US3164464A

US3164464A - Method of introducing magnesium into galvanizing baths

Info

Publication number: US3164464A
Application number: US81239A
Authority: US
Inventors: John A Heath
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1961-01-09
Filing date: 1961-01-09
Publication date: 1965-01-05
Anticipated expiration: 1982-01-05

Description

United States Patent No Drawing. Filed Jan. 9, 196 1, Ser. No. 81,239 2 Claims. (Cl. 75-135) This invention relates to zinc galvanizing and more particularly is concerned with an effective method for introducing magnesium into zinc galvanizing baths.

The desirability of adding magnesium to galvanizing baths for the purpose of reducing to elemental zinc the zinc oxides formed and present on the bath surface has been known for some time. Additionally, recent developments indicate that zinc galvanized coatings containing small amounts of magnesium exhibit marked improvement in corrosion resistance over that shown by conventional galvanized coatings.

However, to date widespread use of magnesium in the galvanizing art for both of these purposes has not been realized because of the difficulties encountered in adding the magnesium to the galvanizing bath.

These difficulties arise primarily from the fact that (1) magnesium and magnesium-based alloys are less dense than the molten zinc galvanizing bath and therefore float on the .haths surface; (2) the galvanizing bath temperature of from about 800 to about 900 F. is such that only those magnesium-zinc binary alloys which contain from about 43 to about 65 weight percent zinc or those binaries which contain more than about 95 weight percent zinc are molten and miscible in the bath at these temperatures; and (3) ordinarily there is present on the magnesium-based materials surface films, produced by atmospheric oxidation, which tend to keep the magnesium and zinc separated thereby preventing the metals from making the intimate contact needed for ready reaction to produce the low melting magnesium-zinc binaries which are liquid at the bath temperature.

Now unexpectedly it has been found that the difiiculties normally encountered in adding magnesium to galvanizing baths can be overcome by the method of the instant invention wherein a chemically cleaned magnesium base'd metal source is introduced into the bath.

It is the principal object of the present invention, therefore, to provide a simple, effective and rapid method for introducing magnesium into a galvanizingbath.

It is another object of the present invention to provide an efficient method for introducing magnesium into a galvanizing bath wherein there is verylittle metal loss through surface oxidation of the magnesium.

It is a further object of the present invention to pro vide a safe method for introducing magnesium into a galvanizing bath whereby the operational hazards normally encountered in such an addition virtually are eliminated.

It is an additional object of the method of the present invention to substantially eliminate the danger of fires as magnesium is introduced into a galvanizing bath.

These and other objects and'advanta'ges will become apparent from the detailed description presented hereinafter.

In'carrying out the method of the instant invention, a magnesium metal source .is (1) chemically treated by acid pickling; (2) the pickled magnesium is water rinsed and dried; (3) the cleaned magnesium is placed on the surface of a galvanizing bath whereupon there forms a magnesium-zinc alloy which is fluid at the bath temperature and (4) the molten alloy is stirred, as it forms, thereby dispersing it into the zinc galvanizing bath.

3,164,464 Patented Jan. 5, 1965 The term magnesium or magnesium metal as used herein is meant to include chemically pure magnesium, cell magnesium (i.e. magnesium combined with any element or combination of elements which are considered as manufacturing impurities), magnesium-zinc binary al-. loys containing up to about 65 weight percent zinc and magnesium-based alloys wherein magnesium constitutes at least more than 50 percent by weight of such alloy. Preferably, a high magnesium content material will be used in order to keep the size and weight of the addition metal to a minimum.

In pickling the magnesium ingot, bar, plate or other form used prior to its placement in the galvanizing bath, the residence time of the metal in the pickling bath will be determined by the amount of surface contamination present on the magnesium. The magnesium will be immersed in the acid pickle and maintained therein until its surfaces are visually clean, i.e. bright and shiny. Ordinarily, satisfactory cleaning is achieved by about a one minute immersion in the acid pickle.

The pickle solution itself can be selected from any of the chemical solutions which will remove surface consolid magnesium metal source.

tamination from the magnesium metal but not leave residues detrimental to the instant process thereon. Particularly effective pickling solutions, with concentrations expressed as volume percent, are for example, sulfuric acid (about 0.5 to about 15 percent), hydrochloric acid (about 0.25 to about 15 percent), nitric acid (about 0.5 to about 10 percent), orthophosphoric acid (about 1 to about 10 percent), acetic acid (about 1 to about 10 percent) and chromic acid (about 1 to about 5 percent). These pickle solutions can be used either alone or in combination. The sole limitation in the use of these acid pickles in combination is that the total acid concentration of :amixture should not exceed that shown to be the maximum desired concentartion for any of the given acids in the mixture;

Following the acid pickle, the magnesium is given a conventional water rinse and dried prior to its introduction into the galvanizing bath.

Introduction of the magnesium into the galvanizing bath is made before undesirable oxidation compounds are reformed on the magnesium surface. Formation of such compounds defeats the purpose of the pickle in the instant method. Ordinarily, the magnesium will be used as soon after pickling as is practical. However, such pickled and cleaned magnesium metals can be stored for indefinite periods of time prior to their introduction into a galvanizing bath if they are properly protected, as by placement in gas and moisture-proof Wrappings and/ or kept in an inert atmosphere.

Ordinarily the so pickled magnesium is placed on the surface of the galvanizing bath. Almost immediately, the surface of the floating magnesium which is in contact with the molten zinc bath starts to melt yielding a low melting magnesium-zinc alloy containing from about 43 to about 65 Weight percent zinc. (This action is in direct contrast to the action of a non-pickled metal source where in extremely slow melting excessive surface oxidation and burning are encountered.) The resulting molten alloy runs out onto the surface of the bath adjacent to the This molten alloy is rapidlydispersed into the bath as by stirring or other means of agitation. By such dispersion of the molten magnesium-zinc binary alloy into the bath as it is formed, further reaction with the zinc of the bath to yield the higher melting, more concentrated 65 to weight percent zinc containing binary alloys is either completely prevented or is held to a minimum. Any small quantities of the high melting point zinc-magnesium binary which may form using the instant method are not detrimental zinc binary alloy 'within a short period of time.

In the instant method utilizing the pickled magnesium metal source, the dissolution and introduction .of the magnesium into the galvanizing :bath proceeds so rapidly that there is formed substantially no detrimental amounts of undesirable surface oxidation and no hazardous metal burning occurs. 1 1

The following example will serve to further illustrate the method of the present invention but is not meant to .limit it thereto.

. Example A 40 pound ingot of ASTM-B92 grade commercially pure cell magnesium was immersed in a pickling solution consisting of water and 6 percent by volume sulfuric acid. After about 1 minute in the pickle, the resulting shiny, clean ingot was removed from the solution and rinsed in hot water. The ingot then was dried. The surface of a 230,000 pound molten galvanizing bath maintained at about 850 F. was skimmed and the freshly pickled, dry ingot was placed thereon.

Within about 30 seconds, the sides and bottom of the ingot in contact with the bath started to melt by formation of low melting magnesium-43 to about 65 the bath was about 4 minutes and subsequent analysis indicated that less than about 5 percent of the metal was lost through oxide formation. No burning of the metal was observed during the addition.

Ina control study, a 20 pound ingot of the ASTM-rB9 cell magnesiumwas taken in the as received condition and floated on the surface of a similar galvanizing bath in the manner described above. (This ingot was metallic in appearance with no pronounced amounts of surface film visually apparent.)

Substantially no fluid allow formed even atfer several,

minutse contact of the ingot by the bath. After about 4- minutes however, a black powder began spreading over the bath surface outward from the magnesium bar. vAs the bar continued to be heated by the bath, a heavy oxide layerrformed on the exposed surfaces and sporatic burning of the ingot was observed.- Simultaneously, a;

solid mass (magnesium-zinc binary containing from about 65 to about 95 percent zinc by weight) wasobserved forming and floating in the bath. After about 30 minutes no further reaction between the magnesium and bath was observed. 1

The bath was skimmed to remove the magnesium oxides therefrom and the solid high zinc containing magnesiumzinc binary also was removed from the bath. Analyses indicated that about half of the magnesium had gone into the formation of magnesium oxide and the other half had gone into formation of the high melting magnesium-zinc alloy. Essentially no magnesium was found to have been dissolved in the galvanizing bath. v

A 4, inch by 4 inch by-1 inch thick sample of the soproduced bath insoluble magnesium-zinc binary alloy was taken and placed onto the skimmed surface of the galvanizing bath. This material slowlywas diluted by reaction with the zinc of the bath thereby forming a bath soluble magnesium-zinc'binary containing more than 95 percent zinc. The actual time for complete dissolution of the small mass of material into the bath was found to be over 1 hour, and was accompanied by undesirable zinc oxide (ash) formation.

In a manner similar to that described for the foregoing example, the ASTM-designated commercial magnesium- 5 based alloys such as for example A231, AZ91, ZK60, A280, A261, AZ92, ZE41 and A3A, a 50 Weight percent magnesium- 50 weight percent zinc alloy, a 30 weight percent aluminum-70 weight percent magnesium alloy,

21 50 -weight percent magnesium-30 Weight percent alu- 10 minum-ZO weight percent zinc alloy and a 60 Weight percent magnesium-5 weight percent aluminum-35 weight percent zinc alloy and the like all can be used satisfactorily in the method of the instant invention. 7 Various modifications can be made in the instant invention without departing from the spirit or scope thereof for it is understood that I limit myself only as defined in the appended claims.

I claim: 1. A method for the introduction of magnesium into a galvanizing bath which comprises; (1) immersing for about one minute a magnesium metal source in an acid pickling solution, saidpickling solution being an aqueous acid solution selected from the group consisting of (a) sulfuric acid of from about 0.5 to about 15 volume percent concentration,

(b) hydrochloric acid 'of from about 0.25 to about 15 volume percent concentration, (0) nitric acid of from about 0.5 to about 10 volume percent concentration, (d) orthophosphoric acid of from about 1 to about 10 volumepercent concentration, i (e) acetic acid of from about'l to about 10 volume percent concentration, (f) chromic acid of from about 1 to about 5 volume percent concentration, and

(g) mixtures thereof, said acid mixtures being characterized in that the total acid concentration of a mixture does not exceed the maximum concentration for any of the given acids in said mixture, said pickling solution removing surface contamination from said magnesium metal but not leaving residues detrimental to the process thereon and said magnesium being a member selected from the group consisting of chemiv cally pure magnesium, cell magnesium, commercial magnesium-based alloys, magnesium-zinc alloys containing up to 65 weight percent zinc and other magnesium alloys wherein magnesium is present in a concentration of at least weight percent; (2) water rinsing and drying the cleaned magnesium metal; (3) placing the chemically cleaned magnesium into a molten zinc galvanizing bath prior to the time detrimental oxidation products again reform on the surface of the cleaned and pickled magnesium metal sourcewhereupon a magnesium-zinc alloy forms which is molten at the bath temperature; and, (4) dispersing said alloy into the zinc galvanizing bath.

The method as defined in claim l wherein the acid pickling solution is a dilute aqueous sulfuric acid solution, said acid having a concentration of about 6 volume percent sulfuric acid.

References Cited in the file of this patent f UNITED STATES PATENTS Cook et a1 Feb. 18, 1958 nRoschen Sept. 29, 1959 Ihrig Aug. 9, 1960 Steenbeck et al. Apr. 24, 1962

Claims

1. A METHOD FOR THE INTRODUCTION OF MAGNESIUM INTO A GALVANIZING BATH WHICH COMPRISES; (1) IMMERSING FOR ABOUT ONE MINUTE A MAGNESIUM METAL SOURCE IN AN ACID PICKLING SOLUTION, SAID PICKLING SOLUTION BEING AN AQUEOUS ACID SOLUTION SELECTED FROM THE GROUP CONSISTING OF (A) SULFURIC ACID OF FROM ABOUT 0.5 TO ABOUT 15 VOLUME PERCENT CONCENTRATION, (B) HYDROCHLORIC ACID OF FROM ABOUT 0.25 TO ABOUT 15 VOLUME PERCENT CONCENTRATION, (C) NITRIC ACID OF FROM ABOUT 0.5 TO ABOUT 10 VOLUME PERCENT CONCENTRATION, (D) ORTHOPHOSPHORIC ACID OF FROM ABOUT 1 TO ABOUT 10 VOLUME PERCENT CONCENTRATION, (E) ACETIC ACID OF FROM ABOUT 1 TO ABOUT 10 VOLUME PERCENT CONCENTRATION, (F) CHROMIC ACID OF FROM ABOUT 1 TO ABOUT 5 VOLUME PERCENT CONCENTRATION, AND (G) MIXTURES THEREOF, SAID ACID MIXTURES BEING CHARACTERIZED IN THAT THE TOTAL ACID CONCENNTRATION OF A MIXTURE DOES NOT EXCEED THE MAXIMUM CONCENTRATION FOR ANY OF THE GIVEN ACIDS IN SAID MIXTURE, SAID PICKLING SOLUTION REMOVING SURFACE CONTAMINATION FROM SAID MAGNESIUM METAL BUT NOT LEAVING RESIDUES DETRIMENTAL TO THE PROCESS THEREON AND SAID MAGNESIUM BEING A MEMBER SELECTED FROM THE GROUP CONSISTING OF CHEMICALLY PURE MAGNESIUM, CELL MAGNESIUM, COMMERCIAL MAGNESIUM-BASED ALLOYS, MAGNESIUM-ZINC ALLOYS CONTAINING UP TO 65 WEIGHT PERCENT ZINC AND OTHER MAGNESIUM ALLOYS WHEREIN MAGNESIUM IS PRESENT IN A CONCENTRATION OF AT LEAST 50 WEIGHT PERCENT; (2) WATER RINSING AND DRYING THE CLEANED MAGNESIUM METAL; (3) PLACING THE CHEMICALLY CLEANED MAGNESIUM INTO A MOLTEN ZINC GALVANIZING BATH PRIOR TO THE TIME DETRIMENTAL OXIDATION PRODUCTS AGAIN REFORM ON THE SURFACE OF THE CLEANED AND PICKLED MAGNESIUM METAL SOURCE WHEREUPON A MAGNESIUM-ZINC ALLOY FORMS WHICH IS MOLTEN AT THE BATH TEMPERATURE; AND, (4) DISPERSING SAID ALLOY INTO THE ZINC GALVANIZING BATH.