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
  • C23C22/34—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 fluorides or complex fluorides
  • C23C22/36—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 fluorides or complex fluorides containing also phosphates

Definitions

• ABSTRACT A zinc phosphate coating process for coating a metal surface consisting of aluminum, iron or zinc or alloys of aluminum, iron or zinc which comprises treating said metal surface with an aqueous solution containing as its essential ingredients, (a) at least about 0.7 gram per liter of zinc ion, (b) at least I gram per liter of phosphate ion 3as PO (c) at least about 1 gram per liter of nitrate ion (calculated as NO;,), (d) between about 0.006 gram per liter and about 1 gram per liter of nitrite ion (calculated as N0 (e) between about 0.025 gram per liter and about 2.5 grams per liter of fluoride added as a combination of fluorides and bifluorides of sodium and potassium, such that the molar ratio of potassium to sodium is about 2: l and the ratio of fluoride to bifluoride is about 1:1.
• 2,487,137 which discloses solutions containing as essential ingredients, a dihydrogen phosphate, an oxidizing agent, ferrous ion and fluoride ion provided exclusively by hydrofluoric acid and its salts. These solutions are adjusted to exhibit a total acidity" and free acidity" within defined pointage ranges, these properties referring to the acidity of the solution as measured by titrating a predetermined sample against standard alkali using, respectively phenolphthaleir (about pH 9 and brom-phenol blue (about pH 4 as the end-point indicators.
• the make-up solution (a) is added to replenish the zinc and the phosphate ions depleted from the coating solution by the fonnation of the zinc phosphate conversion coating.
• Sodium bifluoride (b) is added as needed to complex and precipitate the dissolved aluminum formed during the coating process.
• the boric acid (c) is required to complex the excess fluoride.
• sodium carbonate (d) is necessary to neutralize the excessive acidity caused by the addition of the acidic sodium bifluoride.
• a further difficulty is that the excess aluminum is precipitated from solution at a relatively slow rate owing to the nature of the Na AlF precipitate so that the proportion of free aluminum ions remaining dissolved in the coating solution must be capable of removing free aluminum from the solution as fast as it enters, since the free aluminum ions have a poisoning effect on the formation of the phosphate coating.
• aqueous acidic solutions for the deposition of zinc phosphate conversion coatings on substrates comprised of aluminum or its alloys and zinc or iron, and particularly such solutions which will function even when a substantial portion of the total surface area treated is aluminum or an alloy of aluminum.
• the coating solutions of this invention comprise essentially zinc, phosphate, nitrate, nitrite and fluoride ions, so adjusted as to exhibit a certain free acidity" and total acidity" as will be more fully set forth hereinbelow.
• Solutions such as these are the so-called coating phosphate solutions, wherein the cation of the phosphate employed (in this case,zinc) is actually incorporated into the coating formed, and can be maintained by adding to the solution sufficient amounts of a concentrate of zinc ion, phosphate ion, and nitrate ion to replenish the zinc and phosphate removed by the coating and maintain the free acidity and total acidity within their respectivelydesired ranges. Maintenance of the acid concentration sufficient to produce the desired coating while avoiding excess phosphate which causes sludging is accomplished by adding nitric acid.
• nitrite ion is added, preferably for economic considerations in the form of sodium nitrite, to avoid the accumulation of iron in the coating bath when processing steel and to consume hydrogen when processing aluminum.
• Fluoride ion is added as alkali metal bifluorides of sodium and potassium whereby the undesirable aluminum ion is readily removed from solution as a dense precipitate of K,NaAlF,,.
• this invention pertains to; a process for coating metal surfaces with a phosphate coating, coating solutions and premixes used in such process, replenishing solutions and premixes for maintaining the composition of the coating solution within optimum parameters and metal products coated according to the process of this invention.
• the invention relates to coating compositions for use in treating either aluminum, iron, or zinc surfaces, which compositions comprise solutions containing as essential ingrcdients zinc ion, phosphate ion, nitrite ion, nitrate ion, fluoride ion and both sodium and potassium ions.
• the quality of the coatings fonned is as high as, if not higher than, any coatings hitherto attainable using a bath specifically adapted to coat either iron, zinc or aluminum individually.
• the addition of fluorides in the manner of this invention allows the bath to be operated successively or simultaneously with either of these metals utilizing the same bath composition. it will, therefore, be appreciated that the coating compositions as more specifically described hereinbelow, particularly as regards the nitrate, nitrite and fluoride components are especially important to the success of the process.
• this invention provides an aqueous acidic zinc phosphate solution adapted for the formation of phosphate conversion coatings on the surfaces of iron, zinc and/or aluminum, as well as alloys thereof, which in a preferred embodiment had the following characteristics:
• nitrate/phosphate weight ratio of at least about 1 to l.
• a coating composition and process having these characteristics can be established and maintained in operation to provide high quality zinc phosphate coatings of aluminum, zinc or steel.
• the solutions should contain at least 0.7 g./l. of zinc, otherwise the coatings formed, particularly on iron and aluminum, are of low weight and poor quality.
• zinc concentration there is no real upper limit for the zinc concentration save that dictated by solubility and economic considerations although very high zinc concentrations necessitate a high free acidity and that tends to give poor quality coatings.
• a zinc concentration of 20 g./l. is a practical limit and ordinarily it is unlikely that 6 g./l. need be exceeded.
• As the source of zinc for the bath, or for the preparation of a concentrate for bath make-up there can be conveniently employed zinc oxide or carbonate, through any of the other commonly used zinc derivatives may also be employed.
• the solutions of this invention must contain at least I g./l. of phosphate ion, in order to assure that the coating is of an acceptable weight and quality.
• the coating itself is concerned there is no real upper limit for phosphate, apart from that set by solubility and economic factors.
• the higher the phosphate concentration the greater the tendency for sludge formation, particularly with low quantities of nitrate, and therefore a maximum of 30 g./l. phosphate is recommended,
• the solutions of this invention operate satisfactorily with from 1 to 20 g./l. (phosphate), and the preferred range is from 1.5 to 10 g./l.
• the phosphate can be conveniently added to the bath or incorporated into a concentrate, as phosphoric acid.
• the solutions of this invention contain nitrate, which prevents the formation of tertiary zinc phosphate, nevertheless if the amount of phosphate relative to the amount of zinc is greater than about L5 to l a weight basis-l to l on a molar basis) undesired hydrolysis will occur. Accordingly, the phosphate/zinc weight ratio must be less than at least about 1.0 'to 0.6 L5 to l and preferably about 1 to l.
• a coating composition having the above listed characteristics will provide inter alia a means for avoiding sludging by replacing a part of the phosphate with nitrate in the coating bath.
• Test results given hereinafter indicate that, to maintain a bath acceptably sludge-free, the solutions must contain at points total acid (as herein defined);
• g./l. of nitrate less is not suflicient to satisfactorily prevent the formation of an unacceptable amount of sludge. It should be pointed out, however, that this g./l. minimum is not an absolute limit since some sludge formation is still encountered, even with somewhat higher amounts of nitrate but at least 1 g./l. will reduce sludge formation sufficiently to permit continuous operation without the periodic stoppages necessitated by sludge build-up in the prior art processes currently in use. While no absolute upper limit for the nitrate concentration can be established, any amount greater than about 30 g./l. would be impractical. A preferred range is from 1.5 to 10 g./l. nitrate.
• the nitrate is conveniently added to the bath or incorporated into a bath concentrate, as nitric acid. Since the nitrate used in the solutions of this invention replaced part of the phosphate, it will be appreciated that the ratio of nitrate to phosphate plays some part in determining sludge prevention.
• the test results indicate that the nitrate/phosphate weight ratio must be at least 1 to l (1.5 to l on a molar basis); less nitrate is insufficient to give an acceptable reduction in sludge formation.
• the ratio can, of course, be higher and a value of 4 to l is recommended as an upper limit; beyond this, good results can still be obtained provided the absolute phosphate concentration is kept above the minimum.
• sodium nitrite as an oxidizing agent provides a means for replenishing the coating bath without incorporating the use of a neutralizing agent such as sodium carbonate.
• a neutralizing agent such as sodium carbonate.
• the coating solution contain from 0.006 to l g./l. of nitrite ion.
• the nitrite ion will oxidize any ferrous iron in the solution (dissolved 011' an iron or iron-containing surface) to fenic iron, which can in turn react with the phosphate to form insoluble ferric phosphate which will precipitate out of solution thereby removing the undesired ferrous iron from the bath. If less than 0.006 g./l. nitrite is used the ferrous iron is not oxidized fast enough, so that the concentration of ferrous ion in the bath rapidly builds up to a level at which the bath becomes inoperative. A preferred minimum nitrite concentration is 0.03 g./l.
• the upper limit of l g./l. does not relate to the oxidizing function of the nitrite, but rather to the fact that, in the solution, which is quite acidic, large concentrations of nitrite are unstable, breaking down to form toxic oxides of nitrogen. Accordingly, a low nitrite concentration is desired.
• a preferred range for the nitrite concentration is from 0.03 to 0.3 g./l. it is, of course, theoretically possible to employ other oxidizers instead of nitrite.
• Other conventional oxidizers are for example the peroxides, chlorates, bromates and so on.
• nitrite can be conveniently added to the bath. or incorporated into a com centrate as an alkali-metal nitrite, preferably sodium nitrite. Potassium nitrite can also be used or a mixture of sodium and potassium for example in a 1:2 molar ratio can also be used.
• compositions of this invention are as indicated useful for phosphating aluminum as well as iron and zinc and their a1- loys.
• one of the problems encountered in coating aluminum is that following a brief period of operation aluminum ions are dissolved from the surface being treated. The dissolved aluminum poisons the bath significantly hindering, if not substantially preventing, further operation.
• the addition of fluoride to a zinc phosphate coating bath for treating aluminum is known to cause an etching action on the metal surface and also to form a complex with dissolved aluminum to give a soluble fluoaluminate. As a result the bath in operation will build up a measurable, and detrimental quantity of free aluminum in association with dissolved fluoaluminate.
• the solubility of the fluoaluminates depends significantly on the cation in the complex and on the crystal form which the precipitating complex can take up as it comes out of solution. Only certain cations can be made available to complex with the fluoaluminate, generally speaking, only the alkali-metal fluorides and bifluorides (and including also the ammonium fluorides) are soluble enough to remain in the solutions as simple fluorides and yet form fluoaluminates which are sufficiently insoluble to provide for removal of dissolved aluminum.
• Sodium has been particularly preferred for this purpose, not only because sodium fluorides and bifluoride are comparatively inexpensive and readily available, but also because the fluoaluminate fonned can be forced" to precipitate out in the form of cryolite, Na3AlF6, which is acceptably insoluble.
• the fluoride be added as a 1 to 2 molar mixture of the sodium and potassium salts.
• the quantity of fluoride employed in the solutions of this invention is from 0.025 to 2.5 g./1.
• a preferred lower limit is 0.1 g./l. If more than 2.5 g./1. of fluoride is employed then the bath may be in such an acid condition as to cause an undesirable etching effect. For this reason a concentration of 2.5 g./1. is ordinarily not to be exceeded since at this level the etching action is kept to an acceptable minimum while providing rapid complexing and precipitation of any aluminum in the solution.
• a preferred fluoride concentration maximum is 0.5 g./l.
• the fluoride concentration may be measured by the standard etching of glass method, or a technique such as is disclosed in US. Pat. No. 2,814,577; 3,129,148; and 3,350,284 may be employed.
• the removal of aluminum from solution according to this invention lies in the use of a mixture of sodium and potassium bifluorides as described above. Similarly there can be used a mixture of the nonnal fluorides of sodium and potassium or mixtures of nonnal fluorides and bifluorides. Such mixtures are also used throughout the operation of the bath to maintain the fluoride content within the range specified above.
• the bifluoride mixture the continued addition of such a mixture to a coating solution of the type described when used with aluminum tends to render the solution gradually more acidic, particularly in free acidity.” This increased free acidity leads to coatings of an unsuitable sort, or in the extreme case, to no coating at all.
• replenishment of the fluoride content of the zinc phosphate coating baths of this invention is preferably accomplished by the use of a combination of the fluorides and bifluorides maintaining a KzNa molar ratio of 2:1.
• the fluorides and bifluorides are preferably present in equal proportion. In this way there is achieved the desired optimum aluminum removal, with the added advantage of having no undesirable efiect of either lowering or raising the free acidity.”
• the coating solutions of this invention require a free acidity in the range of 0.2 to 5 points.
• the free acidity of a solution is defined as the number of ml. (points) of N/10 NaOH that will neutralize 10 ml. of the solution, using bromphenol blue (endpoint at about pH4) as an indicator.
• the free acidity is, as its name suggests, a measure of the free, unassociated hydrogen ion in the solution. If the free acidity of the solutions is less than 0.2 points, then the solutions are not acid enough to ef fect the initial etching of the metal surface before a coating can be formed.
• the preferred range of free acidity (FA) is from 0.3 to 1.5 points.
• the overall pl-l should preferably be in the range of about 3.0 to 4.0 (FA of 1.0 down to 0.2) especially in the pH range 3.1 to 3.3 (FA of0.8 down to 0.4)
• the solutions of this invention require a total acidity (TA) of at least 4 points.
• the total acidity of a solution is defined as the number of ml. (points) of N/ 10 NaOl-l required to neutralize 10 ml. of solution, using phenolphthalein (end point about pH 9) as an indicator.
• the total acidity is, as its name suggests, a measure of the total available hydrogen ion in the solution. In particular it includes the free acid and hydrogen ion formed when dihydrogen phosphate dissociates into monohydrogen phosphate. it is thus, as will be appreciated, a rough measurement of phosphate concentration. 1f the total acidity is less than 4 points the solutions do not contain enough phosphate for the fon'nation of a coating.
• There is no particular maximum limit set for the total acidity just as there is none for the phosphate. However, a recommended maximum is 35 points, and a preferred total acidity range is from 6 to 20 points.
• the solutions of this invention must have a total acid to free acid ratio of at least 10 to 1. 1f the ratio is less, then regardless of the absolute amounts of either quantity, the solutions will be too acid to provide a coating. There is, however, no particular maximum value for this ratio, although with a very high value the absolute quantity of free acid should be above its minimum. As a practicality though a ratio of greater than 40 to 1 is unnecessary. A preferred range of ratios is between about 12 to 1, and about 20 to l.
• Nickel is particularly useful when coating zinc surfaces (galvanized iron, for example) to promote adhesion of the ultimate paint layer to the coating, and to darken the color of the coating itself.
• the nickel ion content of the bath solution may vary from about 0.05 to about 1 g./l.
• the nickel may be conveniently added to the bath. or to a concentrate, as nickel oxide, nickel carbonate or even nickel nitrate.
• the solutions of this invention are conveniently operated at a temperature of from about 45 C. (113 F.) to about 60 F. F.), at which temperatures good coatings can be obtained with contact times of between about 15 seconds to 3 minutes. Temperatures lower than 45 C. can of course be employed though in such instances reaction time is really too long. Above 60 C. there is an increasing tendency for zinc to precipitate out as tertiary zinc phosphate (at 65 C. this becomes noticeable, and becomes even more evident as the temperature increases). The time for which any surface is allowed to remain in contact with the solution depends primarily upon the solution temperature and on the weight of coating required. Generally speaking, the more acceptable coatings are obtained at lower temperatures with longer contact times.
• the bath solutions of this invention when in operation, sufi'er depletion caused by, for example, actual use of chemicals to form the coating, the formation of sludge, i.e. iron phosphate and by drag-out on the surface being coated. Accordingly, it is periodically necessary to replenish the bath by adding the required chemicals in amounts appropriate to counteract depletion.
• the replenishment procedure for the solutions of this invention is extremely simple and easy to carry out. This in itself provides a clear advantage over prior art solutions, which require a complex replenishment procedure.
• the solutions of this invention can be replenished with three liquid concentrates regardless of the metal being treated and furthermore the addition of each of these liquid concentrates can be closely controlled by automated techniques.
• these three materials are; l) the makeup concentrate (sufficient of which is added to restore the total acidity and free acidity to the desired values); 2) nitrite ion, conveniently added as sodium nitrite (the amount required can be determined by a standard titration against permanganate); and 3) sufficient fluoride to restore the fluoride content to within the desired range.
• the makeup concentrate sufficient of which is added to restore the total acidity and free acidity to the desired values
• nitrite ion conveniently added as sodium nitrite (the amount required can be determined by a standard titration against permanganate)
• 3) sufficient fluoride to restore the fluoride content to within the desired range As specified by the ranges given above and as further indicated by the evaluations to be given with examples hereinafter, monitoring and replenishing of the bath are necessary to keep the solution in good coating condition.
• the makeup concentrate (which, as its name implies, is used to make up the original bath) contains zinc, phosphate, nitrate and nickel ions in the appropriate proportions.
• the nitrite concentrate contains only nitrite (usually as the sodium salt) conveniently at a concentration of about 2 lbs/gal.
• the fluoride mixture concentrate contains a mixture of sodium and potassium fluorides and sodium and potassium bifluorides.
• a typical fluoride concentrate contains about 0.7 1b./gal. sodium and potassium fluorides and bifluorides.
• the sodium and potassium are preferably present in a molar ratio of about 1 to 2.
• the relative amounts of fluoride and bifluoride is not critical though for best results and being adjusted in order to provide for the suitable pH and free acidity in the bath the fluorides and bifluorides are usually employed in about equal amounts.
• Such mixtures of fluorides and bifluorides of potassium and sodium can be prepared as a stable, nonlumping powder or as liquid concentrates in which the fluoride mixture is dissolved in water, such powder or concentrate is useful for makeup and replenishment of phosphating baths constitutes a part of this invention.
• the powder formed is substantially nonhygroscopic and has a long shelf life.
• a bath set up and maintained according to this invention can be used for any and all metal combinations, provided that into the basic phosphating solution, containing ions of zinc, phosphate, nickel, nitrate and nitrite,
• the metal surfaces to be phosphatized should first be cleaned, and it is preferred to clean the surfaces with an alkaline rinse. After the phosphatizing process, the coated surface is preferably given an after-rinse with a hexavalent chromium or other final rinse, to improve its corrosion resistance.
• the metal surface to be coated is precleaned with a conventional alkaline rinse at elevated temperatures, followed by a water rinse.
• the conversion coating is then formed, using a solution according to this invention and the coated surface is given a water rinse and a conventional after-rinse.
• the precleaning and after-rinse solutions can be of the type ordinarily employed in the art in connection with the formation of chemical conversion coatings on iron, zinc or aluminum.
• solution A for preparing a zinc phosphate coating bath according to the prior art was prepared as follows:
• Solution A Contents Amount in Grams ZnO 143 No, 233 H,Po,-. 75% 205 Water to a specific gravity of 1.48 at 60 F.
• such a solution can be used to treat a succession of aluminum articles or its alloys, depositing a zinc phosphate conversion coating on the surfaces thereof, provided that there is incorporated into the bath sufficient fluoride salt (say sodium bifluoride) to provide a concentration of 0.2 to 2 g./l. of fluoride.
• the purpose of the fluoride salt is to combine with the free aluminum dissolved during the coating process and to remove it from the solution as Na AlF (cryolite).
• a replenishing solution prepared according to the prior art comprising sodium bifluoride and a neutralizing agent, such as sodium carbonate.
• a convenient means for measuring the content of fluoride ion is by the simple technique disclosed in U.S. Pat. No. 3,129,148 or more preferably by the more sophisticated techniques disclosed in Pat. Nos. 3,329,587 or 3,350,284.
• Solution C was prepared by adding 0.5 g./l. fluoride to solution B prepared as above.
• the solution was used to coat a suc- (3005 alloy) by spray impingement for a period of one minute each.
• the temperature of the solution was maintained at F. during operation.
• the coating solution C was replenished with,
• sodium bifluoride to maintain the fluoride ion concentration at its initial level, as indicated by the device described in U.S. Pat. No. 3,350,824 and I 3. sodium carbonate, as a neutralizing agent to maintain the free acidity and or slightly above its initial level.
• solutions H and .1 containing higher concentrations of phosphate, exhibit greater sludging and more rapid loss of zinc than solutions K and L, which contain lower amounts of phosphate.
• EXAMPLE 3 A solution identical to coating solution K, heretofore mentioned, was prepared, and a 3 inch X 4 inch soft glass slide was immersed therein. The weight losses of the glass slide at 15, and 60 minutes was noted to be 0.4, 2.2. and 3.8 mg., respec tively. Sufficient boric acid was then added to solution K to complex the fluoride therein, (solution M) substantially no further weight loss (0.3 mg.) was noted on 3 inch X 4 inch soft glass slides immersed in the solution for periods of one hour or more. Further evidence of the presence of free fluoride in the case of coating solution K on the one hand.
• EXAMPLE 4 o 2 4 l r E 2 a z
• the following zinc phosphate coating solution base was F 2 2 3 re ared: o 2 l 4 p p Gals. Lbs '1 by WI Z- ox'de 1.359 11.74
• a 2% percent (volume/volume) bath solution was prepared 40 zi :idc 0098 0'84 from each concentrate. 15% Phosphate acid 0. we 2.l8l 18.84 25 ml. of each of the solution concentrates were diluted, 0172 Base 2 0.001 0.0a: 0.72 respectively and individually, to l liter and there was added 75% phusphom and 0.5 g./l.
• mixture R is such that the ratio of fluoride and bifluorides is 1:1; and the ratio of potassium to sodium is 2: l.
• Mixture R is prepared as follows:
• a succession of cleaned aluminum panels (3003 alloy) was subjected to impingement with solution S, heated to F. by spray for a period of one minute each.
• the coating solution S was maintained in total acidity and fluoride concentration in a manner similar to that described in example 1, using solution F, heretofore described, to maintain the total acidity and mixture R to maintain the fluoride content, conveniently measured by the device described in US Pat. No. 3,350,284.
• the nitrite content was determined by a titration against 0.042 Normal permanganate, 25 cc. of solution S, requiring 3.1 ml. of permanganate, and replenished to maintain this concentration of nitrite, or slightly less, with a solution of sodium nitrite. Continuous evolution of volatile nitrous acid was detectable.
• the solution S was able to successfully coat a long succession of aluminum panels, the essential parameters of the test being as follows:
• EXAMPLE 6 coating weight was determined by measuring the weight loss of a sample specimen after chemically removing the coating. The coated panels were subjected to the following series of tests:
• Base W 0.33 Comprised of: potassium fluozirconate 18.8% by wt. potassium fluoborate 6
• All chemicals should be tree-flowing powders or granules.
• EXAMPLE 7 Spray Application Good coatings were obtained on aluminum steel and galvanized surfaces when the coating solution was applied by spray application according to the following procedure.
• anhydrous sodium fluoride OJ 1 I l L! anhydrous
• the caustic soda solution was prepared by dissolving 2 lbs. of caustic soda per gallon of water.
• the work was cleaned using a suitable alkaline cleaner. If the work is heavily soiled and additional cleaning power is required a detergent cleaner additive may be added to the alkaline cleaning bath.
• Maintenance of the Bath The bath was manually controlled in plant by a Pointage, A Free Acid, :1 Nitrate Titration and Fluoride Reading using a fluoride activity meter such as the commercially available Llneguard Meter No. 101A.
• the other components of the bath were monitored either electronically or chemically. chemical monitoring can be carried out as follows:
• Acid Ratio Determination Determine the acid ratio by'dividing the pointage (Tptal Acid) by the Free Acid. For optimum results, the acid ratio should not fall below 12. if the acid ratio is too high, but pointage is normal and marginal coatings are produced, a portion of the bath is dumped and sufficient water and bath makeup solution are added to restore the bath to proper pointage. If the acid ratio is low, but pointage is nonnal and marginal coatings are produced, the nitrite content is increased. 4. Nitrite Test and Titration a. Dip a strip of ferrous iron test paper No. 2005 into a sample of the bath. If a blood-red color develops, ferrous iron is in the bath, and there is a b. of nitrite solution. The bath is replenished with the starting concentration of nitrite solution and the test is repeated.
• Replenishment Add approximately 3 fl. oz. of NaNO solution per 100 gallons of bath for each milliliter lacking. Whenever a portion of the bath is discarded or lost by sludge removal or leakage, the volume should be restored with the same proportion of chemicals and water as used in the original bath. Bath strength is best maintained using a volume-regulating feed pump.
• the fluoride content (active fluoride component) was determined by using Lineguard Meter No. lOlA (described in US. Pat. No. 3,350,284).
• the Lineguard Meter No. l0lA reading showing the initial fluoride activity in the bath was recorded and served as basis for comparison with subsequent fluoride acidity readings/concentration to determine the amount of fluoride replenisher needed for continued operation.
• Additional component for inclusion with zinc phosphate coating bath Flouridc concentrate (alkali nitrito.
• the treated metal is given a water rinse and an acidulated rinse.
• Coated materials coming from the final acidulated rinse should be dried as soon as possible in an indirectly fired oven or by other means which will not contaminate the metal with fumes, oil, or partially burnt gases. In many cases, heavyguage metal will retain enough heat to dry completely and rapidly without using an oven.
• compositions and processes of this invention are readily adapted to conventional power-spray processing equipment.
• the equipment for the coating stage is ordinarily constructed of stainless steel. All other stages may be constructed of mild steel. All heated tanks are preferably equipped with steam plate coils and side heating (preferred for a more even temperature distribution) or other heat sources capable of rapidly heating the bath to the specified temperature.
• a phosphate coating solution capable of producing paint receiving coatings on surfaces of iron, zinc and aluminum wherein the coating solution is an aqueous acidic solution consisting essentially of water, at least about 0.7 gram per liter of zinc, at least about 1.0 gram per liter of phosphate (calculated as P0 at least about 1.0 gram per liter of nitrate (calculated as N0 between about 0.006 and about l.0 gram per liter of nitrite (calculated as N0 between about 0.025 and about 2.5 grams per liter of fluoride (calculated as F), at least about 0.005 gram per liter of sodium, and at least about 0.017 gram per liter of potassium; such that the ratio of nitrate to phosphate is at least about l:l and the ratio of zinc to phosphate is greater than lzl and wherein the potassium to sodium molar ratio is about 2: l and the fluoride is present as a result of adding a mixture of fluorides and bi
• a stable, nonlumping, substantially nonhygroscopic powder consisting essentially of anhydrous NaF and about I part by weight of anhydrous NaF-HF per part of NaF and about 2 parts by weight of each of anhydrous KF and anhydrous KF'HF per part of NaF.
• a method for producing zinc phosphate coatings on a metal surface consisting of aluminum, iron or zinc or alloys of aluminum, iron or zinc which comprises treating said metal surface with an aqueous solution containing as its essential ingredients, (a) at least about 0.7 gram per liter of zinc ion, (b) at least about I gram per liter of phosphate ion (calculated as PO.
• a method according to claim 5 wherein the metal surface being treated is aluminum or an alloy of aluminum and the solution is maintained substantially free of dissolved aluminum by maintaining the solution content of fluorides and bifluorides of sodium and potassium such that dissolved aluminum is precipitated from solution as K NaAlF 7.

Landscapes

• 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)

Applications Claiming Priority (5)

Publications (1)

Family

ID=27507750

Family Applications (1)

Country Status (5)

Cited By (23)

Families Citing this family (3)

Family Cites Families (2)

• 1968
  • 1968-11-13 US US775517A patent/US3619300A/en not_active Expired - Lifetime
• 1971
  • 1971-07-09 FR FR7140182A patent/FR2159181B1/fr not_active Expired
  • 1971-11-08 GB GB5185571A patent/GB1324460A/en not_active Expired
  • 1971-11-09 BE BE775093A patent/BE775093A/xx not_active IP Right Cessation
  • 1971-11-09 DE DE2155670A patent/DE2155670C3/de not_active Expired

Also Published As

Similar Documents

Legal Events