METHOD OF APPLYING ZINC-PHOSPHATE CONVERSION CRYSTAL COATING

Description

FIELD OF THE INVENTION

The present invention relates to a method of applying phosphate conversion coating, and specifically zinc-phosphate conversion crystal coating (ZPCCC), on steel, cast iron, various metals and alloys, and on different types of Zinc coatings on metals.

Phosphate conversion coating, and specifically zinc-phosphate conversion crystal coating, is used for improving paint coatings, rubber coatings, organic, and inorganic coatings to metal surfaces. In addition, zinc-phosphate coating is applied to increase corrosion protection for metals, where the coating works as a carrier for the specific oil or wax film that is applied over it.

BACKGROUND OF THE INVENTION

An excellent overview of the Zinc Phosphating processes is presented in the article by John Donofrio (Metal Finishing, v. 98, N 6, 2000, pp 57-73). For the reader's convenience, a portion of this article is presented here:

• • “Zinc phosphate is a crystalline conversion coating that is formed on a metal substrate, utilizing the chemical reaction between metal ions that have been dissolved in mineral acids and then diluted with water to form the process solution. The mineral acids that are normally used to dissolve the metal ions are nitric acid and phosphoric acid. Metals such as Zinc, Nickel and Manganese are dissolved depending on the process necessary. Accelerators may be added to phosphating processes to increase reaction speed, modify hydrogen elimination and control sludge formation.
  • Three primary reactions take place:
  • The first reaction that occurs when the zinc phosphate solution comes into contact with the metal surface is the pickling reaction, in which some metal is dissolved from the surface. In this reaction a chemical cleaning of the surface takes place. This cleaning affects the adhesion of the coating to the base metal. The free acid of the solution close to the metal surface is consumed because of the dissolution of the metal surface. Metal ions are transferred into the process solution. The type of metal depends on the type of substrate mix being treated.
  • The second reaction is the coating reaction. Due to free acid consumption in the liquid-metal interface, pH rises and the metal cations can't stay soluble in the solution. They react with the phosphate in the solution and deposit on the metal surface as crystalline Zinc Phosphate.
  • The third reaction is the sludge formation reaction. The metal ion (Fe++) that is dissolved from the pickling reaction is oxidized using the accelerator and will precipitate out as sludge. The sludge, created in the process, is normally filtered out from the solution utilizing some sort of filter media or equipment.
  • Special prerinses, applied to the metal surface prior to phosphating, provide a considerable increase in the number of nuclei for phosphate crystallization. This is termed activation of the phosphate coating formation.
  • Several different processes can be utilized with Zinc Phosphating. Straight spray installations are popular, as well as straight dip operations. Because the spray utilizes the kinetic energy of the spray pressure, concentrations and treatment times can be kept lower in spray operations compared to dip. Heavy Zinc Phosphating processes are applied in dip operations, either in bulk processes utilizing tumblers, or in a specific rack design.”

In general, the ZPCCC process proceeds as follow:

• • 1. The parts, or substrates, to be coated, are immersed in a solution that contains soluble zinc-phosphates (or soluble phosphates of iron, nickel, or manganese).
  • 2. The solution reacts with a base metal, and as result an insoluble layer of phosphate crystals is created on the part's surface. The rate of reaction of this process, and the properties of the created layer depend on: the concentrations of the free and bonded Phosphoric Acid; the concentration of the metal ions; the temperature and the acidity of the solution.

The coating application processes are similar for the various types of phosphate solutions. Relatively small substrates are loaded into special immersion baskets, or rotated drums, and immersed in the phosphate solution baths. They are immersed for a set dwelling time, and then transferred to rinsing baths. (Several baths, containing different solutions are sometimes required.) The coated parts/substrates are then dried. Large substrates may often be immersed as they are.

Another popular phosphate coating method is spraying the parts, with a spray solution taken from the phosphate solution tank, to coat them.

There are a number of serious technical problems associated with the aforementioned coating application processes. The attempts to solve these problems, to date, involve: the addition of complex equipment; the addition of manpower to operate and maintain the complex equipment; and emission control of toxic waste byproducts.

The coating process problems include the following:

• • Applying uniform ZPCCC thickness on all areas of the substrate.
  • Maintaining long-term bath solution reactivity.
  • Controlling toxic material emissions.
  • Removing precipitates from the coating immersion bath.
  • Increasing the process cost, due to the addition of expensive process control components.
  • Maintaining phosphate coating crystal size stability

Applying uniform ZPCCC thickness on all areas of the substrates requires transferring fresh solution to all surfaces of the part. To date, movable or rotated immersion baskets are used. Alternatively, phosphate solution spraying is used.

Maintaining long-term bath solution reactivity is difficult. Since all the base metals react, to varying degrees, with the phosphate solution, they reduce the ZPCCC reaction rate. Some of the base metals, such as Aluminum, significantly reduce the ZPCCC thickness. To minimize these negative effects often requires the addition of toxic materials, such as Fluorine, whose products must later be dealt with. Solution temperature and acidity control becomes critical.

In addition, the concentration of phosphate is diminished during the reaction with the substrate metal, and consequently, the solution reactivity and its acidity are reduced. Insoluble phosphates are generated in the solution, as well as on the substrate surface. Precipitation removal from the bath poses an additional problem.

To maintain phosphating bath reactive stability requires the addition of expensive process control components and computerizing in order to measure and control a large number of bath parameters, as discussed in U.S. Pat. No. 5,117,370 to DeCello et al (1992). Phosphating baths are generally cleaned every 4-5 weeks, and their coating material totally exchanged every 9-10 weeks [Surface Engineering, ASTM Handbook, V. 5, p. 386].

The process problems, discussed above, are less acute when dealing with thin phosphate coatings, 2-4 g/m2, used as a base for painting or oiling.

The process problems, discussed above, are acute when dealing with corrosion resistance phosphate coatings that require 10 g/m2 and more. In that case, the coating process parameters, such as phosphate concentration, acidity, temperature, and activator concentration, must be carefully controlled, in order not to reduce solution stability, and as a result solution reactivity.

The phosphate coating rate and phosphate crystals size depend, both, on solution composition and the surface condition of the substrate. For example, grinding or polishing the substrate surface results in small phosphate coating crystals. Small size phosphate coating provides better corrosion resistance and paint adhesion.

Maintaining phosphate coating crystal size stability is an additional phosphate coating process problem. Special activators, such as Titanium Phosphate, are used to help create phosphate coating with specific crystals size. The activator stability and its lifetime in the phosphate solution depend on several parameters, including: acidity, salt concentrations, temperature, and surface roughness.

The ZPCCC Mechanism

The ZPCCC mechanism is described in the article by John Donofrio (see above). The main principle of this process is that the zinc-phosphate solution reactions take place in a thin layer above the metal surface. The solution reacts with the metal. The acidity in this thin layer diminishes. The solution is shifted from its equilibrium state, and consequently, an insoluble crystal layer is formed on the metal surface.

The bath solution is constantly agitated. As a result, the solution, situated in the thin layer above the metal surface, is constantly renewed. Solution that has undergone reaction is mixed in with the bulk bath solution. This solution transfer process decreases the coating formation rate. It also disturbs the equilibrium of the bulk bath solution, resulting in the precipitation of insoluble phosphates in the bath solution, and its contamination.

Thick ZPCCC layer creation requires increasing the temperature and adding special activators, which in turn increase the bath solution contamination rate.

U.S. Pat. No. 5,399,208 to Sobata et al (1995), suggests performing the ZPCCC layer formation in an additional separate bath reactor. The required components are added to this separate reactor. After a set period of process time, the used solution is removed, cleaned from contaminations, and then transferred to the main bath.

This method, utilizing an additional separate bath reactor, increases the bath solution lifetime and the ZPCCC process stability. However, the reactor volume must be large enough to maintain a stable process. The real, bulk density of steel items is usually 1-4 kg/L. Since the steel density is 7.8 kg/L, the unfilled volume of the bulk volume is, on the average, 0.7 L/kg. To provide ZPCCC process uniformity, all this volume should be filled with solution. Therefore, for a 200 kg batch of substrate, the minimal reactor volume should be 140 L, and in real industrial situations a volume of 500 L. is reasonable. Therefore, the volume of solution, required for the process, is a hundred times more than the volume of solution, actually required for ZPCCC formation. For example:

• Relative surface of treated parts is 0.1 m²/kg
• Batch weight is 200 kg.
• ZPCCC thickness is 4 g/m²
• Total weight of created phosphate salts is 80 g.

Usually the phosphate salt concentration in bath solution is 100 g/L. Therefore, 500 L of solution contain 50,000 g phosphate salts, which is 600 times more than was needed for the ZPCCC formation.

Using too much volume of solution results in increased cost due to loss of unusable chemicals and increased ZPCCC bath solution aging.

Therefore, it would be desirable to provide a method of applying zinc-phosphate conversion crystal coating that possesses the advantages of the standard ZPCCC process, but utilizes a minimum of phosphating solution.

SUMMARY OF THE INVENTION

The above-mentioned process drawbacks are eliminated using a modified ZPCCC technology development.

The preferred embodiment of the present invention deals with corrosion resistance phosphate coatings that require 10 g/m2 and more.

In accordance with a preferred embodiment of the present invention, and to achieve the goal of providing a uniform coating thickness, while utilizing a minimum of phosphating solution, there is provided a process reactor volume containing a quantity of relatively small, chemical inert filler particles. The shape and size of these filler particles may be varied according to the application. The particle size ranges between 1 to 20 mm (usually 2-5 mm). The quantity of phosphate solution, required for the modified ZPCCC process, depends on the filler's solution absorption ability.

According to the present invention, the modified ZPCCC process is performed, using either continuous or periodic movement of treated parts and filler particles, continually exposing substrate surfaces to fresh solution. This method is herein described as Absorbed Solution Layer Phosphating (ASLP).

For the example, described previously, the required filler volume should be 140 L., and the filler particles are present as spheres having a 3 mm diameter. Experiments show that the phosphate solution volume, absorbed on this quantity of filler is approximately 3-5 L. This volume of phosphate liquid contains enough phosphate salts for providing the required thickness of ZPCCC. The ZPCCC reaction is carried out in a small volume of solution, in an interface layer between the treated substrate and the filler particle. The solution reacts rapidly not only on the border between the solution and the substrate, but also in all the interface layer volume. During the period of time when there is movement of treated parts relative to filler particles, fresh phosphate solution is introduced to the interface layer, enabling rapid coating layer creation and uniform coating thickness.

As a result, a thick, uniform ZPCCC layer may be created without having to add any special activators.

Another positive result of the movement of treated parts and filler particles is uniformity of the ZPCCC crystals' size. This effect results from the nucleating of many small, already deposited, phosphate crystals.

ASLP is carried out in a reactor, which provides either continuous or periodic movement of treated parts and filler particles. The simplest types of reactors use a drum rotated at a speed of 0.2-1 r.p.m., or a vibrating machine. Other mixing options involve using the magnetic properties of treated substrates being passed through the filler.

Units for reactor loading and reloading, and for phosphate solution addition and removal/exchange are generally required.

Other features and advantages of the invention will become apparent from the description and experimental data contained herein below.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the invention, reference is made to the following drawing, in which like numerals designate corresponding elements or sections throughout, and in which:

FIG. 1 shows the ASLP batch process diagram

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, two possible ASLP batched process options are described below:

• • Option 1: Heated phosphating solution is provided from tank 1 to the ASLP reactor 2, which is already filled with the required amount of filler particles, during the time required for filler heating and rinsing. The solution, used for rinsing, is removed from the ASLP reactor 2 to the cleaning system 4, by gravitation or by pumping. Cleaning is performed using a filter, precipitator, or hydro-cyclone. The treated solution is returned to tank 1, as required.
    • Before phosphating, valve A is turned off and all the solution from the reactor is removed, except for the solution absorbed on the filler particle surfaces. A batch of the substrate, to be treated, is loaded and treated for the period of time required to attain the required thickness of ZPCCC. During the ASLP process, periodic or continuous movement of the treated parts and filler particles is carried out. After phosphating, the coated substrate may be rinsed in the reactor, or transferred for required finishing, for example, rinsing, neutralizing etc., to other equipment. Finally, the coated substrate undergoes a drying operation.
  • Option 2: Tank 1 contains heated washing liquid for substrate washing, before and after phosphating. After the washing liquid is removed from reactor 2 to the tank, as in the previous process option, valve A is turned off, and a small quantity of phosphating solution concentrate is added to the reactor 2. This concentrate is mixed with a washing solution, which was absorbed on the filler particles' surface. The quantity of phosphating solution concentrate is calculated, so that the final composition of solution absorbed on the filler particles is that required for the phosphating process. The batch of substrate, to be treated, is loaded into the reactor and the process continues as in the previous process option.

EXPERIMENTAL DATA

For a better understanding of the invention, reference is made to the following experimental data:

Phosphating Solution Preparation

Concentrate of phosphating solution (CPS):

• 1. To prepare 1 liter of CPS the following amounts of each component is used:
  • ZnO—130 g
  • Water—270 g
  • H₃PO₄—510 g
  • NaOH—42 g.
• 2. The density of the CPS liquid was increased to 1.300 g/cm³ by adding water.
• 3. Phosphating solutions were prepared by mixing CPS with water or washing solution
• 4. Washing solution was prepared by mixing CPS with water to obtain a density of 1.040 g/cm³; the pH of this solution was 2.80.

Chemicals Used in Phosphating Solutions

• ZnO: pigment grade, producer—“Numinor Ltd.”
• H₃PO₄: technical grade, density 1.65 g/cm³, supplier—“Chemilab Ltd.”
• NaOH: technical grade, producer—“Glüsan”, Turkey, supplier—“Chemilab Ltd.”
• Water: deionized.

Control of Solutions and Phosphating Process:

• 1. Density of phosphating liquid was measured by hydrometer.
• 2. Density of CPS was measured by weighing.
• 3. Acidity of phosphating liquid was measured by pH-meter.
• 4. Crystal size was measured by microscope (average of ten measurements).

Equipment for Controling Phosphating Process:

• • pH-meter model “CyberScan 500 pH”, producer “EUTECH Instruments”, Singapore, supplier—“M.R.C. Ltd.”
  • Hydrometer with scale in limits 1.000-1.200 g/cm³, supplier—“Bein Z. M. General Laboratory Equipment”
  • Analytical balance GF-200, producer “A&D Co., Ltd.”, Japan
  • Metallographic microscope model OPTIPHOT-1005, producer “Nikon”, Japan, supplier “Prisma Ltd.”
    Process Equipment used in the Experiments
    The experiments were carried out using the following equipment:
  • Plastic drum with volume of 1 liter and diameter 70 mm.
  • Industrial vibrator model CV 250 SG, producer PMG, Japan.

Process Parameters in the Experiments

• • 1. The method of phosphating was based on the known process of phosphating in a bath, simulated in laboratory plastic beaker, having a volume of 0.5 liter.
  • 2. In all experiments, the phosphating temperature was 35° C. and was maintain by thermostat with accuracy +/−2° C.
  • 3. Duration of phosphating process (except for specific cases) was 10 minutes.
  • 4. After the phosphating process, the control samples were washed in running water and dried using paper towels.
  • 5. Initial concentrate of phosphating solution (CPS) was constant.
  • 6. Plates, each with an area of 17 cm², produced from steel AISI 220, were used as control samples.
  • 7. Before phosphating, the plates were diffusion saturated in Zinc powder at 410° C. during 1 hour.
  • 8. The thickness of Zn—Fe layer that was received by the above-mentioned method was approximately 40+/−5 micron.
  • 9. The phosphate coatings that were obtained in different experiments on these samples were described using the following parameters:
    • thickness of phosphate coating, in g/m²
    • crystals size of phosphate coating, in microns
    • corrosion resistance, determined by SST data (in hours from time that white corrosion began). The results were averaged no less than by three samples.
  • 10. The phosphating solution was prepared via mixing one part of CPS and three parts of water.
  • 11. The density of the phosphating solution was 1.090 g/cm³, pH was 2.48.
  • 12. Part of the solution was used for simulating the phosphating process in the bath and another part—for the selection of ASLP process mode.

Experiment Descriptions

The results of experiments are tabulated in Table 1.

Experiment #1:

• 1. 300 ml of solution, prepared as described above, was added to a plastic beaker, having a volume of 0.5 liter.
• 2. The quantity of metal for phosphating was 100 g.

Experiment #2:

• 1. 700 g of porcelain chips, having triple-edged prism shape, with sides 3*3*4 mm and height 4 mm, was added to a plastic drum.
• 2. 300 ml. of solution was added to the drum, covered with a lid, and was rotated at a rotation speed of 2 rpm, in a horizontal position during 5 minutes.
• 3. The lid was then removed and solution fully removed, except the solution absorbed on the filler particle surfaces.
• 4. The results of weighting show that in the drum, about 28 ml of phosphating solution was absorbed by chips and drum' sides.
• 5. 300 g of metal for phosphating, including three control samples, were added to the drum.
• 6. The drum was closed and revolved with rate 0.3 rpm during 10 minutes.
  Experiment #3: Repeated Experiment #2 with the difference being that the rate of revolution during phosphating process was 2 rpm.
  Experiment #4: Repeated Experiment #2 with the difference being that the rate of revolution during the phosphating process was 5 rpm.
  Experiment #5: Repeated Experiment #2 with the difference being that the rate of revolution during the phosphating process was 1 rpm.
  Experiment #6: Repeated Experiment #1 with the difference being that the density of phosphating solution was 1.0150 g/cm³, pH was 2.35. The solution was prepared by mixing one part of CPS and one part of water.
  Experiment #7: Repeated Experiment #5 with the difference being that used the solution prepared for experiment #6.
  Experiment #8: Repeated Experiment #5 with the difference being that 1 kg of corundum powder, with average grain size 850 micron, was added to the drum. The solution quantity absorbed by corundum powder was 240 ml.
  Experiment #9: Repeated Experiment #5 with the difference being that 7 ml of CPS was introduced into the chips before and 21 ml of water after.

Experiment #10:

• 1. The vibrator was filled with 150 kg of chips (the same size used for Experiments ##2-9).
• 2. Phosphating solution was added to the vibrator and 50 kg of metal, including three, control samples, were added. The frequency of the electrical current supplying the vibrator motor was used as an indicator of the rate of moving chips in the vibrator.
• 3. Preliminary experiments showed that the thickest phosphating layer was formed at a frequency of 20-25 Hz. At lower frequencies the chips didn't move. At higher frequencies, a decrease of coating thickness was observed. As the frequency was increased, it was observed that the coating thickness decreased even further. Consequently, a frequency of 22 Hz was used.
• 4. The time of phosphating was 10 minutes.
• 5. The phosphated articles were un-loaded at a frequency of 30 Hz, during 5 minutes.
• 6. At process end, the solution was emptied.
• 7. The chips had absorbed 4.5 liter of solution.
  Experiment #11: Repeated Experiment #10 with the difference being that 0.8 liter of CPS and 3.7 liter of water was added to the vibrator.

Experiment #12:

• 1. To create a closed-cycle system, 600 liters of washing solution was prepared.
• 2. The vibrator containing 150 kg of chips and 50 kg of metal was washed by this solution during 10 minutes.
• 3. After this, the tap providing the washing solution was closed.
• 4. The washing solution was pumped into the bath with a device for sludge settling.
• 5. 0.8 liter of CPS was added to the vibrator, and was phosphated during 10 minutes at a frequency of 22 Hz,
• 6. The metal was washed by this washing solution during 2 minutes and the metal was unloaded from the vibrator and was loaded into the dryer.
  Experiment #13: Repeated Experiment #12 with the difference being that the phosphating time was 15 minutes.
  Experiment #14: Repeated Experiment #12 with the difference being that the phosphating time was 7 minutes.
  Experiment #15: Repeated Experiment #12 with the difference being that 1.5-liter of CPS was added to the vibrator.
  Experiment #16: Repeated Experiment #12 with the difference being that 2.5-liter of CPS Was added to the vibrator.
  Experiment #17: Repeated Experiment #12 with the difference being that 0.6-liter of CPS was added to the vibrator.
  Experiment #18: Repeated Experiment #12 with the difference being that 0.3-liter of CPS was added to the vibrator.
  Experiment #19: Repeated Experiment #12 with the difference being that a solution, diluted to a density of 1.030 g/cm³, was used during washing.
  Experiment #20: Repeated Experiment #12 with the difference being that the vibrator was filled with chips of cylindrical form with diameter 8 mm and length 6 mm.

TABLE 1
Characteristics of ZPCCC obtained in experiments

	Phosphating	Stability	Average size
	layer thickness,	in SST,	of crystals,
#	g/cm2	hours	microns	Notes

1	9	144	5
2	6	72	2	The surface color is not
				uniform. There are areas
				without a phosphating layer
3	10	192	1
4	2	48	<1	Wide areas of un-coated
				surface
5	12	240	1
6	10	168	8
7	13	288	1
8	11	264	<1
9	12	240	1
10	13	312	<1
11	12	264	<1
12	13	312	<1
13	15	336	1
14	8	192	<1
15	14	312	1
16	15	312	2
17	12	264	<1
18	8	144	<1
19	13	312	<1
20	13	312	<1

As follows from data in the Table 1, the ASLP process performs consistently in a wide range of varying phosphating solution compositions (Experiments #7, 9, 10, 11, 12, 16, 17, 19), varying phosphating time (Experiments #10, 13, 14), varying filling grains size (Experiments #2, 8, 10, 20), varying speeds of substrate and filler movement (Experiments #2, 3, 5, 10). At optimal ASLP conditions, the process, without adding activators, obtains a very good thickness of phosphate layer (up to 15 g/cm²), small sizes of crystals (1 micron and less) and high corrosion stability (336 hours in SST).

Having described the invention with regard to certain specific embodiments and examples, it is to be understood that the description is not meant as a limitation, since further modifications may now suggest themselves to those skilled in the art, and it is intended to cover such modifications as fall within the scope of the appended claims.