US2505785A - Method of producing a corrosion inhibitive coating on ferrous metals - Google Patents

Description

148 CROSS REFERENCE .MayZ. 1950 2,505,785

H. R. MOORE METHOD OF PRODUCING A CORROSION INHIBITIVE COATING ON FERROUS METALS Filed Sept. 1'7, 1945 POLYGLYOOL ETHER OF NORMAL AUPHATIC CATONIC SURFACE ACTIVE ASENT WATER AT 200 To 2l2F c EOUALS 72 GM./ GAL. OFACID BATH FIG. I

TANK I STAGE l INORGANIC ACID PICKLE BATH (DESCALING) 5% BY VOLUME OF ACID 8.66% BY WEIGHT INLET.

FIG. 2

OUTLET TANK Z STA GE 2 WATER OIRCULATING TEMPERATURE, |70'-l80l'-.'

SOLID POWDER CATALYST |.o ETA/GAL. 0F BATH POLYGLYCOL ETHER OF NORMAL ALIPHATIC ALCOHOLS .02). on VOL. OF BATH G v (Mcr)(Po T b TANK 3 sum-:3 Fib- PRETREATMENT BATH SODIUM mcHRoMATE, 2a.? GIL/GAL; OR GHROMlC ACID, |9.3 em. AL.

PHOSPHORIC A010, 75%, 25.2 GM./ GAL.

TEMPERATURE |95-- 205 F.

HOWARD R. MOORE EXAMINER Patented May 2, 1950 UNITED STATES PATENT OFFICE METHOD OF PRODUCING A CORROSION INHIBITIVE COATING ON FERROUS METALS (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 2 Claims.

This invention relates to a process of treating metals i3; proyide a corrosion inhibiting surface coating there and more particularly to a proceg in which the successive steps cooperate to produce a desired result. It also relates to a process wherein the final step produces rust inhibition on certain metals such as cold rolled steel which possesses little or no mill scale.

An object of the present invention is to provide a process whereby a metal may be prepared and mridwmsa le w of oxide which is integral with the metal and corrosion resistant by virtue of its extremely low solution tension, nonconducting and electrically insulating properties.

A second object is to provide a continuous nonporous inorganic coating which is sufiiciently corrosive resistant to dispense with slushing compounds or other oils, greases and solvent combinations hitherto used to protect descaled metals from corrosion during a variable storage or stocking period before fabrication and assembly. This improved method of descaling low carbon mild steel and various grades of alloy steels, rinsing and treating the same in a pretreatment bath of unique composition produces a hard, dry, impervious coating of metallic nature which avoids the use of slushing compounds and their attendant disadvantages; namely, welding hazards due to incomplete removal of the materials with inflammable solvents, embrittlement of welds due to carbonization, dirt collection, and material and labor costs in the application of siushing oils and their final removal.

A third object is to provide an inhibitive surface suitable for the application of all types of primer paints and surface coatings, as a consequence of which ultimate failure due to the gradual transfer of water through films subjected to outdoor exposure is greatly delayed and retarded. d

A fourth object is to provide a brightfclean, inert surface of passivated steel, free from loose crystalline deposits, so that electroplated coatings, vitreous enamels and organic baking finishes can be directly applied.

A fifth object is to provide an inhibitive coat- "Ing suitable for the direct application of various metals by hot dip processes, such as zinc in galvanizing.

" A general objective is to provide a process which is rapid, economical and efficient in operation, which is easily maintained and in which the reactants are readily available materials.

In the. art of pickling metals it is customary to 2 use an inorganic acid such aww hydrochloric acid or various combina ions of these acids inhibited by the addition of complex organic sulphur derivatives and thereafter rinsing the steel in hot or cold circulating water. Occasionally a rust preventive treatment of very limited value is obtained by immersing the steel in a third bath containing 0.5 to 2.0% by volume of phosphoric acid. This is due to the fact that the ferrous phosphate coating formed is water soluble and hence readily removed by moisture or light rains. If the steel is painted with a rust inhibitive primer after pickling, early blistering and rusting occur due to the solvent action of water, gradually transferred through the coating. Heretofore there has been no attempt to correct this condition by adding an oxidizing acid such as chromic acid or salts thereof to the phosphoric acid bath, although it is recognized go that such combinations have been used for other purposes not based on immersion in baths of relatively high temperature.

In evaluating this invention for its effectiveness as a rust preventive measure, and in comparing its performance relative to other processes, the following classes of tests were carried out of unpainted test specimens:

(1) Outdoor exposures.

(2) Total immersion in 3.5% water solution.

(3) Salt spray box operated with 4% sodium chloride.

(4) Water condensation in a humidity box operated at deg. F. with cold water flowing through the coils supporting the test panels.

(5) Salt water of 3% concentration flowed at hourly intervals on panels maintained at a 30 deg. angle to horizontal.

The process In order to have a clearer understanding of one embodiment of the invention, reference is N now made to the drawing which is a flow chart showing the process in three stages. Each stage is designated as a tank i. e. tank I, tank II andR LE-" synthetic sea corrosion inhibition is still obtained by departing measurably from these optimum conditions.

The three stages are further described or defined as follows: Stage I is the descaling bath; stage II, the rinsing bath and stage III, the pretreatment bath.

The preferred composition of the solution employed in the descaling bath is a to by weight (of the solution) of 100% hydrochloric acid although 5 to 10% by weight of sulphuric acid may be used satisfactorily as the inorganic pickling acid.

The flexibility of the process is indicated by the fact that it is operable with acid concentrations varying over the range of 1.5 to 36% by weight, depending on the thickness of the scale, temperature of the bath and allowable time for descaling. A combination of hydrochloric and sulfuric acids in higher concentration is preferred for special alloy and high carbon steels whose scale is difficult to remove. The use of hydrochloric acid obviates the possibility of a partial reduction of the positively charged sulfur in sulfuric acid to sulfur dioxide, sulfur and finally hydrogen sulfide, whose sulfide ions accelerate corrosion if occluded within any fine pore structures present on the surface of the steel. To the acid solution is added an inhibitor consisting of a cationic surface active agent of the cationic nitro en bearinm Usually this type of material functions satisfactorily by itself, but in certain cases of restricted solubility it has been found advantageous to add a dispersing Qagent comprising various polyglycol ethers of normal or polyhydric aliphatic alcohols. A preferred cationic material is a aqueous emulsion of cetyldimethylbenzylammonium chloride, although it has been found that a wide range of high molecular alkyl-dimethylbenzyl ammonium chlorides are applicable. The chemical structure of these non-metallic organic compounds is C Ha CH3 in which R represents alkyl radicals ranging from CaHrz to C18H37, as contained in the corresponding fatt I coconut oil. The radical for the cetyl derivative, supra, is C16H33.

In addition to the above preferred class of compounds, the following materials of similar structure and selected quaternary ammonium types, together with amids containing amine groups, show satisfactory performance in the cycle of treatments prescribed for this invention:

(1) Trimethylbenzylammonium chloride.

(2) Octadecyl dimethyl benzyl-ammoniumchloride.

(3) P-tertiary-octyl-phenoxy-ethoxy-ethyl-dimethyl-benzyl ammonium chloride.

(4) Lauryl and/or cetyl pyridinium chloride or bromide.

(5) Cetyl dimethyl ethyl ammonium bromide.

(6) Octadecyl trimethyl ammonium bromide.

(7) Lauryl stearyl trimethyl ammonium bromide.

(8) Dimethyl ethyI-Q-octadecenyl ammonium bromide.

(9) Dodecylisoquinolinium bromide.

(10) Sapamines manufactured by the Ciba Co., namely, diethyl-amino-ethyloleyl-amide hydrochloride and diethyl-amino-ethyl-oleyl-amide methyl sulfate.

(11) Intracpl, man'ufactured by Synthetic Chemicals, Inc., lon chain fatty acid amides containing multiple amide groups.

Not all of the foregoing materials function as well as the alkyl-dimethyl-benzyl ammonium chloride types, especially the cetyl derivative.

While this invention is most expeditiously carried out with the foregoing classes of organic nitrogen bearing cationic surface active agents as inhibitors in the descaling bath, it must be explicitly understood that the composition of the pretreatment bath chemicals is so efficient that other surface active types belonging to the be described presently. Hence, while the cationic surface active agents of the type alkyl-dimethylbenzyl ammonium chloride are preferred, I do not desire to be limited thereto.

As mentioned above, in using certain cationic nitrogen bearing types, particularly the cetyl derivative of dimethyl benzyl ammonium chloride, it is desirable to add a dispersing agent comprising a water soluble wax pro uce y he reaction of a large number of 'ethylene oxide molecules with normal aliphatic and/or polyhydric alcohols.

A preferred dispersing agent is the reaction product of twenty mols of ethylene oxide with one mol of lauryl alcohol. However, it is understood that other polyglycol ethers, based on normal alcohols containing eight to eighteen carbon atoms and four to ninety ethylene oxide groups, may be used. Similarly, it has been found that other types of water soluble waxes, such as those manufactured by the Carbide and Carbon Chemicals Corporation, may be used. These materials are known as Carbowax and are formed by the polymerization of 34 to 137 molecules of ethylene oxide. They are not as effective, however, as the ethers formed from ethylene oxide, but contain one or more alkyl groups (polyglycol ethers of normal or polyhydric aliphatic alcohols).

While the reaction products of normal aliphatic alcohols with variable amounts of ethylene oxide are preferred, it has been found that reaction products based on ethylene oxide with secondary alcohols such as 2-ethyl hexanol and polyhydric alcohols such as ethylene glycol, glycerine and pentaerythritol are also satisfactory.

For the preferred cationic surface active agent, the 25% aqueous emulsion of cetyl dimethyl benzyl ammonium chloride, 8, ratio of four parts of this agent to one part of the polyglycol ether based on the reaction of 20 mols of ethylene oxide with lauryl or coconut oil fatty alcohols has been found most effective. In terms of solid inhibitor,

this quantity of dispersing agent is 0.125%; hence the inhibitor and dispersing aid stand in a 1:1

ratio for this particular case. However, repeated trials have shown that this ratio is not absolute for other types of nitrogen bearing inhibitors of this class, since inhibitor to dispersing agent ratios vary over the range :10 and 10:90 for CROSS REFERENCE The recommended quantity of the aqueous emulsions of the cetyl compound is 0.5% of the weight of anhydrous acid present. This corresponds to 1.81 gms. per gallon of a solution of 5% by volume of sulfuric acid. The quantity of dispersing material is 0.125% of the weight of acid or 0.45 gms. per gallon of a solution of 5% by volume of sulfuric acid.

The foregoing quantities of cationic material and dispersing agent are applicable only to this specific compound cited above since the merit value of the mixture remains consistently within the range of 80 to 85% for new and used baths, a value insuring a rapid descaling rate for mild steel and optimum conservation of both acid and steel. In order to maintain this optimum merit value with other cationic materials of the alkylated quaternary ammonium base type, it is frequently desirable to add as much as 2.5% or as little as 0.0625% cationic inhibitor of the weight of anhydrous acid in the bath, depending on the extent of adsorption of the material on the steel. Likewise dispersing agents may be omitted entirely, or used in quantities varying over the range 0.0625 to 2.5% depending on the chemical structure and ease of dispersibility of the organic nitrogen surface active compounds.

For the purposes of explanation, inhibitor merit value is defined in terms of loss of weight, or hydrogen gas evolved, by clean descaled steel in the presence and in the absence of cationic surface active inhibitor. Specifically, inhibitor merit value is given by the following equation expressed in terms of weight changes, since the weight loss method of evaluation is used most commonly in the industry:

Merit value A Wu where AWu and AWi represent the weight losses in uninhibited and inhibited acid, respectively. A relatively high merit value of 85 indicates that the waste of acid by attack on the steel is reduced 85%. Conversely, the waste of steel by attack of acid is reduced by the same amount.

One method of incorporating the alkylated quaternary ammonium base inhibitor is illustrated by the flow chart, Figure 1, although this method must not be considered restrictive if obvious variations in dispersion technic prove more satisfactory. Thus it has been found that a hot water concentrate, prepared by first dissolving mether of normal aliphatic alcohols in hot water and then adding the cationic material, comprises an alternate equally effective method.

The rinsing bath, stage II, consists of hot circulating water. Cold circulating water may be used, but it has little effect in accelerating the removal of hydrogen occluded within the steel. The circulation should be sufficiently rapid to prevent the pH from decreasing below 3.0. If circulation facilities are not available, the rinse water should be replaced when the pH approaches 3.0.

The freshly prepared pretreatment bath, stage III, comprises a solution of sodium dichromate and phosphoric acid containing the equivalents 0.51% of the weight of the solution of chromic Sodium dichromate, technical grade, is

namely, various polyglycol ethers of normal aliphatic alcohols and green powder catalyst. In the case of sodium dichromate (technical grade), the convenient starting weight percentages are 0.78% by weight of sodium dichromate and 0.59% by weight of phosphoric acid of the 85% C. P. grade or 0.67% of the commercial grade. The respective quantities required for one gallon of solution are:

Grams Sodium dichromate, NazCrzOmZH-fi, techninical grade 28.7 Phosphoric acid, C. P. grade 22.3

or Phosphoric acid, 75% commercial grade 25.2

For the chromic acid bath 19.25 gms. of chromic acid anhvdride per gallon of solution are required.

An optional constituent may be added as a catalyst in the amount of 1 gm. per gallon of solution. If not added, this catalyst automatically forms in the bath after the passage of a definite amount of steel. Although this material is a. product of the reaction, it has the unique property of speeding up the forward reaction so that a depleted bath frequently shows better corrosion inhibitive, or oxidizing properties, than the original bath or an artificially prepared depleted bath. While the most significant differences in reaction rate have been noted when the concentration of this material is 1.0 gram per gallon, appreciable differences are also obtained over the range 0.1 to 10 gm. per gallon. This catalyst is a double phosphate of chromium and iron with the alloy elements present in the steel, and is designated by the formula (MCr) (PO02. M designates MCr(PO4)2 where M designates Fe (approximately 99%) and the alloy elements (Cu, Cr, Mn, Ni, Si, Al, etc.) approximately 1%. While this constituent does not greatly improve the performance of a fresh bath, it has been found to be of considerable value in promoting the oxidispersion of the cationic surface active inhibitor in th descaling bath. This material is added in amount of 0.02% on the weight of the bath, or 0.76 gram per gallon. Quantities as high as 0.2% or as low as 0.002% have been found to give a detectable improvement in corrosion inhibition; indeed with different organic nitrogen cationic inhibitors it may be desirable to vary the quantity of organic accelerator to determine the optimum effect. These water soluble waxes containing an organophilic group evidently possess surfac active properties since they improve the oxidizing efficiency of both fresh and aged pretreatment baths. Organophilic means organic non-polar. This material possesses surface active properties and greatly improves the performance of both fresh and aged pretreatment baths. However, it has been found that polyglycol ethers based on .the reaction products of EXAMINE? wan ,with lauryl alcohol. Other products of this type containing as many as 90 mols ethylene oxide per molecule of normal aliphatic alcohol are also effective.

There is a lower limit to the concentration of alkali metal chromates or dichromates and phosphoric acid below which the operation of the process will cause a measurable falling off of the rust resistance of the pretreated steel. This lower limit has been found to be one third of the initial concentration of these two chemicals although even lower concentrations give a degree of rust resistance not approached by other processes. At this point, determined preferably by colorimetric comparison, the quantities of alkali metal chromates, dichromates or chromic acid and of phosphoric acid to be added are calculated by difference between the depleted concentration and that given above. Thus at one third concentration the solution will contain 9.6 gms. of sodium dichromate per gallon or 6.42 gms. of chromic anhydride and 7.4 gins. per gallon (the 85% grade) or 8.4 (75% grade) of phosphoric acid. 19.1 gms. of sodium dichromate of 12.83 gms. of chromic anhydride, and 14.9 gms. (85% grade) or 16.8 gms. (75% grade) of phosphoric acid per gallon of solution will be required to restore the solution to the original concentration of these two chemicals. It is possible to restore a used bath to the original strength by these simple calculations since both constituents, chromic acid or sodium dichromate and phosphoric acid, are depleted at equal rates by the passage of steel through the bath liquid. This is a unique property of bath III compositions based on equal percentages by weight of chromic anhydride and anhydrous phosphoric acid.

The operation of the process is as follows:

The steel to be treated is immersed in the descaling bath (stage I). The bath temperature should be maintained within the range 160-180 F. for the sulphuric acid bath and 110-150 F. for the hydrochloric acid bath. The time of descalding should be from 10 to 20 minutes for new and aged inhibited baths, respectively.

In practice, the acid concentration should never be allowed to drop below 1.5% by volume for sulphuric acid or hydrochloric acid baths, nor should the iron content of ferrous sulphate or chloride rise above 6%. Observance of this pre-' caution will insure rapid and constant descaling rates. is exhausted, proportional amounts of the cationic quaternary ammonium surface active compounds and polyglycol ether of normal aliphatic alcohols should be added as they are depleted along with the acid.

The steel is slowly withdrawn from bath I and allowed to drain for approximately one half minute over the bath in order to conserve acid in the descaling bath as well as to prevent an accumulation of acid in the rinsing bath in which the steel is next immersed. This bath comprises fresh circulating water at 170-l80 F. The period of immersion here is approximately two minutes. Longer periods of immersion effect a more complete removal of hydrogen occluded within the body of the steel, but are not required in a cycle requiring a high production of pickled metal.

The steel is next immersed in pretreatment bath 1211 which shouldbe maintained at a temo- P I When renewing the acid before the bath perature range of 195-205 F. for optimum results. A temperature of 200 F. should be preferably maintained. If both the polyglycol ether and solid powder catalyst above mentioned are 5 used, an immersion period of two minutes is sufficient to develop a coating of high rust resistance on the steel; otherwise, a longer period of reaction is required. However, when the catalyst and polyglycol ether are not available the period of 10 immersion should be from 10 to 20 minutes. Convection currents in the bath solution usually provide sunicient contact of the solid particles with the steel, but if this is inadequate air agitation or mechanical stirring should be provided.

The process results in formation of a coating in situ on the steel. By formation in situ is meant the reaction between material oriented on the steel surface from the descaling solution and material in the pretreatment bath.

Advantages of this process Hydrochloric and sulphuric acid solutions inhibited with cationic nitrogen containing surface acitve agents in accordance with this process show the following advantages over acids inhibited with orthodox pickling inhibitors:

1. Remove oily residues from the steel due to the emulsifying action of the dispersing agent.

2. Foam slightly, trapping acid spray and remain clear throughout the pickling cycle.

3. Facilitate rapid descaling due to the wetting out action of the dispersing agent, at the same time conserving steel and acid by maintaining inhibitor merit values within the range 75 to 85.

4. Prevent the deposition of carbon smut, de-

tached mill scale and other harmful residues on the surface of the steel because of the repellent nature of the organophilic absorbed layer.

5. Give effective inhibition due to the attraction of the negatively charged steel for the positively charged alkylated cationic nitrogen ions. The transient negative charge on the steel is neutralized in the act of absorption forming a very thin layer of hydrophobic, and hence rust resistant properties. The organophilic properties of this layer are due to the oriented adsorption of the positive organic nitrogen ion to the steel surface and the alkyl group to the aqueous phase. Further attack on the steel is reduced because of its newly acquired water repellent properties.

6. Prevent incipient corrosion effects due to localized adsorption on the steel by means of the polar nitrogen atom present in the molecule, rather than through the sulphur linkages char- 5 acteristic of the usual commercial inhibitors. Furthermore, alkylated cationic nitrogen bearing compounds are comparatively immune to the reducing efiect of nascent and molecular hydrogen evolved by the steel as contrasted to orthodox 5o inhibitors whose sulphur linkages are activated by these processes. These orthodox inhibitors contain labile or mercaptan linkages which stimulate the corrosion of steel. Moreover the number of labile or organic sulphur linkages is increased by thermal decomposition of the entire inhibitor molecule in the hot pickling bath and also by the chemical effects of the hydrogen originating from the steel in the act of dissolving. Inhibitors containing sulphur in higher stages of oxidation, such as sulphates and sulfonates are less harmful, but ultimately they are converted to corrosion inciting molecular fragments in the descaling process.

7. Prevent the embrittlement of the steel by promoting the combination of nascent hydrogen wuss KU'LHtNUl: WWIHU atoms to molecular hydrogen gas which escapes from the solution. Sulphur bearing inhibitors accelerate the embrittlement of steel because they retard this reaction of recombination of the hydrogen atoms initially evolved. As a consequence the hydrogen atoms are driven back into the steel, causing embrittlement. This unfavorable eflect is due to the poisoning of the steel surface by sulfonium ions or sulphur bearing reduction products of the inhibitor molecule. The use of cations formed by the electrolytic dissociation of nitrogen bearing organic molecules avoids this undesirable result, leading t further applications of pickling in the industrial arts than was hitherto considered possible.

8. Permit a lower descaling temperature in the case of all inorganic acids due to the fact that cationic surface active agents are preferentially adsorbed, and consequently less pitting of the steel is encountered due to the action of the acid.

It has been found that there are desirable limits to the pretreatment bath composition and to the operating conditions. As described, supra soluble hexavalent chromium oxygen compounds or chromic acid and phosphoric acid are used in equal proportions by Weight relative to their content of chromic anhydride and anhydrous phosphoric acid. If this condition is met, superior properties are conferred on the bath solution as an agent for passivating steel.

While a 1:1 ratio is favored, the separate elements of this invention are so effective that good results are still obtained with measurable departures from this ratio. Thus significant corrosion inhibitive properties continue to be developed when the anhydrous chromic acid to phosphoric acid ratio varies over the range 80:20 and :80. Hence it is intended that these significant departures from the favored 1:1 ratio be considered within the scope of this invention.

The detailed conditions specified for the preferred economic manner of operating the pretreatment bath are as follows:

1. The working concentration range of the reacting chemicals are 0.78% sodium dichromate 5 use concentrations higher than 2% of the basic and 0.59% pl'losphml 3 acld chemicals, with or without the wetting agents grade) in a fresh bath. These concentrations polyglycolethers of aliphatic alcohols, because of happen to m accorfi W quantities 119095 the necessity of a final rinse as a fourth step in sary to Satlsfy the Stolchmmetnc equation the process, the use of these higher strength solutions is valuable nevertheless in furnishing a Na2Cr2O7+H2O+2H3PO4 2NQH2PO4+2HZCIO4 source for the manufacture of an activated preand to prbvide an initial pH range of 1.7 to 22. treatment concentrate which can be furnished to When used in equivalent concentrations by users of process- The Steps m the manufac weight, the pretreatment of large quantities of ture of this pretreatment concentrate are as steel causes each ingredient of the solution to be follows: depleted at equal rates, so that a bath ready for ((1) Prepare an aqueous S u of he bas renewal maintains its two components in the same Chemicals Comprising al metal C te d fixed ratio of 1:1 and the same mechanisms are chromates and/01 chromic acid and phosphoric operative in yielding the passivated coating on the acid other inorganic acids of Sui able dissteel. However, the use of chromic acid is favored 50 sociation constants whose anions are capable of since the hydrogen ion concentration (pl-I) has acting as acceptors for trivalent chromium) in a been found to vary less in depleted baths than 1:1 ratio based on chromic acid and anhydrous when sodium dichromate is used. This is due to phosphoric acid and in concentration of each the fact that no sodium or other non-volatile anhydrous ingredient exceeding 5.0% by Weight, alkali ions are free to accumulate as the bath but ot mor tha 40%, becomes gradually exhausted 0f hexavalent Chro- (b) Raise temperature of solution concentrate mium due to the passage of large quantities of t 195.205 steel- (c) Descale steel sheets of maximum size that The same cPndltlons are aPphcable t0 Q P can be accommodated by the pretreatment bath anhydnde whlch can be m Smaller quantlt les container with the preferred bath I composition due to the fact that the ratio of molecular equive ified by this invention alents 1s gwen by the moment (6.) After rinsing steel sheets as prescribed for 200 bath II, attach as many as can be accommodated 298.05 2 mols Croa 1 mol Nazcrzol-zmo by a stainless steel or other suitable mandrel in Incidentally, it has been found that the same 1:1 ratio is advantageous when the starting concentration of the two basic inorganic chemicals is tenfold the recommended concentration for economic operation. This tenfold concentration corresponds to 5.1% anhydrous chromic acid (or equivalent concentration of soluble compounds of hexavalent chromium) and 5.0% anhydrous phosphoric acid. When high concentrations are used, a one to two minute dip in circulating water maintained at to degrees F. is desirable to remove unreacted chemicals from the surface or the metal. This fourth stage dip in water is unnecessary for a bath containing a total concentration of 2% active chemicals, however. Such a bath has the advantage of furnishing a higher degree of corrosion inhibition when depleted to the 1% level than a fresh 1% bath, due to the accumulation of inorganic catalyst and the peculiar ionic equilibrium accompanying its formation.

2'. The minimum time of immersion, 1 minute, in the presence of the inorganic catalyst and polyglycol ether of normal aliphatic alcohols above mentioned and 10-20 minutes in the absence thereof. An extremely beneficial effect due to heavier deposit of oxide film is obtained by conducting the process for 10 to 20 minutes in the presence of both accelerators.

3. Temperature of immersion of pretreatment solution 205 F. A detectable improvement is obtained by conducting the process at the boiling temperature, 212-214 F., although this is not commercially feasible. At the opposite end of the scale, measurable protection to the metal is obtained by operating the pretreatment tank at temperatures as low as 125 F., a circumstance which makes possible the spray application of the bath composition to previously descaled objects fabricated from cold rolled steel, such as refrigerators and automobile bodies. Descaling is not necessary for certain articles fabricated from cold rolled steel which has little or no scale.

While it has not been found advantageous to' solution containing the maximum yield of green powder catalyst in the shortest possible time.

(I) Recirculate the treated sheets through the descaling bath I to dissolve off the oxide coating, allowing a minute pickling time for this process, thereafter rinsing in bath II and immersing again in bath III until a minimum of 0.5 gram of catalyst is prepared per gallon of activated pretreatment bath concentrate.

Since the catalytic activity of the activated bath is due in large measure to the characteristic ionic equilibrium prevailing in the presence of green powder catalyst MCr(PO4)2, it is only necessary to pass through bath III enough steel to produce a very small quantity of this material because of its high insolubility and the rapidity with which equilibrium conditions are set up. M designates MCr(PO4)2 where M designates Fe (approximately 99%) and the alloy elements (Cu, Cr, Mn, Ni, Si, Al, etc.) approximately 1%. The

surface area of steel necessary to accomplish this result varies between 100 to 900 square feet per gallon of solution concentrate treated by this process.

Since a high concentration of ions in the pretreatment bath, rather than undissociated molecules, are most active in the process resulting in the formation of catalysts, it has been found most desirable to operate the above process with concentrations of basic reactants not exceeding each, thereafter concentrating the resulting activated retreatment solution by evaporation. A solution concentrate can then be made available with solids concentration in the 40-90% range, so that the user needs only to add a small amount to a pretreatment bath tank to secure the optimum passivation of descaled hot or cold rolled steel.

The invention described herein may be made or used by or for the Government of the United States for governmental purposes without the payment to me of any royalties thereon or therefor.

What is claimed is:

1. A process of pretreating a ferrous metal surface, said process comprising immersing said surface in an aqueous solution including: sodium dichromate in the amount of about 378% by weight of solution, phosphoric acid about 59% by weight of solution, reaction product of 29 to mols of ethylene oxide with 1 mol of lauryl alcohol about .002 to .2% by weight of solution, ferric chromic phosphate about .1 to 10 gm./gal. of solution; maintaining said solution at a temperature of about to 205 F.; and withdrawing said surface from said solution after about 1 to 20 minutes.

2. A process of pretreating a ferrous metal surface, said process comprising immersing said surface in an aqueous solution including: sodium dichromate in the amount of about 378% by weight of solution, phosphoric acid about 59% by weight of solution, reaction pr oduct of 29 to 90 mols of ethylene oxide with 1 mol of lauryl alcohol about .002 to .2% by weight of solution; maintaining said solution at a temperature of about 195 to 205 F.; and withdrawing said surface from said solution after about 1 to 20 minutes.

HOWARD R. MOORE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,719,464 Cole July 2, 1929 2,006,216 Macarthur et a1. June 25, 1935 2,030,601 McDonald Feb. 11, 1936 2,114,151 Romig Apr. 12, 1938 2,127,202 Boyle Aug. 16, 1938 2,296,884 Thompson Sept. 29, 1942 2,403,426 Douty et a1 July 2, 1946 2,413,495 Given Dec. 31, 1946

Claims (1)

1. 2. A PROCESS OF PRETREATING A FERROUS METAL SURFACE, SAID PROCESS COMPRISING IMMERSING SAID SURFACE IN AN AQUEOUS SOLUTION INCLUDING: SODIUM DICHROMATE IN THE AMOUNT OF ABOUT .78% BY WEIGHT OF SOLUTION, PHOSPHORIC ACID ABOUT .59% BY WEIGHT OF SOLUTION, REACTION PRODUCT OF 29 TO 90 MOLS OF ETHYLENE OXIDE WITH 1 MOL OF LAURYL ALCOHOL ABOUT .002 TO .2% BY WEIGHT OF SOLUTION; MAINTAINING SAID SOLUTION AT A TEMPERATURE OF ABOUT 195* TO 205*F.; AND WITHDRAWING SAID SURFACE FROM SAID SOLUTION AFTER ABOUT 1 TO 20 MINUTES.

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