CA1187064A - Vanadium-cobalt corrosion inhibitor system for sour gas conditioning solutions - Google Patents

Abstract

VANADIUM-AMINE CORROSION INHIBITOR
SYSTEM FOR SOUR GAS CONDITIONING SOLUTIONS

ABSTRACT

The corrosion of iron and steel surfaces by an aqueous alkanolamine conditioning solution used to remove CO2 from a gas stream is effectively inhibited by a combination of (a) a vanadium containing ion and (b) a cobalt salt or a diamine or triamine.
A surprising fact is that some of the amine co-inhibitors have been identified by others in the art as promoting ferrous metal corrosion.

Description

D.75,795-FB

VANADIUM-AMINE CORROSION INHIBI~OR
SYSTEM FOR SOUR GAS COWDITIONING S01,UTIONS

The present invention relates to i~hibitor comp-ositions useful for preventing corrosion of ferrous metal surfaces by alkanolamine solvents used in treating sour gas streams and more particularly relates to such inhi~itox com-positions which contain (a) vanadium and (b) a cobalt salt or a di- or triamine.
It has been a long standing commer ial practice to use agueous alkano~amine solutions (e.g. a monoethanolamine solution) to absorb acidic gases such as CO2, H2S, COS and ~CN to condition naturally occurring and synthetic gases.
These treated gases may include feed synthesis gases, nat-ural gas and flue gas. Frequently, ~he conditioning process is pxacticed by passing a 5 pex cent to 30 per cent alkanol-amine solution ~ountercurrent to a gas stream in an absorp-tion column to remove the acid gases. The absorbed acid gases may b~ later forced out of the conditioning solution at higher temperatures and the alkanolamine solution re-cycled for more absorbing.
~ gueous alkanolamine so~utions are not themselves very corrosi~e toward ferrous metal eguipment, ~owever, they become highly corrosive when acid gases are dissolved ~here- .
in, particularly ~hen the solution ls hot. It has been found that both general a~d local corrosive attack.can oc- ¦
cur. This is a particular problem in reboilers and heat ex- ¦
changers where the steel is exposed to a hot, protonated alk-anolamine sol~tion. A heat transferring metal surface ap- I
pears to be especially vulnerable. Previous investigation by others have revealed that under conditions corrosive ;'" : '' ' products such as aminoacetic, glycolic, oxalic and formic acids uere ormPd. The monoethanolamine salts of these acids present the possibility of increased attac~ upon fer-rous metals.
One of ~he most economical and efficient methods of traating this corrosion problem is by including small guantities of corrosion inhibitors. Various me~al compou~ds have been used by others alone or in co~bination with co-inhibitors, for e~ample, such as compounds of arsenic, an timony and vanadium. These metal compounds seem to be much more effective against CO 2 -promoted corrosion than they are when H2S has been ~bsorbed in the conditioning svlution.
A number of U. S. patents have been granted re-lating to the use o~ corrosion inhibitor additives. For ex-1~ ample, the use of antimony was described in U. S. Patent

2,715,605. A number of amine compounds were found to be use-~ul in pre~nting corrosion by addition to petroliferous oil well fluids containing carbon dioxide or hydrogen sulfide brines, as disclosed i~ U. S. Patents 3,038,856; 3,2~9,999 20~ and 3,280,097. U. S. Pa~ent 3,808/140 relates to a combi natlon i~hibitor system using v~nadium and antimony. Nitro-substituted aromatic acids and acid salts, s~annous salts, organo-tin compounds, benzotriazoleJ vanadium and antimony were used in various combinations as i~hibitor systems for conditi~ning solutions as described in U. S. Patents

3,8~6,0~* and 3,959,170. The use of vanadium compounds as corrosion inhibitors for agueous amine gas sweetening re-agents is well Xno~n; for e~ample see ~. Ratchen and C. Kozarev, Proceedinqs of the International Congress on ~ C~rro~on 5th, 1972 and ~. Williams and `

.8t7 H. P. Lackie, Material Protection, July 1968, p. 21.
Pyridinium salts were found to be useful corrosion inhibitors when used together with lower alkylpolyamin~s, thioamides, thiocyanates, sulfides and cobalt as noted in U.S. Patents 4,100,099; 4,100,100 and 4,102,804; as well as U.S. 4,096,085 and 4,143,119. Still another U. S. patent, 2,826,516, uses soluble silicates as effective corrosion in-hibitors~ ~owever, many of these corrosion inhibitor sys~
tems ha~e not found industry acceptance because of factors such as cost and toxicity~
Qther cases related to monoethanolamine gas scrub-bing operations are U.S. Pate~t 4,184,855 which uses inter-coolers and flash heat exchangers to increase the energy efficiency of the method and U.S. Patent 4,1~3~903 which describes using cyclic ureas as anti foaming agents in the alkalin~ absorption solution.
It is an object of ~his in~ention to provide an agueous alkanolamine conditioni~g ~olution inhibitor system using components which are nontoxic relative to some of the prior art ~ystems and which permit relatively higher amine concentrations and thus higher carbon dioxide loading making for a more efficient process. What i particularly surpris-ing about the sy~tem of ~his invention is that some of the amine co-inhibitors used herein have been described by the prior art as corrosion promo~ers. For example, U. S. Pat-ents 3,535,260 and 3,535,263 find that N-(2-hydroxyethyl)- 1 e~hylenediamine (~EED), also known as N aminoethylethanol-amine or AEEA, i5 a degradation product of monoethanolamine.
Previously mentionqd U0 S. Patents 3,808,140; 3,896,044 and .3,959,170 state that AEEA was found to increase corrosivity .

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towards steel, particularly under heat transfer conditions. However, this comp-ound was found to be an effective corrosion co-inhibitor in accordance with the method of this invention.
In one aspect the inven-tion relates to a corroslon inhibited acid gas-absorbing composition which comprises an aqueous alkanolamine solution which add-i.tionally contains (a) an anion containing vanadium in the -~4 or +5 valence state, and (b) a co-inhibitor comprising a cation containing cobalt in the +2 val-ence state or an amine selected from N-aminoe-thylethanolamine, ethylenediamine, propylenediamine, piperazine, N-aminoethylpiperazine, methyliminobispropylamine and alkyl and N-hydroxyalkyl substituted derivatives thereo~.
In another aspect the invention relates to a method for inhibiting the corrosion of ferrous metal surfaces by an aqueous alkanolamine solution employed in acid yas remova]. service which comprises adding to said aqueous alkanolamine solution ; (a) an anion containing vanadium in the +4 or +5 valence state, and ~ b) a co-inhibitor comprising a cation containing cobalt in the +2 val-ence state or an amine selected from N-aminoethylethanolamine, ethylenediamine, propylenediamine, piperazine, N-aminoethylpiperazine, me-thyliminobispropylamine and alkyl and N-hydroxyalkyl substituted derivatives thereof.
The use of aqueous solutions of alkanolamines and particularly monoeth-anolamine for sour gas conditioning solutions is well known in the art. The sur-prising aspect of the instant invention is that vanadium-containing anions and di- and triamines together form a corrosion inhibitor sys-tem much better than the vanadium or the amines alone.
Vanadium-containing compounds are thought to act as oxidant-type inhi-bition catalysts which undergo a redox reac-tion at the ferrous metal surface. It .~ ~

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is thought that the iron needs to be somewhat corroded Æor the vanadium to be effective. The limited corrosion would place the iron in the proper valence state for protective film formation.
The choice of vanadiu~ co~pounds is not critical since it is the vanad-ium-containing anion, particularly vanadium in the plus 4 or 5 valence state, which provides - 4a -.

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this unusual corrosion inhihiting property in combination with the amines. Thus, for example, one can employ vanadatPs including orthovanadates, represented by the generic formula M3V04~ pyrovanadates, represented by the general formula M4 V2 07 and metavanadates, represented by the general formula MV03 and the like where M represents a cation. The condensed v~nadate ions which form in aqueous solutions, such as V60174 are also useful in this invention.
For convenience in introducing vanadate ions into the inhibiting systems of this inven~ion the alkali metal ammonium and alkaline earth vanadates including orthovanadates, pyrovanadates and metavanadates are preferred. Exemplary of such vanadates are the following: sodium metavanadate, potassium metavanadate, lithium metavanadate, ammonium metavanadate, sodium pyrovanadate, potassium pyrovanadate, lithium pyrovanadate, ~mmonium pyrovanadate, sodium orthovanadate, potassium ortho~anadate, lithium orthovanadate, calcil~m pyrovanadate, calcium metava~adate, magnesium orthovanadate, magnesium pyrovanaAate, magnesium metavanadate, ferrous orthovanadate, ferrous pyrovanadate, ferrous metavanadate, and the like.
Other forms of vanadium that can be used in this invention include: the vanadovanadates, double vanadates, i.e. 9 heteropoly acids containing v~nadium and the peroxy vanadates having the general formula:
MVO4.

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Generally, the amine co-inhibitor may be any di- or triamine which may serve as a chelating agent.
It is thought that this chelating effect may contribute to corrosion inhibition by somehow a~ecting the surface layer of iron molecule~. It is especially preferred that the amine used be selected from the group of ~mines consisting of N-aminoethylethanolamine, ethylenediamine, propylenediamine, piperazine, N-aminoethylpiperazine, methyliminobispropylamine as well as lower alkyl and N-hydroxyalkyl substituted derivatives thereof. For the purpose of this invention, "lower alkyl" is defined to be an al~yl moiety having one to four carbon atoms. It is anticipated that one skilled in the art could use more than one of the amines to optimize this inhibitor method.
~ssentially any cobaltous compound which is sufficiently soluble in the aqueous alkanolamine solution to provide the desired concentration of divalent cobaltous ions can be used. Inorganic salts 20 such as CoC12 , CoBR2 , CoC03 , CoS04 or Co(NO~ )2 ;
and organic salt~ such as cobaltous acetate and cobaltous benzoate are all suitable sources o~ cobaltous ions.
Salts such as the sulfate, nitrate; carbonate, or ch]oride are particularly preferrcd.
As will be seen in the Examples, the corrosion inhibitor system is effectlve even i~ very small amOUllt8 of additi~es are used. For example, the vanadium-containing anion and cobalt compound is seen to be effective in conoentra~ions as low as 100 parts per million while the amines may be effective in concentrations lower than 0. 5 weight per cent.
Of course, now that this particular corrosion system has been discovered, it is merely a matter for one skilled in the art to optimize the system for a particular application. Upper limits on the inhibitors might be 600 ppm for vanadium and cobalt, and 1. 0 weight per cent for the amines, but these limits would vary depending on co-inhibitor concentrations and the application. The precise concentrations must be ~et as a balance , ~ ~ ~ 7 ~ ~ ~

between the needs of the conditioning solution and the eco-nomi~s of using relatively high inhi~itor concentrations.
The inhi~itor combination is particularly usef~l in aqueous lower alkanolamine solutions kno~n as sour gas scrubbing solvents. Preferred lower alkanolamines can be defined as those having the formula:
R~ R R

~.
. R" R R
wherein R' and R" independently represent hydrogen or ---CR2C~2---OH a~d wherein each R may be hydrogen or an alkyl radical of 1-2 carbon atomsO Representative alkanol-amines are ~thanol~mine, die~hanolamine, triethanolamine, isopropanolamine, diisopropanolamine, and N-methyldiethanol-ami~. Related alkanolamines which are useful acidic gas absor~nts are ~ethicol (3-dimethylamino-1,2-propanediol3 .
and DIGLYCOLAMINE~ ~2 (2-aminoethoxy)ethanol~] age~nt, the latter being a p~oduct of Texaco Chemical Co. Other gas treating absorbents in which this inhibitor combination is effective include sulfolane ( te~rahydrothiophene~ dioxide ) and aqueous potassiu~ carbonate. These absorben~s can be employed alone or in combination of two or more, usually in aqueous solution although the water may be replaced in part or wholly by a glycol.
The followi:pg examples will illustrate the method of this invention as well as diss~lose t~e method of corros- !
ion testing employed.

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In this example the equipment involved were a set of copper strip corrosion test bombs that met ASTM D130 specifications. The covers were modified with valves and dip tubes to allow sampling of the liquid phase when the vessel was pressurized due to autogenous pressures. A Teflor~3 coupon mount was attached to the dip tube and a polypropyle-ne liner was fitted to the vessel in a manner so that the test solution was not in direct contact with the body of the vessel. In a typical experiment, 90 ml of a 50 weight per cent aqueous mono-ethanolamine was premixed with carbon dioxide, ammonium metavanadate and N-amino-ethyl-ethanolamine. The solution was placed in the liner of the vessel. A piece of 37.6 x 10.4 x 3.1 mm. 1020 mild steel (hereinafter referred to as "coupon") with a 6.35 mm diameter hole for mounting was freshly polished with fine Emery cloth (#JB5R, RED-I-CUT~Carborundum), followed by rinsing with water and acetone.
The dried clea~ coupon was then weighed and attached to the Teflon mounting in a manner such that when the vessel was closed the coupon would be totally immersed in the test solution. The vessel was sealed and placed in an 115-~ 1C shaker bath for a period of 96 hours. Then the coupon was recovered and cleaned by scrubbing with a bristle brush. ~len needed, a mild abrasive, PUMACE~3FFF
(supplied by Central Texas Chemical Co.), was employed for post-test cleaning.
After the coupon was clean and dried, weight loss was determined. A series of such experiments provided the results listed in Table l.

TABI.E I

Monoethanolaminea, CO2b AE;EAC Vd, Corrosion Rat~
wt . % m/m wt % ppm mm 0 per ~ear 1.
50 . 0 o . 30 0 0 1 . 12 50.0 0.30 l.0 0 0.55 ' 5~ 0 . 30 0 lO0 1 0 5 50-0 0.30 0 200 ~0.~)2 5~.0 0.30 l.~ lO0 c0.~25 50.0 0.30 l.0 lO0 ~0.025 50.0 0.39 0 0 0.
5~ 0 0 0 . 39 0 ~ 87 ~ 0 . 75 50 . ~ 0 . 39 0 . 87 0 0 . 55 50 . 0 ~ . 39 0 ~00 ~ . 75 50 O 0 . 39 0 300 0. 27S
50 . 0 0~39 0 .137 100 < 0.025 50.0 0.39 0.~7 200 < ~.025 50.0 0~39 0.~37 300 ~ 0.025 aMonoet:hanolamine, low iron grade, ~lO ppm Fei made by Texaco Chemical Co.
bMole C02 per mole o ~.
1$ CN-aminoethyle'ch~nolamine; available from Altlrich Chemical Co., dInt:roduced as ammoni~Q metavanadate, used in all examples~
eThe corrosion r~te is a measurement of linear penetration ir~ ~ousandths of an inch per year as computed by the , followin~ formula:

Rate (mils/year~
87.6X weiqh~ loss of coupon, mqs (coupon de~sity, g/cc)(c:oupon sur~ace, s~.cm. . )(hrs) J

The surprising fact about ~;~ample ~ is that the amine co-inhibitor, AE~;A, has been found by other investi-gators to increase the corrosion rate of ferrous metal sur-2S faces under heat transfer ~:ondition~O For instance, see the first columrl of U. S . Pat:ents 3, 808 ,140, 3, 896, 044 and 3,959,170 and ~:as PurificaLion by Fred C. Riesenfel~l and A~thur L. Kohl, Houston: Gulf Publishing Co., 1974, p. 85.
.

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~' I
Combinations o.ethylenediamine (EDA) and vanadium inhibitors were tested in the manner described in Example I.
Results are listed in Table I I .

TABI~
Monoethano~ amine, Cû2, EDA , V, CorrQsion Rate wt . % m/m wt . % ~ mm . E~er Year 50, 0 0 . 39 0 0 0 . 6 ' 50 . 0 ~ . 39 0 ~oo 0. 75 50 0 0 . 39 0 200 0 . 525 ~0 . 0 0 .39 0 300 0. 325 50.~ 0.39 0.87 0 0.55 S0.0 0.39 0.~7 100 0.375 .
50-0 0.39 0.~7 20~ C 0.û25 .
50 0 39 ~7 300 C 0 . 025 , ~3'7~

E X~ 1. E II I
Combinations of propylenediamine (P3A~ and vanadium inhibitors were tested in the manner described in Example 1, Results are listed in Table I I I .

TABLE I I I
Monoethanolamine, C02, PDA , V,Corrosion Rate w~ ~ m/m wt O ~ ~m :~ e 50.0 0.3g 0 Q 0.6 ` 0 . ~9 ~ 200 0 .
39 0 200 0. 525 0 0 . 3g Q . 87 0 0 . 85 0 . 39 ~ . ~7 100 0 . ~
0.39 0 ~7 200 0.3 S0 0 0 . 39 0 87 300 0 . 22~ .
S0.0 0.39 0.43 lO00.525 50.0 0.39 0.43 2000.45 50.0 0.39 0.43 30~<0.025 E X A M P L E IV
Coll~binations of N-hydroxyethylpiperazine (~:P ) and vanadium inhibitors were tested in the same ma~ner as des~
cribed in ~:xa~ple I. Results are lis~ed in Table IV.

l'~BLE IV
Monoethanolamine, CO2 EEP, V,Corrosion Rate wt . % _ ~ wt . ~ ~ mm . ~er Year 50~0 0.39 o ~ 0.6 50 . 0 0 . 39 0 200 0 ~ 75 50 . 0 0 . 39 0 200 o . 525 50 . 0 0 . 39 0 . 87 0 0 . 775 2550 0 û . 39 0 . 87 100 0 . 6 50 0 0.39 0.87 200 0.65 50.0 0.39 0.87 300 0.3 0 . ~.39 0.43 20~ 0.175 5~ 0 0 . 39 0 .~3 300 o. 075 3~ ` .
.~

E X A M P L E V
ColT~ination of N-aminoethylpiperazine tAEP ), and vanadium in~ibitor were tested in 'che same manner as des- i cribed in :;xample I. ~esults are listed in Table v.

TABLE V
Monoethanol~nine, CO2, AEP, V, Corrosion Rate ~:. % _ m~'m ~ E~ mm . ~ r Year 50.0 0.39 0 0 0.6 50 0 0 . ~g 0 200 0. 75 ~0 ~ 0.39 0 200 0. 525 50 0 0 .39 0 .87 0 0. 6 0 50 0 0 . 39 ~ . 87 100 0 . 425 50 0 . 0.39 0.87 200 0O375 S0 0 0 . 39 0 . ~7 300 0 . 42~
50.0 0.3g 0.43 100 0.475 50.0 0.39 0.43 ~00 ~0.025 50.0 0~39 0.43 30~ <0.025 15E X A M P L E ~7I
.
Combinations o methylaminobispropylamine (MIBPA~ .
and vanadium compounds were tested in ~h~ same manner a~ des cribed in Exampl~ I. The results are listed in Tabl~ VI.

20~31E: V:~
Monoethanolamine, CO2t ~qIBPA V Corrosiorl Rate _ m~m wt~ P~2m mm per Year 50.0 ~.39 ~) o 0.6 50 0 0 . 39 0 ~00 û ~ 75 50 0 0 .39 ~ 2~0 0. 525 ~0 0 0.39 0.87 0 1.125 50 0 0 . 3~ 0 . 87 1~0 0 . 6~
50 . 0 0 . 39 0 . 87 2Q0 o . ~75 S0 0 0 . 39 . 0 . 87 300 o . 625 50 0 0 .39 0 .43 100 0. 25 S0 0 0 .39 0 .43 200 0. 525 ....
50 0 0 39 0.43 300 o.z7s ..87~

EXA~LE VI I

The procedure of Examples I to VI was repeated, using nickel, copper, cobalt and zinc as co-inhibitor 5 in place of the amine. The results obtained are set out in Table YII below.

TABLE VII
.

Corrosion M~Aa CO b~ ~fanadiumd~ Ratee % mole~mole Inhibitor AC, ~ ppm nunpy 155Q. OO . 3g A = Mi 101) 0 4 . 25 50 . 00 . 39 A = Ni 100 100 0 . 65 50 . OO . 39 A = Cu 100 0 0 . 975 50 . OO . 39 A = Cu 100 100 1.175 50 . OO ~ 39 A = Co 100 C) O .175 2050 . OO . 39 A = Co 100 100 0 . 275 50 . O O .. 39 . A = Zn 100 0 1.1;25 50. 00 . 39 A - Zn 100 100 O. ~75 50.0 0.:~9 ~ 0.6 25 a Monoethanolamine, low iron grade, ~10 ppm Fe; made by Texaco Chemical Co.
b Mole C02 per mole of ~:A.
c Nic:~cel was introduced as nicl~el nitrate, copper was introduced as cupric nitrate, cobalt was introduced as cobalt nitrate, . and zinc was introduced as zino nitrateO

d Introduced as ammonium metavanadate, used in all examples.

~' E X A P L E Vlll The effect of soluble iron on an ammonium meta- ~
vanadate inhibited system was tested in a 30% aqueous mono- i ethanolamine loaded with 0.30 moles of carbon dioxide per mole of a~ine reagent according to the same procedure given in Example Io Results given in Table Vlll indicated that in-creasing soluble.iro~ in the test solution reduced the effec-tive soluble vanadium in the test solution.

TABLE Vlll EFFl;CT OF SOLUBLE IRON ON TEIE
VANP~II~M INHIBITED SYSTEM

Post-tes~
_~ditive~ Ana~ysis_, j MEA, C02~ Fe~, V~, Fe, V, corrosion ~ate _~_ mole~mole ` 30.0 0.30 1~0 100 ~2 ~7 ~0.025 30.0 0.30 200 100 d 19 0.3 30.0 0.30 300 - 100 d 11 0.175 30.0 0.30 400 100 d 9 . Q.7 30.0 0.30 500 100 d 8 0.2 30.0 0.30 50 20~ 3 234 C0.025 30.0 0.30 100 200 8 197 C0.025 30.0 0~30 ~50 20~ 3 15~ ~0.025 30.0 0.30 200 200 5 ~42 ~0.025 30.0 0.30 250 200 3 110 <0.025 aIron was introduced as freshly preparad aqueous solution of ferrous ammonium sulfateO
bVanadium was introduced as ammonium metavanadate.
CBy atomic absorption analysis.
~ ot analyæed.

E X A M P L E IX _ ~ he effectiveness of the cobalt-vanadi~m inhibitor system was further tested in a 50% aqueous monoethanolamine loaded with 0.39 moles of carbon dioxide per mole of amine reagent. To further increase the corrosiveness of the test, the bath t~mperature was increased tQ 120C. Results of these tests indicated the combi~ation of cobalt and va~adium provided protection to mild steel coupon while eith~r co-balt or vanadium alone was not effective.

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The effectiveness of the corrosion inhibitor sys-tem of this invention may be readily seen from ~he examiples where the inhibiting effect of both co-inhibitors is greater than eithex inhibltor singly. In Example I, every instance where vanadium and AEEA were both used ~ave a corrosion rate of less than 0.025 multimetr~s ~mm) per year.
The corrosion rates given are generally good over a ten unit range or plus or ~inus 0.1 mm. /year. In E~amples IV, V
and VI, it may bei seen that the sy~tems with less than 0.5 1 wt.% amine wor~s better or just as well as th~ systems with twice as much that amine co~centration. This surprising re-sult suggests ~hat there may be a threshold concentration for some of these amines beyond which the addition of amine gives diminishing return~. ~t is also noted that in all ex-amples, the monoethanolamine concen~ratio~ was 50 weight per cent which is much higher than the ~ to 30 per cent used in ~he prior art methods. As a result, the sour gas condition-ing solution can be more concentrated and ~ore effective in removing CO2 than ~urrent solutions and provide corrosion protection in addition.
.

.

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A corrosion inhibited acid gas-absorbing composition which comprises an aqueous alkanolamine solution which additionally contains (a) an anion containing vanadium in the +4 or +5 valence state, and (b) a co-inhibitor comprising a cation containing cobalt in the +2 val-ence state or an amine selected from N-aminoethylethanolamine, ethylenediamine, propylenediamine, piperazine, N-aminoethylpiperazine, methyliminobispropylamine and alkyl and N-hydroxyalkyl substituted derivatives thereof.

2. A composition as claimed in claim 1 wherein the amine co-inhibitor has a concentration in the composition of at least 0.4 weight per cent.

3. A composition as claimed in claim 1 wherein the cation containing cob-alt in the +2 valence state has a concentration in the composition of at least 100 parts per million.

4. A composition as claimed in claim 1 or 3 wherein the cation containing cobalt is derived from CoC12, CoBr2, CoCO3, CoSO4, Co(NO3)2, cobaltous acetate or cobaltous benzoate.

5. A composition as claimed in claim 1, 2 or 3 wherein the anion contain-ing vanadium in the +4 or +5 valence state has a concentration in the composition of at least 100 parts per million.

6. A composition as claimed in claim 1, 2 or 3 wherein the vanadium-contai-ning anion is derived from an orthovanadate, metavanadate, pyrovanadate, vanadium oxide or vanadium halide.

7. A composition as claimed in claim 1, 2 or 3 wherein the alkanolamine is monoethanolamine.

8. A method for inhibiting the corrosion of ferrous metal surfaces by an aqueous alkanolamine solution employed in acid gas removal service which compri-ses adding to said aqueous alkanolamine solution (a) an anion containing vanadium in the +4 or +5 valence state, and (b) a co-inhibitor comprising a cation containing cobalt in the +2 val-ence state or an amine selected from N-aminoethylethanolamine, ethylenediamine, propylenediamine, piperazine, N-aminoethylpiperazine, methyliminobispropylamine and alkyl and N-hydroxyalkyl substituted derivatives thereof.

9. A method as claimed in claim 8 wherein the concentration of amine co-inhibitor in the composition is at least 0.4 weight per cent.

10. A method as claimed in claim 8 wherein the concentration of cobalt cat-ion in the composition is at least 100 parts per million.

11. A method as claimed in claim 8 or 10 wherein the cobalt cation is deri-ved from CoC12, CoBr2, CoCO3, CoSO4, Co(NO3)2, cobaltous acetate or cobaltous benzoate.

12. A method as claimed in claim 8, 9 or 10 wherein the concentration of vanadium-containing anion in the resulting composition is at least 100 parts per million.

13. A method as claimed in claim 8, 9 or 10, wherein the vanadium-contain-ing anion is derived from an orthovanadate, metavanadate, pyrovanadate, vanadium oxide or vanadium halide.

14. A method as claimed in claim 8, 9 or 10, wherein the alkanolamine is monoethanolamine.