CA1085614A - Reducing hot corrosion due to alkali metal and vanadium contamination - Google Patents

Abstract

REDUCING HOT CORROSION DUE TO ALKALI
METAL AND VANADIUM CONTAMINATION

ABSTRACT OF THE DISCLOSURE
Hot corrosion of metal alloys in a high temper-ature gas stream, due to the presence of alkali metal contaminants (sodium and potassium) and vanadium is exhibited or reduced by the additional combined presence of chromium and magnesium in the gas stream. Petroleum fuel oils containing more than 0.5 ppm alkali metal and more than 0.5 ppm vanadium produce such corrosive gas streams when combusted. Magnesium compounds and chromium compounds are dissolved in such fuels to inhibit hot corrosion.
Ratios of Cr: Na + K of about 4.5:1 and Mg:V of about 3:1 are preferred. Corrosion due to the presence of lead is also reduced by the presence of magnesium.

Description

BACKGROUND OF THE INVENTION
High temperature resistant nickel base and cobalt base superalloys are employed as materials for casting or forging gas turbine blades and vanes. The gas stream in turbines can contain in the order of 15 per cent oxygen and have inlet temperatures, for example, in the order of 1700-2300F, creating surface temperatures in the first : stage blades and vanes averaging 1650F and peaking at about 1750F. This relatively high speed stream of oxidizing combustion produced gas is a rigorous environment and requires parts to be fabricated from these sophistlcated high temperature superalloys.
Varlous grades of petroleum fuels are avallable for use in gas turbines~ varying from crude oils to high distillates and varying in impurity levels of certain trace .
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46,411 ' iL4 elements that become part of the gas stream and add additional corrosion problems to the materials exposed to the stream. Alkali metals (sodium and potassium) and vanadium fuel contaminants are known to create such corrosive problems. The higher grade fuel oils, e.g.
the higher distillates, will ordinarily contain reduced amounts of these contaminating elements. Water washing can be employed to reduce the alkali metal concentration, primarily prior to distillation but also thereafter, since these are typically present as soluble chloride salts.
Where hot section corrosion is to be held to a minimum, gas turbine fuel specifications typically limit both the ..
alkali metal elements and vanadium to a maximum 0.5 ppm by weight. Reducing the hot corrosion in this manner can be expensive because such fuels may require more treatment than the more contaminated lower distillates, residuals and crudes but this expense is balanced against the less , .
frequent replacement of blades, vanes and other parts, particularly those in the first stage of a turbine.
Reducing the temperat~re of the gas stream may also reduce corroslon rates but again must be balanced ; economlcally against lower gas turbine efflciencies.
It should also be noted that alkali metal contaminants are also known to find their way into the gas stream from sources other than the fuel. Alkali metal salts are, of course, present in salt water operating environments. These salts may be present in the combustion air or combustion cooling water, particu~arly in marine equipment, equipment installed on coastal sites or other salty environments.
Studies of the individual high temperature corrosion ~ -2-: '~

46,411 `
1~856 mechanisms ~or (1) alkali metals and (2) vanadium have resulted in another use~ul way o~ reducing corrosion due to these di~ferent individual contaminants. It is known that alkali metals, whether introduced into the combustion process in the fuel or inlet air, combine with the sulfur normally present in the fuel and form alkali metal sulfates (e.g. Na2S04). When these sul~ates contact the hot alloy parts, the alloy deteriorates by a mechanism known as sul-~idation. The addition o~ chromium and certain other metal element compounds to provide metal oxides (e.g. Cr203) in -~
the gas stream are d~sclosed in the prior art as ef~ective in reducing or inhibiting hot corrosion of coated and uncoated superalloy parts due to the aforesaid sulfidation.
The chromium oxide is not known to be effective in inhibiting the degradation or corrosion due to vanadium ~uel contamination and intolerable degradation would occur i~
only chromium oxide is added to gas streams resulting from fuels containing, for example, several ppm of vanadium.
Degradation o~ the superalloys occurs through a dlf~erent mechanism, best described as gross oxldation, because o~`
this vanadium contamlnatlon. Vanadium ln the ~uel is oxidized to ~orm molten vanadium pentoxide (m.p. 1274F).
When the pentoxide ~luxes exposed hot metal surfaces such as turbine blade and vanes or even boiler tubes, rapid ~ ~
oxidation o~ the alloys takes place. The uninhibited ~-rapid oxidation will rapidly degrade and require early, costly replacement o~ metal parts so exposed. However, ;
it is known that this oxidation degradation due to vanadium contamination can be remedied by lntroducing a magnesium compound into the gas stream. It is believed that magnesium r 46,411 S~

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oxide formed in the hot gas stream reacts with the vanadium pentoxide to form magnesium orthovanadate (3 MgO.V2O5), a compound inocuous to the superalloys at normal operating temperatures of gas turbines and boilers. When alkali metal , contaminants, such as sodium and potassium, are also present, the effectiveness of the magnesium additive is significantly diminished.
It has been a common practice, therefore, to remove the alkali metal contaminants from fuels also con- ;-taining a relatively high level of vanadium contamination by water washing to a level below which sodium does not extensively add to the degradation process. Typically, a maximum of 0.5 ppm of sodium plus potassium is specified for fuels entering gas turbine combustors. The level of ~
the alkali metal contaminants in the fuel or, more import- `
antly, in the gas stream~ is even more critical if the vanadium exceeds the typically specified maximum limit of 0.5 ppm in the fuel. The corrosive degradation of hot section alloy parts when both alkali metal and vanadium are present in concentrations of more than 0.5 ppm for each is not merely equal in rate to the rates encountered by ;~
each alone at the given concentration levels but may be - ;
several orders greater than the individual rates added ; together. This undesirable corrosive synergism is believed to be caused by the formation of extremely corrosive low melting sodium vanadates, although the study of these combined reaction mechanisms at high temperatures is diffi-cult. The aforesaid increased degradation rates, however, have been experienced. Lower melting corrosive compounds, it should be noted3 also cause corrosion in a greater number ; .

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-~ S614 of gas turbine stages.
PRIOR ART ~ -U.S 3,581,491 discloses ~he attenuation of sulfidation attack of nickel-base and cobalt-base alloys~:~
in gas turbine engines by treating the gas stream con-~` taining alkali metal salts with oxides of chromium, tin, ; :: -samarium and columbium. -An article by R. C. Farmer entitled IlRx for : , .
Sodium Sulfidation," appearing at pages 32-35 of Gas 10Turbine Internat~onal, (Vol. 15, No. 5), September-October 1974 discloses Cr2O3 as an inhibitor of sulfidation degradation due to the sodium contamination of hot gas - streams in gas turbines and the efficacy of adding to the fuel a chromlum containing organic compound in a fuel-soluble solvent. `
An article by S. Y. Lee, W. E. Young and G. Vermes entitled, "Evaluation of Additives for Prevention of High Temperature Corrosion of Super Alloys in Gas Turbines,"
appearing at pages 333-339, Transactions of the ASME, 20Journal of Engineering For Power, Vol. 95, Series A, No. 4, October 1973, dis¢loses certain additive elements, ;~
including magnesium, for reducing corrosion due to vanadium.
An article by S. Y. Lee, W. E. Young and C. E.
.
Hussey entitled, "Environmental Effects on the High-Temperature Corrosion of Super Alloys in Present and Future Gas Turbines," appearing at pages 149-153, Transactions ; of the ASME, Journal of Engineering for Power, Vol. 94, Series A, No. 2, April, 1972, discloses the high temperature corrosion of certain superalloys due to contaminants in - 30 fuels.
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An article by F.J. Wall and S.T. Michael entitled "'Effect of SuLate Sal~s on Corrosion Resistance of G~s Turbine Alloys," appearing at ; . pages 223-245 o Hot Corrosion Problems Associated ~with Gas Turbines, ASTM Special Technical Publication No. 421 (lg67) discloses corrosion test'results on . . .
' alloys.'exposed ta sodium.sulfate and mag~esium sulfate which ~orm a low melting eutectic. .' ~, ' ., The present invention provides a'fuel additive composition for petroleum fuel oils con~
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,taminated with alkali metal and vanadium which com~
prises a solution of a petroleum fuel soluble solvent and compounds of.,chromium and magnesium. Acc~rdi'ng to' a specific embodiment ~he chromiu~ com~ound, is '- .
chromium octoate... The magnesium compound may be '. '.~
magnesium sulfonate. According to one embodiment o~ ' .;~`
the invention, the composition comprises a petroleum . '.
fuel oil containing more than 0.5 ppm alkali metal, more than 0.5 ppm vanadium and both chromium and magnesium in amounts e~fecti~e to inhibit or reduce m~tal degrad-. ;
ation by a gas stream produced by the combustion of the fuel oil.
The present invention also provides a method of reducing the corrosion of nickel base and cobalt base alloys traversed by a hot'gas stream containing both alkali metal and vanadium contaminants, which method '`

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comprises the step of exposing the stream to a com-bination of chromium and magnesium. According to one ~ ~-embodiment, the ratio of chromium:alkali metal in the fuel is about 4.5:1 and the ratio of magnesium:vanadium is about 3:1.
It is the primary object of this invention to provide a composition that can be added to petroleum fuels having relatively high concentrations of contam-inating alkali metal salts and vanadium compounds and thereby decrease the degradation caused by the presence `;
of these contaminants or products thereof in hot gas ~` streams impinging on superalloy combustion chamber or t ` gas turbine parts. Another object of the invention is to provide a reduced rate of degradation on turbine vanes and blades by gas streams produced by combusting petroleum i : .
uels containing a relatively high concentration of sodium and vanadium particularly high grade crudes containing more than G.5 ppm o sodium plus potassium and more than 0.5 ppm of vanadium.
~ hese and other ob`jects are preferably accomplished with a combination o at least two compounds, one compound containing chromium and one containing magnesium which are dissolved in petroleum fuel oils. When the fuel is com-busted, the added compounds will provide chromium and magnesium oxides in the hot gas stream. The combination ;~

of these oxides will inhibit or reduce the rapid degradation otherwise produced because o~ the combined presence of - 6a -.
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46,411 :3L tD8 5614 ,~ , .~ .
relatively high but typical levels of both sodium and vanadium in Saudi crude fuel (in the order of 3 ppm ; sodium and 5 ppm vanadium). Thus, Saudi crude or other economical ~ut contaminated fuels containing more than 0.5 ppm alkali metal and more than 0.5 ppm vanadium may be combusted and employed in gas streams to drive gas turbines with reduced corrosion or degradation of uncoated .
: or coated blades, vanes or other components made from nickel base or cobalt base superalloys. :~
BRIEF DESCR:I:PTION OF THE DRAWINGS : ::
Figure 1 is a graphic plot illustrating the degradation of samples of X-45, a cobalt base alloy, in several different ~as streams at 1650F; and Figure 2 is a graphic plot illustrating the degradation of samples of U-500, a nickel base alloy, in several different gas streams at 1650F.

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DESCRIPTION OF THE INVENTION
We have dlscovered that the unusually high rate of degradation of nickel base and cobalt base superalloys when sub~ected to gas streams resulting from the combustion of petroleum fuel oils containing more than 0.5 ppm of alkali metals (sodium plus potassium) and more than 0.5 ppm of vanadium can be reduced, preferably by the addition of a combination of a soluble chromium compound and a soluble magnesium compound to the ~uel oil prior to combustion.
While water washing is a relatively convenient and inexpenslve way of reducing the alkali metal concentrations to levels be-low 0.5 ppm (because the alkali metals are present as water soluble salts), there may be areas where the readily avail-able wash waters are themselves contaminated with alkali metal salts, thus precluding effective water washing.
Already washed ~uel may be later contaminated by residual alkali metal salts due to the purging of tank cars, tanks and pipelines with sea water. An ability to use fuels such as several light Arabian crude oils and particularly Saudi 20 crudes whlch have more than 0.5 ppm o~ sodium plus potassium ~;
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and more than 0.5 ppm o~ vanadlum ln the absence o~ potable wash waters would be desirable if the high temperature corrosion could be reduced.
In accordance with our invention both chromium and magnesium are added to or preferably dissolved in the -contaminated ~uel oil. Magnesium, in the form of oil soluble organic salts such as magnesium naphthenate or magnesium sulfonate is added to the ~uel oil to preferably provide a weight ratio o~ about 3:1 o~ Mg:V. (A11 concentra-`30 tions and proportio~s herein are on a weight basis unless .

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46,411 8 ~ 6 ,~
otherwise stated.) Chromlum, also preferably in the form of an oil soluble organic salt such as chromium octoate, is also added to the fuel oil, preferably in an amount to pro-vide a ratio of about ~.5:1 of Cr:Na+K. The ratios of Mg:V and Cr:Na*K may vary considerably ~rom the preferred ratios and yet provide the advantages of reduced corrosion or degradation. The use of fuels with lower than preferred ratios of Mg:V and Cr:Na+K may result in an undeslrable more frequent gas turbine blade and vane replacement but the presence of even these lower ratios of our combined additives ; -will provide a long~r life than would be obtained wlthout our additives. Ratios higher than the expressed preferred ratios should provide the full advantages of reduced corrosion, indeed may permit even higher than currently recommended inlet gas and blade temperatures, but suffer the dis-advantages of more copious deposlts within the turbine.
Consequently more frequent washing away of deposits on the gas turbine parts would be requiredO The disadvantages of these copious deposits would also be present if, for example, fuel oil~ containing 10 or even 100 ppm o~ vanadium was present in the fuel oil and even the preferred ratio of Mg:V was employed. The advantages of reduced corrosion would nonetheless be obtained.
The most convenient method of in~ecting our additives into the fuel oil is by means of a solution of the magnesium and chromium in a solvent which is in turn soluble ~ ;

in the fuel oil. This solvent may be a high petroleum fuel distillate orJ preferably, a fuel soluble solvent such as kerosene, mineral spirits or benzene. These latter solvents have significantly lower pour points and viscosities and _g_ ~ .

Ll6, 411 1a~8561~

'`':" ' provide solutions which can be more easily pumped, poured and otherwise handled. That ls why we pre~er as an additive composition a solution of chromium and magnesium compounds in a fuel soluble solvent~ The concentration of chromium and magnesium in the solution is not critical. For convenience in reducing the volume of solution to be handled, relatively high concentrations are advantageous.
Chromium octoate and magnesium sulfonate can be dissolved -~
- in relatively common and inexpensive oil soluble solvents to provide a solution containing about 8 percent chromium and 8 percent magnesium. Solubility of various organo-metallic compounds containing either chromium and/or magnesium can be easily determined by trial. Petroleum solubility of .
solvents ~or these compounds can be similarly determined.
It should also be understood that the relative proportions of chromium and magnesium in the oil solublesolution may be formulated or des~gned to provide the preferred ratios o~ 3 Mg:lV and 4.5 Cr:l Na+K where the proportlon of V:Na+K
in the fuel is known~ More specifically, for example, ~or a Saudi crude consistently containing 3 ppm sodium plus potassium and 5 ppm vanadium, the weight proportion o~' chromium to magnesium would be 13.5:15 in the oil soluble solution. That one solution could be added to the Saudi i; . . .
crude fuel oil, in a predetermined measured amount, with the knowledge that the two preferred ratios are accommodated so long as the concentration and proportion o~ sodium plus potasslum and vanadium did not significantly change. Where :` the contaminating constituents vary extensively either in ~`
conc~ntration or proportion it may be more convenient to employ two separate oil soluble solutions, one containing :' -1O- ;
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46, 4 ~L~856~4 :~

magnesium, the other chromium and then varying the pro-portions of additions in response to the changes.
The possibility of alkali metal contamination in the hot gas stream from sources other than the fuel oil or in addition thereto should also be recogni3ed. In coastal and marine installations~ alkali metal contaminants may be introduced in the air employed for combustion or in water that may be in~ected into the combustion chamber to control the flame temperature. Regardless of source, the alkali metal contamination in the gas stream, together with the vanadium contamination, will cause rapid degradation of the superalloy parts. This degradation or corrosion can, of course, be reduced or inhiblted by adding the combination of soluble magnesium and chromium compounds to the ~uel oil.
` ~he concentrations and proportions of the magnesium and chromium in the fuel shouId be selected to provide the hereinabove described preferred ratio of Cr:Na~K taking into account all of the alkali metal which may find its way into the gas stream regardless of source. This may be done, for example~ by measuring the sodium plus potassium in the com-bustion air and converting that into an equivalent concentration in the fuel. This back calculation is relatively simple f;
since fuel:air ratios are known.
Although we believe that the addition of the combination of chromium and magnesium is most conveniently effected with soluble compounds in the fuel, it should be understood that other modes of introduction and other com-pounds may be employed so long as the result is the inclusion or in~ection of the combination of the chromium and magnesium in the hot gas stream to negate the corrosiveness ', , ~856:1.4 r o~ the alkali metal and vanadium present in the stream~
It is, of course, the contaminants in the hot gas stream that cause the co.rrosion of the superalloy components. - The combined chromium and magnesium additions could with greater ;~`
difficulty be made by injecting the compounds directly into -~
thç combustion chamber or with the stream of air for the ~:
comhustion. The benefits of our invention would also be iprovided, for example, by coating the exposed metal par~s ;~
;with a material containing both chromium and magnesium. .~:
Inaeea, the benefits of red~ced corrosion may be obtained . ;~
wherever combustion product gases containing both alkali ;~
~metal and vanadium con~aminants (a~ individual l~vels . :
equivalent to about O.S ppm in the fuel) impinge on compo~ents fabricated ~rom nickel base or cobalt base supe~alloys.
Returning now to the gas turbine speciically.and the use of fuels containing more than 0.5 ppm of alkali metal and 0.5 ppm of vanadium ~or gas streams containing ~hese - ~-elements in equivalent levels~, we prefer to provide ~hromium in.the fuel i~ an amount so that the ratio of chromium to alkali metal is about 4.5:1 and magnesium in ~he fuel in amounts to provide a ratio of magnesium to ~anadium o~ about .3:1. Employing these fuels in gas turbines with blades o nickel base superalloys and vanes of cobalt base alloys, satisfactory first stage life can be obtained at average blade and vane temperatures of 1650F and spot peaks o 1750F.
In present day commercial gas turbine technology nominal inlet gas stream temperatures of about 2070F are consistent with the above first stage metal component temperatures and the .
gas stream temperatures may vary from nominal ~300F.
We have conducted corrosion tests in a laboratory .

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turbine simulator where samples of commonly used gas turbine nickel base and cobalt base superalloys are exposed to gas streams containing vanadium and sodium contaminants.
The fuel employed in these tests was a No. 2 petroleum fuel oil distillate containing, according to specification, less than 0.5 ppm of either sodium or vanadium. To simulate a crude of the nature heretofore described, the actual concentration of both of these elements in the test distillate -were determined analytically. Then, test sample levels of 10 sodium and vanadlum were provided by appropriate additions ~ ;
of sodium carboxylate and vanadium carboxylate to the test fuel oils. The test fuels also contained 0.5 percent sulfur. Chromium was added to the fuel test samples as chromium naphthenate. Magnesium was added as a fuel oil soluble solution known as KI-16, commercially available from the Tretolite Division of Petrolite Corp. Dosages of the ; additives provided fuel test samples havi.ng Cr:Na ratios of 4.5:1 and Mg:V ratios of 3:1. Oxidation results were obtained with natural gas which does not contain sodi.um or vanadium. The alloy test samples were 1/4" dlameter pins.
The pins were mounted in a high pressure corrosion test passage where they were exposed to test gas streams ;
; characteristic of those encountered in gas turbines. Additional details of the test facility and operating procedures are disclosed by S.Y. Lee, S.M. De Corso and W.E. Young in an article entitled "Laboratory Procedures for Evaluating High- ;
Temperature Corrosion Resistance of Gas Turbine Alloys"
appearing at pages 313-320, Transactions of the ASME, Journal of Engineering for Power, Vol. 93, Series A. No. 1, July, 1971 3~ and incorporated herein by reference. The article also ~ ' '' ~L~856~4 ,` . ':

sets forth the compositions of nickel base and cobalt basP
superalloys. Corrosion results in descaled metal weight loss and metal diameter recession rates were measured.
Results of tests at 1650F on X-45, a typical gas ; turbine vane alloy and ~-500, a typical rotating blade alloy are illustrated graphically in Figures 1 and 2, respectively.
The test resul~s confirm a significant reduction of corrosion rate due to vanadium and sodium ~uel contaminants when our -combination of magnesium and chromium was also included in the-fuel. Indeed r the combination of chromium and magnesium , additions give some degree of protection to superalloys rom the simple oxidation that occurs in tests employing con-taminant-free natural gas. The e~feçtiveness of the adaitives applies equally well to various nickel base and cobalt base .
;superalloys used in gas turbine engines. The additives are efficacious with coated blades and vanes as well.
. . ~ . ' ~ dditional test results confirming the e~ficacy of the combined magnesium and chromium ~dditions in the pre~erred Cr:Na and ~g:V ratios are summarized in Tables 1 and 2.

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TABLE I - CORROSION RATE IN WEIGHT LOSS

~a) Result at 1~50F with fuel containing 3 ppm sodium an 5 ppm vanadium ~reated with chromium and magnesium additives.

Corrosion r~te expressed as weiqht loss, Mq/cm~
A~LOYS 115 hour result 300 hour resul~
_ . , - .
-X-4~ 4.74 or 0.041/hr 3076 or 0.013/hr :: : , , . ............ . _ . . _ : :
U-500 4.72 or 0.041/hr 9.70 or Q.032jhr :
' . ~ , _. '~'' ' U-520 4.75 or 0.041/hr ~.13 or 0.007/hr. . ,, _ , ~ . _ .
~ ~ ECY-768 4.62 or 0~04/hr 5.68 or 0.019/hr '~ . , . ._ ._, , , . _ (b) Result at 1500F with fuel containing sodium and vanadium not treated wit~ any additive.
Sodium and aorrosion rate i~ 150 hours of Vanadium testing 2 Level ALLOYS Weight loss, Mg/cm : _ _ . . . r - . _ ~ . . .~ :
5 ppm Na X-45 36.0 or 0.240!hr ~ ;
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2-ppm V U-500 56.6 or 0.377/hr .', ~ ,. . . ,, ._ . , 2 ppm Na X-45 _ 12.2 or 0.081/hr 2 ppm V U-500 12.1 or 0.81/hr . . , ..
~c) Result at 1600F with uel containing no sodium or ; ~anadium. Additives not used.

Corrosion rate expressed as weight loss, M~cm2 ALLOYS
115 hour result 1 300 hour result 500 hour,result , _ . .
X-45 7.5 or 0.06S/hr 13.9 or 0.046/hr 16.6 or 0.033/hr i. . . .: . I
U-500 7.2 or 0.063/hr 1~:`6 or G.034/hr 15.5 or 0.031~hr .~ _,, . . ' _ 1. ' ' '.' ' ~ :
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56~4 . ` :' (a) Result at 1650F with fuel containing 3 ppm soaium and ~;
S ppm vanadiu~ treated with chromium and magnesium ; additives.
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~__ _ _Diametçr metal recession, mils j ALLOYS 115 hour result 300 hour resul~
~ . ,~ . , ;~ , ~ 45 0.7 or 0.0061~hr 1.2 or 0.004~hr ~
: . . -................. . '. '. ~'; ' .
U-500 0.9 or 0.0078/hr 1.2 or 0.004/hr ~ .. ,.... ~ , ~, . . _ _ . -~-520 1~3 or 0.0113/hr 1.2 or 0.004/hr _ _ .T . ' , ~ , .
- ECY-768 1.3 or 0.0113/hr 1.3 or 0.0043/hr !
~b) Result ak 1650F with a fuel containin~ 5 ppm Na and ~ ;
2 ppm V without additive treatment. - -Diameter metal recession, mils -ALLO~S 102 hour result 250 hour xesult 400 hour result ~
. , . .................. ... _._ . , .~ I ' .. ~: .
~ ~ X-45 13.8 or 0.1353/hr __ 26.1 or ~.0653/hl .. ...... ___ , ~........... _ ,, , ~ ... .. ~,.; i .
U-500 12.3 or 0.1206/hr 37.6 or 0.1504/hr ~___- j _ . . , , _ .. .
~c) P~esult at 1650F with a uel containin~ no sodium or ~anadium. Additives not used.
~LL~YS Diameter metal recession in 298 hours, mils X-454.1 or 0.0138/hr ~~
_ . A _ .. _.. : . .. . ' U-500 5.0 or 0.0168/hr '.

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An additional advantageous feature o~ our invention applies to fuels which al~o contain lead as a contaminating element. Lead contamination may occur where petroleum fuel oils are shipped or transpoIted in tanks where gasoline residuas remain. The magnesium additive is effective in re2ucing corrQsi~n degradation ~ue to lead fuel con~amination. If present, the concentration of .
the lead can be added to the concentration of the vanadium and the amount of magnesium to he added . will be determined ~rom that sum.

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Claims (16)

We claim:

1. A fuel additive composition for petroleum fuel oils contaminated with alkali metal and vanadium comprising a solution of a petroleum fuel soluble solvent and compounds of chromium and magnesium.

2. The composition of claim 1 wherein the chromium compound is chromium octoate.

3. The composition of claim 2 wherein the magnesium compound is magnesium sulfonate.

4. The composition of claim 3 wherein the weight proportions of chromium and magnesium are substantially equal.

5. The composition of claim 4 wherein the solvent is kerosene.

6. A petroleum fuel oil containing more than 0.5 ppm alkali metal, more than 0.5 ppm vanadium and both chromium and magnesium in amounts effective to inhibit or reduce metal degradation by a gas stream produced by the combustion of the fuel oil.

7. The fuel oil of claim 6 wherein the proportion of Cr:Na+K is about 4.5:1.

8. The fuel oil of claim 7 wherein the proportion of Mg:V is about 3:1.

9. The fuel oil of claim 6 wherein the ratio of Cr:Na+K is about 4.5:1 and the ratio of Mg:V is about 3:1.

10. The fuel oil of claim 9 wherein the ratio of Mg:V includes the weight of lead as V.

11. A method of reducing the corrosion of nickel base and cobalt base alloys traversed by a hot gas stream containing both alkali metal and vanadium contaminants comprising the step of exposing the stream to a combination of chromium and magnesium.

12. The method of claim 11 wherein the hot gas stream contains the combustion products of a petroleum fuel oil which contains more than 0.5 ppm of alkali metal and more than 0.5 ppm vanadium

13. The method of claim 12 wherein said fuel oil contains chromium and magnesium.

14. The method of claim 13 wherein the ratio of chromium:alkali metal in the fuel is about 4.5:1 and the ratio of magnesium:vanadium is about 3:1.

15. The method of claim 11 wherein said alloys are fabricated components of a gas turbine and those com-ponents are traversed by said stream.

16. The method of claim 15 wherein the temperature of said stream is about 2070°F.