US3684588A - Metal treating process - Google Patents

Abstract

THERE IS DISCLOSED A METHOD FOR APPLYING PROTECTIVE CHEMICAL COATINGS TO METAL SURFACES TO IMPROVE THEIR PAINT ADHESION PROPERTIES AND/OR CORROSION RESISTANCE BY SEQUENTIALLY SPRAYING ONTO SAID SURFACES, THROUGH A NOZZLE SYSTEM ON A SINGLE HAND WAND, A CLEANING SOLUTION AT HIGH PRESSURE SUCH AS 300-600 P.S.I., A CHEMICAL COATING SOLUTION AT A LOW PRESSURE SUCH AS 80 P.S.I., AND A FINAL PASSIVATING RINSE. WATER RINSES MAY ALSO BE APPLIED THROUGH THE SAME HAND WAND MOUNTED NOZZLE SYSTEM. THE METHOD IS CAPABLE OF APPLYING ZINC PHOSPHATE COATINGS TO FERRIFEROUS AND ZINCIFEROUS SURFACES, AS WELL AS OTHER TYPES OF CHEMICAL COATINGS TO A WIDE VARIETY OF METAL SURFACES.

Description

United States Patent 01 fice 3,684,588 Patented Aug. 15, 1972 3,684,588 METAL TREATING PROCESS John Patrick Curran, Doylestown, Pa., assignor to Amchem Products, Inc., Ambler, Pa. No Drawing. Filed May 20, 1970, Ser. No. 39,160 Int. Cl. C23f 7/08 U.S. Cl. 148-615 Z 6 Claims ABSTRACT OF THE DISCLOSURE There is disclosed a method for applying protective chemical coatings to metal surfaces to improve their paint adhesion properties and/or corrosion resistance by sequentially spraying onto said surfaces, through a nozzle system on a single hand wand, a cleaning solution at high pressure such as 300600 p.s.i., a chemical coating solution at a low pressure such as 80 p.s.i., and a final passivating rinse. Water rinses may also be applied through the same hand wand mounted nozzle system. The method is capable of applying zinc phosphate coatings to ferriferous and zinciferous surfaces, as well as other types of chemical coatings to a wide variety of metal surfaces.

This invention relates to the art of applying chemical coatings to metal surfaces to prepare them for painting or to increase their corrosion resistance. It particularly involves a novel method of applying such coatings. The method is capable of use with a wide variety of protective coatings for metals, but it is of particular utility in the field of applying zinc phosphate coatings to ferriferous and zinciferous surfaces. For this reason, the following discussion will be cast principally in the context of zinc phosphate coatings.

Various application systems have been used to apply protective coatings, including zinc phosphate coatings, to metals, and while these vary in some respects, practically all systems involve a cleaning step in which cleaning solution is contacted with the surfaces, a coating step in which coating solution is contacted with the surfaces, and a final rinse step in which a passivating rinse is applied to the coated surface. In addition, water rinsing steps are often included preceding or following each of the three principal steps.

When the metal parts which are to be coated are formed into individual articles, the foregoing sequence of steps is usually performed in a spray tunnel having a number of separate stations for each step in the process. The parts to be coated are carried through the spray tunnel on a conveyor and as they pass through each processing station, one step in the process is performed. Thus, the equipment involved includes not only the spray tunnel and its conveyor equipment, but also a solution tank, spray risers, and pumping equipment for each processing step, including the water rinsing steps. The solution tanks for each processing stage are very large, compared to the volume of solution in contact with the metal parts being processed at any given moment, and for reasons of economy, the solution falling out of contact with the parts is recycled into the solution tank even though it is partially chemically depleted. In order to maintain uniformity of coating quality it is necessary to make control measurements of the solutions in the various solution tanks and to add make-up material to those tanks as it is needed.

Sometimes one or more of the spray coating stations in a tunnel-type system is replaced by a dip tank large enough to accommodate parts being coated as they move along the conveyor. Such systems still involve the use of large volumes of solution and require control measurements and solution replenishment.

From the foregoing it can be seen that the spray tunnel system for applying protective coatings to metal parts involves a large capital investment, and requires careful operational control. These factors, as a practical matter, limit its use to mass production operations where many parts of moderate size are to be substantially continuously given protective coatings. This has limited the field of use of protective coatings for metals and has effectively denied their advantages to many potential users. For example, very large parts cannot be treated in a tunnel system unless a commensurately large tunnel is provided, one so large that it is economically impractical. It is also impractical to use such a system in the field, such as at a construction site, or at the location of an installed machine which is being repaired and refurbished. The tunnel technique is difiieult to adapt to production situations where parts of varied size and shape must be processed and the number of any given type part is relatively small. Finally, some plants have a need to operate a metal coating installation only on an intermittent, sporadic basic and cannot justify the large capital investment in tunnel-type equipment.

Attempts have been made in the past to devise methods for applying protective coatings "to metal which do not have the disadvantages of the tunnel or dip tank techniques and which forego as few of the advantages of those techniques as possible. However, such methods have not had wide-spread success. One method which has been employed is to introduce the coating solution into the steam line of a steam cleaning apparatus. The solution is discharged with the steam through a nozzle onto the surface. The nozzle can be mounted on the end of a hand wand which is passed over the surface of the metal being treated. Thus, this system can be used in the field, or in intermittent operations, or with very large parts.

The steam application method has severe limitations, however, which have circumscribed its success. For one thing, chromate coatings and zinc phosphate coatings cannot be suitably applied, and the method is therefore effectively limited to the application of iron phosphate to ferriferous surfaces. Since the solutions are applied at very high temperatures, any volatile components are driven from the solution. Thus nitrite ion will be evolved as oxides of nitrogen. One further characteristic of zinc phosphate coatings which makes them unsuitable for application by the steam process is that such coatings, when fresh, are particularly susceptible to the rapid formation of light rust in a hot humid environment, such a phenomenon being termed rust blushing. When the application of a zinc phosphate coating is attempted by the steam method, it is found that the surfaces rust blush severely.

In accordance with the present invention a new method of applying protective coating is provided which does not require the large capital investment, and complex op erating procedures of the spray tunnel and dip tank systems, and which overcomes the disadvantages and limitations of the steam application system. The method involves the application of cleaning solution, coating solution, final rinsing solution, and any desired Water rinses sequentially through a nozzle system mounted on a single hand wand. Thus the method is adaptable to field use, to use with large parts, to short or intermittent production runs, and to coating situations where flexibility in coating time, cleaning time, and the like is required. The method can be used with a wide variety of protective coating systems, and in this respect is more flexible than the steam application method, which, as noted, can only be used with iron phosphate coatings. In particular, the method the invention is capable of applying good quality zinc phosphate coatings to ferriferous and zinciferous surfaces.

In accordance with the method of the invention the metal to be coated is first cleaned by a cleaning solution passed through the nozzle of the hand Wand at high pressures in the range of 300-600 p.s.i., preferably near 500 p.s.i. Then the surface may be rinsed, if desired, and coating solution is sprayed onto the surface at a relatively low pressure, preferably about 80 p.s.i., through the same hand wand mounted nozzle system. Following application of the coating chemicals the surface may again be rinsed, if desired, and then a final passivating rinse solution is sprayed onto the surface through the nozzle of the hand wand, at a relatively low pressure, preferably about 80 p.s.i.

The application pressures which have been outlined generally above have been found to be quite important to the successful operation of the method. Performance of the cleaning step at a high pressure is of particular importance since such pressures are necessary to obtain quick, thorough and uniform cleaning of the metal. When cleaning is performed at lower pressures, not only is more time required for the cleaning step, but the likelihood of producing non-uniform low quality coatings is substantially increased.

The use of relatively low pressures during application of the coating material itself is also of considerable importance in the application of the method inasmuch as the use of high pressures, above about 80 p.s.i., has been found to result in the formation of sparse, non-uniform coatings, low coating weights, and at best, greatly increased application times to obtain a useable coating.

The application pressure for the final passivating rinse is not as critical as that for the application and cleaning steps, but since completely satisfactory results are obtained at low pressures, it is preferred for reasons of convenience to use such low pressures.

The temperatures of application are also important factors in the method of the invention. It is preferred that the cleaning step be performed at a relatively low temperature between about room temperature and 150 F. both for reasons of economy and to minimize the contribution of the cleaning step to rust blushing when zinc phosphating is being done. It is preferred that the coating materials be applied at relatively low temperature also, for example between 110 F. to 150 F. The use of coating temperatures toward the high end of this range results in more rapid coating formation, although uniform coatings can be obtained throughout the temperature range. If coating temperatures above 150 F. are used in the coating step of the method, the likelihood of rust blushing is increased, as is the likelihood of evolution of volatile components such as the loss of nitrite ion as oxides of nitrogen. The temperatures of the passivating rinse step and of water rinses are preferably about 150 F. In field use the method can thus be readily practiced at a single temperature which is suitable for all of the processing steps including the water rinse steps. The considerations outlined above are illustrated by the following examples.

EXAMPLE 1 The effect of the application pressure for the cleaning step of the method is illustrated by the following tests. A mild titanated alkaline cleaner was sprayed for 30 seconds onto a series of lightly oiled steel panels at varying pressures for 30 seconds. The application temperature was 110 F. After water rinsing, a conventional zinc phosphate solution was then applied to the panels for one minute at 70 p.s.i., and the coating appearance was rated visually. Table 1 reports the results of this series of tests. In Table 1 the column headed water break represents an evaluation of the cleanliness of the work as measured by the uniformity with which the water left the surface without beading up.

From the foregoing it can be seen that high cleaning pressures are required to obtain satisfactory coatings.

EXAMPLE 2 pressure, p.s.i.

Coating appearance No coating. Sparse coating.

hin, uniform coating. Uniggrm coating.

Table 2 shows that in the low pressure ranges of about 20 to about p.s.i. good phosphate coatings are produced. It should also be noted that at the application temperature used in this test, no rust blushing was encountered.

EXAMPLE 3 A further series of tests was made to illustrate the effect of coating solution application temperature upon coating quality. In this series of tests the coating application pressure was maintained at 80 p.s.i., while the coating solution temperature was varied. A conventional zinc phosphate coating solution was employed, and the coating time was one minute. The cleaning step was performer at 500 p.s.i. and at 150 F. The results are reported in Table 3.

TABLE 3 Coating temperature, F. Coating appearance Rust blush 70 Thin uniform coating None. Good uniform coating. Do. do.. Do. .do Slight. do Severe.

The foregoing tests show that good coatings can be ob tained over a relatively broad range of application temperatures without encountering rust blushing.

In accordance with the method of the invention the large solution tanks characteristic of the spray tunnel and dip tank systems are eliminated. Cleaning material, coating material, and passivating rinse material are provided in small tanks or drum of concentrated pre-mix solution and are metered into water supplied from the water mains as it is pumped to the nozzle system on the hand wand. The pre-mix tanks or drums need not be heated, when the water from the mains is hot or is heated by an auxiliary heater. Mixing valves or similar devices associated with the spray Wand feed equipment may be utilized to adjust the proportion of each of the pre-mixes fed to the water from the mains in order to deliver to the nozzle system solutions of the appropriate dilution.

As was pointed out above, the method may be used with the various kinds of coating solutions known to the art. Generally speaking, the proportions of components of any particular type of coating solution at use dilution are preferably approximately the same as those used in spray tunnel systems. For particular coating situations, it may be desirable to alter the relative proportions or concentrations somewhat within the general ranges taught by the art. When zinc phosphate coating solutions are used, and when nitrite ion is used as an accelerator, in accordance with the art, care should be taken to maintain the phosphate solution in the pre-mix tank at a sufficiently low concentration that the nitrite ion is not dissipated as evolved nitrogen dioxide gas. When titanated cleaner, such as a mild alkaline cleaner containing col- I loidal titanium salts, is used for the cleaning step, the cleaner pre-mix solution should be stirred to maintain the colloidal salt in good suspension.

Treating times for each of the steps of the method, including water rinsing steps, may be varied over a considerable range, in accordance with the judgment of the operator as he observes the results of his work. Generally speaking, adequate final coatings can be obtained by contacting each section of the surface to be coated with cleaning solution for about 30 seconds, with the coating solution for about 30 seconds, and with the final rinse for about 30 seconds. Similarly, the water rinses, if employed, may be applied for about 30 seconds.

Since the method of the invention involves the sequential spraying of solutions through the Wand at widely differing pressures, it may be desirable to employ a nozzle having alternately useable orifices of different sizes, or to employ a multiplicity of nozzles on the wand which are alternately operated. For this reason, the terms nozzle and nozzle system are employed herein in a broad sense to connote not only a single nozzle, but also such specially refined nozzle systems.

I claim:

1. A method for applying a protective chemical coating to a metal surface comprising sequentially delivering to said surface through a nozzle mounted on a hand held wand a cleaning solution at a pressure between about 300 p.s.i. and about 600 p.s.i. and a temperature between about 70 F. and about 150 F., a chemical coating solution at a pressure between about 20 p.s.i. and about 80 p.s.i. and a temperature between about 110 F. and about 150 F., and a final passivating rinse solution.

2. A method in accordance with claim 1 in which a portion of the aqueous component of each of said solutions is supplied from a main, in which the remaining components and the remainder of the aqueous component are supplied as concentrated premixes, and in which each of said solutions is formed by admixture of its premix and said aqueous component portion in the course of delivery thereof to said hand wand mounted nozzle.

3. A method for applying a zinc phosphate coating to ferriferous and zinciferous surfaces comprising sequentially deliverying to the surface to be coated through a nozzle mounted on a hand held wand an alkaline cleaning solution at a pressure between about 300 p.s.i. and about 600 psi. and a temperature between about F. and about 150 F., a zinc phosphate coating solution at a pressure between about 20 psi. and about psi. and a temperature between about F. and about F., and a final passivating rinse solution.

4. A method according to claim 3 and further comprising delivering water rinses to said surface between delivery of said cleaning and said coating solutions, and between delivery of said coating and said final rinse solutions.

5. A method according to claim 4 in which said solutions and rinses are delivered to said surface at substantially the same temperature.

6. A method according to claim 3 wherein said hand held wand is provided with a multiplicity of nozzles which are alternately operated for delivering said solutions in the sequential order set forth.

References Cited UNITED STATES PATENTS 2,228,836 1/1941 MacQuaid 117104 X 3,207,165 9/1965 Durrant 134-36 X 3,167,797 2/1965 Hergonson 134-99 X 3,615,912 10/1971 Dittel et al. 148--6.15 R

RALPH S. KENDALL, Primary Examiner WESTON, Assistant Examiner US. Cl. X.R.

1l7--104 B, 104 R; 134--36