CA1080956A - Corrosion inhibitor - Google Patents

Abstract

A B S T R A C T

A corrosion inhibiting composition for removal of acid gases from natural or synthetic gas streams com-prising a solvent selected from alkanolamines, tetra-hydrothiophene - 1,1-dioxide, potassium carbonate and diglycolamines, and an inhibitor system consisting essentially of (1) a source of copper ion selected from copper metal, copper sulfide and copper salts, and (2) a source of sulfur atoms selected from (a) sulfur and (b) hydrogen sulfide and/or carbonyl sulfide in combination with an oxidizing agent that will produce in solution the sulfur atom, at least some of which is the polysulfide.

Description

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The conditioning (sweetening) of gases, natural and synthetic, i.e., the removal of acidic gases such as ~CO2, H2S and COS, by absorption in a li~uid absorbent medium, has been practiced commercially for many years. Various absorbentsr such as the alkanolamines, "sulfolane" (tetra-h~drothiophene~ dioxide), "sulfinol" (tetrahydrothiophene--l,l-dioxide plus diisopropanolamine) and potassium carbonate have been used commerciall~. Each of these systems is plagued by corrosion problems, some of which result from i decomposition of the absorbent, some from reaction between the acidic components of the ~ases treated and the absorbent, and commonly, from attack by the acidic components of the gases treated upon the metals of construction of the equipment. Generally, the corrosion occurs in the regenerator, heat exchangers, or in pumps and piping associated with these elements of the gas treating units.
Numerous compounds have been suggested as additives to the absorbents to prevent the corrosion and/or the formation o corrosive elements. For example, copper sulfate was used for three years in a 15 percent monoethanolamine gas processing plant for removing 10 percent CO2 from the gas. Corrosion was observed as a decrease in reboiler and heat exchange tube life and on analysis of the amine solution, only a few parts per million copper was found thus indicating excessive copper deposition in the unit.
(Gas Conditioning Fact Book, ~he Dow Chemical Company, Midland, Michigan, 1962, pp. 157-158, Case Number 8).

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In United States Patent No. 1,989,004, Fife teaches using an alkanolamine or-mixture of mono-, di-q and trialkanolamines in a concentration of 15 to 30 percent in water and less than 1 percent of a soluble metal salt, e.g., copper or nickel sulfate or oxide.
This does not teach maintaining sulfur, that is, elemental or the necessary sulfur compounds and oxidizing agents to produce sulfur atoms.
Fife absorbs the H2S and organic sulfur compounds from the gas using the complexing activity of each of the components, viz., the amine, the amine-metal complex and the metal. These complexes are nonthermally treated in -two steps to release the amine for reuse and convert the sulfur to a solid for removal from the system.
Fife treats the spent solution (aqueous .
alkanolamine containing the amine-sulfur compound complex and the amine-metal sulfur compound complexj not with ~-:~ : :
heat to regenerate but with air (page~L, column 2, llnes ~ -20-38) to oxidize the sulfur compounds (thiosulfates) and sulfates, then the oxidized solution is treated ., with lime to precipitate the sulfur as calcium sulfate and thiosulfates. It is welL known that the alkanoLamine .
also undergoes oxidation and that aeration increases the loss of alkanolamine, as is evidenced by the fact that the 15 to 30 percent amine solution used in Example II

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(page 2, column 1, lines 49-56) after regeneration con-stituted only a 10 to 13 percent amine solutlon, a loss of about 30 percent of the original amine.
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In recent years, both natural and synthetic gases are being produced which contain high concentra-, tions of CO2 and/or H2S. As the demand for neutral gases increases, the size of the units for treating these gases increases, thus for economic reasons, an increase in the concentration of absorbent in the system and/or an increase in the loading on the system seems desirable. ~oth of these increases, although most desirable, increase the corrosion in the unit, resulting in more frequent un-scheduled downtimes for repair and replacement of ~ ~
major elements. :~ :
. It, therefore, would be desirable to provide an inhibitor system for use in equipment in contact with acid-gas environments which reduces the corrosion of the metals of construction of equipment used under the conditions.
Accordingly, there has been discovered, according to the present invention a corrosion inhibiting composition i:
for ferrous metal and its alloys when in contact with acid-gas stripping absorbent solutions prepared from :~
solvents selected from the group consis-ting of (a) alkanol-amines having the formula~
'~:
Rl ~ ,R31R
N-C-C-OH
R2~ ' ' :.
R3R3 :::
wherein each of Rl and R2 is hydrogen, or -CtR3)2C(R3)2-OH
and R3 is hydrogen or Cl_3 alkyl radical, tb) tetrahydro-thiophene-l,l-dioxide, (c) potassium carbonate or (d) di-glycolamines, each alone or in combination with one or .
more of each other as aqueous solutions or glycol solutions, said inhibiting composition consisting essentially of:

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te) a source of copper ion selected from the group consisting of copper metal, copper sulfide and copper salts, and (f) a source of sulfur atoms selected from the group consisting of 1) sulfur, or 2) hydrogen sulfide and/or COS in combination with an oxidizing agent which will :~
produce in solution the sulfur atom, at least ~ome of which is the polysulfide, under the conditions of operation in an acid-gas stripping plant which employs ~
10 thermal regeneration techniques for separating the ~ ~ `
com~ined acid-gas from the stripping absorbent, and a -solvent for (e) and (f) selected from (a), (b), (c) or (d).
The present invention also resides in a ferrous metal corrosion inhibiting composition con-sisting of (a) copper as the copper metal, copper sulfide or copper salt; and (b) an oxidizing agent .
which, in the presence of sulfur or a sulfur containing . ::
compound, will convert the sulfur to the sulfur atom : . ~
20 andjor oxidize copper to the copper ion, and an . -alkanolamine of the formula :

Rl\ R3R3 N-C-C-OH

25 wherein each of Rl and R2 is hydrogen, or -C(R3)2C(R3)2--OH and R3 is hydrogen or a Cl_3 alkyl radical.
The present invention further resides in a :
corrosion inhibited acid-gas absorbing composition con-sisting of a solvent selected from the group consisting of alkanolamines having the formula:

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Rl ~ R3R3 ,,,N-C-C-OH
R2 R3~3 wherein each of Rl and R2 is hydrogen or -C(R3)2C(R~)2--OH and R3 is hydrogen or Cl 3 alkyl radical, tetra-hydrothiophene-l,l-dioxide, potassium carbonate, or diglycolamines, each alone or in combination with one or more of each other as aqueous solution containing from 1 to 5000 ppm copper and 1 to 5000 ppm sulfur, 10 both based on the weight of the solvent, at least some ~
of which is present as the polysulfide. ;j;
The quantity of copper ions necessary to reduce corrosion will vary with each plant and may vary from day to day in a particular plant. Therefore, the practical manner for determining the proper quantity of copper ions is to place in the equipment at places where the most severe corrosion occurs, generally in the reboiler and cross-exchangers to the reboiler, a ~
metal coupon which can be examined periodically for ;
signs of bright copper plate and corrosion. Such a control design will be explained and described in detail in the examples. Further, when the inhibitor composi-tion of the present invention is first put into use in a gas-treating unit, a larger quantity of copper must be supplied since much is consumed in remo~ing the corrosive elements and corrosion products at the sites corrosion has or is likely to develop. Once a degree of passivation to corrosion has been obtained, it has been found that as little as about 15 parts of inhibitor per million parts of absorbent solution will 17,884-F ~4a-"".

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maintain a corrosion rate of less than l mil/yr. (0.025 mm./yr.). It is to be understood that in most instances , where initially high concentrations of inhibitor are required it is common for the inhibitor to eventuàlly passivate the unit by removing the innocuous components and thus enable reduction of inhibitor to more conven-tional quantities, i.e., in some instances even as low as l to lO ppm. However, even in those instances where corrosion is only slightly reduced, downtimes due to corrosion are minimized and even eliminated between normal plant shutdowns.
Copper (metal, oxide, hydroxide or salt) is used to provide, in situ under the conditions found ~ --in a gas treating or conditioning process using thermal regeneration, copper ions and sulfur or a sulfur yielding :
compound which, under the conditions of operation (particularly thermal regeneration) will produce sulfur atoms and preferably polysulfide moieties. The copper - ~ ions form a copper polysulfide with the copper atoms ~20 and polysulfide moieties:

Cu++ + S-Y-2 ~CuSnY

where y is an integer from 2 to 6 and n is an integer from 2 to about 50. The copper polysulfide is available to react from solution with the Fe~+ in solution to form at the surface of the vessel, heat exchanger, etc. a chalcopyrite, i.e., CuFeS2. The Fe is the primary product of the cathodic corrosion reaction. Further, 17,8g4-F ~5~
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in some instances, the copper will be reduced to copper atoms which alloy with the ferrous metal below the chalcopyrite s surface layer and form a copper-ferrous alloy. These phenomena provide an effective physical and electrical barrier to the corrosive environment (the alkanolamine, alkanolamine decomposition products, ~I2S, CO2, Cu 2, S , Fe~3 and 2); that is, the chalcopyrite acts as a -kinetic barrier to the corrosion reactions by hindering ~
the electron and mass transport required to support the ;
anodic and cathodic-corrosion half reactionsD
The phenomenon is, o course, based on theoretical concepts supported by the results of surface analysis of the ferrous surfaces of both passivated and corroded - surfaces using depth profiling, Auger spectroscopy and the electron microprobe.
The copper ions are conveniently introduced into an absorbent medium both during and after the initial period as a solution of, or by dissolution in, said absorbent.
There is employed a sufficient amount of one of the following to maintain the equivalent of 1 to 15 or more parts of copper per million parts of absorbent medium: -I copper metal when accompanied by sulfur, or an o~idizing agent which will produce sulfur from the H2S dissolved in said absorbent;
II copper sulfide (either cupric or cuprous);
III a copper salt (cupric or cuprous) in combina-tion with an o~idizing agent, and if little or no H2S or COS is present in the acid gas, then sulfur or a sulfur-generating compound. The copper salts may be, amony others, cupric or 17,884-F ~6-1 ' ' - .: . . ~. ,: , , , ,: ,. , . ' .

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cuprous carbonate, benzoate, stearate, acetate, acetylacetonate, chloride, oxalate, oxide, s molybdate, chromate, perchlorate, sulfa-te or tetrafluoroborate.
It is to be understood that the quantity of inhibitor in the circulating system may be, and frequently is, greater than that dissolved in the absorbent.
Such a condition will occur during the initial stages of introduction of the inhibitor to a plant, or at any ;
upset of the plant or on start-up after any scheduled - or unscheduled shut down. During these periods, the inhibitor is reacting with the metal of the plant, pass-ivating the sites of corrosion, and reacting with corrosion products to passivate these products; thus, the inhibitor ; 15 is being consumed. Therefore, the inhibitor components must be present to replace those consumed. And where the solubility of one or more of the components i5 limited, ~ an excess of that component or components must be present ; ~ to enable reaction and/or dissolution in order to effectively inhibit corrosion. It has been found that initialIy as much as 500 to 2000 ppm inhibitor must be present to passivate a plant on start-up in order to obtain satisfactory readings on the monitoring coupons. This condition can exist for from several hours to several days, depending upon the condition of the plant. Slmilarly, during operation of a plant which has experienced operating difficulties it may be necessary to increase the amount of components circulating, dissolved or undissolved, until the monitoring unit indicates a non~corroding condition.
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The oxidizing agents which have been found to give satisfactory results when used in combination with one or more of the above sources of copper are: sulfur, potassium permanganate, calcium permanganate, sodium perman-ganate, potassium persulfate, potassium iodate, calciumiodate, sodium bromate, sodium persulfate, potassium meta periodate, strontium permanganate, sodium perborate, zinc permanganate, hydrogen peroxide, sodium dichromate, potassium dichromate, sodium perborate, sodium peroxide and oxygen.
The absorbents for acid-gas stripping which are satisfactory for use in accordance with the present invention are the alkanolamines having the formula Rl R3 R3 ~N-C - C-OH

wherein each Rl and R2 represent independently hydrogent or -C(R3)2C(R3)2-OH and where R3 represents independently hydrogen or a Cl 3 alkyl radical. This absorbent may be used alone or in combination with each other or one of the following, also useful, gas~stripping absorbents:
sulfolane (tetrahydrothiophene-l,l-dioxide), potassium carbonate or diglycolamines. The presence of hydrogen sulfide (H2S) is required if the inhibitor is to be effective in absorbent systems of potassium carbonate and the dialkanolamines. The inhibitor has limited effec tiveness in the presence of CO2 alone in these absorbents.
The preferred absorbents are monoethanolamine, diisopropanol-amine-sulfolane, diethanolamine, diglycolamine(2-(2-amino-ethoxy)ethanol), 3-dimethylamino-1,2~propanediol (Methicol) all as aqueous solutions. It is to be understood that instead of water glycols may be employed.

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GENEl~L TEST PROCEDU~
Various aqueous solutions of monoalkanolamine,with or without inhibitors were prepared and the solutions saturated with H2S at 25C. Mild steel coupons, which had been pickled in HCl (15 percent) for 30 minutes, rinsed with water, scrubbed with pumice soap and a toothbrush, rinsed first with water, and then with acetone and air dried, were weighed and immersed in the test solutions, with or without inhibitor. Then the solution jars were sealed and placed in a pressure vessel and maintained at 120C. for 15 hours under a pressure which just exceeds the vapor pressure of the stripping solution (absorbent) at the 120C. temperature used for stripping the acid gases from the absorbent. The metal coupons were removed, pickled for 10 minutes in 10 percent HCl which was inhibited with a commercial acid corrosion inhibltor, scrubbed, rLnsed and dried as before. The so~prepared coupons were weighed and the difference in weight was conver-ted - : :
to corrosion rate in mils penetration per year (mpy) ~ (mm. x 39.37). Such a procedure determines the corrosion rate of the solutions on AISI 1010 or 1020 Steel. This test determines whether a compound or mixture of compounds has potential as an inhibitor, e.g. it is a screening test. The coupons are similar to the metal in a freshly cleaned, new plant and thus simulate start-up in such . . .
a plant. However, in an operating plant, there is an .. .. .
~ additional problem of corrosion products which act as ~ ' ' , '.:' ' ' ;~ ' ,;,' .' ':
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a copper sink (absorb copper by chemical-combination), e.g., FeS will rapidly absorb copper ions from solution . to form chalcopyrite, CuFeS2, a poor inhibitor.
Example 1 Solutions, prepared containing various compounds and mixture~s of compounds, were tested in accordance with the above. Procedure to determine the effectiveness of the compounds as inhibitors. The results are set forth below:

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Example 2 . .
In a similar manner, the following compounds were tested yielding the results set forth in Table II.
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The numbers under "Percent Inhibition" represent the average results of replicates. The data hereafter presented establish the utility of many of the copper salts listed as well as the effectiveness of the inhibitors of the present invention to reduce corrosion in various gas treating solutions used commercially. It is to be understood that these data do not define the lower limits of the compositions of the present invention, but only the indication that these compositions have utility as a corrosion inhibitor. The lower limits are determined by the test methods later set forth in this application.
It will be readily observed, for example, that Cu~ and S
added to maintain about 90 ppm and 45 ppm, respectively, will substantially inhibit corrosion in a badly corroded commercial plant, whereas most of the data from the laboratory - hereafter set forth indicates 100 ppm of any combination usually yives less than 50 percent protection~
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TABLE II
80% Monoethanolamine in Water Saturated with H2S at R.T.
Copper Source, ppm Oxidizing Agent, ppm ~ Inhibition(1) Cu2S 1000 KMnO4 1000 84,28 CuS 100 K persulfate 100 corrosion(2) CuS 100 KMnO4 100 corrosion CuS 500 KMnO4 500 90 CuS 1000 KMnO4 1000 96 CuS 500 K iodate500 91 CuS 1000 K iodate1000 90 CuS 500 K persulfate 500 88 CuS 500 K persulfate 1000 92 CuS 500 K persulfate 100 92 CuS 1000 Na bromate 1000 93 CuS 500 Na ~romate 100 93 CuS 500 Na chlorate 1000 93 CuS 500 Na chlorate 100 93 . .
CuS 1000 H22 (30%)looo 60 Cu (oxalate)2 10~00KMnO4 1000 85 -~Cu (oxalate)2 2000 KMnO4 1000 94 Cu (oxalate)2 5000 KMnO4 1000 94 Cu (oxalatej2 1000 K persulfate 2000 corrosion Cu (oxalate)2 2000 K persulfate 5000 94 ;
Cu (oxalate)2 1000 H2O2 (30~) 5000 88 Cu (oxalate)2 2000 H22 (30~) 1000 85 Cu (oxalatej2 5000 H22 (30~) 1000 85 Cu (acetate)2 1000 ~nO4 1000 gl Cu (benzoate)2 2000 KMnO4 1000 80 )As calculated in Table I

2)Corrosion means no inhibition, i.e. same or greater corrosion rate as that observed with no inhibitor :

17,884-F -13-TABLE II (continued) Copper Sourcer ppm Oxidizing Agent ~pm ~ Inhibition(l) 47 1000 KMnO4 1000 , 85 CuI(P~3)4 2000 KMnO4 1000 80 CuI(P~3)4 1000 KMnO4 1000 35 CU(CF3C2)2 1000 KMnO4 1000 77 CuSO4 1000 K persulfate 1000 87 - ~:
CuSO4 1000 Na bromate 1000 60 CuSO4 1000 KMnO4 1000 66 Cupric chromatelO00 KMnO4 1000 77 - :
Cu (stearate)2 1000 KMnO4 1000 62 Cu (stearate)2 2000 KMnO4 1000 81 ~ ~:
Cu (stearate)2 2000 KMnO4 2000 85 Cuprous per- 1000 KMnO4 1000 64 ~ ~
chlorate ~:
Cu(BF4)2 1000 KMnO4 1000 84 .
Cupric per- 1000 KMnO 1000 75 chlorate 4 . ~
Cupric per- 2000 KMnO4 1000 83 :
chlorate . . :::
Cupric per- 5000 KMnO4 1000 95 chlorate Cupric 1000 KMnO4 1000 51 acetylacetonate Cuprous 1000 KMnO4 1000 84 acetate Cu2O . 1000 Sulfur 1000 49 Cu2O 2000 Sulfur 1000 82 Cu2O 1000 Sulfur 2000 70 Cu2O~ 2000 Sulfur 2000 75 CuCO3 1000 Sulfur 1000 37 CuCO3 2000 Sulfur 1000 77 17,884-F -14-' .' :

: TABLE II (continued) Copper Source, ppm Oxi izi g Agent~ ppm ~ Inhibition(l) . . .
CuCo3 1000 Sulfur 2000 72 -CuCo3 2000 ~ulfur 2000 83 Cu(NO3)2 1000 Sulfur 1000 60 Cu(NO3)2 ~ 1000 Sulfur 5000 46 CuCO3 1000 ~5nO4 1000 9o CuCO3 1000 KMnO4 2000 80 CuCO3 1000 KMnO4 5000 7~
CuCO3 1000 KMnO4 500 corrosion -CuCO3 1000 KMnO4 100 corrosion :~ .
CuCO3 2000 KMnO4 1000 91 CuCO3 2000 . KMnO4 500 92 CuCO3 2000 KMnO4 100 87 CuCO3 2000 ~5nO4 50 86 CuCO3 5000 XMnO4 1000 91 CuCO3 5000 X~5nO4 500 90 CuCO3 5000 KMnO4 100 : 88 CuCO3 5000 ~5nO4 50 86 CuCO3 10000 XMnO4 1000 90 CuCO3 10000 KMnO4 100 89 CuCO3 1000 K persulfate 2000. 80 CuCO3 2000 X persulfate 1000 81 CuCO3 2000 K persulfate 2000 85 CuCO3 1000 K iodate1000 79 CuCO3 1000 Na bromate1000 85 CuCO3 500 Na persulfate 500 corrosion CuCO3 1000 Na persulfate 500 corrosion 1: ~
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TABLE II (continued) .
Copper Source, ppm Oxidizing Agent, ppm % Inhibition(l) CuCO3 1000 Na persulfate 1000 corrosion ~ :
CuCO3 2000 Na persulfate 100G 86 CuCO3 1000 Na persulfate 2000 84 CuCO3 2000 Na persulfate 2000 86 CuCO3 1000 Na persulfate 5000 82 .
CuCO3 1000 K meta- 1000 71 .
-periodate .
CuCO3 1000 Na permanga- 1000 26 :
nate ~
CuCO31000 Na permanga- 2000 33 . :
nate .
CuCO32000 Na permanga- 1000 75 ~ :
nate ~ .
CuCO32000 Na permanga- 2000 . 81 ~ :
nate . -:
~ .
CuCO31000 Sr permanga- 1000 66 :
~: : nate CuCO31000 Sr permanga- 2000 82 nate CuCO31000 Sr permanga- ~5000 - 94 nate CuCO32000 Sr permanga- 2000 96 nate CuCO35000 Sr permanga- 2000 94 nate - CuCO31000 Na perborate 5000 68 CuCO31000 Zn permanga- 1000 72 nate CuCO32000 nate 80 CuCO31000 Zn permanga- 2000 59 nate .
CuCO31000 Ca io~ate 1000 75 .~:
: CuCO31000 NaHSO3 1000 corrosion CuCO31000 Na chlorate 1000 corrosion :
CuCO31000 Na chlorate 2000 corrosion .,~ .

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TABLE II (continued) .
Copper Source, ppm Oxidizing Agent, ppm % Inhibition(l) :~-CuCO3 1000 KI 1000 77 .
~nO4 1000 CuCO3 1000 H2O2 (30~)5000 63 Cu2O 1000 K persulfate 1000 corrosion Cu2O 1000 K persulfate 2000 90 Cu2O 1000 K persulfate 5000 93 :~
Cu2O 2000 K persulfate 2000 89 Cu2O 2000 K persulfate 5000 93 Cu2O 1000 KMnO4. lQ00 corrosion Cu2O 2000 KMnO4 1000 44 .
Cu2O 2000 KM~04 100 38 Cu2O 1000 Ca iodate1000 87 .
CuO : 1000 KMnO4 1000 90I: . :
CuO 1000 Ca iodate1000 91 .
CuO 1000 K iodate1000 ~ 95 ~
CuO 1000 Na permanga- 1000 59 :-:.
nate .
Cu(NO3)2 1000 nate 63 .

CuO 1000 K persulfate 1000 94 . -CuO 1000 Na ~romate1000 92 .
Cu(NO3)2 1000 Na dichromate 1000 40 :
Cu(NO3)2 1000 Na dichromate 5000 49 ;
Cu(NO3)2 1000 2 1000 corrosion :
Cu(NO3)2 1000 I2 1000 corrosion :
Cu(NO3)2 1000 I2 2000 . 6 .
Cu~NO3)2 1000 I2 5000 corrosion . . . .
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TABLE II (continued) Copper Source, ppm Oxidizing Agent, ppm ~ Inhibition(l)~Cu(NO3)2 1000 K persulfate 5000 corrosion Cu(NO3)2 1000 K persulfate 1000 corrosion Cu(NO3)2 1000 Na persulfate 1000 42 Cu(NO3)2 1000 Na persul~ate 5000 50 ~:
Cu(NO3)2 1000 Na chlorate500 corrosion Cu(NO3)2 1000 Na chlorate100 corrosion ~.
Cu(NO3)2 1000 Na chlorate1000 corrosion Cu(NO3)2 1000 Na chlorate5000 corrosion :
CU(N3)2 5000 Na chlorate5000 88 :
Cu(NO3)2 1000 La perchlorate 1000 corrosion .
Cu(NO3)2 1000 Na perborate 1000 30 Cu(NO3)2 1000 Na perborate 5000 35 1-Cu(NO3)2 1000 Sr permanganate 1000 . 63 Cu(NO3)2 2000 Sr permanganate 2boo 91 :
Cu(NO3)2 5000 Sr permanganate 2000 95 CU(NO3)2 1000 5r permanganate 2000 67 Cu(NO3)2 1000 Sr permanganate 5000 79 Cu(NO3) 2 looo Na peroxide1000 corrosion Cu(NO3) 2 looo Periodic acid 1000 20 CU(NO3)2 1000 NH~ persul~ate 1000 . 16 Cu(NO3)2 1000 NH4 persulfate 2000 corrosion Cu(NO3)2 1000 NH~ persulfate 5000 28 Cu(NO3)2 1000 KMnO~ 1000 72 Cu(NO3)2 1000 Ca permanganate 1000 80 Cu(NO~)2 1000 Ca iodate1000 13 Cu(NO3) 2 looo K-meta 1000 78 :
per:iodate .
Cu(NO3) 2 looo H2O2 (80%)5000 45 "

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TABLE II (continued) :
Copper Source, ppm Oxidizin~ ~gent, ppm ~ Inhibition(1~ .
Cu(NO3)2 1000 H22 (30~)1000 37 :
Cu(NO3)2 1000 H2O2 (30%)5000 68 Cu(NO3)2 1000 Cu selenate1000 corrosion . .: -Cu(NO3)2 1000 Na chlorate1000 93 Cupric 1000 KMnO 1000 77 - .
chromate 4 . .
Cupric 2000 KMnO4 1000 70 molybdate . . .
Cupric 5000 KMnO4 1000 41 tungstate . - :
Cupric !5000 X~qnO4 1000 28 .:
phosphide CuC12 1000 4 92 CuC12 1000 KMnO4 1000 84 . . .
CuC12 1000 K persulfate 500 92 ~
CuCl 1000 K iodate 500 92 . .
2 . .
CuI2 1000 KMnO4 1000 71 ~.
CuBr2 1000 KMnO~ 100~ 77 To demonstrate the effectiveness of the corrosion inhibitor compositions of the present invention in other commercial solvents used in gas-treating plants a series ::
. .. .
of tests were run where various solvents containing CuCO3 .- ~.
5 or CuS, an oxidizing agent and one or more acid gases were :
evaluated.
Procedure: .
- The conditions for testing the various solvents, including gas ratios, as volume ratios, are described in the .
10 following tables. The saturated solutions, at the. .
concentration used, were poured into 4 oz. bottles ~118 ml~) ~
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which had the inhibitors and the 1020 mild steel coupons (prepared as in the general procedure) already weighed and placed in them. The bottles were capped with loose fitting caps and placed in a pressure vessel which was then held at 125C with a total pressure of 40 psig (2.8 kg./cm.2) of either H2S, or CO2 or both, for 16 hrsO All inhibitor components are reported in parts inhibitor component per million parts o-f absorbent medium.

Solvent used: N-Methyldiethanolamine 45% ~ water 55%
Gas Loading ratio: 0.2 vol. CO2/vol. H2S

MPY
CuCO3_ KMnO4 (mm./yr.)% Inhibition - - 38.5 0.0 (0.98) 1000 1000 11.2 71.0 (0.29) 1000 - 12 68.8 (0.30) - 1000 61 -58.5 ~ (0.16) Example 4 ~ -SoLvent used: Diisopropanolamine 45%, Sulfolane 35%, Water 20%
Gas Loading ratio: Saturated with H2S only MP~ , :~
CuCO3_ KMnO4_ (mm./yr.) % Inhihition - - 18.4 0.0 ~0.47) 11.1 39.7 (0.28) 500 3.1 ~3.
(0.08) 500 500 5.56 70 (0.14) I

MPY
CuCO3 KMnO4 (mm./yr.) % Inhibition 2500 50 8 56.5 (0.20) 2500 2500 5.74 68.8 (0.15) 5000 5000 7.46 59.4 (O .19) Example 5 Solvent used: Diglycolamine: (2-(2-aminoethoxy)ethanol) 60% * water 40%
Gas loading ratio: Saturated with H2S only MPY
CuCO KMnO (mm./yr ) % Inhibition

- 3 ----4 - - 20.4 0.0 (a.s2) ~ 37.5 - -84 ~
( 1 . O ) , . . . .
2500 6.21 69.5 (0.16) ~-500 500 3.22 ~83.7 (0.08) 2500 50 3.19 - ~.3 (~.08) . .
2500 2500 3.1 84.8 (0.08) 5000 5000 4 80.4 ( 0 . 10 ) . , " ,~
Example 6 :~
Solvent used: 3-dimethylamino-1,2-propanediol ;
50% + water 50%
Gas loading ratio: CO sparged 30 min., 1I2S sparged the 20 min Ex~ernal H~S gas pressure was applied to to establi~h2the pressure at 40 psig~
~ ~2.8 kg./cm. ) :

.
., .
17,884-F -21-.
.
.

MPY
CuCO3 ~lnO~_ (mm./yr.) % Inhibition - - 25.55 0.0 1~00 1000 11.11 56.5 (0.28) Example 7 Solvent used: Potassium carbonate Gas loading ratio: 50/50 CO /H S saturated MPY
CuCO3 KMnO4_ (mm./~r~) % Inhibition - - 27.63 0.0 (0.70) 1000 100 20.0 27.6 (0O50) 1000 1000 18.632 D 6 (0.~7) 1000 2000 15.1 45.0 (0.38) The following three tables illustrate that a high --CO2 content, i.e., CO2 alone or ratios greater than 1 to 1 C2 to H2S, in potassium carbonate or diethanolamine systems ~ are not effectively inhibited. ~Iowever, when the CO2 to - 5 H2S ratio is 1 to 1 or less than 1 to 1, e.g., 1/4.5 or H2S alone, inhlbition is efected.

- ,: .
: . .

.:

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17,884--F 2~

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Comparative Example 1 Solvent used: Potassium Carbonate :
Gas ratio: CO only CuS S (mm./yr.) ~ Inhibition . -- .
_ - ~2 (1.07~

(1.42) - 500 80.2 -86.6 (2.04) -100100 60.2 -40 ~ ;
(1.53) (1.75) 10001000 87 -104 , (2.21) (1.37) (2.67) -,'""
, , ~: .' , , . . ~ , .. . .

. .
.

17,884-F -23-: - , . . ; . : .
,.. ' '. . '' ' " ,'. ' " ' ~ ' Comparative Example 2 Solvent used: Diethanolamine: (DEA) 30% ~ 70~ water Gas ratio: 9/1 CO2/H2S `

~lPY
CuCO3 S (mm./yr.) % Inhibition _ _ 9.3 .
(0.24) :
100 5000 22.76-148 :
(0.58) 500 5000 27.8 -205 (0.71) :
1000 5000 26.2 -185 .
(0.67) 5000 5000 33.02-260 :
(0.84) :

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..... ~ . .
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17,~84-F -24- ~

~" ' ' ' ' . ', ' , , : , , Example 8 Solven~ used: Diethanolamine: (DEA) 40% + 60% water Gas loadin~ ra-tio: CO2/H2S 1/4.5 MPY
CuCO3 KMnO4 (mm./yr.) % Inhibition - ~ - 14.35 0.0 (0.37) 16.8 -17.07 ~ -(0.43) 5000 11.6 19.2 (0.29) 500 25 8.05 44.0 (0.20) , 500 1000 7.91 45.0 (0.20) 500 5000 8.84 3804 (0.22) 1000 50 9.27 35.4 (0.24) 1000 2000 11.0 23.3 (0.28) 5000 50 10.1 29O6 -(0.26) ,' .. ..

17,884--F -25-. , .
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Exa~ple 9 -:~
Solvent used: Diethanolamine: ~DEA) 80% -~ 20% water .-:
Gas ratio: Saturated with H2S only ' MP~
CuCO KMnO (mm~/yr.) % Inhibition - 3 ~ - :
_ - 17.25 0.0 (0.44) 1000 1000 5.66 67.2 (0.14) 1000 2000 6.5 62.3 (0~16) 1000 5000 6.00 65~2 (0.15) 2000 1000 6.85 60.0 (0.17) ~000 2000 4.8 72.2 (0.12) , 5000 1000 7.~6 59.07 ~G.18) 5000 5000 6.7 61.2 (0.17) , , 17,884-F -26 s~ :

It is apparent Erom the above data that the ~ .
copper inhibitor composition of the present invention is not a satisfactory inhibitor for diethanolami.ne solutions of less than 50 percent diethanolamine when used for swee~ening acid-gases high in carbon dioxide content.
Si~nilarly the inhibitor composition has limited utility in potassium carbonate solutions of greater than 50 percent CO2. Ho~Jever, as H2S content increases, the inhibitory effect is apparent.
Example 10 An off-gas stripper in a mediurn siæe refinexy operating at 250~F. (121C.) on the reboiler and with 17 to 19~ aqueous monoalkanolamine absorbent, 6000 U.S. gallons .
(22.7 kiloliters) was seriously corroded and recording 37.4 mpy (0.95 mm./yr.) corrosion rate on an electrical .resistance corrosion monitoring probe at the reboiler outlet, and 74.6 mpy and 95.5 mpy (1.89 and 243 mm./yr.) corrosion rate on coupons at the reboiler outlet and the reboiler cross-exchange~ respectively. This rate :~
of corrosion was experienced during a five-day period ollowing a periodic shutdown. On the sixth day, copper sulfide and sulfur were added to the aqueous amine solution -~ontinuously at a rate of approximately 50 pounds (22.7 kg.) CuS e~ery 6-8 days and approximately 24 pounds (10.9 kg.) su~ur every day.
Beginning the sixth day of the test, khe day the corrQsion inhibitor was added, throughout the twelEth day the corrosion rates were ~.75 mpy, 4.77 mpy and 16.08 rnpy (0.25, 0.12 and 0.41 mm./yr.) at the probe at the reboiler outlet and the coupons at the reboiler outlet : -. . '., . 17,884-F -27-: ' . .: . . . . .
. . . . . . ...

s~ :

and reboiler cross-exchanger, respectively. From the twelfth day to the end of the test the corrosion rates were as follows:
Probe Coupons Reboiler Reboiler Cross-Outlet, Outlet, Exchanger DayMPY (mm/yr) MPY (mm/yr) MPY (mm/yr) 12-14 1.85 1.26 9.44 (~05) (0.03) (0.24) 19-26 1.85 1.25 14.6 (0O05) (0.03) (0.37) 27-39 1.0 10.5 13.7 (0.03) (0.27) (0.35) 39-45 1.0 4.7 18.1 (0O03) (0.12) (0.46) On several instances the corrosion rate on the probe was allowed to increase to about 24 mpy (0.61 mm/yr) by discontinuing the addition of inhibitor, then inhibi-tor addition resumed. Within one hour after the addition was resumed, the probe registered 1 ~.py (0.03 mm/yr) or less corrosion rate.
METHODS FOR DETERMINING THE CORROSION
RATES AND INHIBITOR EFFECTIVENESS
There are two methods which can be used to measure the corrosion rate and also the effectiveness of the copper based inhibitors of the present invention at various points throughout plants which are using solvents such as monoethanolamine to remove mixtures of acid gases, such as H2S and CO2, from liquid or gas process streams. The first is by using metal corrosion test coupons. The second is by using a metal resistance probe and an electrical resistance measuring bridge such as that manufactured by Corrosion Monitoring Systems, Inc., 33 Lincoln Rd., Springfield, Ne~ Jersey 07081. A very ,.

17,884-F ~ ~ -28~
.
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110~3~

good probe, the F. Jefferies probe, is especially useful because of its all metal construction and essential freedom from temperature fluctuation sensitivity. This ..

17,884-F ~28a- .

.

.
prObe is also sold by the above named corporation.
Another probe which is also useful is one made by Magna Corporation, who also manufactures a resistance bridge which will measure the corrosion experienced by their probe.
It is not unusual to have different sections of any gas treatment plant made from different metals. Thus, if the corrosion rate of a particular plant section is sought, it is necessary to measure the corrosion on a probe or coupon made of that same alloy which has been placed in the plant at that section. -A convenient coupon test method for mild steelis the one described below. Similar tests are available for other alloys.
First, a coupon of the metal of interest is machined, A convenient size is 1/2 in. x 2 in. x 1/16 in. (1.27 cm.
x 1.27 cm. x 0~16 cm.). A 3/8 in. (0.95 cm.) hole is drilled near one end so that the coupon can be mounted on a coupon holder. This holder is used to place and hold the coupon, in electrical isolation from the plant's base metal, in the plant at the point of interest.
Second, the machined coupon is cleaned as described under General Test Procedure, weighed accurately on a balance to a tenth of a milligram and mounted on the coupon holder and placed into the plant's solution.
Entry to the corrosive environment is achieved by one of several techni~ues widely known in the art.
One such method is to in5ert the coupon and its holder through a packing gland and then through an open valve, :: :
: ' 17,88~-F ~29-.

.. . . .

Third, after the coupon has been in the corrosive environmen-t for several days, it is removed. The exact number of days is not important except that, in order to be able to place more significance in the measured corrosion rates, times of 20-30 days are desirable; but times as short as one day can be used. Very short test periods generally result in higher apparent corrosion rates than ~
would be expected from longer time period tests. The -coupon is then recleaned according to the following procedure:
Place the corroded metal coupon in 15~ ~ICl solution containing a good acid corrosion inhibitor. Then, follow the same instruc-tions as listed above for the first cleaning of the coupon.
When the coupon is reweighed, subtract to determine the difference in weight from the weight of the untested coupon. The original weight, minus the final weight, will give the weight loss in grams. ~
By using the following method, a corrosion rate ... . .
for the solution-alloy system can be calculated.
CR (mpy) = 0.183 x Wt. Loss (mg) (mm. x 39.37) Strip factor x Coupon Weight Before (gm) x Time (days) Where: 0.183 is the conversion factor from milligrams/
square decimeter/day (mdd) to mils penetration per year (mpy) for mild steel (102`0).
,.
Strip Factor = Area in Decimeters of Reference Cou~on Weight of Reference Coupon in Grams ~For coupons used 0.0176 was slrip fac~cr~

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17,884-F -30-.

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Fourth, inhibitor in the way and amount described in accordance with the present invention is added to the s plant and a new corrosion rate determined by the method outlined just above.
Fifth, a percent inhibition is calculated for the particular inhibitor concentration used as follows:
Uninhibited Corrosion Rate % Inhibition = Inhibited Corrosion Rate Uninhibited Corrosion Rate Sixth, the amount of inhibitor is increased or decreased to achieve the desired corrosion protection as measured by the coupon method just outlined. Usually, 5 mpy (0.13 mm./yr.) is achievable; but corrosion has been reduced in the hot, corrosive environments from 20 mpy (O.Sl mm./yr.) to values less than l mpy (0.03 mm./yr.) as measured by this technique which is ~ound to correlate well with plant inspection and measurements made on specific elements by a state licensed boiler inspector.
The corrosion rate of a plant's solution can ;~- also be measured by using metal probes such as~those described earlier. In this case, the probe is desiyned -to replace the coupon as described above. The probe can . , be used directly as received from the manufacturer where it was sand-blasted before shipment or it can be cleaned with uninhibited HCl as described in the above by the coupon cleaning method.
After the corrosion probe is cleaned, it is ~ ~
inserted into the plant's solution at the location in ~ -question and the temperature allowed to equilibrate.
' ~ .: . .
, ~ .. . ..... ..

17,884-F -31-,' ' A resistance reading is taken with a resistance bridge.
The bridge sold by the probe manufacturer is ver~ convenient.
By drawing a graph of the resistance probe vs. time, the corrosion rate can be calculated from the slope o the line using the formula supplied by the probe manufacturer.
When the uninhibited corrosion rate has been determined, inhibitor is added according to the present invention and the corrosion rate is measured and calcula-ti~ns m-de as ab~ve outlined ..

17,88~- F -32-' !

Claims (9)

1. A corrosion inhibiting composition for ferrous metal and its alloys when in contact with acid--gas stripping absorbent solutions prepared from solvents selected from the group consisting of (a) alkanolamines having the formula:

wherein each of R1 and R2 is hydrogen, or -C(R3)2C(R3)2--OH and R3 is hydrogen or C1-3 alkyl radical, (b) tetra-hydrothiophene-1,1-dioxide, (c) potassium carbonate or (d) diglycolamines, each alone or in combination with one or more of each other as aqueous solutions or glycol solutions, said inhibiting composition consisting essentially of:
(e) a source of copper ion selected from the group consisting of copper metal, copper sulfide and copper salts, and (f) a source of sulfur atoms selected from the group consisting of 1) sulfur, or 2) hydrogen sulfide and/or COS in combination with an oxidizing agent which will produce in solution the sulfur atom, at least some of which is the polysulfide, under the conditions of operation in an acid-gas stripping plant which employs thermal regeneration techniques for separating the combined acid-gas from the stripping absorbent, and a solvent for (e) and (f) selected from (a), (b), (c) or (d).

2. A ferrous metal corrosion inhibiting composition consisting of:
(a) copper as the copper metal, copper sulfide or copper salt; and (b) an oxidizing agent which, in the presence of sulfur or a sulfur containing compound, will convert the sulfur to the sulfur atom and/or oxidize copper to the copper ion, and an alkanolamine of the formula:
wherein each of R1 and R2 is hydrogen, or -C(R3)2C(R3)2--OH and R3 is hydrogen or a C1-3 alkyl radical.

3. The ferrous metal inhibiting composition of Claim 1 consisting of copper metal and a compound yielding sulfur atoms by reaction with an oxidizing agent in solution of an acid-gas stripping absorbent under the conditions of acid-gas stripping and regenera-tion of stripping absorbent.

4. The ferrous metal corrosion inhibiting composition of Claim 1 consisting of copper sulfide.

5. The ferrous metal corrosion inhibiting composition of Claim 1 consisting of copper sulfide and an oxidizing agent.

6. The inhibitor composition of Claim 5 wherein the oxidizing agent is sulfur.

7. A corrosion inhibited acid-gas absorbing composition consisting of a solvent selected from the group consisting of alkanolamines having the formula:

wherein each of R1 and R2 is hydrogen or -C(R3)2C(R3)2--OH and R3 is hydrogen or C1-3 alkyl radical, tetrahydro-thiophene-1,1-dioxide, potassium carbonate, or diglyco-lamines, each alone or in combination with one or more of each other as aqueous solution containing from 1 to 5000 ppm copper and 1 to 5000 ppm sulfur, both based on the weight of the solvent, at least some of which is present as the polysulfide.

8. A corrosion inhibitor composition comprising the composition of Claim 6 wherein said medium is an alkanolamine.

9. In the method for removing acid-gases from natural and synthetic gases containing the same by contacting the acid-gas containing gases with gas absorbing solution prepared from solvents selected from the group consisting of alkanolamines having the formula:

wherein each of R1 and R2 is hydrogen, or -C(R3)2C(R3)2--OH and R3 is hydrogen or C1-3 alkyl radical, tetrahydro-thiophene-1,1-dioxide, potassium carbonate or diglyco-lamines, each alone or in combination with one or more of each other as aqueous solutions or glycol solutions, and freeing the medium of the acid-gases by heating, the improvement comprising maintaining in solution in said medium the composition of Claim 1 thereby to reduce the corrosive attack of the acid-gases on the metallic components of the equipment in which the acid-gas removal and regeneration of medium is carried out.