Liquid concentrate composition for preparing phosphating solutions containing manganese

Abstract: This invention relates to a method of coating metal surfaces including zinc-coated steel with manganese phosphate crystals for the purposes of improving paint adhesion, corrosion resistance, and resistance to alkali solubility. Potassium, sodium, or ammonium ions present as a phosphate salt are combined with manganese ions in relative proportions to cause the manganese ions to form a crystalline coating on the surface in combination with the phosphate.The coating solution contains a nitrogen-containing reducing agent. The invention includes liquid concentrates for preparing the phosphating solutions.

Description: The present invention relates to a concentrate composition and method of preparing thereof. EP-A-135 622 discloses a concentrate aqueous compositions which are conveniently diluted to prepare acidic aqueous phosphatizing solutions containing from 0.1 to 2.0 g/l of zinc ion, from 5 to 30 g/l of phosphate ion, from 0.2 to 3 g/l of manganese ion; and a conversion coatings accelerators such as sodium nitrite, ammonium nitrite and others. Weight proportion ratio in the concentrate solution of zinc ion, phosphate ion, manganese ion, and optionally nickel ion and a preferable concentration range of the zinc ion are given. Conversion coatings are used to promote paint adhesion and improve the resistance of painted substrates to corrosion. One type of conversion coating is a zinc phosphate conversion coating which is composed primarily of hopeite [Zn₃(PO₄)₂]. Zinc phosphate coatings formed primarily of hopeite are soluble in alkali solutions. Such conversion coatings are generally painted which prevents the conversion coating from dissolving. However, if the paint coating is chipped or scratched, the zinc phosphate coating is then exposed and subject to attack by alkaline solutions such as salt water. When the conversion coating is dissolved, the underlying substrate is subject to corrosion. It has also been proposed to include other divalent metal ions in phosphate conversion coatings such as manganese. However, one problem with the use of manganese is that it is characterized by multiple valence states. In valence states other than the divalent state, manganese tends to oxidize and precipitate, forming a sludge in the bath instead of coating the substrate. The sludge must be filtered from the bath to prevent contamination of the surface. Prior attempts to manufacture a manganese phosphate concentrate encountered a serious problem of unwanted precipitation that formed sludge which is turn must be removed. Adding manganese alkali, such as MnO, MN(OH)₂ or MnCO₃ to phosphoric acid results in the formation of a brownish sludge. The objective of the invention is to improve the control of the phosphate coating process so that an effective coating, which is both corrosion-resistant and adhesion-promoting, can be consistently applied to steel, aluminum and zinc-coated panels. As part of this general objective, the control of a phosphate coating process including manganese is desired wherein sludge formation is minimized. This object is attained by a liquid concentrate composition consisting essentially of water and (i) divalent manganese salt, (ii) phosphoric acid, (iii) a nitrogen reducing agent selected from hydroxylamine sulfate, hydrazin sulfate, sodium nitrite, potassium nitrite or ammonium nitrite, and having the following molar proportions 0,001 to 0,388 manganese : 1 phosphoric acid, at least 0,05 nitrogen containing reducing agent : 1 manganese and wherein the manganese concentration is less than 2,24 moles per liter and by a process forpreparing a liquid concentrate composition for subsequent dilution to form a mangangese containing phosphatizing solution by the steps mixing water, phosphoric acid and a nitrogen containing reducing agent selected from hydroxylamine sulfate, hydrazin sulfate, sodium nitrite, potassium nitrite or ammonium nitrite until said nitrogen containing reducing agent is dissolved, adding a divalent manganese salt in such an amount that the concentration is less than 2,24 moles per liter and the molar proportions are 0,001 to 0,388 manganese : 1 phosphoric acid and at least 0,05 nitrogen containing reducing agent : 1 man- ganese. Three essential components of a conversion coating bath are maintained within relative proportions to obtain a preferred crystal structure, referred to as "Phosphonicollite" [Zn₂Ni(PO₄)₂] or "Phosphomangollite" ([Zn₂Mn(PO₄)₂], which are considered trademarks of the assignee. A Phosphonicollite is a zinc-nickel phosphate which has superior alkaline solubility characteristics as compared to hopeite crystals characteristic of other phosphate conversion coatings, the essential constituents being grouped as follows: A - potassium, sodium, or ammonium ions present as a phosphate; B - zinc ions; and C - nickel and manganese. The quantity of zinc ions in the coating composition at bath dilution is between 300 ppm and 1000 ppm. The ratios in which the essential constituents may be combined may range broadly from 4-40 parts A : two parts B : 1-10 parts C. A preferred range of the ratios of essential ingredients is 8-20 parts A : two parts B : 2-3 parts C, with the preferred quantity of zinc being between 500 to 700 ppm. Optimum performance has been achieved when the essential constituents are combined in the relative proportions of about 16 parts A: 2 parts B : 3 parts C. All references to parts are to be construed as parts by weight unless otherwise indicated. A third divalent metal ion may be added to the coating solution to further improve the alkaline solubility characteristics of the resulting coating. The third divalent metal ion is manganese. When manganese is included in the bath, the nickel content of the coating drops because the presence of manganese in the boundary layer competes with nickel for inclusion in the phosphate coating. Manganese is considerably less expensive than nickel and therefore a manganese/nickel/zinc phosphate coating solution may be the most cost-effective method of improving resistance to alkaline solubility. Alkaline solubility of manganese/nickel/phosphate coatings is improved to the extent that the amnonium dichromate stripping process generally used to strip phosphate coatings is ineffective to remove the manganese/nickel/zinc phosphate coating completely. According to the present invention, nitrogen-containing reducing agents such as sodium nitrite, hydrazine sulfate, or hydroxylamine sulfate eliminates the unwanted precipitation in the concentrate composition. The precise quantity of reducing agent required to eliminate precipitation depends upon the purity of the manganese alkali. The reducing agent must be added prior to the manganese and prior to any oxidizer. Examples including manganese are prepared by adding the specified quantity of the nitrogen-containing reducing agent to a phosphoric acid/water mixture. To this solution, a manganese-containing alkali, such as MnO, Mn(OH)₂, and Mn(CO₃) is added. If an oxidizer, such as nitric acid, added to the bath, it is added subsequent to the addition of the manganese-containing alkali. EXAMPLE 1 Name of Raw Material CONCENTRATE M1 CONCENTRATE MB Water 29% 34% Phosphoric Acid (75%) 36% 28% Nitric Acid (67%) 19% 5% Zinc Oxide 10% -- Nickel Oxide 1% -- Manganese Oxide 4% -- Sodium Hydroxide (50%) -- 13% Potassium Hydroxide (45%) -- 19% Hydroxylamine Sulfate < 1% -- Sodium Salt of 2 Ethyl Hexyl Sulfate < 1% -- Ammonium Bifluoride -- 1% Ammonium Hydroxide < 0.1% -- Nitro Benzene Sulfonic Acid < 0.1% -- EXAMPLE 2 Name of Raw Material CONCENTRATE M2 CONCENTRATE MB Water 24% 34% Phosphoric Acid (75%) 36% 28% Nitric Acid (67%) 23% 5% Zinc Oxide 9% -- Nickel Oxide 3% -- Manganese Oxide 4% -- Sodium Hydroxide (50%) -- 13% Potassium Hydroxide (45%) -- 19% Hydroxylamine Sulfate < 1% -- Sodium Salt of 2 Ethyl Hexyl Sulfate < 1% -- Ammonium Bifluoride -- 1% Ammonium Hydroxide < 0.1% -- Nitro Benzene Sulfonic Acid < 0.1% -- The above concentrates were diluted to bath concentration by adding 5 liters of concentrate M1 or M2 to 378.5 liters of water, to which was added a mixture of 10 liters of Concentrate MB combined with 378.5 liters of water. The above concentrates, after dilution, were combined and a sodium nitrite solution comprising 50 grams sodium nitrate in 3478.5 liters of water which is added to the concentrate as an accelerator. The coating was spray-applied for 30 to 120 seconds or immersion-applied for 90 to 300 seconds in a temperature 46.1°C - 54.4°C of (115-130° F). When noMB concentrate is used, a total of 7 liters of concentrate is added to 378.5 liters of water. All the rest of the procedure is the same. Comparative examples 10 and 12 : Comparative EXAMPLE 10 Name of Raw Material CONCENTRATE A9 CONCENTRATE B Water 35% 34% Phosphoric Acid (75%) 33% 28% Nitric Acid (67%) 16% 5% Zinc Oxide 8% -- Nickel Oxide 4% -- Sodium Hydroxide (50%) -- 13% Potassium Hydroxide (45%) -- 20% Sodium Salt of 2 Ethyl Hexyl Sulfate < 1% -- Ammonium Bifluoride 1% -- Ammonium Hydroxide < 0.1% -- Nitro Benzene Sulfonic Acid < 0.1% -- Comparative EXAMPLE 12 Name of Raw Material CONCENTRATE A10 CONCENTRATE B Water 36% 34% Phosphoric Acid (75%) 39% 28% Nitric Acid (67%) 11% 5% Zinc Oxide 11% -- Nickel Oxide 1% -- Sodium Hydroxide (50%) -- 13% Potassium Hydroxide (45%) -- 20% Sodium Salt of 2 Ethyl Hexyl Sulfate < 1% -- Ammonium Bifluoride 1% -- Ammonium Hydroxide < 0.1% -- Nitro Benzene Sulfonic Acid < 0.1% -- Comparative Examples 10 and 12 were prepared in accordance with Example 1 above. TESTING A series of test panels were coated with combinations of two-part coating solutions. The test panels included uncoated steel panels, hot-dip galvanized, electrozinc, galvanneal, and electrozinc-iron. The test panels were processed in a laboratory by alkaline cleaning, conditioning, phosphate coating, rinsing, sealing and rinsing to simulate the previously described manufacturing process. The panels were dried and painted with a cationic electrocoat primer paint. The panels were scribed with either an X or a straight line and then subjected to four different testing procedures, the General Motors Scab Cycle (GSC), Ford Scab Cycle (FSC), Automatic Scab Cycle (ASC), Florida Exposure Test, and the Outdoor Scab Cycle (OSC). TEST METHODS The GSC, or 60°C (140° F) indoor scab test, is a four-week test with each week of testing consisting of five twenty-four hour cycles comprising immersion in a 5% sodium chloride solution at room temperature followed by a 75 minute drying cycle at room temperature followed by 22.5 hours at 85% relative humidity at 60°C (140° F). The panels are maintained at 60°C (140° F) at 85% relative humidity over the two-day period to complete the week. Prior to testing, the test panels are scribed with a carbide-tipped scribing tool. After the testing cycle is complete, the scribe is evaluated by simultaneously scraping the paint and blowing with an air gun. The test results were reported as rated from 0, indicating a total paint loss, to 5, indicating no paint loss. The FSC test is the same as the GSC test except the test is for ten weeks, the temperature during the humidity exposure portion of the test is set at 48.9°C (120° F) and the scribe is evaluated by applying Scotch Brand 898 tape and removing it and rating as above. The ASC test is comprised of 98 twelve hour cycles wherein each cycle consists of a four and three-quarter hour 95 to 100° humidity exposure followed by a 15 minute salt fog followed by seven hours of low humidity (less than 50 percent humidity) drying at 48.9°C (120° F). The ASC test is evaluated in the same way as the FSC test. The Florida exposure test is a three-month outdoor exposure facing the south and oriented at 5° from horizontal at an inland site in Florida. A salt mist is applied to the test panels twice a week. Panels are scribed per ASTM D-1654 prior to exposure and soaked in water for 72 hours following exposure. The panels are crosshatched after soaking and tested according to ASTM D-3359, Method B. The most reliable test is the OSC test wherein a six-inch scribe is made on one-half of a panel and the other half is preconditioned in a gravelometer in accordance with SAE J 400. The panel is then exposed to salt spray for twenty-four hours which is followed by deionized water immersion for forty-eight hours. The panel is then placed outside at a forty-five degree angle southern exposure. A steel control panel, treated with the same conversion process except for the final rinse which was chrome (III) final rinse, is treated simultaneously in the same manner. When the control panel exhibits a corrosion scab of about six millimeters, the panels are soaked for twenty-four hours. The OSC is evaluated according to the same procedure used for the FSC and ASC tests as described previously. The panels scribed with a crosshatch grid were used to evaluate adhesion performance. After cyclical testing, the panels were contacted by an adhesive tape which is removed and qualitatively evaluated depending upon the degree of removal of non-adhering film by the tape. The numerical rating for this test is based upon a five-point scale ranging from a rating of 0 for no adhesion to 5 for perfect adhesion. The above examples were tested for corrosion resistance and adhesion by the above-described test method. ZINC MANGANESE NICKEL PHOSPHATE COMPOSITIONS Testing has been conducted to determine the effectiveness of adding manganese and nickel to zinc phosphate coating solutions having preferred ratios of zinc to nickel. Also, formulations incorporating nitrite, hydrazine and hydroxylamine have the effect of reducing the manganese precipitation and producing a clearer bath solution. The compositions were tested as previously described and are listed above as Examples 1 and 2 TEST RESULTS OF MANGANESE ZINC PHOSPHATES Comparative examples 10, 12, and examples 1 and 2, were compared to determine the effect of the addition of manganese to both a low zinc/low nickel composition as represented by Example 12 and and a low zinc/high nickel composition as represented by Example 10. The nickel and manganese contents of manganese-containing zinc phosphate coatings and comparable panels from non-manganese baths are shown in Table I below: Composition of Manganese Zinc Phosphates Type of Phosphate Low Zinc Low Zinc Low Zinc Low Zinc Low Nickel Low Nickel High Nickel High Nickel High Manganese High Manganese Concentrates Used Comp. Example 12 Example 1 Comp. Example 10 Example 2 Nickel Content Steel 1.0% 0.6% 1.5% 1.0% Hot Dip Galvanized 0.9% 0.7% 1.6% 1.1% Electrozinc 0.8% 0.7% 1.2% 1.0% Electrozinc-iron 0.9% 0.7% 1.4% 1.0% Manganese Content Steel ---- 3.0% ---- 2.6% Hot Dip Galvanized ---- 2.9% ---- 2.8% Electrozinc ---- 2.7% ---- 2.0% Electrozinc-iron ---- 3.3% ---- 2.4% When manganese is included in the bath, the nickel content of the coating drops. This is because the manganese in the boundary layer also competes with the nickel for inclusion in the phosphate coating. As will be shown below, the addition of manganese to the bath does not cause a drop in performance, but in some instances actually shows improvements. Since manganese is generally less expensive than nickel, a manganese/nickel/zinc phosphate bath may be the most cost-effective method of improving resistance to alkaline solubility. Quantitative testing of the alkaline solubility of manganese/nickel/zinc phosphate coatings is not possible since the amnonium dichromate stripping method was not effective in removing the coating. However, qualitatively the decrease in alkaline solubility of manganese/nickel/zinc phosphate is clearly shown by the increased resistance to the alkaline stripping method that was effective on nickel/zinc phosphate coatings. CORROSION AND ADHESION TEST RESULTS The maganese/nickel/zinc phosphate coatings were tested by the indoor scab test with the results shown in Table II below: 60°C (140° F) IDS TEST RESULTS Type of Phosphate Low Zinc Low Zinc Low Zinc Low Zinc Low Nickel Low Nickel High Nickel High Nickel High Manganese High Manganese Concentrates Used Comp. Example 12 Example 1 Comp. Example 10 Example 2 Scribe (mm) Cross Hatch Scribe (mm) Cross Hatch Scribe (mm) Cross Hatch Scribe (mm) Cross Hatch Steel 3mm 5 4mm 5 3mm 5 3mm 5 Hot Dip Galvanized 4mm 5 4mm 5 3mm 5 3mm 5 Electrozinc 4mm 4+ 3mm 5 2mm 5 2mm 5 Electrozinc-iron 1mm 4 1mm 4+ 0mm 4+ 1mm 4+ Table II shows that the test results for low zinc/flow nickel and low zinc/high nickel compositions having manganese added thereto are substantially equivalent as applied to steel, hot-dip galvanized, electrozinc and electrozinc-iron substrates. The exception is that electrozinc shows improvement with additions of manganese to the low nickel bath. The test results were obtained on panels that were coated by immersion phosphating. NITROGEN-REDUCING AGENTS Substantially equivalent phosphate concentrate having manganese oxide were prepared using a reducing agent to limit precipitation during manufacture. Some effective reducing agents were nitrite, hydrazine, hydroxylamine when added in the proportions shown below in Table III: Effect of Nitrogen-Reducing Agents on Manganese Phosphate None Nitrite Hydrazine Hydroxylamine Water 46.4% 46.4% 46.0% 46.2% Phosphoric Acid 40.2% 40.2% 39.9% 40.0% Sodium Nitrite ---- 0.38% ---- ---- Hydrazine Sulfate ---- ---- 0.75% ---- Hydroxylamine Sulfate ---- ---- ---- 0.75% Manganese Oxide 9.10% 9.10% 9.03% 9.06% Nitric Acid 3.72% 3.49% 3.76% 3.47% Nickel Oxide 0.45% 0.45% 0.45% 0.45% Solution Clarity muddy brown slightly cloudy clear clear Precipitate heavy brown slightly brown none none Table III and all other concentrates in this section show the ingredients in the order added. The results of the above comparative test indicate that the hydrazine and hydroxylamine reducing agents were completely effective in obtaining a clear solution and eliminating precipitation from the baths. The sodium nitrite was moderately effective in clarifying the solution and partially effective in that it reduced the degree of precipitation. Therefore, the addition of sufficient amounts of nitrogen containing reducing agents can eliminate or greatly reduce the precipitation and clarity problems. The quantity of reducing agent required is expected to be dependent upon the purity of the manganese alkali. The quantity of reducing agent is limited primarily by cost considerations. The reducing agent is preferably added prior to the manganese and prior to any oxidizing agent. Another key factor is the ratio of manganese to phosphoric acid. Table IV shows the effect of variations of the manganese/phosphoric acid ration on the clarity of the concentrate. EFFECT OF MANGANESE: PHOSPHORIC ACID RATIO Name of Raw Material Example 3 Example 4 Example 5 Example 6 Water 41.1% 42.3% 43.5% 46.5% Phosphoric Acid (75%) 48.0% 46.8% 45.5% 42.3% Hydroxylamine Sulfate 0.52% 0.52% 0.52% 0.53% Manganese Oxide 10.4% 10.4% 10.5% 10.7% Clarity Clear Sl. Cloudy Cloudy Voluminous White ppt. Mn:H₃PO₄ Molar Ratio 0.378:1 0.388:1 0.403:1 0.441:1 Clearly, the manganese:phosphoric acid molar ratio should be between 0.388:1 and 0.001:1. As in all concentrates, the less water added the better as long as no precipitate is formed. Table V shows the effect of increasing the concentration of the concentrate. One of the traits of manganese phosphate concentrates is that they form moderately stable super-saturated solutions. Thus, in order to determine whether or not a solution has been formed that will not precipitate during storage, the concentrates must be seeded. EFFECT OF CONCENTRATION Name of Raw Material Example 7 Example 8 Example 9 Water 31.8% 36.4% 41.1% Phosphoric Acid (75%) 55.6% 51.8% 48.0% Hydroxylamine Sulfate 0.60% 0.56% 0.52% Manganese Oxide 12.0% 11.2% 10.4% Manganese Concentration 2.42 m/l 2.24 m/l 2.06 m/l Mn:H₃PO₄ Molar Ratio 0.388:1 0.388:1 0.388:1 Initial Solubility All Soluble All Soluble All Soluble Solubility after Seeding Massive Precipitation All Soluble All Soluble Thus, the concentration of manganese should be 2.24 m/l or below.

Claims: A liquid concentrate composition consisting essentially of water and (i) divalent manganese salt, (ii) phosphoric acid, (iii) a nitrogen reducing agent selected from hydroxylamine sulfate, hydrazin sulfate, sodium nitrite, potassium nitrite or ammonium nitrite, and having the following molar proportions 0,001 to 0,388 manganese : 1 phosphoric acid, at least 0,05 nitrogen containing reducing agent : 1 manganese and wherein the manganese concentration is less than 2,24 moles per liter. The liquid concentrate composition of claim 1 wherein the divalent manganese salt is selected from manganese oxide, manganese hydroxide manganese carbonate. A process for preparing a liquid concentrate composition for subsequent dilution to form a mangangese containing phosphatizing solution by the steps mixing water, phosphoric acid and a nitrogen containing reducing agent selected from hydroxylamine sulfate, hydrazin sulfate, sodium nitrite, potassium nitrite or ammonium nitrite until said nitrogen containing reducing agent is dissolved, adding a divalent manganese salt in such an amount that the concentration is less than 2,24 moles per liter and the molar proportions are 0,001 to 0,388 manganese : 1 phosphoric acid and at least 0,05 nitrogen containing reducing agent : 1 manganese.

Inventors: CAPE THOMAS WILSON, CHARLES HARRY RANDOLPH, MILES DONALD LEE, CAPE, THOMAS WILSON, CHARLES, HARRY RANDOLPH, MILES, DONALD LEE
Applicants: PPG INDUSTRIES INC, PPG INDUSTRIES, INC.
Application year: 1987
Main classification: C23C22
IPC codes: C23C22