PATENT office 2,035,380 UNITED STATES. Ernest John Wilhelm, Palmerton, Pa., assignor. Patented Mar. 24, 1936

Transcription

1 Patented Mar. 24, ,0,380 UNITED STATES PATENT office 2,0,380 METHD F CATING ZNC R CADMIUM BASE METALS Ernet John Wilhelm, Palmerton, Pa., aignor to The New Jerey Zinc Company, New York, N. Y., a corporation of New Jerey No Drawing. Application May 13, 1933, Serial No. 6,992 Thi invention relate to the coating of zinc or cadmium bae metal, or zinc or cadmium coated metal, and ha for it object the proviion of an improved method of producing a ueful coating or film on uch metal. By zinc or cadmium bae metal' I mean metal and alloy compoed principally of zinc or cadmium, including met al (e. g. ther than zinc) coated with a zinc or Cadmium bae metal a for example by galvaniz 0 ing, electroplating, etc. In my co-pending application, Serial No.,138, filed February 21, 1936, I have decribed and claimed generally the production of viible pro tective coating on malleable non-noble heavy metal by method analogou to thoe herein de Scribed. The invention i particularly concerned with the production of a viible film or coating on zinc or cadmium metal, including zinc or cad 17 Claim. (C. 1486) mium bae alloy, for the purpoe of retarding corroion and particularly the formation of white corroion product reulting from atmopheric or. moiture expoure. rdinary commercial zinc (in particular, rolled or trip zinc), zinc bae al loy uch a die-cating alloy containing alumi num, magneium and/or copper (for example, the alloy of United State Patent No. 196,761, 1,663,21, 1779,, 182,434, 182,441 and Re. 18,600), and rolled zinc-bae alloy of United o State Patent No. 1716,99, may be advanta geouly coated by the method of the invention. It ha been uggeted heretofore to treat zinc with chromate or dichromate olution, or with dilute olution of chromic acid, to paivate the metal and increae it reitance to corroion. It ha alo been uggeted to treat zinc or cadmium. with a olution containing chromic and ulphuric" acid to produce a o-called "bright etch' or bright metallic finih. In treating zinc or cad mium with chromic and ulphuric acid it ha been the object to employ a olution having uch a ratio of ulphate ion to chromate or dichro mate ion that a "bright etch' Would reult with out forming a film or coating on the metal. So far a I am aware, uch heretofore uggeted im merion method have not intentionally pro duced a viible or corroion retarding film. During the coure of my invetigation I have -found that if ulphate anion (in the form of 0 ulphuric acid or a oluble ulphate that doe not form a precipitate with. chromic acid) be added in gradually increaing amount to a chromic acid olution in which zinc i dipped, that the action of the olution on the zinc varie. with the ulphate anion concentration. When the amount of ulphate anion i very mall, very little zinc i diolved and the zinc urface i found to be lightly paivated. A the concen tration of the ulphate i increaed the amount of zinc diolved increae, and the zinc begin 10 to be viibly etched. Thi etching action con tinue until a certain ulphate concentration i reached, when a viible coating begin to form. Within the range of ulphate concentration which produce the mot deirable coating, the i amount of zinc diolved no longer increae ap preciably with the increae in ulphate concen tration. Within a narrow range of ulphate concentration beyond that at which the coating firt form, the coating i emi-tranparent and to ha a noticeable blue tinge. After thi range ha been paed, the coating become iridecent and finally develop a golden-brown (bronze or bra color). Further increae in the ulphate con centration caue the film to diolve, o that the i action on the zinc i merely an etching. I have found that the appearance of the coat ing a to color and iridecence varie lightly with the metal employed for a given concentration of chromic and ulphuric acid. Moreover, the o range of ulphate concentration, within which coating are formed, varie lightly with the metal employed, whether it be cadmium, zinc, or a zinc-bae alloy, and cloely parallel that de cribed for zinc. If the metal treated be a zinc-bae alloy containing copper, for example a rolled zinc-bae alloy containing 1 per cent copper and 0.01 per cent magneium, the diap pearance of the film due to the preence of an exceive amount of ulphate ion i indicated by a change in color of the treated metal from the golden-brown of the film to black due to the o lution of copper from the alloy and it reprecipi tation infinely divided black form. If a Solution of odium dichromate be em- ployed in place of chromic acid, and ucceive addition of Sulphuric acid be made, the reac tion of the metal immered in the olution are in general imilar to thoe hereinbefore de cribed for chromic acid olution. With a odi- 0

2 2,08,880 um dichromate olution, however, the amount of zinc diolved i much le than with a chromic acid olution. Moreover, in the range of Sull phuric acid concentration within which the mot deirable coating are formed, the amount of zinc diolved continue to increae a the ulphuric acid concentration i increaed. In carrying out the treatment according to the invention the metal appear to undergo the foll lowing action: (1). Etching of the metal, (2) paivation of the metal, and (3) the formation of a viible protective coating. I attribute thee action which appear to occur in the formation of a protective coating, to the character of Solu tion employed in the treatment. A a reult of my invetigation I have di covered that a viible, colored, protective film can be formed on zinc or cadmium bae metal that i reitant to the corroive action of the atmo phere or moiture. I have found that if the met all be immered for a relatively hort time in an acid olution containing chromate or dichromate ion and the anion of a mineral acid, that a vi ible film or coating will be formed thereon. By mineral acid' I mean Sulphuric acid, hydro chloric acid, and nitric acid. Baed upon thee dicoverie, my preent invention involve, in it broad apect, the formation of a viible coating on zinc or cadmium bae metal by immering the metal in an acid olution containing chromate or dichromate ion and the anion of a mineral acid. Solution for practicing the invention may contain chromic acid and the anion of the Salt of a mineral acid, or a dichromate and a free mineral acid. In carrying out the invention the article of zinc or cadmium bae alloy to be coated are ubjected to an immerion treatment in an acid olution having an appropriate ratio of chro mate, or dichromate, ion to the anion of a min eral acid to form a viible coating. The film or coating formed may vary from a golden-brown (bronze or bra color) to black and tend to di play iridecence. An appropriate Solution for carrying out the invention may be formed of chronic acid and a alt of a mineral acid uch, for example, a di um ulphate, odium chloride, ammonium chlo ride or odium nitrate. Free mineral acid may be added if deired. A Satifactory olution may contain from about gram of chromium tri xide per liter of water up to the Saturation point, but the range from about to 0 gram per liter of water ha been found to be highly Satifactory. The relatively dilute Solution produce le attractive film than the olution of higher concentration. With relatively highly concentrated Solution the lo by drag-out i un neessarily high. The quantity of mineral acid anion in the olution hould be proportional to the amount of chromic acid preent. With o lution containing, for example, about 0 gram of chromium trioxide per liter of water, from 2 to 10 cc. of Sulphuric acid (pecific gravity 1.84) may be added per liter of Water, and in the cae of a olution containing about 0 gram of chro mium trioxide per liter of water, about 8 to 0 cc. of ulphuric acid may be added per liter of water. The ulphate ion may be added in the form of a alt, uch a odium ulphate, in amount equiv alent to the above indicated amount of ulphuric acid. The film formed on Zinc or cadmium bae metal when immered in Solution of the above type poe a golden brown (bronze or bra) color and tend to diplay iridecence, epecially when the amount of ulphuric acid i low in pro portion to the amount of chromic acid. In cae the content of ulphuric acid i very low, the amount of Sulphate anion being jut above the amount at which only etching occur, the film formed are emi-tranparent having a noticeable blue tinge. If hydrochloric acid or an equivalent amount of a Salt uch a odium chloride be added to the chromic acid, the amount neceary produce a film having a given color or appearance i lightly greater than if Sulphuric acid or odium Sulphate be employed. If nitric acid be added to the chromic acid, very much larger amount of nitric acid mut be ued in order to obtain Satifactory coating, and in general the coating are le deirable than thoe produced by the addition of ulphate r chloride ion. : The invention may be carried out by employing a olution containing a dichromate, for example Sodium or potaium dichromate, in which the acidity i provided by a mineral acid, for example Sulphuric, hydrochloric or nitric acid. If the Sodium Salt be ued, the olution may contain from about to 0 or more gram of odium dichromate (Na2Cr27.2H2) per liter of water, the range from about 0 to 0 gram per liter being preferred. The film formed on zinc by mean of uch olution may vary from a pale and iridecent nature to a uniform, very lightly iridecent bronze color, depending upon the amount of Sulphuric acid in olution. Thu, with a Solution containing about 0 gram of odium dichromate per liter of water, the quantity of Sulphuric acid (pecific gravity 1.84) required to be added to the olution varie from about 1.0 to 19. cc. per liter of water; however, an amount of acid within the range of from 1. to 7. cc. per liter of water i generally preferred. When uing a olution containing about 0 gram of Sodium dichromate per liter of water, the quan tity of ulphuric acid (pecific gravity 1.84) re quired to be added to the olution may vary from 2 to 80 cc, per liter of water, while a pre ferred amount of acid will generally be found within the range of from 3 to 19 cc. per liter of Water. Thi latter range of acid content alo applie to Solution containing from about 100 to about 0 gram of odium dichromate (Na2Cr27.2H2) per liter of water. If hydrochloric acid be added to the dichromate Solution intead of, ulphuric acid, ufficient hy drochloric acid mut be added to make the chlo ride ion concentration omewhat greater than. the concentration of the ulphate ion a indicated in the example immediately preceding. If nitric acid be added to the dichromate olu tion, the amount required to produce a ati factory film i much greater than that of either Sulphuric or hydrochloric acid. - Before immerion the metal article to be treated hould be cleaned to remove dirt or greae. Thi i preferably done by a cathodic treatment of the article for A to 3 minute in a hot olution of triodium phophate, uing enough current to caue copiou evolution of ga. At the concluion of the cathodic treatment the article i rined in hot and then in cold water to remove any adhering impuritie. The coating are formed in all modification of the invention by imply immering the article in the olution for period ranging from 2 econd to 3 minute, while period of from 2 to econd S 60

3 , are uually of ufficient duration to produce at ifactory reult. The immerion time depend Somewhat upon the compoition of the olution being ued, but chiefly upon the compoition of the metal being treated. The longer immerion period may be ued with rolled cadmium or zinc, galvanized iron, or a rolled zinc-bae alloy con taining, for example, about 1 per cent copper and 0.01 per cent magneium. With zinc-bae alloy of the type uitable for die-cating, for example thoe containing aluminum, magneium and/or copper, the immerion period mut be con iderably reduced in order to prevent looening of the film which firt form. Thu, with a olu tion containing about 0 gram of chromic acid per liter of water. and an appropriate amount of ulphuric acid, it may be, deirable to limit the immerion time to a little a 2 econd. With a olution containing about 0 gram of chromic acid per liter of water, the immerion time may be extended to about 10 econd. When employ ing a olution containing odium dichromate and ulphuric acid, the immerion time for the die cating alloy may be extended to Second or more. The metal and olution may be at normal room temperature when carrying out the im merion treatment. The immerion Solution gradually become pent, mainly becaue of the exhaution of ion containing hexavalent chromium in the produc tion of the coating, which latter appear to con tain a ubtantial amount of trivalent chromium along with ome hexavalent chromium. After the immerion treatment, the article i cleaned of adhering olution by rining in cold water, and then dried by any appropriate mean a in an air blat. The coating i fragile imme diately after it formation and mut be aged for a hort time before being ubjected to abra ion. A an illutration of the enhanced protection againt corroion which i offered by the pre ence of the above decribed type of viible film on cadmium, zinc and zinc-bae alloy, the fol lowing example may be given: If a clean piece of rolled zinc be expoed ut door, the urface i rapidly tarnihed, and white corroion product form in a few day. If a ini lar pecimen be immered in a olution of chromic acid containing 0 gram of chromium trioxide (Cr3) per liter of water, ed, dried, and then be expoed outdoor, the rate of tarnihing may be very lightly retarded, but white corroion product will form in a few day a before. If, to the above decribed chromic acid olution containing 0 gram of chromium trioxide (Cr3) per liter of water, a mall amount of ul phuric acid be added (for example,. cc. per liter of water of ulphuric acid of p. gr. 1.84), a rolled zinc pecimen immered, therein will be lightly etched. Thi amount of ulphuric acid i ufficient to produce on the zinc a condition which i on the borderline between that of imple etching and that of viible film formation. If a pecimen of zinc be treated in thi olution and then expoed outdoor, trace of white corroion product will be oberved in a few day, epecially if a rainfall occur, and the urface of the peci men gradually become dull, and in from three to four month i covered with a gray film of Cr roion product. If the amount of ulphuric acid per liter of water in the above olution be increaed (for example, to 17. cc. of ulphuric acid of p. gr. 1.84), a rolled zinc pecimen immered therein 9,08,880 acquire an iridecent, brown coating. If uch a treated pecimen be expoed outdoor it ha been found that after a much a ix month the coat ing till retain it original appearance and that no white corroion product have formed. A imilar olution, in which the ulphate wa. added in the form of odiuim ulphate intead of Sulphuric acid, wa ued to treat a pecimen of a rolled zinc alloy containing 1 per cent cop per and 0.01 per cent magneium. After ex psure of thi pecimen outdoor for more than two year, it wa found that, except for adhering particle of dut and dirt, the pecimen had changed very little in appearance and no white corroion product were to be een. The protection of zinc againt corroion ac cording to the method of the preent invention ha been further tudied by determining the change in weight which occur upon outdoor expoure of treated and untreated pecimen. With untreated pecimen it i found that dur ing the firt month or more of outdoor expoure there i an increae in weight due to the ac cumulation of white corroion product; with continued expoure the pecimen lowly loe in weight, correponding to the olution of zinc and the removal of the enuing corroion prod uct. The ame phenomena regarding change in Weight were oberved in the cae of pecimen of rolled zinc paivated by immerion for three minute in a chromic acid olution (0 gsm of chromium trioxide (Cr3) per liter of water) before expoure utdoor. Specimen treated by immerion in a chromic acid olution containing ufficient ulphuric acid to produce a viible film, according to the method of the preent invention, how no increae in weight due to the formation of white corroion product upon expoure outdoor; on the con trary thee pecimen loe in weight from the beginning of the expoure, but at a rate, which wa very much le, over a period of month,. than the rate at which untreated pecimen loe weight. The following example illutrate the practice of the invention with different olution and dif ferent zinc-bae alloy. Solution I Water cc. Sulphuric acid (94% H24, p. gr. 1.84) -- 6 cc. Sodium dichromate (Na2Cr22H2)-- 0 gram Solution II * Water cc. Sulphuric acid (94% H24, p. gr. 1.84) -- 3 cc. Chromic acid (Cr3) gram Nature of metal treated Metal Compoition %. Al % Cu. % Mg No. 1, die cat No. 2, die cat- - 4, 10 No. 3, die cat No. 4, die cat i No., rolled zinc Balance zinc 3 i0. Zinc with about 0.0% lead. No. 6, rolled zinc containing about 0.0% lead. The following table how the type of Cat ing obtained by immering the alloy and the rolled zinc in the above decribed olution at 7 3) 0

4 10 No Dull iridecent coating No ridecent coating with bra tinge No ridecent coating No D0. No Slightly iridecent bra colored coating No Sightly iridecent bronze colored coating SS TS 4. 2,08,880 room temperature. All the coating were firmly adherent and afforded good protection againt corroion: Solution. I Duration of Metal immerion decription of coating (in econd) Solution II No Iridecent coating No Iridecent coating with yellow tinge No ridecent coating No D0. No ridecent bra colored coating No Slightly iridecent bronze colored coating The film or coating of the invention not only protect the zinc or cadmium bae metal from cor roion but are alo ueful for decoration. The coating produced in accordance with the in vention retard the formation of corroion prod uct that form on bright zinc or cadmium bae metal urface expoed to the atmophere or moiture. Zinc coated according to the method of the invention can be oldered a readily a uncoated zinc. I claim: 1. The method of producing a viible protective coating on a zinc or cadmium bae metal which comprie ubjecting the metal to immerion treatment in a olution containing about gram or more of chromium trioxide per liter of water, and a mineral acid of the group com priing ulphuric acid, hydr chloric acid and nitric acid. 2. The method of producing a viible protec ion treatment in a olution containing about gram or more of chromium trioxide per liter of water, and 2 to 0 cc. of ulphuric acid (pecific gravity 1.84) per liter of water. 3. The method of producing a viible protec ion treatment in a olution containing from to 0 gram per liter water of a dichromate, for example odium or potaium dichromate, and a mineral acid of the group compriing Sulphuric acid, hydrochloric acid and nitric acid. 4. The method of producing a viible protec ion treatment in a olution containing from to 0 gram of odium dichromate, and from 1 to 80 cc. of Sulphuric acid (pecific gravity 1.84) per liter of water.. The method of producing a viible protec ion treatment in a olution containing about 0 gram of chromic acid plu 2 to 10 cc. of ul phuric acid (pecific gravity 1.84) per liter of Water. 6. The method of producing a viible protec ion treatment in an aqueou olution contain ing from to 0 gram of chromic acid plu 2 to 80 cc. of ulphuric acid (pecific gravity 1.84) per liter of water. 7. The method of producing a viible protec ion treatment in an aqueou Solution contain ing about 0 gram of odium dichromate and from 3 to 19 cc. of ulphuric acid (pecific grav ity 1.84) per liter of water. 8. The method of producing a viible protective coating on a zinc or cadmium bae metal which comprie ubjecting the metal to immerion treatment in an aqueou olution containing to 0 gram of odium dichromate and 1 to 80 cc. of ulphuric acid (pecific gravity 1.84) per liter of water. 9. The method of treating an object having a urface of zinc or cadmium bae metal which comprie ubjecting the urface of the object to the action of an aqueou Solution containing Sull phuric acid and from to 0 gram of odium dichromate per liter of water until a viible cor roion-retarding coating i formed n Said Sur face. 10. The method of forming a Crroion-re it ubjecting the urface of the object to the action of an acid olution containing chronic acid ion and the anion of a mineral acid of the group coniting of ulphuric acid, hydrochloric acid and nitric acid, regulating the ratio between chromic acid ion concentration and the concen tration of mineral acid anion in aid Solution within predetermined limit, and leaving the Sur face of the object in contact with the Solution until a color ranging from yellow to black ap pear on aid Surface. 11. The method of forming a corroion-re itant coating on an object having a Surface of ubjecting the urface of the object to the action of an acid olution containing chromic acid and a alt of a mineral acid of the group coniting of odium ulphate, odium chloride, ammonium chloride and odium nitrate, regulating the ratio between the chromic acid concentration and the concentration of the alt of a mineral acid in aid olution within predetermined limit and leaving the urface of the object in contact with the olution until a color ranging from yellow to black appear on aid Surface. 12. The method of forming a corroion-re itant coating on an object having a Surface of ubjecting the urface of the object to the action of an acid olution containing chromic acid, a alt of a mineral acid, and a mineral acid of the group coniting of ulphuric acid, hydrochloric acid and nitric acid, regulating the ratio be tween the concentration of chronic acid, the concentration of the Salt of a mineral acid, and concentration of the mineral acid in Said olu tion within predetermined limit and leaving the object in contact with the olution until a color ranging from yellow to black appear on aid Surface. 13. The method of forming a corroion-reit ubjecting the urface of the object to the ac tion of a olution containing dichromate and a mineral acid of the group coniting of a Sul phuric acid, hydrochloric acid, and nitric acid, regulating the ratio between the concentration 10 7

5 2,08,880 of the dichromate and the concentration of the mineral acid in aid olution within predeter mined limit and leaving the urface of the object in contact with the Solution until a color rang ing from yellow to brown appear n Said ur face The method of forming a corroion-reit 10 ubjecting the urface of the object to the ac tion of an aqueou olution containing a dichro mate and hydrochloric acid, regulating the ratio between the dichromate concentration and the hydrochloric acid concentration in aid olu tion and leaving the urface of the object in con tact with the Solution until a color ranging from yellow to black appear on aid urface. 1. The method of forming a corroion-reit Zinc or cadmium bae metal which comprie ub jecting the urface of the object to the action of an aqueou Solution containing a dichromate and nitric acid, regulating the ratio between the dichromate concentration and the nitric acid concentration in aid olution within predeter mined limit and leaving the object in contact S with the olution until a color ranging from yel- w low to black appear on aid urface. 16. The method of forming a corroion-reit Zinc r cadmium bae metal which comprie ub jecting the urface of the object to the action of an acid olution containing dichromate ion and the anion of ulphuric acid, regulating the ratio between the dichromate ion concentration and the concentration of the anion of Sulphuric acid in aid olution within predetermined limit and leaving the urface of the object in contact with the olution until a color ranging from yell low to black appear on aid urface. 17. The method of forming a corroion-reit ubjecting the urface of the object to the ac tion of a olution containing chromic acid and a ulphate, regulating the ratio between the chromic acid concentration and the concentra tion of the ulphate in aid olution within pre determined limit and leaving the urface of the object in contact with the Solution until a color ranging from yellow to black appear n Said Surface. ERNEST JHN WILHELM, 10