US2877148A - Method of phosphate coating surfaces of metals - Google Patents

Description

March 10, 1959 R. F. DRYSDALE ETAL 2,87

METHOD OF PHOSPHATE COATING SURFACES OF METALS FiIed May 2, 1955 tion.

United States Patent METHOD OF PHOSPHATE COATING SURFACES OF METALS Royston Fraser Drysdale and Stanley Arthur Charles Burton, 'Croydon, England, assignors to The Walterisation Company Limited, Croydon, England Application May 2, 1955, Serial No. 505,430

'Claims priority, application Great Britain June 4, 1954 6 Claims. (Cl. 148-615) This invention is concerned with a process and apparatus for the surface treatment of metals and is more particularly concerned with a process and apparatus for the production of phosphate coatings on metallic surfaces by spraying such surfaces with an aqueous solution containing phosphate ions and the cations of a coating metal. In the production of phosphate coatings, solutions commonly employed have as their basic ingredient a primary orthophosphate of zinc, ferrous iron or manganese or mixtures of such phosphates. In addition it is usual for the solutions to contain an accelerator, such as an oxidizing agent, in order to cut down the treating time required to produce an acceptable coating weight; the most commonly used accelerators are nitrates which have a pronounced beneficial effect upon the speed of the phosphating process. Metallic surfaces commonly treated by the phosphating process are those of iron, zinc, aluminium, cadmium and alloys of these metals.

Hitherto the application of phosphating solutions by the spray technique has been done in a continuous manner, e. g. by subjecting the surface to be coated to the action of a spray for a continuous period which is generally for accelerated solutions in the region of 50-80 seconds. According to conventional practice, the surface to be sprayed is passed on a conveyor belt or the like through a tunnel or chamber provided with a directional spray system, the rate of passage of the surface through the tunnel being controlled to give the requisite total spray treating time. In systems of this type theused solution which drains off the surface under treatment is recovered, usually filtered, and then resprayed. The spray phosphating treatment is in practice preceded by spray cleaning and water-rinsing operations and is fol lowed by spray water-rinsing and chromate-rinsing operations.

It has now been discovered that substantially improve ments are obtained in a spray phosphating process if the application of the spray to the surface under treatment is carried out in an intermittent manner, by which is meant that the surface is subjected to an initial spray application and is thereafter maintained out of impingement contact with the spray, such cycle of operation being repeated at least once and preferably several times. Surprisingly enough, it has been found that when thus intermittently operating the spray process, the Weight of the coating produced is substantially greater than that produced in the same total period of time by a continuous spray, notwithstanding the fact that with intermittent operation the total period of spray application is but a small fraction of that involved in the continuous opera- Quite apart from the improvement in coating weight obtained in a given processing time, intermittent operation is more economical in that less solution is used to process a given surface area which of course leads to a saving in heat requirements since the solution is almost always employed at an elevated temperature and also in that it leads to a decreasein the'amount of used solution to be recovered and recirculated. Furthermore, since 2,877,148 Patented Mar. 10, 1959 the solution is sprayed under pressure, usually about 20 p. s. i. g. at the discharge jets, the load on the pumping system required is substantially reduced. Of course, in cases where the coating weight obtained by a conventional continuous spray operation is regarded as satisfactorily high, the intermittent process of the invention is equally advantageous in that it will produce the same coating weight in a substantially reduced processing time.

According to preferred procedure, the initial spray application is carried out for such a period and with such force that the surface, if not already chemically clean, is subjected to a surface cleaning action and is then provided with a continuous film of the solution. Using conventional spray jet pressures of about 20-25 p. s. i. g., and a conventional design of jet and spacing of jet from the work, a period of five seconds usually sulfices for the initial spray application. Subsequent spray applications are then carried out with such force and for such a period as are conducive to the provision of a substantially completely fresh film of solution upon the surface; at the conventional jet pressures etc., indicated above a period of from 1-5 seconds usually serves this purpose. With regard to the force with which the sprayed solution impacts against the surface under treatment, this normally will be chosen so as to be just sufficient to achieve the functions above stated and spray jet pressures, design of jets and spacing of jets from the work will be arranged accordingly. The solutions should not of course be sprayed with such excessive force as may have a deleterious effect upon the coating formed after a previous spraying operation. With regard to the initial spray application which may serve a surface cleaning function, it may be desirable that this be applied with greater force than the subsequent spray applications.

As regards the duration of the periods between each spray application, or in other words the quiescent periods, this is the most important variable in the process and this variable should be selected for best results by means of a few routine experiments bearing in mind the following factors. In general the optimum duration of the quiescent periods depends primarily (a) upon the elficiency of the phosphating solution (including temperature of operation) as measured by speed of coating action, and (b) upon the physical characteristics of the solution insofar as such characteristics affect the thickness of the film left by a previous spray application and the length of time the film tends to be retainedon the surface under treatment after a previous spray application. The faster the action of the coating solution the less will be the optimum duration of the quiescent period; on the other hand, the thicker the film and the longer the film tends to be retained on the surface under treatment, the longer will be the optimum quiescent period up to a maximum value determined by the speed of the coating solution. The position may be summarised by stating that the length of the quiescent period will be selected so that throughout the whole of such period there will be a chemically active film of solution upon the surface, the quiescent period being terminated either when the film ceases to be maintained on the surface or, if this occurs first, when an existing film becomes so depleted in the active ingredients as to materially reduce the speed of the coating action. In some cases it may be desirable to increase successively the length of the quiescent period after each individual spray application, the reason for this being that the speed of the coating action after the first spray application is relatively fast but thereafter usually slows down after successive spray applications. For so-called accelerated phosphating solutions, e. g. solutions which give an acceptable coating weight in 2 or 3 minutes when used by the immersion technique, it has been found in practice that the quiescent periods, for best results, range from about 5 to 20 seconds. Where equal duration quiescent periods are employed throughout the process, times of from 5-15 seconds have been found to give best results. Where, on the other hand, the length of the quiescent periods is successively increased, the first period may be about 5 seconds is duration, subsequent periods being increased gradually in duration to about 25 or 30 seconds depending on the number of complete cycles of operation, each cycle involving a spray application followed by a period of quiescence. It should be added that what is required during the quiescent periods is the absence of an impinging spray on the surface; the exposure of the surface to a mist of the treating solution during the quiescent periods does not matter and in practice such a mist will usually exist at any rate for a portion of the quiescent periods.

The number of complete cycles of operation in the improved intermittent process of the invention will of course depend on the final coating weight desired. In practice, however, an upper limit on the total time of the process is often imposed by economic considerations. Most commercial spray phosphating systems are arranged so that the actual processing time is the minimum required to produce an acceptable coating weight, this time usually varying from about 50 seconds to 80 seconds when accelerated solutions are employed. If a total processing time of, say, about one minute is aimed at, the total number of cycles of operations in the intermittent process of this invention will, for accelerated solutions, usually vary from about 3 to 8 complete cycles depending .of course upon the duration of the quiescent period of each cycle and to a lesser extent, because this is usually very short, upon the duration of the spraying periods.

While the intermittent process of the invention is of advantage as applied to any phosphating solution capable of forming a coating when in the form of a sprayed film on the metal surface, it-is contemplated that its principal application is to accelerated phosphating solutions, particularly those based on zinc dihydrogen phosphate and containing nitrates and/or simple fluoride ions as accelerating agents and those based on manganese dihydrogen phosphate together with nitrates. Preferred solutions for use in the process are zinc phosphate solutions containing both nitrate and simple fluoride ions. Solutions based on ferrous dihydrogenphosphate are less preferred since when sprayed they tend to undergo oxidation which results in the formation of insoluble ferric phosphate; this tendency towards oxidation may however be counteracted to some extent by including a reducing agent in the solution. Usually the solutions will be employed at elevated temperatures of around 60-70" C.

The outstanding improvements provided by this invention will be appreciated from a consideration of the results of the following comparative tests in which sample plates of clean CRCA steel were spray phosphated in a continuous and then in an intermittent manner with a 20 points strength solution made up by mixing 97 parts of water with 3 parts of a concentrated aqueous phosphate solution containing 250 gms./litre P 260 gms./litre N0 and 201.6 gms./litre of zinc, such concentrated solution containing also 55 gms./litre of zinc fluoride. In the first test, the clean sample plate was continuously spray phosphated for a period of one minute, using a solution temperature of 70 C. and a conventional jet pressure of 20 p. s. i. g. At the end of the spraying treatment, the sample plate was quickly washed free of 'solution and the coating weight was determined as 0.028 gram per square foot. 7 the same manner except that the sample was subjected .to six cycles of operation, eachcomprising a, one second spray period and a 9 second quiescentperiod, the coating weight was determined as 0.4 gram per, square foot which represents a fourteen-foldincrease in the coating Weight produced by the continuous operatipn. further. tests 'l means will usually be provided for heating and mainasr mes In the second test, carried out in I cases.

i which were undertaken to compare the efficiency of the intermittent spray operation with a conventional immersion operation surprisingly indicated that the former operation produced a coating weight as great as the latter. These further tests were carried out under comparable conditions, the immersion operation, which in fact produced a coating weight of 0.403 gram per square foot, being carried out at 70 C. with the same solution and with an immersion time of one minute.

As indicated above, the improvements obtainable by the intermittent process of the invention are not restricted to any particular type of phosphating solution and examples of various solutions which may be employed in the process are tabulated below, quantities given being expressed in grams per litre of aqueous solution.

The above-quoted solutions are not as fast in operation as that employed in the experiments described above and therefore for the obtainrnent of a given coating weight a greater number of cycles of operation of the intermittent process are required.

A particular commercial application to which the process of the invention opens the way is the coating of steel articles prior to deep-drawing, particularly the coating of steel work-pieces to be formed by deep-drawing into shell Hitherto in such deep drawing operations it has been thought necessary to employ an immersion operation to provide phosphate coatings of a sufiiciently high weight per unit area; however, as evidenced by the experimental data quoted above, it is possible by the interrnittent process of the invention to obtain coating weights of the same order of magnitude as those obtainable by the immersion technique so that the process combines the obvious advantages of high coating weight and ease .of rapidly processing a continuous series of work-pieces.

The invention also provides novel apparatus for carrying out the intermittent spraying process as described above and such apparatus in one form thereof comprises supporting means for the work to be sprayed, spray forming means, means for supplying liquid under pressure to the spray forming means, and automatic spray control means whereby the work may be subjected to a spray of the liquid in a predetermined intermittent manner. Preferably, with such form of apparatus the articles are supported in a continuous line by a conveyor such as a belt or moving monorail, and are continuously moved through the apparatus, the spray control means being timed to produce the desired lengths of spray and quiescent periods.

Alternatively, apparatus for carrying out the intermittent spray process may comprise conveyor means for supporting and continuously moving a succession of work-pieces through the apparatus, spaced spray-forming means, and means for continuously supplying the liquid under pressure to the spray-forming means, the

number and arrangement of the spray-forming means beingsuch as to provide at least two narrow and welldefined spray zones spaced apart from each other. Preferably the first of the spaced spray-forming means is arranged so that the spray zone produced thereby is several times as long as that of succeeding spray zones .and the conveyor means is arranged to travel at a constant-speed. Also, it will be desirable to provide baflles or the like to limit the zones of spray application proare used.

In each of the forms of apparatus described above,

taining the liquid at a predetermined temperature as well as means for recirculating previously sprayed liquid.

A preferred form of apparatus according to the invention will now be described by way of example with reference to the accompanying schematic drawings in which:

Fig. 1 is a side view of a spray-phosphating plant;

Fig. 2 is a cross-section of the tunnel shown in Fig. 1 along the line II-II;

Fig. 3 is a view of a spray control valve with cover removed, and

Fig. 4 is a cross-section along the line IV-IV of the valve shown in Fig. 3.

The plant of this embodiment is intended to operate with an initial S-second spray application followed by a number of cycles each consisting of a l-second spray application followed by a nine-second quiescent period.

Referring now to Figs. 1 and 2, flat, steel articles 1, 1. suspended from an overhead conveyor 2 are arranged to travel slowly through the tunnel. The tunnel comprises a primary alkali spray cleaning section 6, a hot water spray rinse section 8, a phosphating spray section 3, and a furtherhot water spray rinse section 11, the tunnel thereafter including other sections, e. g. for drying and painting, which are not relevant to this invention. A fume trap 4, with an exhaust vent 5 is located at the entrance of the tunnel, and the spray sections are separated from each other by fume traps 7, 9, and 12. Each spray section is provided with an electric motor driven centrifugal pump 13, 14, and 16 having pressure gauges 17, 18, 19 and 20, and is also provided with thermostatically controlled heating coils to maintain the various liquids at the requisite temperature.

Considering the phosphating section 3 in more detail, on either side of the conveyor 2 extending downwards from the level thereof are two sets of spraying nozzles 21 and 22 respectively. The pump 15 is arranged to continuously deliver, under a pressure of about 20 lbs. per sq. in., phosphate solution drawn from a reservoir 3a in the base of this section, through nozzles 21 while the supply of solution under the same pressure to nozzles 22 is controlled by an automatic valve 23.

In this embodiment it is proposed to operate the spray phosphating section by firstly subjecting the articles to a continuous spray of solution for about 5 seconds, followed by a succession of spraying periods of about one second duration separated by a nine-second quiescent period. The conveyor is arranged to travel at a speed of 18 inches per minute this being a convenient speed for conveyors and apparatus of this type. Under these conditions, the width of the spray zone produced by nozzles 21 should be about 1 /2 inches to produce a fivesecond preliminary spraying period. If the width of the spray zone produced by nozzles 22 is arranged to be 12 inches the articles, during their passage therethrough, will be subjected to nine cycles, each consisting of a one second spray period and a nine-second quiescent period.

Referring to Figs. 3 and 4 which show in detail the automatic valve 23 which is arranged through reduction gearing to be driven at 6 R. P. M. by the electric motor which drives the pump 15. A' disc-like plate valve 25, housed in a valve body 24, is non-rotatably secured to a driving shaft 26, such valve plate 25 being seated upon 'a valve face 27. A cover 28 with a liquid supply connection 29 is secured to the body 24 by screws threaded into tapped holes 30. The valve plate 25 has a valve port 31 which coincides with a similar delivery port 32 in the valve body 24 for one-tenth of a revolution of the valve plate in the valve body. Liquid under pressure from the pump 15 is supplied to connection 29 and enters space 33 above the valve plate 25; since the valve plate rotates at 6 R. P. M. and the ports are open for onetenth of a revolution the liquid is allowed to flow through the valve for one second and is stopped for 9 seconds.

As an additional safeguard against leakage, a gland 34 is fitted, although in fact little liquid should leak past the seating surfaces of the valve plate and body to the shaft.

The valve shown in Figs. 3 and 4, pump 15 and nozzles 22 are suitable for intermittently spraying stationary articles, the duration of the spraying periods being equal likewise that of the quiescent periods. Alternatively, the valve may easily be modified so that in operation it will give a succession of predetermined spraying periods of different duration if desired, for one revolution of the valve plate 25 and still be applicable to the spraying of individual articles not moving and mounted on a conveyor as above described. Such modification would include the provision of a number of spaced ports 31 around the plate 25 instead of only one and the driving of the plate 25 at a much slower speed.

The tunnel described above with reference to Figs. 1 and 2 of the drawings may easily be adapted according to the second aspect of the invention by discarding the automatic valve 23, extending the phosphate spraying section 3 of the tunnel, arranging nozzles 22 at spaced intervals along the conveyor within the phosphating section 3, and increasing the conveyor speed so that in operation any one point on the conveyor takes about one second to pass through a spraying zone and nine seconds to travel from one spraying zone to the next. Preferably in this latter embodiment the spacing of the spraying nozzles 22 are of different extent so that the quiescent period is progressively longer. Also bafiles are provided to limit and more positively define the spray zones and quiescent zones.

In a further modification of the apparatus shown in the drawings, a tripping device may be fitted whereby an article 1 on conveyor 2 entering the phosphating zone is arranged to control each series of spray and quiescent periods. Such device may comprise a mechanical trip lever or a beam of light focussed on a photo-electric cell and arranged to be interrupted by the articles 1.

Two types of control systems operated by the tripping device are envisaged: in the first type, the tripping device, which is arranged to reset itself automatically after a predetermined interval of time, is arranged to open a hydraulically balanced valve in the solution delivery pipe between the valve 23 and the nozzles 22. In the second type of control system, the tripping device is arranged to initiate each series of spray and quiescent periods for each article in question and such system may comprise for example, a disc or drum whose movement is initiated by the tripping device, the disc or drum being provided with projections thereon operative to open and shut a hydraulically balanced valve in the solution delivery pipe. In this latter system the automatic valve 23 will of course be dispensed with.

As thus far described, the intermittent spray process of this invention has been particularly described in relation to a phosphating treatment. The process has, however, a quite general application to other operations involving the spraying of metal surfaces with liquid compositions which react therewith and produce a physical and/ or chemical change in the nature of the surface. Examples of such other operations include the following, viz. the production of protective coatings by means of the chemical action of solutions containing the acid radical of aliphatic or aromatic dicarboxylic and hydroxycarboxylic acids, such as oxalic, malonic and tartaric acids; solutions containing aromatic sulphonic acids; and solutions containing chromates.

We claim:

1. A method of treating a metal surface which comprises applying to the surface, by the impingement contact of a spray thereon, an initial film of a liquid composition which reacts therewith to produce a change in the nature of the surface, maintaining the thus sprayed surface out of impingement contact with the spray, and in a substanass- 1 as tially ,nndisturbedsstate, rm- ,a period sufficient to permit reaction ofthe 'film with the surface, repeating this sequence of operations at least once to maintain on the surface a reactive film of the same nature as that of said initial film during each interval between successive spray applications, and controlling the duration of each spray application so that the total period of time between the first and last spray applications during which the surface is out of impingement contact with the spray is several times the total period of time during which the surface is in impingement contact with the spray.

2. A method according to claim 1, wherein said sequence of operations is repeated at least twice.

' 3. A method of treating a metal surface which comprises applying to the surface, by the impingement contact of a spray thereon, an initial film of a liquid phosphating composition which reacts with the surface to form thereon an adherent phosphate coating, maintaining the thus sprayed surface out of impingement contact with the spray, and in a substantially undisturbed state, for a period sufiicient to permit reaction of the film with the surface, repeating t-hissequence of operations at least once to maintain on the surface a reactive film of the same nature as that of said initial film during each interval between successive spray applications, and controlling the duration of each spray application so that the total period of time between the first and last spray applications during which the surface is out of impingement contact with the spray is several times the total period of time during which the surface is in impingement contact with the spray.

4. A method according to claim 3, wherein said sequence of operations is repeated at least twice.

5. A method according to :claim 3 whereinsaid SQ? quence of operations-is repeated at least three times.

6. A methodof treating a metal surface which comprises applying to the surface, ,bythe impingement-contact of a spray thereon, an-initial film of a hot aqueous solution which contains zinc dihydrogen phosphate and an accelerator and which is reactive with the surface to form thereon an adherent phosphate coating, maintaining the thus sprayed surface out of impingement contact with the spray and in a substantially undisturbed state for a period of time, inthe range from 5 to 20 seconds, sufficient to result in a phosphate coating, repeating this sequence of operations at least twice to maintain on the surface a reactive film of said hot aqueous solution during each interval between successive spray applications, and controlling the duration of each spray application so that the total period of time during which the surface is in impingement contact with the spray is less than the total time between thefirst and last spray application during which the surface is out of impingement contact with the spray.

References Cited in the file of this patent UNITED STATES PATENTS 2,245,609 Romig June 17,1941

2,470,580 Schneider May 17, 1949 2,505,179 Gaythwaite Apr. 25, 1950 2,552,874 Snyder et al May 15, 1951 2,554,139 Drysdale May 22, 1951 FOREIGN PATENTS 52,942 France July 10, I944 (Addition to No. 882,041)

Claims (1)

1. A METHOD OF TREATING A METAL SURFACE WHICH COMPRISES APPLYING TO THE SURFACE, BY THE IMPINGEMENT CONTACT OF A SPRAY THEREON, AN INITIAL FILM OF A LIQUID COMPOSITION WHICH REACTS THEREWITH TO PRODUCE A CHANGE IN THE NATURE OF THE SURFACE, MAINTAINING THE THUS SPRAYED SURFACE OUT OF IMPINGEMENT CONTACT WITH THE SPRAY, AND IN A SUBSTANTIALLY UNDISTURBED STATE, FOR A PERIOD SUFFCIENT TO PERMIT REACTION OF THE FILM WITH THE SURFACE, REPEATING THIS SEQUENCE OF OPERATIONS AT LEAST ONCE TO MAINTAIN ON THE SURFACE A REACTIVE FILM OF THE SAME NATURE AS THAT OF SAID INITIAL FILM DURING EACH INTERVAL BETWEEN SUCCESSIVE SPRAY APPLICATIONS, AND CONTROLLING THE DURATION OF EACH SPRAY APPLICATION SO THAT THE TOTAL PERIOD OF TIME BETWEEN THE FIST AND LAST SPRAY APPLICATIONS DURING WHICH THE SURFACE IS OUT OF IMPINGEMNT CONTACT WITH TH SPRAY IS SEVERAL TIMES THE TOTAL PERIOD OF TIME DURING WHICH THE SURFACE IS IN IMPINGEMENT CONTACT WITH THE SPRAY.

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