US2964432A

US2964432A - Process for treating metals and product

Info

Publication number: US2964432A
Application number: US702538A
Authority: US
Inventors: Howard T Francis; Frank H Roebuck
Original assignee: National Steel Corp
Current assignee: National Steel Corp
Priority date: 1957-12-13
Filing date: 1957-12-13
Publication date: 1960-12-13
Anticipated expiration: 1977-12-13

Description

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This-invention relates to a process for treating metals to retard'corrosion, and more particularly, to a process for treating material having a zinc surface to inhibit white rus't formation and generally improve corrosion resistance.

The invention further relates-to the novel corrosion inhibited product produced by the process described herein,

The'invention will be described and illustratedhereinafter with specific referencetoa process for inhibiting white rust formation on materials, havlng a zlnc surface. However, it will be recognized by those skilled in.

the art that the process of the presentinvention also may be useful in retarding or inhibitingother types of corrosioh and generally improving corrosion resistance. The

term zinc surface as used in the specification and claims is intended to includega surface of a material subject to white rusting which is composedaof zinc or predominantly of zinc. Examples ofmaterials having a zinc surface sub ject'to white rusting and suitable, fortreatment in accordance with the present invention includemetallic forms or articles composed'of zinc or predominantly of zinc, base metals such: as.ferrous metal or articles fabricated therefrom whichare provided with a protective coating of zinc or zinc alloyscontaining a predominant amount of zinc, etc. Itis understood that a suitable material for treatmentmay have a zinc surface as defined herein on only a portion of its surface area, and that such materlals maybe treatedin-accordance with the present invention for the'purpo'se of improving corrosion resistance of at a least that portion of the surface area provided .with the 'zinc surface. 1 I

A fresh, bright, untreated zinc, surfaced material ex- United t s P nt posed:to the atmosphere soon develops a surface film composed of corrosion products. which are produced by :the action of atmospheric substancessuch as ;carbon dioxide,--oxygen and moisture. :As corrosion proceeds, the surface tends to increase, somewhat in thickness and the layer of corrosion products becomes a white deposit whichlrenders the appearance of the zinc surface less pleasing to the eye." ,However, *a zincsurfaced material directly exposed-tto weather isgenerally capable of giving long. service. without great loss of its i appearance value since thin films of the. 'corrosionproducts are more or I lesscontinuously removed byt-he elements, thereby preventing an appreciable build-upWin the thickness of the layer of deposited corrosion products and leavingthe zinc surface more or less uniform in appearance. I j

White rust is a term commonly used in the art to refer to a specific form of the above mentioned type of corrosion. It is generally considered to be a relatively Y thick white deposit of corrosion products composed largely of .zinchydroxideand basic zinc carbonate which forms .on unprotected zinc surfaced materials exposed to air and excessive amounts of moisture or water. The development of whiterust occurs most rapidly when water is confined-against thezinc surface by an overlying member, and it is particularly severe where fresh, brig-ht, un- ..treated zinc surfacedsheets or shapes are arranged during storage or shipment in such a way that water can I formation.

, r 2,964,432 Patented Dec. 13, 196p r I 2 V accumulate between adjacent surfaces and remain for extended periods of time. For example, galvanized sheets useful in the construction of metallic buildings are can: monly stacked in open railway cars, either'packaged'or without packaging, and shipped over long distances, or storedfor long periods of time awaiting shipment or use. Water accumulates between adjacent sheets 7 due to sweating and white rust quickly forms on those portions of V the sheets exposed to the water. 'Over a periodof time, the coating of white rust gradually thickens and eventually the appearance of the originally bright galvanized surface iscompletely destroyed and-it takes on the appearance of aninferior product. T 3" The art has long sought a' suitable process for treating zinc surfaced materials to effectively inhibit whitel rust In general, the prior artprocesses involve the application of either a relatively thick: protective coating such as oils, waxes, greases, varnishes, paints, etc., or various chemical treatments which deposit'a thin protec 'tive coatingintegral with the zinc surface: In every'instance, the prior art processes areunsatisfactory due; for

example, totheir beinginetfectivein substantially completely eliminating white rust formation for a reasonable period of time, too expensive, tootimeconsumin'glfor use in high-speedcontinuous galvanizing operations; or

the treatment results in a protective coating which must be removed prior to use ofthe treated zinc surfaced material. Also, in some instances, the coating substance either destroys or mars the originally bright pleasing finish of the fresh zinc' surface.- zments, in-partic'ular, require careful control of operating The prior art chemical-treatconditions such as concentration of ingredients and temperature of treatmentpand often exhibit a pronounced ,tendencytoward formation of undesirable coloredfilms onthe treated zinc surface. V U

It is an object of -the present invention to provides.

.novel process for treating materials having a zinc surfacetoimprove corrosion resistance" which overcomes the above mentioned disadvantages of priorart-processes'.

' It is still a further object ofv the present invention'to provide an improved process for inhibiting white rust formation on a zinc, surface which does not discolor or substantially change its appearance. v 1" It isstill a further object of'thepresent inventionto provide material having a zinc surfacewhichis treated .in accordance .with' the process of the present invention corrosion resistance j 1:

to inhibit white rust formationeand; generally improve Still .other'objects of the p-resentinvention and th e' advantages thereof willbe apparent to those skilled in the art by reference to the. following detailed description-Q:

InIaccordance withthe present invention, material having a zinc surface is treated to deposit a corrosioni'nhibiting .film on the zinc's'urface and thereby inhibit-white rust formation and generally improve corrosion resistance by immersing the material in ajbody of treatingrsolution consisting "essentially of anaque'ous solution of zinc -ni- 'trate andchromic acid, removing the treated material from the body of treating solution, and then subjecting the treated material to an aqueous rinse to remove excess treating solution. It will be appreciated from the'following discussion'that certain of theforegoing steps of the process of the invention must be performed under conditions to be fully described hereinafter for generally acceptable results.

The treating solution'ofthe present invention consists essentially of an aqueous solution of zinc :nitrate .and chromic acid (chroinic oxide) having a molar ratio. of

zincnitrate to chromic acid between- 4:1 and 1:9. However, better results usually are-obtained when the .zinc

nitrate-and, chromic acid are presentjin about 'equimolar quantities. The total molar concentrationv Of the zinc 5. 1i: r f 3' nitrate, and chromic acid may vary over a considerable range. For example, satisfactory results may be obtained when the total molar concentration of zinc nitrate and chromic acid is about 0.5 to 2.0 moles,: but better results usually are obtained whenthe total molarconcentration of zinc nitrate and chromic acid is about 1.0. 'For optimum results, generally the total molar concentration of zinc nitrate and chromic acid should be about 1.0 mole and the zinc nitrate and chromic acid should be present in about equimolar amount.

j, The treating solution maybe prepared by dissolving zinc nitrate and chromic acid in water in the desired molar ratio and amount. The water used as a solvent does not require a special treatment to remove dissolved substances responsible for hardness of water, but generally soft water is preferred over hard water. The resulting solution is acidic and since acidic'solutions of chromic acid are genrinse since otherwise it is not possibleto produce asub: stantially colorless corrosion, resistant film on the zinc surface. For best results, the aqueous rinse should follow immediately after removal of the treated material from the treating solution and before the excess treating solution has had an opportunity to dry.

Whenpracticing the present invention, thezmc surfaced material to be treated may be heated to the desired initial temperature and then immersed in the treating solution in any suitable manner. For example, .while it is generally more convenient to effect immersion of the material in the solution by passing the material to be treated 7 into a body of treatingsolution, a bodyof'the treating erally considered to contain both chromate ion and dichromate ion, and perhaps other species of chromate ion, it is understood that such species of chromate ion may be present in the treating solution of the present invention. Thus, where the treating solution of the present invention is referred to as being an aqueous solution of zincnitrate and chromic acid, it is understood that these substances are dissolved in water when preparing the treating solution of the present invention and that actually other substances may be present in the resulting solution. However, the substancesto be dissolved in water when preparing the treating solution consistessentially of zinc nitrate and chromic acid.

The initial temperature of the zinc surface to be treated is of importance. For example, at least the zinc surface ofthematerial to be treated must be initially at a temperature substantially above room temperature and not over 450 F. at the time of immersion in the treating solution. ,At temperatures above about 450 F., or at temperatures only moderately above room temperature, generally acceptable results are not obtained. A very satisfactory temperature range which produces excellent corrosion inhibition has been found to be between about .200". F. and 450. F., with even better results usually being obtained at a temperature of about 300 F. The temperature of the treating solution does not appear to .be of importance and may vary anywhere between room temperature and substantially the boiling point. In instances where the temperaturerof the zinc surface to be ;treated is initially at a temperature substantially above the boiling point of the solution and thustends to raise 4' thetemperature of thetreating solution above its boiling point, the treating solution may.berecycled ifdesired, for cooling to a temperature below its boiling point. 1 The periodof treatment may vary considerably. How- .ever,. once a corrosion inhibiting film is deposited, ex-

tended periods of treatment are of no advantage and are -to be avoided particularly when treating galvanized steel and the like since the. relatively thin zinc coating may be attacked by the .treatingsolution. In'most instances, periods of treatment varying between about .2 seconds and 45*sconds'are satisfactory, with shorter. periods of treat- -ment'such as about 3-5 seconds usually being preferred. When the zinc surface to be treated is at an initial temperature between 200 F. and 450 F., the reaction re- :sponsible for the effectiveness of the process of the inventionappears to take place very rapidly. Often, periods of treatment less than 2 seconds may be as effective as 'longer periods of treatment where the initial temperature -of the zinc surface is approaching the maximum temperature limit.

'After'the zincsurfaced material has been immersed in the treating solution for a period of time sufficient to re- Isultin the formation of a corrosion inhibiting film on the zinc surface, it is then-removed fromthe body of treating solution and subjected to an aqueous rinsefor-the'purpose of removing excess treating solution. It is essential that the treated material be-subjected-to such an aqueous solution may befiowed over the surface of the material or other suitable methodmay be used to immerse the material in a body of the solution for the period of time necessary to produce a corrosion inhibiting film. The treated material is then removed and, preferably, immediately subjected to an aqueous rinse for the purpose of removing excess treating solution and assuring that the coating remaining on the treated zinc surface is substantially colorless. The treated material then may be dried and packa ed for shi ment. The zinc s urfaced' material to be treated should be given a suitable conventional pretreatment for the removal of oils, greases, etc. where such are present. However, the presence of a thin film of oxide on the zinc surface usually is not detrimental to the process of thepresent invention and, in fact, often the presence of a thin oxide film is beneficial. In instances where considerable corrosion has taken place, it may be necessary to remove the majoramount of corrosion prior to' treatment. In general, freshly galvanized surfaces .do not exhibit such a degree of corrosion as to necessitate removal of the oxide 'film in a pretreatment step. While the process of the present invention may be used at any suitable time, it is preferred that the zinc surface be treated shortly after the material or article has been manufactured. If desired, the treatment may be successfully incorporated in the manufacturing process. For example, it is possible to incorporate the present invention in the galvanizing of ferrous metal at any suitable stage after application of the zinc coating, provided the proper temperature conditions are met. Thismay be done at a stage in the galvanizing process after application of the molten zinc coating by first cooling the molten zinc on the ferrous metal base to a temperature suificient to solidify the zinc coating and allow the proper temperature conditions to be met, and then immersing the galvanized ferrous metal in the treating solution for a period of time sufiicient to 'form a corrosion inhibiting film on the zinc surface. The process of the present invention is particularly desirable for use in a continuous galvanizing line for ferrous metal strip. In the operation of such continuous galvanizing lines, it is customary to use relatively high strip speeds and as a result the time available for treatment is generally very short. The process of the present invention requires a minimum ofline space since a corrosion inhibiting film may be deposited on the zinc surface in periods as short as about 2 seconds, thereby allowing economic operation. I

The corrosion inhibitingfilms deposited in accordance with the present invention are substantially colorless and invisible to the naked eye, and do not noticeably change the appearance of the zinc surface. Also, such corrosion inhibiting films have been found to be very resistant to leaching, thereby allowing'the treated material to be subjected to the necessary aqueous rinse following deposition The corrosion inhibiting films deposited in acdepositedby conventional chemical. treatments. Even more important, theypifer almost complete protection to the zinc surface The manner inwhichthe. process of the present invention results in improved inhibition of white rusting is not fully understood at the present time. However, it is assumed that the corrosion inhibiting film contains predominant amountsof-zinc .chromate, but perhaps. some zinc dichromate or still other speciesof chromate may be present. Whatever the theory, the process of the present invention is highly effective in producing'a very thin substantially colorless, invisible'corrosion inhibiting film on zine surfaces in a minimum period of time which is substantially completely effective in inhibiting white rusting. Thelfo'regoi'ng detailed "description and the'fifollowing specificfexampleafe for the purpose of illustration only, and are not intended'as limiting to the spirit or scope of the appended claims.

EXAMPLE (2.) The sample was removed from the muflie furnace and immediately immersed for 30 seconds in an aqueous treating solution molar in both zinc nitrate and chromic acid, the total molar concentration of zinc 'nitrate and chromic acid being 1.0 mole. The aqueous treating solution was at room temperature.

( 3) The treated sample was withdrawn from the aqueous treating solution, subjected to a water rinse to remove excess treating solution, and dried.

(4) The dried treated sample was placed over tepid water and observed for white rust formation after 7 days. Where the sample exhibited substantially no visible white rusting, the results were recorded as Good.

If a small amount of white rusting was visible or if con siderable white rusting was visible, then the results were recorded as Fair and Bad, respectively.

The results of treating and testing samples in accordancewith the above described procedure are tabulated below in Table I. The column headed Temperature of ZiIlC surface refers to the temperature of the heated sam-v ple to be treated immediately before immersion in the treating solution.

Table] 7 WHITE RU$T INHIBITION Temperature White Rust of Zinc Inhibition Sample No.

I v 7 Surface, F.

Bad. Fair. Fair. Good. Good. Good. Good. Good. Good. Fair. Bad.

. 7 Controls were run at each of the temperatures mentioned in Table I by immersing the heated sample in dis tilled water rather than treating solution, rinsing the The F. and above 450 generallyv satisfactory results were not obtained. y

We claim: 1 7 v 1 A process for treating material having a'zinc sur face toimprove corrosion resistance comprising immersing the material in a body of treating solution consisting essentially of an aqueous solution of zinc nitrate and chromic acid having a molar ratio of zinc nitrate to chromic acid between 4:1 to 1:9- to produce a corrosion resistant film on the zinc surface, the solution being free .of a-su'bstance which is a reducing agent for chromate ion, at least the zinc surface of the material to betr eated being initially at a temperature substantially above room temperature and less than about 450 F., removing the treated material from the body of solution, and then subjecting the treated material to an aqueous rinse to remove excessrtreating solution and produce a substantially colorless corrosion resistant film on the zinc surface.

2. The material of improved. corrosion resistance produced by the process of claim 1.

3. A. process for treating material having a zinc surface to improve corrosion resistance comprisingimmersing the material in a body of treating solution consisting essentially of an aqueous solution of zinc nitrate and chromic acid having a molar ratio of zinc nitrate to chromic acid between 4:1 and 1:9 to produce a corrosion resistant film on the zinc surface, the solution being free of a substance which is a reducing agent for chromate ion, at least the zinc surface of the material to be treated being initially at a temperature of about ZOO-450 F., removing the treated material from the body of solution, and then subjecting the treated material to an aqueous rinse to remove excess treating solution and produce a substantially colorless corrosion resistant film on the zinc surface;

4. A process for treating material having a zinc sur face to improve corrosion resistance comprising immersing the material in a body of treating solution consisting essentially of an aqueous solution of about equimolar quantities of zinc nitrate and chromic acid to produce a corrosion resistant film on the zinc surface, at least the zinc surface of the material to be treated being ini-' tially at a temperature substantially above room temperature and less than about 450 F., removing the treated material from the body of solution, and then subjecting solution is about 1.0mole.

the treated material to an aqueous rinse to remove excess treating, solution and produce a substantially colorless corrosion resistant film on the zinc surface.

5. The process of claim 4'wherein the total molar concentration of zinc nitrate and chromic acid in the aqueous 6. A process for treating material having a zinc surface to improve corrosion resistance comprising immersing'the m aterial infa body of treating solution consisting essentially of an aqueous solution of about equimolar quantities of zinc nitrate and chromic acid to produce a 1 corrosion resistant filmon the zinc surface, at least the zinc surface of the material to be treated being initially at a temperature of about '200-450" F., removing the treated material from the body of solution, and then sub- .jecting the treated material to an aqueo rinse to remove about equimolar quantities of zinc nitrate and chromic acid to produce a corrosion resistant film on the-zinc surface, at least the zinc surface of the material to be treated being initially at a temperature substantially above room temperature and less than about 450 F., removing the treated material from the body of solution, and then subjecting thetreated material to an aqueous rinse to remove excess treating solution and 'produce asubstantially colorless corrosion resistant film on the zinc surface. i

9. The process of claim 8 wherein the total molar concentration of zinc nitrate andchromic acid in the aqueous solution is about 1.0 mole.

- 10. A process for treating material having a zinc surface to improve corrosion resistance comprising immersing the-material for about 2-45 seconds in a body of treating solution consisting essentially of an aqueous solution of about equimolar quantities of zinc nitrate and chromic acid to produce a corrosion resistant film on the zinc surface, at least the zinc surface of the material to be treated being initially at a temperature of about 200- '450 F., removing the treated material from the body of solution, and then subjecting the treated material to an aqueous rinse'tolremove-excess treating' solution and produce a substantially gcolorless corrosion-resistant film onthe zinc surface. M v r v r '11. The pr ocessof claim10 wherein the total molar concentration-of zinc nitrate and chromic-acid in 'the aqueous solution is about 1.0 niolel References Cited in ithe file of this patent UNITEDSTATES PATENTS 'Thompson, et al July 2, 1940 2,393,943, Thomas et al. Jan. 29, 1946 2,763,569 Bradstreet et a1 Sept. 18, 1956 2,846,342 Curtin Aug. 5, 1958 2,851,386 Hartman Sept. 9,1958

15 2,904,413 Hampel 'Sept. 15, 1959 FOREIGN PATENTS 570,054 f G reat Britain Iune20,'1945 GreatBr-itain Mar. 30, 1948

Claims

1. A PROCESS FOR TREATING MATERIAL HAVING A ZINC SURFACE TO IMPROVE CORROSION RESISTANCE COMPRISING IMMERSING THE MATERIAL IN A BODY OF TREATING SOLUTION CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF ZINC NITRATE AND CHROMIC ACID HAVING A MOLAR RATIO OF ZINC NITRATE TO CHROMIC ACID BETWEEN 4:1 TO 1:9 TO PRODUCE A CORROSION RESISTANT FILM ON THE ZINC SURFACE, THE SOLUTION BEING FREE OF A SUBSTANCE WHICH IS A REDUCING AGENT FOR CHROMATE ION, AT LEAST THE ZINC SURFACE OF THE MATERIAL TO BE TREATED BEING INITIALLY AT A TEMPERATURE SUBSTANTIALLY ABOVE ROOM TEMPERATURE AND LESS THAN ABOUT 450*F., REMOVING THE TREATED MATERIAL FROM THE BODY OF SOLUTION, AND THEN SUBJECTING THE TREATED MATERIAL TO AN AQUEOUS RINSE TO REMOVE EXCESS TREATING SOLUTION AND PRODUCE A SUBSTANTIALLY COLORLESS CORROSION RESISTANT FILM ON THE ZINC SURFACE.