Patent Application
20100132845
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.
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:
In general, the ZPCCC process proceeds as follow:
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 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 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:
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.
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.
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:
Referring to
For a better understanding of the invention, reference is made to the following experimental data:
Concentrate of phosphating solution (CPS):
The results of experiments are tabulated in Table 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.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IL2004/000631 | 7/14/2004 | WO | 00 | 1/16/2007 |
