US2987427A

US2987427A - Metal coating baths

Info

Publication number: US2987427A
Application number: US711135A
Authority: US
Inventors: Shaw Richard Elliott
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1956-09-25
Filing date: 1958-01-27
Publication date: 1961-06-06
Anticipated expiration: 1978-06-06
Also published as: ES237642A1; DE1165378B; NL107385C; US2987425A; GB865210A; DE1169759B; NL220946A; CH366188A; BE561107A

Description

United States Patent 2,987,427 METAL COATING BATHS Richard Elliott Shaw, Windsor, England, assignor to Imperial Chemical Industries Limited, London, England, a corporation of Great Britain No Drawing. Filed Jan. 27,1958, Ser. No. 711,135 Claims priority, application Great Britain Jan. 30, 1957 5 Claims. (Cl. 148-615) This invention relates to improved acid aqueous baths for coating metal with a tightly adherent layer of a salt, e.g. phosphate, chromate or oxalate, or an oxide.

In particular, the invention is concerned with acid coating baths of improved convenience and thermal efliciency which can be used with a minimum consumption of the chemicals therein.

Such salt or oxide coatings are applied to metal, for example, to improve corrosion resistance and paint adhesion, and to provide lubricant or lubricant-carrying layers in drawing processes, and it is well known that the coatings can be produced by treating the metal surface with acid solutions. For example, metals may be coated with tightly adherent phosphate coatings by treating them with acid solutions based generally on iron, manganese, or zinc phosphates, either together or separately. These metals are generally known as coating metals. The coating reaction may be accelerated by small additions of more noble metals such as copper and nickel to the processing solution. Further acceleration may be obtained by oxidising agents such as nitrates, nitrites, chlorates, bromates, and certain organic nitro-compounds such as nitro-guanidine.

It is also well known that phosphate coatings can be produced by solutions containing metallic phosphates of which the metal does not itself enter the coating. Examples are solutions based on sodium, potassium, ammonium, and magnesium phosphates, which form coatings consisting predominantly of phosphates of the metal being treated, e.g. iron phosphates on ferriferous articles.

Oxidising agents such as nitrates, nitrites and chlorates may also be present in these solutions.

Similarly, oxalate coatings may be produced by treatment with acid oxalate solutions and oxide coatings by treatment with acid oxidising solutions.

A serious disadvantage in the use of acid coating baths of the type mentioned above is the relatively high consumption of the chemicals therein. For example, in producing oxalate coatings on stainless steel or other highly corrosion resistant alloys, the oxalate coating bath is so constituted as to include oxygen bearing sulphur compounds, such as thiosulphates, sulphites, hydrosulphites, thionates and bisulphites, which are capable of yielding sulphur dioxide upon decomposition in acid solution and the consumption of such sulphur compounds is quite high due to the escape of sulphur dioxide from the bath to the surrounding atmosphere. This is both wasteful and costly, and, in addition, the sulphur dioxide escaping from the bath is odoriferous and a hazard to health. A generally similar problem exists with other types of acid coating baths, e.g. loss of nitrites from phosphate coating solutions.

A principal object of the present invention is to provide acid coating baths, e.g. phosphate, chromate, oxalate or oxide coating baths, and methods of using same, which make it possible to obtain highly effective coatings in a 65 more convenient and etlicient manner and with a material reduction in the loss of sulphur dioxide liberating substances and/ or other chemicals.

Another object of the invention is to provide acid coating baths and methods of using same whereby objectionable odours and health hazards are etfectively Additionally, it is an object of the invention Patented June 6, 1961 ice to provide acid coating baths which demonstrate a high degree of thermal etiiciency. It is an additional object of the invention to provideconcentrates suitable on dilution with water for use in the acid coating baths.

The foregoing objects are realised by providing the acid coating bath with a non-aqueous surface layer of oil or the like as hereinafter described in detail. The success of the invention is highly unexpected. For one thing, to enable metallic surfaces to be treated satisfactorily, it

has hitherto been customary to conduct a preliminary degreasing operation and to maintain the surface of the processing bath free from all oil and grease. If oil has been allowed to remain on the bath the acid solution, being unable to react with the oil coated areas of the metal, has produced patchy coatings. It is, therefore, surprising to discover that excellent coatings can be obtained on metallic surfaces dipped into certain acid baths covered with a layer of oil.

Furthermore, in the case of, for example, oxalate coating baths containing sulphur dioxide liberating compounds, it has been found that a layer of mineral oil or the like effectively reduces the consumption of these compounds and liberation of objectionable sulphur dioxide from the bath even though sulphur dioxide is soluble in the mineral oil surface layer. This is surprising because it normally would be expected that the oil layer would not minimise liberation of sulphur dioxide in view of its solubility in the protective layer. Additionally, it has been found that the present invention when applied to baths for producing uniform coatings on metal surfaces provides a bath which may be worked with much less heat loss and less scale formation on heating surfaces than has been possible with processes not employing a surface layer.

The surface layer material must be one which is liquid at the temperature of operation of the bath and substantially immiscible with the acid coating solution. The substance may be, for example, a high boiling point paratiin such as liquid paraflin, petroleum jelly or mineral oil, parafiin wax, an animal or vegetable oil such as lanolin, tallow or castor oil, bitumen, a lower molecular weight synthetic resin such as polystyrene.

The layer forming substance must be inert, i.c. stable against reduction or attack by the acid bath or by atmospheric oxygen at the temperature of the bath. For oxide coating baths non-oxidisable materials should be used. The layer forming substance must also be less dense than the coating solution.

In practice, it has been found that the depth of the layer can be varied between relatively wide limits, these limits being dependent on the viscosity range of the ma terials used, the temperature of operation of the bath, the

amount of foam formed at the interface between the inert substance and the aqueous acid solution, the vigour of any etfervescence in the bath and other operating factors. Desirable results can be obtained using a surface layer having a thickness as low as 3 mm. However, preferably, the layer is at least 10 mm. thick for most efiective results, in, e.g. reduction of acid fumes and spray although a layer thickness as high as 50 mm., or even higher, may also be used subject to economic considerations.

It is preferable that the layer contains very little or no material volatile at the temperature of operation of the bath and it should be of such viscosity at the operating temperature of the bath that the layer will readily reform after being broken by, for example, the entry or exit of articles to the bath, or the exit of bubbles of gas or vapour.

It is preferred that the layer forming material should be of the lowest viscosity consistent with it being subcentipoises are preferred, care should be taken that the flash point of the layer forming substance is above that of the temperature of operation. For this reason, a viscosity range of from 2-10 centipoises at the operating temperature is preferred.

The preferred layer forming materials are, at the temperature of operation of the bath, liquid hydrocarbon materials, such as liquid paraflin and various grades of oils.

A surprising feature of the invention is that any surface layer which may tend to cling to articles entering the bath is readily removable and does not seriously affect the uniformity with which the acidic solution reacts with the metal. In any case, this endency may be minimised by adding one or more surface active agents, preferably non-ionic, to the acid solution. The quantity of surface active agent required is dependent on the working temperature, the duration of treatment, and the amount of gas evolved during the coating reaction. Preferably, the surface active agent should be present in suflicient quantity to lower the surface tension of the bath to less than 40 dynes/cm., desirably less than 35 dynes.

Cationic surface active agents, in particular quaternary ammonium compounds, are suitable but in general nonionie surface active agents are more effective. Typically suitable non-ionic surface active agents include: ethoxylated dinonyl phenols and lauryl alcohols, ethoxylated octyl cresols and cetyl and oleyl alcohols. Anionic surface active agents, such as sulphonated secondary alcohols, or a sulphated cetyl/oleyl alcohol mixture may also be used but may necessitate an increase in the amount of accelerating agent used in the coating process.

There is an advantage in having a surface active agent present in the surface layer to facilitate the removal of any adherent surface layer when subsequently rinsing in water. It is an additional advantage to employ a surface active agent which is soluble both in the acid bath and in the surface layer so that the one surface active agent fulfills both functions. The surface layer can then act as a reservoir to take up any excess of surface active agent which may throw out of solution in the bath at elevated temperatures. Thus the benefits of a surface layer may be obtained with coating baths which do not cause much gassing of the metal surfaces undergoing treatment. 1

The proportion of surface active agent present in the bath to that in the layer is thus dependent on the prevailing temperature and equilibrium should be established for optimum results. Experiments were conducted with surface active agent present in the bath but not in the layer and panels were diflicult to rinse free from layer material. Panels treated or minutes later were much easier to rinse as surface active agent was taken up into the layer material. Equilibrium was established within about 10 to 30 minutes, usually 30 minutes, and no rinsing difliculties were then experienced. Similarly, experiments with the surface active agent present in the layer material, but not in the bath, gave incomplete removal of layer material which adhered to the panel on immersion. Panels treated subsequently had all layer material removed, and rinsed without difliculty, as the surface active agent was then present in both bath and layer. The extent to which the surface active agent is soluble in the bath and layer, respectively, at the operating temperature is important. Alkyl trimethyl ammonium salts have adequate water solubility up to 100 C., to prevent the bath being denuded by solution into the layer. Some agents may be insufiiciently water soluble and be preferentially dissolved by the layer which then adheres towork being processed and may become emulsitied into the treatment bath. Desirably, the surface active agents used herein should just give a clear solution in the aqueous phase at the temperature of operation of the bath and the solubility in the surface layer may vary but 4 should be sufiicient to take up the surface active agent thrown out from the aqueous phase.

It is an advantage to incorporate a dye in the surface layer so that the layer thickness may be readily observed. The dyed layer can also be used to warn operators that the bath underneath may be very hot despite the absence of steam or other fumes. A suitable red dye is Waxoline Red 0.8. (Color Index No. C.I.E. 258 (1924/28l).

As has been stated earlier this invention provides concentrates suitable, on dilution, for use in aqueous baths for coating metal, these concentrates comprise an inert liquid and an inorganic layer-forming substance.

The concentrates may be liquid, pasty, or solid compositions and the proportion of inert liquid in the concentrate may be varied over a wide range. The actual proportion of inert liquid used in any concentrate composition will be related to the thickness of the layer required on the tank in which the concentrate is to beused, and the concentration of active ingredients required in the tank, it being desirable to obtain on the surface of the solution in the tank an inert layer at least 10 mm. thick. Surface active agents may be mixed into the concentrates in amounts appropriate, as detailed earlier.

The invention is illustrated by the following examples.

Example I A phosphate coating solution was made up to the following composition:

Percent by weight The non-ionic surface active agent is an anhydrous condensation product of a long chain fatty alcohol and ethylene oxide. Specifically, the condensation product of 1 part of cetyl alcohol and 3 parts ethylene oxide.

This solution was covered with a 12 mm. layer of a saturated hydrocarbon material having a flash point of 155 C., and a viscosity of 3.5 centipoises at the operating temperature containing 0.1% Waxoline" Red 0.8. The solution and layer were then heated to C. and maintained at that temperature for 10 minutes to allow some of the surface active agent to enter the layer. Cold rolled steel panels were degreased by treatment in a conventional trichloroethylene degreasing plant and were passed through the surface layer and left in the phosphatmg solution at 90 C., for 5 minutes. The panels were then withdrawn through the surface layer, rinsed by im mersion in cold running water, rinsed in hot water containing 0.025% chromic acid and 0.025% phosphoric acid, and then dried. The steel surfaces had a typical umform light grey crystalline phosphate coating. These coatings were free from bare patches which would be caused by adherent oil and were perfectly normal for use as rustproof coatings or as a base for oil, paint or lacquer coatings.

Example 11 A phosphate coating solution was made up of the following composition:

Percent by weight 1.00

This solution was covered with a 12 mm. layer of the hydrocarbon material used in Example I containing 0.1 Waxoline Red 0.8. The solution and layer were then heated to 70 C., and maintained at that temperature for centipoises at 10 minutes to allow some of the surface active agent to enter the layer. Cold rolled steel panels were degreased and processed as in Example I but using phosphating solution temperature of 70 C. Again, perfectly normal uniform light grey crystalline phosphate coatings were obtained, eminently suitable for use under oil, paint or lacquer coatings.

Example III A phosphate coating solution was made up to the following composition:

Percent by weight 08 This solution was covered with the hydrocarbon material used in Example I and heated to 70 C., as in Example II. 0.55% carbon steel wire of A" diameter was pickled in cold hydrochloric acid, rinsed in cold running water and passed through the surface layer and left in the phosphating solution at 70 C., for 5 minutes. The wire was then withdrawn" through the surface layer, rinsed by immersion in cold running water, and dried. The wire was coated with a uniform light grey crystalline phosphate coating free from bare patches whichwould be caused by adherent oil and was perfectly normal for drawing to a smaller diameter using a soap lubricant.

Example IV A phosphate coating solution was made up to the following composition:

Percent by weight 3 Manganese (Mn) This solution was covered with a hydrocarbon material having a flash point of 180 C., and a viscosity of 5 4 the operating temperature and heated to 95 C. Cast iron piston rings were degreased with trichloroethylene and passed through the surface layer into the phosphating solution. After 15 minutes treatment they were withdrawn through the surface layer, rinsed by immersion in cold running water, rinsed in hot water, and dried. The piston rings had. a typical iron/manganese phosphate coating, free from bare patches which would be caused by adherentoil and were perfectly normalfor anti-wear purposes, i.e. to hold lubricant and prevent initial wear during the running-in period in internal combustion engines. Steel panels and nuts and bolts treated in this solution received normal phosphate coatings eminently suitable for rustproof coatings and for sealing in oil, stain or paint.

Example V A phosphate coating bath was made up to the following a composition:

Percent by weight 0.3

This solution was covered with the hydrocarbon material used in Example IV and used at 90 C., as in Example IV. Piston rings, panels, and nuts and bolts, so treated had similar coatings to those in Example IV.

6 Example VI 7 A phosphate coating bath was made up to the following composition:

Percent by weight Manganese (Mn) 0.3

Iron (Fe) 0.1

Phosphate (P0 1.6

, Water remainder A concentrated phosphate coating mixture was made up to the following composition:

Percent by weight Manganese (Mn) 4.0 Iron (Fe) 5 0.25 Phosphate (P0 23.0 Nitrate (N0 1.5 Non-ionic surface active agent 1.0

Hydrocarbon material: Flash point C.

and viscosity 112 centipoises at 21 C 16.0 Water Seven gallons of this mixture added to ninety-three gallons of water gave a phosphating bath with a surface layer to prevent evolution of spray or fumes. The same mixture was used to replenish the bath during use. The surface active agent was an alkyl phenol condensate with ethylene oxide, namely, 1 part octyl cresol with 2 parts ethylene oxide.

Example VIII An oxalate coatingibath was made up to the following composition:

Percent by weight Manganese (Mn) 0.4 Nitrate (N0 1.0 Oxalate (COO); 3.5 Accelerators 0.3 Non-ionic surface active agent 0.1 Water remainder The surface active agent was the same as that used in the preceding example. The accelerators were sodium meta-bisulphite and sodium thiosulphate.

This solution was covered with a 12 mm. layer of hydrocarbon material as used in Example I and heated to 70 C. Stainless steel wire was pickled in a mixture of nitric and hydrofluoric acids, rinsed in water, and passed through the surface layer into the oxalate bath. After 5 minutes the wire was removed through the surface layer, rinsed by immersion in cold running water, rinsed in hot water, and dried. The wire had a typical oxalate coating, free from bare patches which would be. caused by adherent oil, suitable for anti-wear purposes, i.e. to hold lubricant and prevent wear during drawing-to smaller diameters.

Example IX A phosphate coating bath was made up to the following composition:

remainder remainder I from 'bare patches, suitable to prevent rust creep under a protective finish in the event of mechanical damage to part of the protective finish.

Example X A phosphate coating bath was made up to the following composition:

Percent by weight 0.8

Zinc (Zn) Phosphate (P 1.2 Nitrate (N0 1.3 Iron (Fe) 0.1 Cationic surface active agent 0.2 Water remainder The cationic surface active agent was an alkyl trimethyl ammonium salt, specifically, cetyl trimethyl ammonium bromide.

This solution was covered with 12 mm. hydrocarbon material as used in Example I and heated to 85 C. A basket containing small spring steel clips was passed through the surface layer into the phosphating solution. After 3 minutes, the clips were withdrawn through the surface layer, transferred to a second basket and again immersed in the phosphating solution. This process was repeated to give four immersion periods of 3 minutes. The clips were then rinsed in cold running water, rinsed by immersion in hot water and dried centrifugally. The clips were then dipped in spirit stain and oven dried.

By this procedure, the spring clips had received a typical zinc/iron phosphate coating and were adequately protected from corrosion in service.

Example XI A chromate passivating bath for aluminium was made The surface active agent was a sodium salt of perfluoropentane sulphonic acid.

This solution was covered with a 12 mm. layer of hydrocarbon material as used in Example IV and heated to 100 C. Aluminium panels degreased by immersion for 5 minutes at 98 C., in a 3% meta-silicate solution containing surface active agent, were rinsed in cold water and passed through the surface layer into the passivating solution. After 1 hour, the panels were withdrawn through the surface layer and rinsed by immersion in cold running water, rinsed in hot water and dried. Panels coated in this way were more resistant to corrosion than untreated panels.

The surface layer completely suppressed spray and steam from the solution during processing without interfering with the coating process. The degreasing solution may also be covered with a 12 mm. layer of the same oil.

Example XII A chromate passivating bath for magnesium was made up to the following composition:

- Percent by weight Sodium dichromate 8.0 Magnesium sulphate 4.0

8 Manganese sulphate 4.0 Anionic surface active agent (as in Ex. XI) 0.5 Water remainder This solution was covered with a 12 mm. layer of a hydrocarbon material as used in Example IV and heated to C. Magnesium die-castings were degreased by immersion for 5 minutes at. 98 C., in a 3% meta-silicate solution containing surface'active agent, rinsed by immersiouin cold running water and passed through the surface layer into the passivating solution. After 30 minutes, they were withdrawn through the surface layer, rinsed by immersion in cold running water, rinsed in hot water and dried.

The surface layer completely suppressed evolution of steam and spray from the solution during processing without interfering with the coating.

Example XIII An oxalate coating solution was made up to the following composition:

Water, to make 1 litre.

An operating bath can be prepared from the foregoing formula by blanketing or covering the same with suflicient commercial white mineral oil to develop a layer approximately 8 mm. thick.

The resulting bath is highly effective for the oxalate coating of stainless steel. A loss of the chemical components therein is noted both while the bath is in use and while standing. It will be understood from the foregoing that other commercial oils and layer depths may also be satisfactorily utilized although for uniform results the minimum layer thickness of 3 mm. mentioned heretofore should be observed.

Other typical and highly satisfactory examples of coating baths according to the invention are noted below:

Example XIV Manganese carbonate, 38% gms 12 Oxalic acid do 15 Nitric acid, 69% ml 24. Sodium thiosulphate ....gms.... 5

Water, to make 1 litre.

Sun oil No. 591 added to obtain a layer depth of 8 mm. over the bath.

Water, to make 1 litre.

Tall oil (purified) added to obtain a layer depth of 8 mm. over the bath.

Example XVI Oxalic acid gms 50 Sodium chloride do 20 Sodium bifiuoride do 10 Sodium thiosulphate do 2 Water, to make 1 litre.

Lard oil was added to obtain a layer depth of 10 mm.

Example XVII Manganese sulphate, 70% --gms- 15 Oxalic acid (In 20 Phosphoric acid, 75 ..ml-- 10 Sodium sulphite gms.... 2

Water, to make 1 litre.

Sufiicient commercial white mineral oil to cover the bath to a depth of 10 mm.

Example XVIII Manganese sulphate, 70% grns.. 15 Oxalic acid do 10 Phosphoric acid, 75% ml 10 Sodium thiosulphate gms.

Igepal D1710 (Amara Chem. Co.) ethoxylated dinonyl phenol gms- 0.5 Water, to make 1 litre.

Ultrol 5 (commercial white mineral oil) added to obtain a layer depth of 8 mm. on the bath.

I Example XIX Manganese carbonate, 88% -gms l2 Oxalic acid do 15 Nitric acid, 69% ml 24 Sodium thiosulphate gms.. 5

Atlas Brij 35*(Atlas Powder Co.) ethoxylated lauryl alcohol gms Water, to make 1 litre.

Sun oil No. 598 added to obtain a layer depth of 8 mm. on the bath.

Example XX A manganese phosphate concentrate for phosphating steel consists of:

A bath 4 ft. x 2 ft. x 2 ft. deep holding 100 gallons requires 100 lbs. manganese dihydrogen phosphate. This is contained in 120 lbs. concentrate together with 18 lbs. hydrocarbon material and 1.2 lbs. surface active agent. 18 lbs. of the hydrocarbon material is 2.1 gallons and over 8 sq. ft. this gives an inert layer about 15 mm. thick. The bath was heated to 100 C. and degreased piston rings were inserted. After about 45 minutes the rings were removed and rinsed and it was noted that they had received a heavy phosphating coating.

Using comparable formulations, oxalate, chromate, sulphide and other coating bath concentrates may be prepared for use in the various processes disclosed in this specification.

Example XXI A concentrate composition, suitable, on dilution, for coating titanium was made up as follows:

A solution was made up as in Example XX using 100 lbs. of the above composition. Titanium wire treated in the solution at 60 C. for 10 minutes received a good coating suitable for use as a drawing aid with soap.

It will be appreciated from the foregoing examples that the invention may be used with any of the conventional type acid coating baths, e.g. phosphate, oxalate, chromate, or oxide solutions. Baths modified in the manner described herein may be utilized, as shown, in the treatment of stainless steel and other metals such as magnesium, aluminium, titanium, zirconium, zinc and cadmium, which are conventionally treated with acid coating solutions. Usual conditions of temperature and time for coating these materials may be utilized. These conditions vary over a relatively wide range depending upon the type of solution and metal being treated. However, generally speaking. treating temperatures of the order of 25 to C., and immersion times of 1 to 60 minutes are utilized.

The eflectiveness of the oil layer in reducing chemical consumption may readily be determined by heating baths prepared in the manner-described in the foregoing examples with and without the layer and checking the changes which take place in the chemical content of these baths over a period of time. Typically, a series of oxalate coating baths having the composition of, for instance, Example XV, have been prepared, one of the baths having no surface-layer and the others having surface layers of varying thickness. These baths were then heated to 70 C. Samples of these baths were removed periodically and treated with 0.05 N iodine solution. This demonstrated the effectiveness of the surface layer in retarding sulphur dioxide loss from the baths. Comparative tests of this type indicate clearly a highly significant reduction in the loss of sulphur dioxide using oil layers of at least 1 mm. thickness.

Various modifications may be made in the invention described herein without deviating from the scope thereof as set forth in the appended claims.

What we claim is:

1. In a process for the acid coating of a metal by immersing said metal in a heated acid coating solution having volatile or decomposable components therein, the improvement whereby loss of components from said solution by volatilization and decomposition is minimized, said improvement comprising immersing said metal in a hot coating solution covered with a non-aqueous surface layer of inert material which is liquid at the operating temperature, said liquid being substantially immiscible with said acid coating solution, substantially non-volatile and stable at the operating temperature of said bath and inert with respect to the constituents of the bath, both said solution and said layer having a surface active agent dissolved therein, the amount of agent in said solution being sufiicient to lower the surface tension thereof to less than 40 dynes/cm.

2. The process of claim 1 wherein said solution is an oxalate coating solution containing a sulphur dioxide liberating compound.

3. In a process for the acid coating of a metal by immersing said metal in a heated aqueous acid coating solution selected from the group consisting of phosphate, oxalate, and chromate coating solutions having volatile or decomposable components therein, the improvement which comprises initially covering the surface of a heated acid coating solution, prior to immersing any metal therein, with a layer of saturated hydrocarbon which is liquid at the operating temperature and substantially immiscible with said solution, said liquid being stable and substantially non-volatile at the operating temperature of said solution and inert with respect to the constituents of said solution, said surface layer having a viscosity of from 2-10 centipoises and a minimum thickness of 3 mm.; including in both the surface layer and said solution a non-ionic surface active agent, which is soluble in both said layer and solution, the amount of surface active agent dissolved in the solution being sufiicient to lower the surface tension to less than 40 dynes/cm., and give a clear solution at the operating temperature, said surface active agent having sufiicient solubility in said surface layer to take up any surface active agent separated out from said solution; and only thereafter immersing the metal through said layer and into said solution to prepare said coating thereon.

4. An aqueous acid coating bath consisting essentially of an acid coating solution covered with a non-aqeous surface layer of inert material which is liquid at operating temperature, said liquid being substantially immiscible with said solution, substantially non-volatile and stable at the operating temperature of said bath and inert with respect to the constituents thereof, both said solution and said layer having a surface active agent dissolved therein,

compound.

References Cited in the file of this patent UNITED STATES PATENTS Burghardt et a1. June 29, 1897 10 Neilson Jan. 7, 1941 Boyle et a1. Dec. 8, 1942 Bean -5. Dec. 25, 1945 Douty Mar. 19, 1946 Vance Ian. 3, 1950 Douty et al. July 25, 1950 Gibson Dec. 11, 1951 Burnham f May 29, 1956 Piccinclli July 23, 1957' Touches Apr. 29, 1958 FOREIGN PATENTS 7 Great Britain May 12, 1927 France Mar. 23, 1940

Claims

1. IN A PROCESS FOR THE ACID COATING OF A METAL BY IMMERSING SAID METAL IN A HEATED ACID COATING SOLUTION HAVING VOLATILE OR DECOMPOSABLE COMPONENTS THEREIN, THE IMPROVEMENT WHEREBY LOSS OF COMPONENTS FROM SAID SOLUTION BY VOLATILIZATION AND DECOMPOSITION IS MINIMIZED, SAID IMPROVEMENT COMPRISING IMMERSING SAID METAL IN A HOT COATING SOLUTION COVERED WITH A NON-AQUEOUS SURFACE LAYER OF INERT MATERIAL WHICH IS LIQUID AT THE OPERATING TEMPERATURE, SAID LIQUID BEING SUBSTANTIALLY IMMISCIBLE WITH SAID ACID COATING SOLUTION, SUBSTANTIALLY NON-VOLATILE AND STABLE AT THE OPERATING TEMPERATURE OF SAID BATH AND INERT WITH RESPECT TO THE CONSTITUENTS OF THE BATH, BOTH SAID SOLUTION AND SAID LAYER HAVING A SURFACE ACTIVE AGENT DISSOLVED THEREIN, THE AMOUNT OF AGENT IN SAID SOLUTION BEING SUFFICIENT TO LOWER THE SURFACE TENSION THEREOF TO LESS THAN 40 DYNES/CM.