US3822118A

US3822118A - Acid zinc-electroplating process and product thereof

Info

Publication number: US3822118A
Application number: US00331597A
Authority: US
Inventors: T Fukuzuka; H Sakai; T Furuya
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1972-02-12
Filing date: 1973-02-12
Publication date: 1974-07-02
Anticipated expiration: 1991-07-02
Also published as: JPS5617438B2; JPS4884040A

Abstract

A LAYER OF ZINC IS FORMED ON STEEL PLATES BY ELECTROLYSIS FROM AN ACID-ZINC ELECTROPLATING BATH CONTAINING 0.05 TO 0.3 G./L, OF CR+6IONS. THE PLATED STEEL IS CHARACTERIZED BY EXCELLENT CORROSION RESISTANCE, AND PROVIDED EXCELLENT ADHESION TO ORDINARY PAINTS. IN A SECOND EMBODIMENT OF THIS INVENTION, THE STEEL MAY HAVE AN INITIAL PLATE OF ZINC IN A THICKNESS OF AT LAST 1X10**3U PRIOR TO SAID ELECTROLYSIS TREATMENT.

Description

F O E R E H T T C w L 0 mm m M m R AT G m T sm mm m E c N I Z D I c A July 2, l 74 4 Sheets-Sheet 1.

Filed Feb. 12, 1973 ANGLE 0F DIFFRACTION 3600 28002000 I800 I600 "+00 I200 I000 800 600 %00 DISTANCE July 2, 1974 TOSHIO FUKUZUKA ET AL ACID ZINC ELECTROPLATING PROCESS AND PRODUCT THEREOF Filed Feb. 12, 1973 I l -|000 ||00 -l200 I300 mV vs. 80E

4 Sheets-Sheet 2 l000 "H00 -|200 -l300 +0 5 mV vs.SCE

I 0 05 |.0 l5 X10 THICKNESS 1974 TOSHIO FUKUZUKA EI'AL 3,822,118

ACID ZINC BLIGTROPLA'TING PROCESS AND PRODUCT THEREOF 4 Shoots-Shoot 5 Filed Feb. 12, 1973 FIG. 6A

FIG. 68

FIG. 6C

TOSHIO FUKUZUKA E'I'AL 3,822,118

July 2,1974

ACID ZINC suc'morm'rxue rxocsss um rnonuc'r manner- FiledFeb. 12, 1973 4 Sheets-Shoot 4 FIG. 9

Uiii'ted States Patent Ofiice 3,822,118 Patented July 2, 1974 US. Cl. 29-1835 2 Claims ABSTRACT OF THE DISCLOSURE A layer of zinc is formed on steel plates by electrolysis from an acid-zinc electroplating bath containing 0.05 to 0.3 g./l. of Cr+ ions. The plated steel is characterized by excellent corrosion resistance, and provides excellent adhesion to ordinary paints.

In a second embodiment of this invention, the steel may have an initial plate of zinc in a thickness of at least 1 X 10' y, prior to said electrolysis treatment.

BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an acid zinc-electroplating process for use on steel plates or the like, which gives plated zinc layers which have excellent corrosion resistance properties and which have surfaces suitably adapted for painting.

Description of the Prior Art It is known that acid zinc-electroplating processes can be adapted to improve the corrosion resistance of steel plates or the like. The electroplating baths used for the plating of steel sheets usually contain therem a basic agent containing zinc ions, electro-conductive salts, to improve the electrical conductivity of the bath, and a buffer agent, to maintain the pH of the plating medium at a predetermined value. Since an electroplated layer or film, which is obtained from such a bath is usually pure 21110, the plated layer obtained is initially a uniform gray-white color. This layer, however, takes on the undesirable ap pearance of white rust in an extremely short period of time after the plated sheet is stored in an atmosphere with a high level of humidity. In addition, in instances in which the metal is a steel plate, the plates eventually develop rust. The time it takes for the initial appearance of rust on the steel plates in a salt spray test is proportional to the thickness of the plated layer, that is, it takes an additional 6 to'8 hours for rust to appear for each additional micron of the plated layer.

An additional disadvantage of conventional zinc-plating procedures is that the plated layer does not have a surface suitable for painting because of the poor adhesion of the paint to the surface of the layer. This results in peeling of the paint from the layer. The surface of the exposed layer must be further treated, such as by chromating or phosphating treatments to improve the corrosion resistance and to improve the ability of paint to adhere to the layer.

When a baked finish is desired on the electroplated layer, blisters will often develop in the baking process in the interface between the surface of the base metal and the electroplated layer. This results in critical defects in the metal. The blisters are believed to be caused by the presence a very small amount of hydrogen formed during the electroplating process which becomes occluded or otherwise contained on the surface of the base metal.

A need, therefore, exists for an electroplating process which yields a zinc plated base metal which is not easily corroded and to which paint coating can be firmly adhered.

SUMMARY OF THE INVENTION Accordingly, one object of this invention is to provide an improved acid zinc-electroplating process.

Another object of the invention is to provide an improved acid zinc-electroplating process which yields a metal product having highly improved corrosion resistance and an improved 'bondability to paint coatings.

These and other objects of this invention as hereinafter will become readily apparent to one of ordinary skill in the art, is attained by providing, in a first embodiment of this invention, an acid zinc-electroplating process for use on steel plates or the like, which comprises the electrolysis of a metal sheet in an acid zinc-electroplating bath containing 0.05 to 0.3 g./l. of Cr.

In a second embodiment of this invention, an acid zinc electroplating process is provided wherein zinc is plated in a thickness of 1X10 ,u. onto the surface of base metal and then said plated metal is further subjected to electrolysis in an acid-zinc electroplating bath containing 0.05 to 0.3 g./l. of Cr+ ions. The electroplated steel plates thus produced are characterized by excellent corrosion resistance and highly improved adhesive properties for paint films applied thereon.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is an X-ray diffraction diagram of the plated layers produced by the process of this invention (1) and by conventional acid zinc-electroplating procedures (2);

FIG. 2 shows infrared spectra of a layer (3) produced by the acid zinc-electroplating process of this invention, a layer (4) obtained from conventional acid zinc-electroplating procedures, and a layer (5) on a steel plate which has been subjected to an electrolytic chromating process;

FIG. 3 is a plot obtained by an electron probe microanalyzer showing the chromium distribution throughout an electroplated layer produced by the process of this invention;

FIG. 4 is a plot showing the relationship between the electrode potential and the current density for Zn-Cr electroplating a mild steel plate;

FIG. 5 is a plot showing the relationship between the electrode potential and the current density for Zn-Cr electroplating a pure zinc layer 0.1, thick;

FIG. 6a shows the development of blisters throughout the surface of a sample having both surfaces electroplated;

FIG. 6b shows no development of blisters on the surface of a sample having one surface electroplated;

FIG. 60 shows no development of blisters on the surface of a sample which has been subjected to the acid zinc-electroplating process of the present invention;

FIG. 7 is a plot showing the relationship between electrode potential and the thickness of a plated zinc layer;

FIG. '8 is a perspective view of a tester used in testing the present invention;

FIG. 9 is a photograph showing the results of peeling tests conducted on plated metal sheets which have had patterns described on their surfaces with the describing tester of FIG. 8.

DESCRIPTION OF IIHE PREFERRED EMBODIMENTS According to the process of the present invention, a small amount of Cr+ ions is added to an acid zinc-electroplating bath. The amount of ions added is in the range of 0.05 to 0.3 g./l. If Cr ions are present in concentrations of less than 0.05 g./ 1., no appreciable improvements are notic-able in the corrosion resistance of the electroplated steel plates. If Cr+ ions are present in concentrations of greater than 0.3 g./l. the final appearance of the electroplated steel plates will be appreciably spoiled. The chromium ions can be added to the bath in the form of compounds of Cr+ such as CrO K Cr O and/or (NH Cr O The pH of the bath should preferably be adjusted to the range from 2.5 to 4.5. If the pH is less than 2.5, poor effects are noted because of the presence of Cr+ ions, while if the pH is greater than 4.5, zinc hydroxide could precipitate from the bath.

The concentration of zinc ions in the electroplating bath is in the range of 30 to 150 g./l., preferably 40 to 110 g./l. Suitable zinc salts which are added to the electroplating bath include ZnSO -7H O, ZnCl Zn(B'F Zn P O; and the like. The thickness of the zinc layer on the plated metal sheets obtained from the acid zincelectroplating bath is ten microns or less.

The salt spray test reveals, according to the present invention, that the corrosion resistance of the plated layer on a steel plate is three times greater than a layer obtained by conventional processes which do not contain Cr+ in the electroplating baths.

The reasons for the improved corrosion resistance of the layer obtained from the acid zinc-electroplating process of this invention are based on the following facts.

Analysis of the chromium content in the plated layer obtained by the process of the present invention indicates that from 0.05 to 0.6% chromium is contained in the plated zinc layer depending on the electroplating conditions. In an attempt to define the characteristics of the electroplated layers (i) X-ray diffraction tests of placed layers, which reveals the condition of the co-deposits of chromium and zinc contained in the plated layers, (ii) infrared spectra of the plated layers and (iii) electron probe micronanalyzer tests which analyze for chromium in the plated layer have been used. The X-ray diffraction tests ('FIG. 1) show that the layer obtained by the process of the present invention presents a diffraction image (1) of metallic zinc, similar to the diffraction image 2) of the plated layer obtained from a conventional acid zinc-electroplating bath.

On the other hand, the infrared spectrum (3) (FIG. 2) of the plated layer produced by a bath containing Cr+ ions has absorption bands in the neighborhood of 5 90 and 1130 cm.- These absorption bands are considered to correspond to the absorption bands at 610 and 1130 cmf which are obtained from an infrared spectrum of the chromate film on a steel plate which has been subjected to electrolytic chromating. It is felt that these absortion paterns suggest the existence of chromium hydrate. Thus, it can be considered that in the process of zinc electroplating in the presence of Cr+ ions, chromium c0- deposits as chromium hydrate simultaneously with the precipitation of zinc. Furthermore, it is considered that the deformation, and stretching vibration of OH are responsible for the absorption patterns at 1600 and 3200 cm. However, the plated layer obtained from a conventional acid zinc-electroplating bath indicates (4) that the stretching vibration of OH only is in the neighborhood of 3200 cm.

The results of a chromium analysis of the cross section of a plated layer obtained from a bath of the present invention, as determined by an electron probe microanalyzer, are shown in FIG. 3. The spectrum indicates a uniform distribution of chromium depthwise in the plated layer (10) as compared to the spectrum of a base steel sheet (11). In other words, unlike the distribution obtained by the chemical treatment of the surface of a plated layer, chromium is distributed throughout the depth of the layer which results in increased corrosion resistance.

Because of the uniform distribution of chromium on the surface of the plated layer, the as-plated condition of the layer obtained by the process of the present invention can present the same effect as is obtained from electrolytic chromating, thus providing improved corrosion resistance for the zinc plated steel plate.

As can be seen from the foregoing description, electrolysis of a sheet of metal in an acid zinc-electroplating bath containing a specific amount of Cr ions therein affords a plated layer which offers substantial improvements in corrosion resistance as compared with the plated layers obtained from conventional acid zinc-electroplating baths.

In addition, studies have been conducted on the problems associated with the adhesion of paint films to the plated layers as will be described hereinafter. A basic problem exists in that when a finish is baked on a plated layer of a steel plate which has been obtained from an electrolysis bath, there appears blisters on the interface between the base metal and the plated layer from the thermal effects of baking. This gives a defective appearance to the plated surface of the metal sheet.

Further studies by the applicants have revealed, however, that the development of blisters can be attributed to the presence of a very small amount of hydrogen which has occluded or is contained on the base metal, such as a steel plate during electrolysis. In order to solve this problem it has been found that the formation of hydrogen, and hence the formation of blisters which would have appeared in a subsequent baking step, can be completely prevented by first electroplating a normal or pure layer of zinc of a specific thickness on the metal sheet prior to electrolysis in the acid zinc-electroplating bath containing cr ions. This is well substantiated by FIGS. 4 and 5, which show the potential (mv. vs. SCE)current (a./dm. curves meaused for an acid zinc-electroplating bath containing Cr ions therein with regard to the instances where a mild steel sheet is plated and a zinc-plated mild steel sheet having a 0.1;1. thick plated layer is plated, respectively. As can be seen from FIG. 4, when a mild steel sheet is electroplated in an agitated acid zinc-electroplating bath containing Cr ions, hydrogen is evolved in the neighborhood of 1150 mv. vs. SCE (20) prior to the Zn-Cr codeposition at -l200 m-v. vs. SCE (21). On the other hand, when a mild steel sheet having an electroplated layer of pure or normal zinc 0.1,u. thick plated thereto is electroplated in an acid zinc-electroplating bath containing Cr"+ ions, co-deposition of Zn-Cr at l200 mv. vs. SCE (30) takes place prior to the development of hydrogen at 1400 mv. vs. SCE (31) (FIG. 5).

In another set of experiments, three samples of a mild steel plate 0.8 mm. thick, which were identified as plates A, B, and C were prepared. Plate A was electroplated on both surfaces in an acid zinc-electroplating bath containing Cr ions therein to produce layers 10a thick. Plate B was electroplated on one of its surfaces to produce a layer 10p. thick. Plate C was first plated with pure or normal zinc to produce a layer 01;; thick followed by an electroplating step in the acid zinc-electroplating bath containing Cr ions therein to produce an additional layer 10p. thick. The samples were then subjected to a boiling water immersion test. After one hour of immersion, blisters appeared throughout the surfaces of Plate A which had been electroplated on both of its surfaces in an acid zinc-electroplating bath containing Cr ions therein (FIG. 6a). However, only minute blisters appeared on the surface of Plate B which had been electroplated on one side (FIG. 6b). This suggests that in the case of Plate C, the hydrogen produced can escape through the surface of the steel plate which has not been electroplated. However, in the case of Plate A, where both sides had been electroplated, the hydrogen produced cannot find its way out to the surface of the steel plate, because of the presence of blocking surface layers. This results in the eventual formation of blisters.

On the other hand, in the case of a steel sheet which has been electroplated with pure or normal zinc prior to electroplating in an acid zinc-electroplating bath containing Cr ions therein, despite the plating on both of its surfaces, no blisters developed on either of its surfaces (FIG. 60). As can readily be appreciated from the foregoing test results, the blisters produced on the interface between the base metal and the plated layer during the baking step can be attributed to the presence of hydrogen which has evolved during the electrolysis and has occluded or is contained on the base metal. Thus, one of the most effective, preventive approaches to this problem is the electrolysis of a metal sheet in an acid zincelectroplating bath containing Cr ions therein, coupled with the preparatory step of plating the base metal with pure zinc. Furthermore, experiments have shown that the minimum thickness of the plated zinc layer necessary for preventing an outbreak of blisters is 1X10- r. This is well represented by FIG. 7, which is a plot of the potential of an electrode versus the thickness of a plated zinc layer in microns. The plot shows the change in the electrical potential from a base sheet (40) at a potential of -720 mv. vs. SCE to that of zinc plated sheet (41) at a potential of 1020 mv. vs. SCE with respect to the thickness of the plated zinc layer. When the thickness of the zinc layer exceeds about 1X10' ,u, the curve indicates that the potential is completely shifted to that of zinc. The development of hydrogen and hence the occlusion thereof is prevented during the electrolysis, thus indicating that it is necessary to maintain the thickness of the zinc layer greater than 1X M.

Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.

Example 1 shows a conventional art process for electroplating a metal sheet. Examples 2-8 represent the first embodiment of the electroplating process of this invention. Examples 9-11 represent the second embodiment of the electroplating process of this invention.

EXAMPLE 1 A mild steel sheet was electroplated under the following conditions to achieve plated layers of a thickness of 4;/..

Bath Composition:

ZnSO -7H O ....g./l 350 (NH4)2SO4 g./l 3

NH Cl g./l 30 Electrolytic Conditions:

pH of Bath 4.0

Temperature of Bath C 50 Current density ..a./dm. 30

One coating of an acrylic base paint was applied on the surface of the layer obtained under a baking temperature of 180 C. for 30' minutes. The final coat was 20 thick.

EXAMPLE 2 Bath Composition:

ZnS0 -7H 0 350 g./l.

(NI-1.9 50 30 g./l.

Cr0 0.095 g./l. (Cr 0.05 g./l.). Electrolytic Conditions:

pH of Bath 4.2.

Temperature of Bath 45 C.

Current density 20 a./dm.

Electroplating time 42 sec. (corresponding to a layer 4n thick).

The metal sheet was covered with a coating of paint similar to that of Example 1.

EXAMPLE 4 Bath Composition:

ZnSO -7H O 350 g./l.

(NH SO 30 g./l.

CrO 0.19 g./l. (Cr 0.10 g./l.). Electrolytic Conditions:

pH of Bath 4.0.

Temperature of Bath 50 C.

Current density 30 a./dm.

Electroplating time 28 sec. (corresponding to a layer 4, thick).

EXAMPLE 5 Bath Composition:

ZIISO4'7H2O (NI-10 50 40 g./l. K Cr O- 0.36 g./l. (Cr 0.13 g./l.). Electrolytic Conditions:

pH of Bath 3.8.

Temperature of Bath 50 C.

Current density 30 a./dm.

Electroplating time 28 sec. (corresponding to a layer 4 thick).

EXAMPLE 6 Bath Composition:

ZnCl 250 g./l.

NH Cl g./l.

(NH4)2CI'207 0.38 g./l. (Cr 0.15 g./l.). Electrolytic Conditions:

pH of Bath 4.0.

Temperature of Bath 60 C.

Current density 20 a./dm.

Electroplating time 42 sec. (corresponding to a layer 4 4 thick). The metal sheet was covered with a coating of paint similar to that of Example 1.

EXAMPLE 7 Bath Composition:

ZnS0 -7H O 250 g./1.

ZnCl 100 g./l.

NH CI 50 g./l.

Na SO Cr0 0.39 g./l. (Cr 0.20 g./l.). Electrolytic Conditions:

pH of Bath 3.0.

Temperature of Bath 50 C.

Current density 30 a./dm.

Electroplating time 28 sec. (corresponding to a layer 4n thick).

7 EXAMPLE 8 Bath Composition:

ZnSO -7H O 450 g./l. (NI-{Q 80 50 g./l. NH Cl 50 g./l. 5 00;; 0.57 g./l. (Cr 0.3 g./l.). Electrolytic Conditions:

pH of Bath 3.0. Temperature of Bath 65 C. Current density 30 a./dm. 10 Electroplating time 28 sec. (corresponding to a layer 4;]. thick).

EXAMPLE 9 A mild steel sheet was electroplated on its surfaces under the conditions below in which (i) pure zinc layers 1Xl0 ,u thick were first electroplated followed by (ii) a second electroplating step using an electroplating bath containing Cr ions to give a layer 4;]. thick.

The metal sheet was covered with a coating of paint similar to that of Example 9.

EXAMPLE 11 The procedures of Examples 9 and 10 were followed except for the conditions listed under (ii).

(ii) Conditions of acid zinc-electroplating hath containing Cr ions Bath Composition:

ZnSO -7H O 250 g./l. ZnCl 100 g./l. N'H Cl 50 g./l. Na SO g./l. CrO 0.39 g./l. (Cr 0.2 g./l.). Electrolytic Conditions:

pH of Bath 3.0. Temperature of Bath 50 C. Current density 30 a./dm.

Table 1 gives the results from the corrosion resistance tests and the adhesion to paint film tests of the plated layers which have been obtained in Examples 1-11.

TABLE 1 Corrosion resistance (salt spray test) Paintability Adhesion of Initial Initial Helical Cross-out Appearance plated layer development development pattern Cross-out Ericksen test (color (close contact 01 white rust of red rust describing adhesion 5 mm. Example tone) bending) (Zno), hr. (F0203), hr. adhesion test test extrusion Denotes the peeling of a plated layer from a mild steel plate.

(i) Pure or normal zinc electroplating conditions Bath Composition: 40

ZnSO -7H O 350 g./l. (NI-10 80 30 g./l.

Electrolytic Conditions:

pH of Bath 4.0. Temperature of Bath C. 45 Current density 30 a./dm.

(ii) Conditions of acid zinc-electroplating bath containing Cr ions Bath Composition:

One coating of an acrylic paint was applied on the surface of the plated layers to give a layer 20 1. thick after baking at a temperature of 180 C. for 30 minutes.

EXAMPLE 10 The same procedures of Example 9 were followed except for the conditions listed under (ii).

(ii) Conditions'of acid zinc-electroplating bath containing Cr ions Bath Composition:

ZnCl 250 g./l.

NH Cl 100 g./l.

K Cr O 0.43 g./l. '(Cr 0.15 g./l.). Electrolytic Conditions:

pH of Bath 4.0.

Temperature of Bath 60 C.

Current density 20 a./dm.

It is apparent from the data in Table 1, that the corrosion resistance of the plated layers given in Examples 2 through 11 (layers plated according to the process of the present invention), is superior to that of Example 1 (layer plated according to the process of the prior art) and that the time required for the formation of rust on the plated layers formed by the process of the present invention is much longer than in the instance where a layer is formed by a process of the prior art.

Similarly, with regard to the adhesion of the layers to paint films, significant improvements have been noticed for the metals plated by the process of this invention. That is, the results of the helical pattern describing adhesion test and the cross-cut adhesion test materially suggest that significant improvements over the prior art processes have been made in the adhesion of the plated layer to paint films (compare Example 1 with Examples 2 through 8). However, as is clear from Table 1, the best results are obtained when the second embodiment of the present invention is employed, wherein a pure zinc layer is applied to produce a layer on a metal sheet of a specific thickness, followed by electrolysis in an acid zinc-electroplating bath containing Cr ions. This fact is well substantiated by the superior data obtained from Examples 9 to 11 in comparison to the data of Examples 1 through 8.

For a better understanding of the data shown in Table l, the helical pattern describing adhesion test and the cross-cut adhesion test will be described in further detail. The helical pattern describing adhesion test is a test made by the tester 50 shown in FIG. 8, wherein a needle 51 is mounted on the lower tip of the shaft 52 which extends vertically and is adapted to rotate. The needle is designed so that it may describe a helical pattern on a painted surface as shown in FIG. 9. Mounted on the top of the shaft 52 is a loading dish 53 on which is placed a weight 54 weighing 300:1 g. The diameter 9 of the helical pattern described is limited to 10 mm. while the length thereof is over 30 mm. After describing a helical pattern on the painted surface of a plated metal sheet, a piece of adhesive tape (such as Scotch tape) is placed on the sheet over the helical pattern. The tape is suddenly peeled from the sheet to observe how much of the paint film is removed from the metal sheet. FIG. 9 shows the inspection criteria.

The cross-cut adhesion test is a test in which the film of paint on a sample metal sheet is cut into a grid pattern with a blade such as a safety razor to a depth which reaches the electroplated surface of the metal sheet. The spacing between adjacent lines of the grid pattern is 1 mm., with eleven lines, longitudinal and lateral, being described so as to intersect each other at right angles. This provides 100 sections 1 mm. in area. Then, a strip of adhesive tape is placed on the grid pattern and is suddenly peeled from the sample. The paintability or the adhesiveness of the paint coating on the plated layer is determined by the amount of paint removed. If all of the sections remain in tact, i.e., if no paint is removed from any of the 100 sections, then the layer is graded as 10 on a scale of 0 to 10. If paint is removed from all of the sections, the layer receives a grade of 0.

The data in the table for the first and second embodiments of the invention (Examples 2 to 11) indicate that the development of red rust in the plated steel sheets takes about three times longer to develop over the plated steel sheets of the prior art processes.

The paintability test in the table indicates that the best results are achieved when metal sheets are plated by the process of the second embodiment (Examples 9 10 to 11) when compared to the data of Examples 1 to 8.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.

What is claimed as new and intended to be covered by Letters Patent is:

1. A process for improving the corrosion resistance and paintability of steel plate which comprises electroplating a pure zinc layer of at least 1X10- p. onto said steel plate and then subjecting said zinc-coated steel plate to electrolysis as cathode in an aqueous acidic zinc electroplating bath containing 0.05 to 0.3 g./l. Cr+ therein to deposit an additional layer of zinc thereon.

2. The product of the process of claim 1.

References Cited UNITED STATES PATENTS 3,505,184 4/ 1970 Schaedler et al. 204-55 R 2,080,520 5/1937 Westbrook 204-55 Y 2,428,356 10/1947 Chester et al. 204-55 Y X 2,406,072 8/1946 Gaver 204-55 R X FOREIGN PATENTS 1,488,214 6/1967 France 204-55 R 1,496,747 5/1969 Germany 204-55 R GERALD L. KAPLAN, Primary Examiner US. Cl. X.R.