NZ199491A

NZ199491A - Aluminium-containing zinc alloy

Info

Publication number: NZ199491A
Application number: NZ199491A
Authority: NZ
Inventors: S F Radtke; D Coutsouradis; J Pelerin
Original assignee: Int Lead Zinc Res
Priority date: 1981-01-16
Filing date: 1982-01-14
Publication date: 1985-08-30
Also published as: DD220342A5; ZA8291B; YU43509B; KR880002516B1; CS32382A2; AR227220A1; YU5782A; BE887121A; MX161324A; CS261856B2; KR830007872A

Description

<div class="application article clearfix" id="description"> New Zealand Paient Spedficaiion for Paient Number 1 99491 199491 & m Priority Date(s): .. I k~J. ......... C'ompfets Specification Filed: ( b.~.! Class: Publication Date: .. J$.Q. AUG-1985- ..... P.O. Journal No: ... .1271 kr...... Patents Form No. 5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION "ZINC-ALUMINUM ALLOYS AND COATINGS" —2/WE INTERNATIONAL LEAD ZINC RESEARCH ORGANIZATION, INC. of 292 Madison Avenue, New York, New York 10017 U.S.A. o hereby declare the invention, for which -I/we pray that a patent may be granted to me/us, and the method by which it is to be performed, to be particularly, described in and by the following statement — ' 1 (•£© -]*- Descr iption Zinc-Aluminum Alloys and Coatings Technical Field The present invention is directed to the application of zinc coatings to a substrate - commonly sheet steel. The use of zinc as a protective coating has been known for many years. In this regard, hot dip galvanizing, either continuous or batch type, has long been used for a variety of steel products to protect the products from corrosion. Background Art In order to obtain increased corrosion protection as well as other advantages (e.g. better sacrificial protection of steel; improved formability, weldability and paintability) efforts have been undertaken in the field of zinc coatings to develop improved zinc alloys for the continuous or batch application to substrates. Studies carried out in this direction have resulted in the development of new types of coatings such as the alloy Zn-55 Al- . 1.5Si and other zinc alloys having low (i.e., less than 15%) Al-1.5Si content. The Zn-55 Al alloy coating developed by Bethlehem Steel (see for example U.S. Patent Nos. 3,343,930 and 3,393,089) reportedly exhibits a good corrosion'resistance but, in view of its high aluminum content does not provide a satisfactory sacrificial protection of the steel substrate Subsequent studies have been aimed at modifying the composition of molten metal baths in order to form (by hot-dipping) a coating which improves corrosion resistance even in the most varied environments. One of the aspects of these studies was the influence of the preparation of the surface to be coated on the quality of the product obtained. It thus appears that in order to ensure a quality coating, some of the alloy coatings previously developed required expensive preliminary surface treatments involving expensive equipment. For example, this was the case with respect to zinc coatings containing typically about 5% Al and additions of other elements such as Sb, Pb + Mg, and Pb + Mg + Cu proposed by Inland Steel (see for example Inland U.S. Patent Nos. 4,029,478 and 4,056,366 as well as U.S. Patent No. 4,152,472 assigned to Nippon Steel) . There exists evidence showing that compositions of these types are characterized by a pronounced tendency to form bare-spots and similar defects even in the presence of careful surface preparation. In view of the above considerations, there continues a need for a hot-dip metal bath of such composition that no special or expensive surface preparation of the substrate would be necessary and such that the protective coating obtained thereby is substantially free of bare spots or other defects. Disclosure of the Invention Consistent with the above, there have been developed according to the present invention zinc- • containing hot-dip metal baths which yield high quality protective coatings, free of defects such as bare spots. Stated generally, the bath compositions and resultant coatings constitute improvements over known alloy baths and coatings in that they contain additionally mixtures of rare earth elements. More particularly, the present invention is directed to zinc-aluminum compositions or alloys which have added thereto rare earths in the form of mischmetal. In" this regard, it is preferred that the zinc-aluminum alloys be what are commonly referred to as low aluminum zinc alloys which are generally recognized to contain from about 3% to about 15% aluminum. Detailed Description The hot-dip metal baths according to the present invention, and hence the coatings obtained therefrom, may vary considerably just as known zinc-aluminum baths and coatings may vary. In each instance, however, it is essential that the bath have added thereto a rare earth containing alloy in an amount in the range of 5 ppm by weight to less than 0.01% (by weight). As will be understood by one skilled in the art, the term mischmetal refers to a variety of known rare earth alloys. For example, two typical cerium mischmetals might have the following compositions (in weight %): (1) Ce 45-60; other rare earths 35-50, the balance comprising Fe, Mg, Al, Si and impurities. (2) Ce 52.7 other rare earths 47.5, Fe 0.04, Mg 0.28, Al 0.08, Si 0.27 and the balance impurities. Typical Lanthanum mischmetals can be defined by the following (in weight %): (1) La 60-9 0; Ce 8.5; Nd 6.5; Pr 2 the balance comprising Fe, Mg, Al and Si as well as possible impurities. (2) La 83, Ce 8.5, Nd 6.5, Pr 2, Fe 0.2, Mg 0.03, Al 0.18, Si 0.43 and the balance impurities. -4- 1 CO 199491 Thus the term mischmetal, as used herein, refers to the above compositions as well as other mischmetal compositions readily apparent to those skilled in the art. 5 As stated above, the preferred alloy to which mischmetal is to be added is a zinc-aluminum alloy containing from about 3% to about 15% aluminum. Such alloys typically contain about 5% aluminum. While these alloys may contain constituents in addition to 10 mischmetal such as Fe, Pb, Sb, Mg, Sn, Cu and Si, it has been found that such additives, especially Pb, Sb and Sn, generally do not improve and in fact may detract from the quality of the coating. It is thus preferred that the alloy be limited to substantially 15 pure Zn, Al and mischmetal. In other words, the levels of Sb, pb and Sn should not exceed those expected in the following starting materials: Zn - Special High Grade (99.99%) Al - Commercially Pure (99.9%) 20 Mischmetal - Ferrous Grade (Total rare earths - 96%, Fe - 4%) Thus one embodiment of the invention comprises a low aluminum (i.e., 3-15%) zinc bath containing Pb or Sn as well as mischmetal. Pb and Sn are known addi-25 tives to galvanizing baths for modifying the fluidity of the liquid metal or the spangle of the solidified coating. The addition of Sb to a galvanizing bath is disclosed in U.S. Patent No. 4,056,366 to improve the 30 coatability of Zn-Al coatings in a manner similar to lead but without the deleterious effect that lead has upon the intergranular corrosion of the coatings. The addition of Sb to the mischmetal-containing compositions according to the present invention is 35 therefore contemplated. Moreover, a. Zn-Al composition containing Pb together with Sb is within the' scope of the invention. A typical composition might contain 3-15% Al, 0.03-0.15% Sb, less than 0.02% Pb, and the balance Zn to which mischmetal has been added. Zinc-aluminum alloys containing lead and also Mg and Cu are reported to be immune to grain boundary corrosion. In this type of coating alloys, mischmetal additions have been shown to exhibit a pronounced beneficial effect as regards soundness and uniformity. ^ Thus a Zn-Al alloy containing Mg, Pb, Cu and mischmetal is encompassed by the present invention. Here a typical composition might contain 3-15% Al, 0.02-0.15% Mg, 0.02-0.15% Pb and possibly 0.1-0.3% Cu, the balance being Zn with mischmetal additions. Various mischmetals may be advantageously used according to the invention, including mixtures of mischmetals in a single zinc bath or coating. For example, a La-mischmetal and a Ce-mischmetal may be added simultanesouly, in an amount such that the total mischmetal concentration is within the ranges described above, i.e. from about 5 ppm by weight to less than 0.01% by weight. In order to facilitate the addition of the mischmetal to the galvanizing bath, a master alloy may be first prepared and then added to the zinc bath so as to yield the desired mischmetal concentration. Such master alloys might be comprised of 20% Zn and 80% mischmetal or 85-95% Al and 15-5% mischmetal. 195491 Example 1 This example relates to trials carried out with a pilot continuous annealing and galvanizing plant. In these trials 800 kg coils of rimming steel sheet 150 mm wide and 0.25 mm thick were first treated in a Selas type furnace at temperatures ranging from 680 to 860°C. The sheet was then cooled in a controlled atmosphere to about 430°C and then introduced into a seven-ton zinc bath. The sheet was then nitrogen-gas wiped at the exit, jet cooled and finally coiled. Depending on test conditions the speed of the sheet varied in the range 10 to 30 m/min. Several coils were galvanized with a bath containing Zn - 5% Al and a cerium mischmetal content from 0.01% - 0.001%. The cerium content was as low as 0.0008% and the La content was as low as 0.0002%. The resulting coating was bright with a grain size varying from 1 to 5 mm, depending on the cooling conditions, and with thicknesses, varying from 5 to 35 m depending on the gas wiping conditions. The coating was uniform and free of bare spots, uncoated areas or other defects. It is evident that the pilot plant conditions are mentioned as examples only and that other conditions prevailing in continuous annealing and galvanizing lines as regards furnace type, composition of gas, speeds, wiping methods, etc., can be used with advantage with the zinc bath composition according to the invention. Moreover, bath and coating compositions as described herein may be used in non-continuous (e.g. batch) galvanizing methods. </div>

Claims

<div class="application article clearfix printTableText" id="claims"> -7- 199491 Example 2 Specimens from the above pilot plant trials were subjected to various trials for the evaluation of formability and adherence, the corrosion resistance in various environments, the galvanic protection, and the microstructure. The formability and adherence was evaluated by means of bulge tests and Erichsen tests. In both types of tests the coatings obtained with the mischmetal-containing bath exhibited an adherence and formability equivalent to that of standard galvanized coatings. For example a 180° bending gave rise to no cracking and in the Erichsen test a depth of 9 mm was made on 0.25 mm thick sheets without peeling of the coating. The corrosion resistance, in a salt spray test, of the Zn-Al coatings containing mischmetal was more than twice that of a standard galvanized coating of the same thickness. For example, with the coatings of the present invention the time to first rusting was about 900 hours instead of 350 hours with a conventional galvanized coating of the same thickness. Similarly the corrosion resistance in an environment containing 10 ppm SC>2 was shown to be at least 50% greater than that of a conventional galvanized coating. The galvanic protection of the Zn-Al mischmetal coating was also determined by examining the progress of corrosion around scratches machined on specimens exposed to a SC>2 containing environment. The galvanic protection of the mischmetal-containing Zn - 5% Al coating was equal to that of a pure zinc coating and far superior to that of a coating containing Zn-55A1-1.5Si. -a- 199491 WHAT WE CLAIM IS:

1. A protective metal coating adhered to a substrate, said coating being applied, by hot dipping the substrate in a bath of molten metal, so as to comprise from 85 wt.% to 97 wt.% Zn, from 3 wt.% to 15 wt.% Al, and at least 5 ppm by weight but less than 0.01 wt.% of a rare earth-containing alloy.

2. A coating according to claim 1, said coating containing additionally at least one of the elements selected from the group consisting of Fe, Pb, Sb, Mg, Sn, Cu and Si.

3. A coating acc©rding to claim 1 or 2, said coating containing additionally antimony.

4. A coating according to claim 3, said coating containing additionally lead.

5. A coating according to claim 4, containing from 0.03 to 0.15 wt.% Sb and less than 0.02 wt.% Pb.

6. A coating according to claim 1, said coating containing additionally Mg and Pb.

7. A coating according to claim 6, containing from 0.02 to 0.15 wt.% Mg and from 0.02 to 0.15 wt.% Pb.

8. A coating according to claim 7, said coating containing additionally Cu.

9. A coating according to claim 8, containing from 0.1 to 0.3 wt.% Cu.

10. A coating according to any one of claims 1 to 9, wherein said substrate is a steel substrate.

11. A coating according to any one of claims 1 to 10, wherein the rare earth-containing alloy is mischmetal

12. A coating according to claim 11, wherein the mischmetal is Ce-mischinetal or La-mischmetal.

13. A coating according to claim 12, wherein said mischmetal is a Ce-mischmetal comprising from 45 to 60 wt.% Ce, from 35 to 50 wt.% other rare earths, and the balance comprising Fe, Mg, Al, Si and impurities. -9- i S 9491

14. A coating according to claim 12, wherein said mischmetal is a Ce-mischmetal comprising 99.3 wt.% rare earths (said rare earths comprising 52.7 wt.% Ce, other rare earths 47.3 wt.%); Fe^ 0.04 wt.%, Mg 0.2 8 wt.%, Al 0.08 wt.%, Si 0.27 wt.% and the balance impurities.

15. A coating according to claim 12, wherein said mischmetal is a La-mischmetal comprising 60 to 90 wt.% La, up to 8.5 wt.% Ce, up to 6.5 wt.% Nd, up to 2 wt.% Pr, and the balance comprising Fe, Mg, Al and Si and impurities.

16. A coating according to claim 15, wherein said mischmetal comprises 9 8 wt.% rare earths, said rare earths comprising 83 wt.% La, 8.5 wt.% Ce, 6.5 wt.% Nd, 2 wt.% Pr, said mischmetal also containing 0.2 wt.% Fe, 0.03 wt.% Mg, 0.18 wt.% Al, 0.43 wt.% Si and the balance impurities.

17. A method of applying a protective metal coating to a substrate comprising the steps of immersing the substrate in a molten alloy comprised of zinc, aluminium and mischmetal, said bath formulated so as to yield a coating comprising from 85 wt.% to 97 wt.% Zn, from 3 wt.% to 15 wt.% Al, and at least 5 ppm by weight but liess than 0.01 wt.% of a rare earth-containing alloy.

18. A method according to claim 17, wherein said rare earth-containing alloy is added to the alloy in the form of a master alloy.

19. A method according to claim 18, wherein said master alloy comprises 20 wt.% Zn and 80 wt.% mischmetal.

20. A method according to claim 18, wherein said master alloy comprises 85 to 95 wt.% Al and 5 to 15 wt.% mischmetal.

21. A method according to any one of claims 17 to 20, wherein said substrate is a steel substrate.

22. A method according to any one of claims 17 to 21, wherein said bath is formulated to yield a coating according to any one of claims 1 to 16. .# INTERNATIONAL LEAD ZINC RES^A&CH ORGANIZATION, INC ttorne BALDWIN, SON & CARE1 </div>