Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/07—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing phosphates
  • C23C22/08—Orthophosphates
  • C23C22/12—Orthophosphates containing zinc cations
  • C23C22/16—Orthophosphates containing zinc cations containing also peroxy-compounds

Definitions

• This invention relates to the art of applying phosphate coatings to ferrous surfaces and especially to the production of coatings of this type which are to be used as a base for paint or the like. Typical instances are the coating and finishing of automobile bodies and fenders; refrigerator, radio and other cabinets; washing machine parts, etc.
• Oth objects of the invention are to provide a process of the character described which will produce a firmly adherent coating so that the subsequently applied final finish of paint or the like is securely anchored: to produce a coating which will increase the life of the final finish under all of the various corrosive influences which may be encountered in service; to produce a coating which is chemically inert toward the constituents of the usual final organic finish of paint, as well as chemically stable toward the corrosive agents normally met with; to produce a coating which is sufllciently smooth so as not or out,
• My invention makes this possible and at the same time provides a process which can be operated practically continuously onv a twenty-four hour a day basis without interruptions due either to chemical or mechanical difliculties or limitations.
• Another object of my invention is to provide a phosphate coating process which, if desired, can be employed without the necessity of heating the coating solution so as to avoid the cost and other disadvantages incident to the employment of heating elements.
• the invention is intended to provide a process of the character described which will produce a coating with great rapidity. As a practical matter, this means that in order to meet the demands of modem mass production methods, the coating treatment should gennated.
• a concentrated solution may be prepared by dissolving zinc oxide in 75% phosphoric acid according to the-following formula:
• This concentrated solution may be diluted with water to the degree required or desired within the compass of the following instructions.
• the diluted solution of Formula No. 1 may be brought to a condition ofsupersaturation by the addition of a chemical capable of raising the pH of the solution. Any chemical otherwise harmless to the solution and capable of neutralizing phosphoric acid in the bath may be used for this purpose.
• a chemical capable of raising the pH of the solution Any chemical otherwise harmless to the solution and capable of neutralizing phosphoric acid in the bath may be used for this purpose.
• sodium hydroxide, zinc carbonate and sodium carbonate have proven satisfactory as they are inexpensive, harmless to the solution, and readily available.
• the degree of supersaturation desirable will be further discussed below, but it will be pointed out here that the supersaturated solution of zinc phosphate referred to above spontaneously hydrolizes, and if nothing is done to restore the supersaturation hydrolization will continue until the solution reaches a state of equilibrium.
• the pH of a supersaturated solution is higher than that of the solution in equilibrium at the same temperature; that is, the spontaneous hydrolization of the solution is accompanied by a fall in pH.
• the difference between the pH of a supersaturated solution and that of the fully'hydrolized solution is a convenient index of the degree of supersaturation.
• the solution must be -applied to the surface of the work to be coated at a temperature substantially below the boiling point and preferably below 190 F. Indeed, it is desirable to employ the solution at temperatures even considerably below 190 F., especially where, among other things, reduction in sludge is an important consideration.
• the maintenance of a substantially constant working temperature has been found to give the most satisfactory results. Ordinary living or room temperatures will often produce the best results, provided certain factors are properly adjusted in accordance with the instructions to be given below. Incidentally I might say that I have found it to be'practically impossible to maintain'the necessary degree of supersaturation at temperatures higher than approximately 190 F.
• the solution should also contain hydrogen peroxide in an amount and for the purpose to be discussed more fully below.
• composition of a coating solution which will produce the desired type of coating in a short period of time is greatly affected by a variety of factors no one of which can be materially altered without affecting others.
• The-pH at which the solution will coat satisfactorily varies inversely with the temperature and the amount of zinc. But the pH of the solution at equilibrium varies in substantially'the same way, decreasing with an increase in temperature or an increase in zinc phosphate in the bath and vice versa. Accordingly, while the actual pH necessary to produce satisfactory coating action will vary with the temperature, the zinc content and other ingredients of the zinc phosphate bath, the amount which the pH should underlain with a dark oxide layer and to be.
• the amount of hydrogen peroxide may be varied greatly. Any detectable amount produces some improvement in the coating action, but under ordinary commercial conditions, as much as .004% is required to produce a satisfactory coating within the time allowed. Some added hydrogen peroxide continues to improve the results, but equal added increments have successively less effect until a point is soon reached where no further improvement is obtained by increasing the amount of hydrogen peroxide, although still greater amounts may beused with out noticeable impairment of the results.
• the amount of peroxide which can be tolerated without bad effects varies with the temperature at which the solution is used, the amount of zinc phosphate in the solution, and the amount the pH is raised above equilibrium. For example, when coating ferrous surfaces, with a solution containing .865% zinc and neutralized above .02%
• passable coatings may be produced with as much as .096% while if the pH is only 2.75 on the one handor 3.05 on the other, the limit for hydrogenperoxide is about 0.75%, other conditions being the same.
• a solution containing 289% zinc and neutralized to approximately the same extent above equilibrium, that is to a pH 3.13 to 3.43, will tolerate only about .045% hydrogen peroxide.
• a solution with the same amount of zinc and at a. temperature of 175 F. and neutralized to a pH of from 2.32 to 2.62 tolerates only in the neighborhood of .03% hydrogen peroxide.
• the upper limit for hydrogen peroxide is not much
• the process may be employed by immersing the work in the solution, but the preferred method is to spray the solution onto the work,'allow it to run off, collect, recirculate and respray.
• Too little zinc in the solution results in little coating action, so that for the commercial- 1y desirable coating action within one minute, at least .1% of zinc in the solution is desirable. While larger amounts of zinc increase-the coating efiect somewhat and allow larger amounts of hydrogen peroxide to be used without exceeding the beneficial amount,.the greater zinc content results in greater deposit of sludge and is more wasteful of chemicals.
• the hydrogen peroxide in the solution may be added as such, or may be generated within or without the solution, by any means well known to the chemist.
• the hydrogen peroxide may be generated by adding to the solution sodium peroxide, barium peroxide, zinc peroxide, sodium perborate, sodium percarbonate, sodium perphosphate, sodium persulphate,
• any of the above reagents causes an undesired disturbance in the pH of the solution, such disturbance may be avoided by adding along with the reagent or separately,
• Hydrogen peroxide likewise spontaneously decomposes in the solution into oxygen and water, although this process is rather slow at the pHs and temp atures prevailing in my solutions.
• the losses ,ustreferred to in zinc phosphate and hydrogen peroxide must be replaced continually if the bath is to be maintained at its original concentration with respect to these ingredients.
• the hydrogen peroxide may be added to the solution as such, or may be introduced in the form of any chemical, otherwise harmless to the solution, which iscapable of liberating hydrogen peroxide when introduced into the solution.
• any chemical otherwise harmless to the solution, which iscapable of liberating hydrogen peroxide when introduced into the solution.
• a number of examples of such chemicals have been given hereinbefore.
• heating coils In general the cooler the solution the less will be the formation of sludge. Furthermore, whatever sludge is formed is of a different character
• the elimination of heating coils likewise eliminates another serious disadvantage which is otherwise present. Owing to the high temperature of the surface of the heating coils themselves, they rapidly become coated with an adherent, heat-insulating scale which so interferes with the passage of heat into the solution that they must be cleaned at comparatively frequent intervals if the proper temperature of the solution is to be maintained. Moreover, this local heating tends to cause the precipitation of zinc phosphate, with a consequent loss of zinc, and furthermore, a loss of supersaturation.
• the parts to be coated should preferably be cleaned before being subjected to the action of the solution.
• This may be accomplished by any of the well known conventional means, but the type of cleaning known as emulsion cleaning is preferred.
• alkali cleaning when compared to emulsion clean- I ing seems to increase the amount of neutralizing than that which is customarily encountered when sludge produced when the solution is cold avoids plugging of the nozzles and piping and makes it possible easily to flush it out of tanks and pipe lines with a stream of water and eliminates the necessity for mechanical removal of sludge by hammering, chipping, etc.
• the absolute quantity of sludge formed is greatly reduced with the cold solutions of my invention.
• the method is capable of variations as to the amounts of the various ingredients employed in the solution, and the process may be used by immersion or spraying and at any temperature be 'low about 190 F. But it has been pointed out that the spray method is generally found most convenient; that emulsion cleaning is preferable; that practical consideration limits the concentration of zinc phosphate in the solution, the degree of neutralization and the amount of hydrogen peroxide content.
• a neutralizing agent should be used in an amount exceeding to some extent that necessary'to neutralize 'all phosphoric acid more than that required by calculation to form Zn(I-I2PO4)z in the materials added for replenishment.
• reaction may be calculated as follows:
• caustic in addition to that calculated by the above formula gives beneficial results, it has been noted' that it has been preferred, under the commercial conditions so far encountered in the use of this process by spraying, to employ the caustic in an amount not varying more than 20% above or below 1.04 lbs. to the gallon of the concentrated solution given above in which there is 1.2 lbs. 'zinc and 5 lbs. phosphoric acid.
• the method which consists in subjecting the surface to a solution of acid zinc phosphate containing hydrogen peroxide and maintaining the operating pH of the solution by substantially continuous addition of a neutralizing agent for phosphoric acid, the solution being applied at a temperature not above 190 F.
• ferrous surface which consists in spraying on the surface a solution of acid zinc phosphate containing hydrogen peroxide and maintaining the pH of the solution above equilibrium to produce on said surface a paint bonding phosphate coating within one minute, the solution being applied at a temperature not above'190" F.
• the method of coating 8. ferrous surface which consists in subjecting the surface to a solution of acid zinc phosphate containing from .004% to 024% hydrogen peroxide and having an operating pH at least .1. above equilibrium, the solution being applied at a temperature not above 190 F.
• the method of coating a ferrous surface which consists in subjecting the surface to a solution of acid zinc phosphate containing hydrogen peroxide and having a pH above that of equilibrium, and replenishing the solution with materials including ingredients which will replenish the hydrogen peroxide and the acid zinc phosphate and including a neutralizing agent for phosphoric acid in the bath, the ingredients of the replenishing material being selected and proportioned so that the pH of the solution is maintained more above equilibrium than it would be maintained if replenished with pure zinc dihydrogen phosphate and maintaining the solution at a substantially constant temperature not above 190 F.
• the method of coating 9. ferrous surface which consists in subjecting the surface to a As will be seen, this cal-' of coating :3. ferrous surface in the solution, that include as their chief coating chemicals the-ingredients of zinc phosphate, and that include neutralizing material for phosphoric acid in the bath in a quantity greater than the quantity having a neutralizing effect equivalent to the pounds of caustic soda given by solution of acid zinc phosphate containing hydrogen peroxide and having a pH above that of equilibrium, and replenishing the solution with materials that replenish the hydrogen peroxide the following expression: pounds NaOH equals (.4xlbs. rnPoo minus (1.2xlbs. Zn) in which the pounds H3PO4 equals the weight of the P04. compounds in the materials calculated as HaPOi, and pounds Zn" equals the weight of the zinc in the added materials calculated as metallic zinc, and applying the solution at a temperature not exceeding 190 F.
• the method of coating a ferrous surface which consists in subjecting the surface to a solution of acid zinc phosphate containing hydrogen peroxide and having a pH above that of equilibrium, and replenishing the solution with materials that replenish the hydrogen peroxide in the solution, that include as their chief coating chemicals the ingredients of zinc phosphate, and that include neutralizing material for phosphoric acid in the bath in a quantity within 20% plus or minus of the quantity having a neutralizing effect equivalent to the pounds of caustic soda given by the following expression: pounds NaOH equals (Axlbs. H3204) minus (.8 Xlbs. Zn) in which the pounds HaPO4" equals the weight of the P04 compounds in the materials calculated as HaPO4, and pounds Zn" equals the weight of the zinc in the added materials calculated as metallic zinc, and applying the solution at a temperature not exceeding 190 F.
• the method of coating aniron or steel surface which consists in spraying the surface with a solution of acid zinc phosphate containing hydrogen peroxide, and replenishing the solution with materials that replenish the hydrogen peroxide in the solution, that include as their chief coatin chemicals the ingredients of zinc phosphate, and that include neutralizing material for phosphoric acid in the bath in a quantity within 20% plus or minus of the quantity having a neutralizing effect equivalent to the pounds of caustic soda given by the following expression:
• pounds NaOH equals (.4Xlb. HaPOO minus (.8Xlb. Zn) in which the pounds HaPOi" equals the weight of the P04 compounds in the materials calculated as H3PO4, and "pounds Zn” equals the weight of the zinc in the added materials calculated as metallic zinc, and applying the solution at a temperature not exceeding F.
• a supersaturated acid zinc phosphate solution for coating ferrous surfaces in less than ten minutes said. solution containing from .l% to .5% zinc and from .004% to .024% hydrogen peroxide.
• An acid zinc phosphate solution for coating ferrous surfaces said solution containing from .1% to .5% zinc and from 004% to .024% hydrogen peroxide and having a pH from .1 to .4 above GERALD c. ROMIG.

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• Chemical & Material Sciences (AREA)
• General Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Chemical Treatment Of Metals (AREA)

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Cited By (9)

• 1941
  • 1941-01-22 US US375448A patent/US2326309A/en not_active Expired - Lifetime
  • 1941-12-11 CH CH245689D patent/CH245689A/de unknown
• 1946
  • 1946-05-11 ES ES0173545A patent/ES173545A1/es not_active Expired

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