; 2-ethyl-l-amino ethanol; 2-tertiary butylamino ethanol, 2- tertiary butylamino ethanol; 2-amino-2-ethyl- 1,3 -propanediol; 2- [(hydroxymethyl)amino] -2-methyl propanol; hydantoin; 5,5 -dimethyl- 1 -hydantoin; acetaldehyde ammonia; acetalsoxime; 2-amino-2-hydroxymethanol, 1,3-propanediol; 2-amino-l ,3-propanediol; 2-amino-2-methyl 1,3-propanediol, the reaction product of methyl pyrol and hydroxylamine; choline; and amino-spirocyclic borate esters derived by reacting boric acid with glycols, amines and amides.

Components that may be reacted with sterically hindered amines comprising amine heads, selected aliphatic diamines, aliphatic triamines, or amino alcohols to produce compositions of the invention include, for example, aldehydes, aldehyde donors, the reaction products of lower alkanolamines and lower aldehydes, and the family of D aldoses having from 3 to 6 carbon atoms.

Aldehydes believed to be useful for making the subject compositions are preferably selected from the group consisting of monoaldehydes and dialdehydes having from 1 to 6 carbon atoms, and mixtures thereof with formaldehyde, acetaldehyde, glycolaldehyde, glyceraldehyde, hydroxymethyl glyceraldehyde, glyoxal, and methyl formeel (a hemiacetal, 55 percent formaldehyde solution in methanol and methoxy-methanol or water) being particularly preferred.

Aldehyde donors believed useful in making the compositions of the inven- tion are preferably selected from the group consisting of hydantoin; hexamethylene- tetramine; hexamethylolmelamine; 2 - [(hydroxymethyl)amino] ethanol; 5,5- dimethylhydantoin; tris(hydroxymethyl)nitromethane; 2-nitro-2-methyl- 1 -propanol, 2- nitro-2-ethyl-1 ,3-propanediol; 2-nitro-1 -butanol; and acetaldehyde ammonia.

A particularly preferred compound of the present invention is commercially available from Alpha Intermediates located at 4420 S. Flores Road, El Mendorf, TX 78112 originally under the trademark of "Alpha 8716," subsequently changed to "Alpha 8640." The compound is patented by Alpha Intermediates as a hydro- gen sulfide converter under U.S. Patent No. 5,488,103 which is herein incorporated by reference. The patent and commercial literature discloses that "Alpha 8640": (1) finds application as a hydrogen sulfide converter with a scavenging rate of 1.0 to 1.5 ppm per ppm hydrogen sulfide, (2) finds application as an additive to corrosion inhibitors and surfactants to enhance control of microorganisms, (3) is used as a replacement for amine sweetening solutions and ironite sponge systems in a batch system, and (4) the spent solutions are water soluble functioning as surfactants and corrosion inhibitors and normally injected into water disposal systems. Specifically, the public literature discloses that the product is applicable to hydrogen sulfide removal by an irreversible chemical reaction.

The corrosion inhibitor of the present invention may be used in a range of concentrations depending on the particular application. However, generally in MEA type plants, the inhibitor of the present invention may be injected into the system

to give an inhibitor concentration ranging from about 25 to about 5000 ppm in a batch mode of injection, or lower in a continuous mode of operation. A generally preferred concentration for such application is about 500 ppm. It is understood that the factors which effect the rate and occurrence of corrosion, e.g., temperature, velocity, pH, oxidizing and reducing conditions, and moisture, also effect the amount or concentration of inhibitor needed for a particular application.

The corrosion inhibitor is typically added to the adsorbent, such as MEA, and then metered into the gas stream in preselected concentrations. The inhibitor of the present invention may be dissolved in, e.g. the alkanolamine, or it can be separately, but simultaneously, metered into the gas stream with, e.g. the alkanolamine. The particular method of addition is not critical and is, in large part, dependent on the configuration of the gas stream and the specific adsorbent material being used.

EXAMPLE 1 An inhibitor composition of the present invention is produced by charging about 695 grams of amine heads into a 1000 ml filter flask equipped for slight vacuum, and thereafter slowly adding about 405 grams of a mixture of a solution comprising 37 percent formaldehyde and 7 weight percent methanol. The addition is controlled to allow a maximum temperature of 120 degrees F, or external cooling is provided to control temperature. The reaction product is a black, viscous liquid and contains less than about 3 weight percent insoluble polymer.

EXAMPLE 2 An inhibitor composition of the present invention is produced in two stages.

In the first stage, about 40 weight percent of a mixture of 85 weight percent monoethanolamine and 15 weight percent water is added to about 60 weight percent of a mixture of 37 weight percent formaldehyde and 7 weight percent methanol in water. In the second stage, the first stage reaction product is titrated with amine heads obtained from Monsanto Chemical to a pH ranging between about 10.5 and about 12.0, or until polymerization occurs and formaldehyde and formaldehyde donor disappears. This occurs when about 14 weight percent amine heads is added to about 86 weight percent of the first stage reaction product. A black liquid product is produced that contains some insoluble polymer, which may precipitate.

EXAMPLE 3 The product produced by the two-stage reaction of Example 2 is diluted by further mixing with about 5 weight percent of a surfactant, Texaco Surfonic N-150, and an amount of methanol approximately equal in weight to the weight of the product of Example 2.

EXAMPLE 4 The product produced by the two-stage reaction of Example 2 is diluted by further mixing with about 5 weight percent of a surfactant, cocoamidopropyldimethylbenzyl quaternary ammonium chloride, and an amount of water approximately equal in weight to the weight of the product of Example 2.

EXAMPLE 5 Fifteen parts of water are charged to a reaction vessel. Ten parts by weight of hexamethylenetetramine are added slowly with mixing. Alternatively, this aldehyde donor can be purchased as an aqueous solution of about 40 weight percent hexamine. Fifty parts of methanol are then charged to the reaction vessel.

Under vacuum and with cooling, 25 parts of amine heads are slowly added to the solution with vigorous agitation. The reaction is allowed to proceed about one hour and until the solution cools to ambient temperature.

The corrosion inhibitor of the present invention, "Alpha 8716," was used in the following tests. The corrosion rates (mils/yr) for each of the below described tests was calculated using the following formula: corrosion rate (mpy) = 534 WL DAT where WL = weight loss of coupon (mg) D = Density of coupon (gm/ml) A = Coupon area (in2) T = Exposure time (hours) The corrosion inhibitors of the present invention were tested by exposing two duplicate carbon steel specimens in 4 oz. jars filled with CO2 saturated 20% MEA solution and 1000 ppm inhibitor at a temperature of 1900 F and a CO2 pressure of 15 psi. The exposure time was 16 Hours and the density of carbon steel was 7.86 g/cm3. The results of Test #1 were as shown in Figure 1.

Compounds of the present invention, "Alpha 8716," were then tested in 20% MEA solution containing 1000 ppm inhibitor and 100 ppm As+3 to check their

effectiveness as corrosion inhibitors and their compatibility with arsenic (Test 2).

The gas was saturated carbon dioxide. The temperature was 250° F and the exposure time was 40 hours. The density of the carbon steel was 7.86 g/cm3.

The results of Test #2, as shown in Figure 2, showed that compounds of the present invention performed well and were compatible with arsenic containing solutions.

The foaming tendency of inhibitors of the present invention, "Alpha 8640," were tested by bubbling carbon dioxide gas at a very high rate through a twenty percent (20%) MEA solution containing 10,000 ppm inhibitor. No foaming resulted even after 4 hours with inhibitors of the present invention.

To better simulate gas plant conditions, a stirred autoclave was filled with four liters of 20% MEA that were prepared and deaerated overnight with carbon dioxide. The temperature was increased to 2500 E A total pressure of 30 psi CO2 was left in the autoclave. The corrosion rate was measured with a linear polarization resistance (LPR) probe located at the bottom of the autoclave as per ASTM G59 which is herein incorporated by reference. After stabilization of the corrosion rate without inhibition, the desired volume of inhibitor was introduced with an external precision pump and the corrosion rate monitored through a computerized data acquisition and analysis. The corrosion rate of an inhibitor of the present invention decreased steadily with time and with higher concentration of inhibitor as shown in Figure 3.

Monitoring of the corrosion behavior in MEA solutions with and without arsenic consisted of determining the corrosion rate of coupons located in the static zone of the autoclave, impingement coupons subjected to high velocity flow located in the autoclave, and a weight loss tubular coupon located in the flow line and subjected to laminar flow. Corrosion rates ranged from 10 to 30 mpy in the blank test without inhibitor and arsenic. At 250 ppm of inhibitor of the present invention and 10 ppm arsenic, corrosion rates decreased to less than 5 mpy.

Without arsenic and at 500 ppm inhibitor level, the corrosion rates were also very low and in the 5 mpy range. These results showed that the inhibitor of the present invention showed an excellent performance at low levels of arsenic (simula- tion of amine plant with some arsenic still left in the vessels) as well as without arsenic (simulation of amine plant after wash-out of all arsenic).

Slow strain rate ("SSR") specimens were machined for the evaluation of stress corrosion cracking (SCC) of steel base metal and welded specimens in MEA solutions. Baseline in MEA solution and arsenic containing MEA solution without addition of inhibitor were also performed as per ASTM G129 which is herein incorporated by reference. The results of the SSR tests showed that all SSR ratios (environment versus air) were high (>70%) and that the specimens showed essen- tially no susceptibility to cracking. Both welded and non-welded specimens showed excellent behavior at all levels of inhibitors of the present invention.

An inhibitor of the present invention, i.e., "Alpha 8716" or "Alpha 8640" as described in U.S. Patent No. 5,488,103 (incorporated herein by reference), was

injected at a 500 ppm level into fresh MEA solution in an actual MEA refinery plant. The monitoring included periodic chemical analysis (pH, percent MEA, CO2 loading, iron, arsenic, chromium, copper, and total chloride) of rich and lean MEA and the regenerator overhead. Corrosion rates were monitored with the ER and LPR probes. The average corrosion rates (over the field test period of two months) recorded from the ER probes on the exchangers and the overhead accumu- lator were less than 5 mpy. These corrosion rates show that inhibitors of the present invention provided commercially useful corrosion protection and inhibition.

The method of the present invention has been described without any refer- ence to particular apparatus or equipment as those skilled in the art of gas separa- tion and other means of aqueous alkanol amine or hot carbonate solution, or diglyme utilization will appreciate that many variations in equivalent or analogous zones, reactors or units can be employed.