US2288007A - Corrosion resistant film on zinc - Google Patents

Description

June 30, 1942. J. c. LUM

CORROSION RESISTANT FILMS ON ZINC Filed Feb. 26, 1941 Acid andCfi/o/r/aife Boiling i .So/ufion. Sfabi/iziny Washing Solufion 4 Cleaning y 9 /4 /2 l4 INVENTOR Patented June 30,1942

2,288,607. f L I CORROSION RESISTANT Fn -o v ZINC I John C. Lum, Union, N. .l'., assignor to wcstinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsyl Vania Applicat ionFebruary26,1941. Serial.No..380,6 53 l a Claims. 101. 148156)" g l 1 This invention relates to the coating of zinc orcadmium base metals and has for its object the provision of a durable'corrosion resistant coating or film on metals. Throughout the spec ification and claims, by zinc or' cadmium base metals, it is intended to denote metals and alloys composed principally of zinc or cadmium,

including other base metals coated with a zinc or cadmium base metal layer applied for example by galvanizing, electroplating and thelike.

Under certain atmospheric conditions of high humidity and elevated temperatures, certain metals such,'for example, as zinc and cadmium,

tend to corrode rapidly withthe production of-a white chalky surface which givesthe metals a very poor appearance. At temperatures above 60 C; and at high humidity zinc will'breakdown with the formation of awhite powder ina short period of time. Several processes are known to the prior art in which protective coatingsare produced on zinc orcadmium base metals. In particular, the prior art process in which zinc or cadmium base metals are given a protective coating by subjecting the metal to a solution containing a mineral acid and a dichromate has produced coatings which havesome value. However, even this type of coating is unstable when the temperature to which the member is subjected exceeds 60 C. and the humidity is fairly high. Furthermore, this latter process does not produce entirely satisfactory permanent protective coatings on zinc or cadmium base metals.

The Object of this invention is to provide for stable protective coatings on zinc or cadmium base metals.

' Other objects of the invention will be obvious from the specification and claims.

The invention will be understood most readily from'the following detailed description thereof, when considered in connection with the accompanying drawing, in which a schematic layout of the several steps of the process is detailed.

This present invention relates more specifically to the treatment of zinc or cadmium base metals or third metals coated with zinc or cadmium base metals by a two-step process, the first of which is substantially the protective coating process outlined in Patent No. 2,035,380 and is followed by a second process which stabilizes and improves the coating provided by the first: proc-. ess as detailed in this patent. I

It has been found that when zinc or cadmium base metals are subjected to a solution containing from 1 to 80 cubic centimeters-of sulphuric acid and from about 25 ;to.300or more grams base metal object may be of adichromate per liter of water that the metal acquires a protective coating which isvaluable for resisting corrosion within a restricted range of atmospheric conditions. Thisprotective coating lacks adequate corrosion resistance when'subjected to temperatures; above C.

with a relatively high humidity such as is pro duced bysteam at this temperature. Within 24 hours, thesurface finish will break down with the formation of the characteristic whitepowder on zincbase metals. V

The sulphuric acid-dichromate coating may be stabilized and-rendered exceedingly durable over a wide range oftemperatures andzat'high humidities so that under a test similar to that mentioned above, the stabilized finish. merely" turns slightly gray after several weeks-of sus-v pension in a steam chest at 60 C. and with no evidence of any white corrosion products after a one month test.

pearance, being either of alight blue-gray or a greenish-gray color. a g

The stabilizing after-treatment, following the acid and chromate solution treatment, .consis'ts in treating the zinc or cadmium base metalwith a boiling aqueous solution containing 1%. or more of an alkaline metal hydroxide or some waterv soluble chromate r -dichromate salt.

For -a more complete description of'the process, reference is made to the single figureof drawing, in which a schematic arrangement l0 suitable for performing the process is showm In the single, figure of the drawing,,an endless conveyer system is shown as one possiblemode of practicing the process. This is not necessarily the preferred method oftreating zinc Or cadmlumbase metal but is merely an example of processing which is adapted to present-day manufacturing operations. The .endless'conveyor l2 runs on and is guided by pulleys lfl. The

endless conveyors l2 are constructed ,so, that If any desired shape or type of zinc or cadmium the treatment. 1 The first process step to which the objects on conveyor l2 are subjected is acIeaning'Operation at Hi, to free them of 'superficial'di'rt' and grease. Onesatisfactory method is'to submerge.

the objects in a hot solution of trisodium phosphate forone-half to three minutes" while making them a cathode employing'enough current H to cause a copious evolution of gas; Vaporjde- I greasing and similar cleansing-maybe employed in;order to produce a clean surface for subse- H n addition, the s. u v bilized. finish ismuch more attractive in ,ap-

suspended thereon for le e .duces goodresults. The stabilizing solution has quent treatment. Any superficial adherent im-' purities'may be removedat the end oi the oath odic treatment, for example, by rinsing in hot and in cold water.

The cleansed zinc or cadmium base alloys or members are then carried by the conveyor H to the apparatus It in which the acid and chromate solutlonis, applied thereto. The mineral acids which have been foundto be satisfactory for this coating operation are sulphuric, hydrochloric and nitric acids. Sulphuric acid is superior to the other acids in some respects and is generally preierred. From 1 to 80 cubic centimeters of sulphuric acid of a specific ravity oi 1.84 maybe I added per'liter of water. However, from 2 to cc; pe'r'liter have given good results. Soluble chromate and dichromate salts and chromic acid are added to the sulphuric acid-water solution. 11 chromium trioxide is employed, from 25 to 200 grams per liter of water produces satisfactory bilization solution has been used following the. initial treatment. As previously mentioned, the v stabilizing coating exhibits lughlysuperior temperature and humidity resistance characteristics.

1 The order :of the, application of the solutions may not be'ehanged since the protective coating is produced only by applying the solution in'the results, .The amount of sulphuric acid is prei'er ably increased in the case of greater concentratiori oi chromium trioxide- Ii asoluble dichromate salt, such for example, as sodium dichromate 'is'employed, from to 300 or more grams per liter of the salt may be used per liter of water.

Referring to the drawi g the zine-er aha base metals are immersed in the solution at I! for a time period ranging irom2 seconds to 3 minutes. A dip of 10 seconds duration has been found to be satisfactory with a solution containing "grarns 'of chromic acid and 10 grams of sulphuric acid per liter. If this acid and chror mate solution application were depended. upon as the sole protective film forming agent, the time "of application of the solution is quite critical.

Prolonging the immersion beyond acertain opti- "mum time results in a destruction of some of. the good characteristics of the protective coating. By following the acid and chromate solution with the stabilizing treatment of this invenorder given.

. a The stabilized coating is washed in hot water at 22 and dried at 24. may be employed to expedite drying. In some cases wiping with rags or sawdust may be resorted to in order to dry the metal. The articles may be removed from the conveyor l2 and immediately putto usein apparatus. .1; 7

.The protective film produced at-sta geit,

taught by the'prior art, had only temporary protective value and is not of a permanent char acter. .The stabilized film or coating produced by the entire process disclosed in thepresent case is satisfactory for use as a permanentprotectivecoating. It is only under extreme corrosiveconditions suchas exposure to highly corrosive acids'and severe abrasive service that the film isdestroyed. Average conditions of service, suchas result from ordinary weather and ethics pheric effects, are adequately met by .the protective-film produced bythe present invention.

tion, thelength of time of the first dip isnot at all critical. Accordingly, somewhat prolonged immersion'times may be used without sun'ering adverse effects, 7

. The base metal may be lifted out of the acid and chromate solution and the solution allowed todrip ,off before application'of a boiling stabiliz-,

.ing solution, at 20, "It has been discovered that the coating produced by the first treatment is greatly improved by following it with a boiling aqueous solution containing 1% 'ormore of-so- Q jdiumior" potassium hydroxide or of'a soluble. "chromate or dichromate salt. The following three examples of suitable composition for pro-' 7 ducin ig""the stabilizing "oi tl i e'acid dichromate coating have produced good results: (1) 10% so-' dium dichromate, 90% water; (2) 1% sodium hy-r w droxlde, 99% water and (3) 10% sodium 'dichromate, 1% ,,sodium hydroxide, 89% water. I The three examples are not intended to be limitations. ,Any alkaline'metal hydroxide such as tivew hen applied at a boiling temperature. Howtemperatures slightly below boiling also probeenefiective when the timeoi application has been'varied'irom '15 seconds to 5 minutes. A 30 second dip in the boiling solution has been used sodium and potassium hydroxidemaybe used;-.

, for this purpose, Both chromate and dichro matesalts which are soluble in water may be I. employed. The sodium and potassium chromates and dichromates are satisfactory.

'I'hestabilizing solution has been most eiiec- In producing the stabilizing solution for ,use at 20, any or. all of the salts and hydroxideslisted may be mixed and applied. The main requirement is that the aqueous, solution contain 1% or more,

ofthe stabilizing chemical in order to produce rapid and satisfactory. results.

While -I have described and illustrated my lll-'- vention in connection with a single preferred form of embodiment, it will be understood that various changes and, modifications may be made by-those skilled in the art without departing from the essential spirit of my invention.- It is desired, .therefore, that -the appended claims shall be accorded the broadest construction consistent wit their language and the'prlor art.

I claim asmy invention: 1". .The method of, providing a stable protective coating on zinc or cadmium base metals which comprises applying a solution containing achromium compound and a. mineral acid to the metal and thereafterv applying a hot aqueous solution containing at least one solute selected from the group consistingof alkalihydroxides and chromate and dichromate salts to provide m a durable, heat stable protective coating.

2. The method of providing a stable protective coating on metals of the group consisting of zinc,

cadmium and base alloys thereof which com-' prises applying a solution containing a chromium compound and a mineral acid to the metals to produce a protective coating which is relatively unstable under humid conditions at elevated temperatures and subsequently applying. an aqueous solution containing over one percent of a solute selected from the group consisting of alkali metal hydroxides; chromate salts and dichromate salts, the aqueous solution being applied at substantially boiling temperature, the

The same Heat or a hot air blast a solution containing from 25 to 300 grams of a chromium compound selected from the group consisting of chromium trioxide and dichromates and more than 2 cc. of a mineral acid'per liter of water, to provide for a protective coating on the metal, and subsequently applying a hot aqueous solution at about boiling temperature to the metal, the aqueous solution containing 1% or more of one or more selected from the group consisting of alkali metal hydroxides, chromate salts and dichromate, salts, the hot aqueous solution imparting stable properties to the coating whereby its resistance to high temperatures and high humidity is increased.

4. The method of increasing the stability of protective chromate coatings on metals selected thereof which comprises applying to the metal base metals, which comprises applying a hot aqueous solution of 1% or more of a solute comprising one or more members of thegroup cont-i sisting of alkali metal"hydr oxides, chromate salts and dichromate salts..

5. An article of manufacture, comprising in combination, a zinc or cadmium base metal memher and a protective surface thereon composed of the reaction product of the zinc or cadmium base metal andan aqueous solution of a chromium compound and a mineral acid and the reaction product being further treated with a substantially boiling aqueous solution containing 1% or more of at least one selected from the group consisting of alkali metal hydroxides, chromates and dichromates, the final reaction product having greater corrosion resistance and more uni-.

form and attractive appearance than the first reaction product.

- JOHN C. LUM.

from the group consisting of zinc and cadmium

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