US3819406A

US3819406A - Hot-dip lead coating

Info

Publication number: US3819406A
Application number: US00193160A
Authority: US
Inventors: H Tachimori; T Nagahori; Y Nakamura
Original assignee: Mitsui Mining and Smelting Co Ltd; Max-Coater Co Ltd
Current assignee: Mitsui Mining and Smelting Co Ltd; Max-Coater Co Ltd
Priority date: 1968-02-16
Filing date: 1971-10-27
Publication date: 1974-06-25
Anticipated expiration: 1991-06-25
Also published as: FR2002012A1; DE1906931A1; GB1224131A

Abstract

Lead-base alloy, hot-dip coating of articles of iron, iron-base alloys, copper and copper-base alloys is accomplished, with improved hardness, corrosion resistance and appearance, using as a coating alloy one of the following (a), (b), (c) and (d): A. AN ALLOY CONTAINING 0.01-5.0 PERCENT BY WEIGHT OF ARSENIC, THE BALANCE BEING LEAD METAL, B. AN ALLOY CONTAINING 0.01-5.0 PERCENT BY WEIGHT OF ARSENIC AND 0.1-3.0 PERCENT BY WEIGHT OF TIN, THE BALANCE BEING LEAD METAL, C. AN ALLOY CONTAINING 0.01-5.0 PERCENT BY WEIGHT OF ARSENIC AND 0.1-10.0 PERCENT BY WEIGHT OF ANTIMONY, THE BALANCE BEING LEAD METAL, AND D. AN ALLOY CONTAINING 0.01-5.0 PERCENT BY WEIGHT OF ARSENIC, 0.1-10.0 PERCENT BY WEIGHT OF ANTIMONY AND 0.1-3.0 PERCENT BY WEIGHT OF TIN, THE BALANCE BEING LEAD METAL.

Description

Tachimori et al.

[ 1 June 25, 1974 HOT-DIP LEAD COATING Inventors: Hironori Tachimori, Tokyo; Teiji Nagahori, Saitama-ken; Yukio Nakamura, Tokyo, all of Japan Assignees: Mitsui Mining & Smelting Co. Ltd.,

Tokyo, Japan; Max-Coater Co., Ltd., Kawaguchi-shi, Saitama-ken, Japan 7 Filed: Oct. 27, 1971 Appl, No: 193,160

Related US. Application Data Continuation-in-part of Ser. No. 797,263, Feb. 6, 1969, abandoned.

US. Cl 117/114 R, 29/196.6, 117/51,

117/114 B, 117/131 Int. Cl. C23c 1/06 Field of Search 75/166 B, 166 C, 166 D, 75/166 R; 29/1966; 117/114 R, 114 B, 52, 51

References Cited UNITED STATES PATENTS 12/1898 Clamer 75/166 R 900,846 10/1908 Goodson 75/166 R 1,378,439 5/1921 Baskerville 29/1966 1,807,788 6/1931 Kemp 75/166 C 2,184,179 12/1939 Domm 1. 29/1966 2,570,501 10/1951 Snyder 75/166 R 2,678,341 5/1954 Stoertz.... 75/166 B 3,197,862 8/1965 Harvey 1. 75/166 C Primary Examiner-Douglas J. Drummond Assistant Examiner-Michael W. Ball Attorney, Agent, or Firm-Woodhams, Blanchard and Flynn [5 7] ABSTRACT Lead-base alloy, hot-dip coating of articles of iron, iron-base alloys, copper and copper-base alloys is accomplished, with improved hardness, corrosion resistance and appearance, using as a coating alloy one of the following (a), (b), (c) and (d):

a. an alloy containing 0.015.0 percent by weight of arsenic, the balance being lead metal,

b. an alloy containing 0.01-5.0 percent by weight of arsenic and 0.1-3.0 percent by weight of tin, the balance being lead metal,

c. an alloy containing 0.0l5.0 percent by weight of arsenic and 01-100 percent by weight of antimony, the balance being lead metal, and

d. an alloy containing 0.015.0 percent by weight of arsenic, 01-100 percent by weight of antimony and 01-30 percent by weight of tin, the balance being lead metal.

7 Claims, No Drawings HOT-DIP LEAD COATING CROSS-REFERENCE TO. RELATEDAPPLICATION This application is a continuation-in-part of our copending application Ser. No. 797,263, filed; Feb. 6,,

1969, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the use of certain lead-base alloys for the hot-dip lead coating of iron, iron-base alloys, copper and copper-base alloys. It also relates to articles coated with said lead-base. alloyg 2. Description of the Prior Art Iron and copper metals have been generally regarded as among those metals which are very difficult to coat with lead-base alloy by the hot-dip lead. coating process because of their inability to form a solid solution or intermetallic compound with lead. In, spite of such metal.- lurgical and technical difficulties, however, various methods of hot-dip lead coating have been worked out and put to practical use in order to utilize the superior corrosion resistance of lead. These conventional methods of hot-dip lead coating may be divided into two general groups as hereinafter stated.

One conventional method is to plate iron or copper with a third metal (exclusive of iron, copper and lead) which either alloys with both iron and lead or both copper and lead before the iron or copper metal is dipped into the lead bath.

The other method is to immerse iron or copper metal in the molten alloy bath having lead and a third metal as alloying elements. In the latter method, tin, antimony and zinc are used as alloying elements. Of the alloys made with one of these third metals, especially the one alloyed with tin is most widely used because of its excellent properties. However, in the case of the coating method in which a tin-lead alloy is used, an alloy containing 15-30 percent by weight of tin is generally used, since an alloy containing a small quantity of tin produces unsatisfactory coating results. Even in such coatings formed by means of a tin-lead bath, still there are a considerable number of pinholes existing in the coating phases or layers which pinholes reduce corrosion resistance. Since lead does not furnish iron such electro-chemical protection as zinc does, the existence of pinholes in the coating layer means a positive and fatal defect of the lead coating. It is not desirable to add too much other metal to lead because a lead alloy is generally inferior to pure lead in corrosion resistance. Furthermore, in the case of coating in which an iron or copper metal article is immersed in the alloy bath wherein tin and lead are present, since the affinity of tin for lead is much stronger than for iron or copper, tin forms a solid solution with leadprior to iron or copper, thus resulting in the formation of less solid solution with iron or copper. Such being the case, when the amount of tin added to lead is increased for the purpose of enhancing the formation of a solid solution between tin and iron or copper, there will be the unsatisfactory result that such coating has less corrosion resistance compared with a pure lead coating.

A hot-dip coating bath of lead alloy prepared with antimony additions is also widely known. However, the coating layers formed by the use of this alloy, which are resistant to acid and have high hardness, have many pinholes exposing the coatedbase metal, like the coatinglayers formedby the use of the aforementioned alloys and, the results have not always been satisfactory.

SUMMARY OF THE INVENTION The present invention provides a novel method of coating metals and metal articles made of iron, ironbase alloy, copper and copper-base alloys with leadbase alloy so as to produce novel coated metals and metal articles with improved corrosion resistance, hardness and appearance.

The invention is based on the discovery that arsenic, used as an alloy element in the hot-dip lead-base bath, has affinity for iron and copper that is stronger than its aff nity for lead under these conditions and that it fomis a solid solution with iron or iron-base alloys, or copper or copper-base alloys in preference to lead. This discovery has led to the further discovery that, when iron metal or copper metal is dipped in a molten lead bath containing a small amount of arsenic, a very thin intermediate layer of arsenic-iron or arseniccopper solid solution is formed, covering the iron metal or copper metal. This layer of solid solution has a thickness of approximately 0.01;; to 0.511. and is formed by a reaction of the iron metal or copper metal with the arsenic in the molten lead bath. Further, the lead in the lead bath combines firmly with this solid solution intermediate layer, thus making the lead coating perfectly free from pinholes.

The invention is based, moreover, on the further discovery that a lead-arsenic-tin alloy, lead-arsenicantimony. alloy and a lead-arsenic-antimony-tin alloy also produce lead coatings and lead-coated articles with correspondingly improved properties when used in the hot-dip lead coating method in place of the leadarsenic alloy described above. These ternary and quaternary lead base alloys for hot-dip coating can be readily prepared by addingthe required quantity of tin, or antimony, or both tin and antimony to the aforementioned leadarsenic alloy bath.

Thus the lead-base alloys for use in the novel process and to produce the novel coated articles of this invention comprise the fouralloys mentioned in the following:

a. an alloy containing 0.0l-5.0 percent by weight of arsenic, the balance being lead metal.

b. an alloy containing 0.0l-5.0 percent by weight of arsenic and 0.1-3.0 percent by weight of tin the balance being lead metal.

0. an alloy containing 0.01-5.0 percent by weight of arsenic and 01-100 percent by weight of antimony, thebalance being lead metal.

d. an alloy containing 0.0l-5.0 percent by weight of arsenic, 0.ll0.0 percent by weight of antimony and 0.1-3.0 percent by weight of tin, the balance being lead metal.

In connection with the present invention, it may be said that the lead base alloy comprising the composition stated in the above-mentioned (a) has, due to the function of arsenic contained therein, an effect of forming a solid coating layer or coating film on the surface of the metal to be coated; the lead-base alloy having the composition of (b), when made into an alloy bath for coating, has a much improved fluidity as compared with the alloy prepared according to the (a) composition and it is accordingly possible, for instance, to form a very thin (thickness of approximately 514) and bright coating with ease on sheet steel or sheet iron.

The conventional tin-lead alloy bath is made to contain a relatively large proportion of tin -30 percent by weight) for the purpose of improving the adhesion of the coating layer; however, a small quantity (0.1-3 percent by weight) of tin is enough in the (b) composition because the object of the present invention is to improve the fluidity and the brightness of the coating layer. The upper limit of the quantity of tin is set at 3 percent by weight from the economical point of view, and no objectionable result is seen with the coating layer when more tin is used; however, it has been observed that the coating obtained from an alloy which has more than 5 percent by weight of tin, which is far in excess of the upper limit of the tin content of composition (b), shows some decrease in its corrosion resistance.

When the lead base alloy composition provided in the foregoing (c) is used in hot-dip lead coating, arsenic and antimony develop their efficiencies by potentialization and not only increase the adhesion between the formed coating layer and the surface of the metal underneath, but also improve the hardness and corrosion resistance of the coating layer to provide a coating which has no pinholes at all. Furthermore, when the hot-dip lead coating is applied using this alloy, the formation of a thick coating layer is also easily obtained. In the lead base alloy prepared according to the composition formula stated in (d), when used in the hot-dip lead coating, arsenic works to improve the adhesion between the coating layer and the surface of the metal underneath, antimony works to improve the hardness and the resistance to acids and corrosion, tin makes it possible to form a thin coating by increasing the fluidity of the molten lead-base alloy and also produces a decorative and reflective luster by improving the brightness of the surface of the coating layer markedly.

Lead having the purity of between 99.9 and 99.99 percent is best fit for preparing lead base alloys according to the present invention.

When a comparison is made between the dip coatings formed on the articles of iron, iron-base alloys, copper and copperbase alloys by using the lead-base alloys (a), (b), (c) and (d) and the dip coating formed on said articles and alloys by using the conventional tin-lead wt. percent) alloy, notable differences are seen as in the following. When a metal to be coated is immersed in the aforementioned coating alloy bath, the alloy bath prepared according to the present invention is distinguished by its very thin alloy layer formed between the surface of the metal underneath and the coating layer as compared with that formed by the use of the conventional tin-lead alloy. Observations made on the cross section of the coating layers formed on the surface of the metal underneath show that the coated metal article consists of three layers i.e. substrate layer, binding alloy layer (or intermediate layer) and coating alloy layer. The binding alloy layer of the coated metal article prepared according to the present invention has a thickness of approximately 0.01-0.5u. On the other hand the intermediate layer of the coated metal article produced by using a conventional tin-lead (20 wt. percent) alloy has a thickness of approximately l-3,u. The phenomenon of forming a thick intermediate layer that is observed in the coating by the use of a tin-lead (20 wt. percent) alloy which has hitherto been most widely used suggests that, where the metal article to be coated is iron or ironbase alloys, for instance, the amount of tin is so large that the iron component part of the substrate combines with said tin to form an alloy layer comprising an iron-tin alloy, thus depriving the lead-tin bath of its tin element selectively. in other words, in the conventional coating conducted by the use of a lead-tin alloy, if such a thick alloy layer comprising an iron-tin alloy should not be formed, it would not be possible to obtain a sufficiently strong coating. Therefore, while making a practical application of such tin-lead bath in coating, unremitting attention should always be paid to determine the decrease of the tin content in the coating bath and supply the deficiency in order to keep the fluctuation of the bath components as small as possible. On the other hand, in the lead alloy coating formed according to the present invention, the aforementioned intermediate layer is very thin, and is strengthened by a bond between the coating alloy layer and the surface of the metal article to be coated, so that the fluctuation of the bath components during the coating operation is negligibly small, and no phenomenon is observed where a third metal contained in the lead base alloy bath is consumed selectively. As mentioned in the foregoing, the lead base alloy thus prepared according to the present invention exhibits markedly efficient performances in the actual coating operation.

The coating conditions employed in the practice of this invention are in accordance with those followed in the conventional coating of metal articles with leadbase alloys by the hot-dip method.

In general, we prefer that the conditions to be employed in this invention are as follows. We maintain, for example, a lead-base alloy bath temperature in the range of 330C. to 390C. and employ a dipping time of the metal article of 4 to 20 seconds. It is to be understood that a wide variety of coating temperatures and times can be selected depending pertinently, for example, upon the size and desired coating thickness on the metal articles.

Any flux conventionally used in the lead coating art can be used, as for example the chlorides such as NH Cl, ZnCl SnCl SbCl CdCl NaCl, LiCl, AlCl l-lgCl KC] and the like.

The metal articles to be coated can be, in general, steel articles, malleable iron articles, ductile iron or cast iron articles, bronze articles, brass articles, stainless steel articles or articles made of suitable combinations of these metals.

The coated articles according to the present invention can be used in the field of civil engineering and construction; they can be materials such as plates, tubes, wires, bolts, nuts or metal materials for making such articles. The coated articles can be used especially as corrosion resistant members for steel-frame buildings such as acid gas-generating chemical factories DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 Arsenic-containing, lead-base alloy and lead were applied to steel plates to form a coating thereon under the conditions (a) (e) stated in the following and the results obtained from said coating are given in Table l. Coating conditions: I

(a) Lead bath quantity (b) Lead base alloy used 5.2 percent by weight of arsenic was added to commercially obtainable pure lead and was melted.

Finished steel plate. deoiled and pickled. size 300 X 300 X 0.8 mm. Commercially obtainable one for hot-dip lead coating use (principal ingredient: zinc chloride).

(c) Bath temperature (d) Specimen (e) Flux Table l Number of Pinholes Arsenic Luster Content wt)? Coating Hobrlipping U f 1 nl ormi y llmC 0 60 Not satisfactory Good Lusterless 0.01 Nil Excellent do. do. do.

lusterless and rough More than 20 It is to be noted in Table 1 that the best results are obtained when the amount of arsenic contained in lead is between 0.01 and 5.0 percent by weight from the fact that if the arsenic content is less than 0.01 percent by weight, the desired effects are not obtained and if the arsenic content is more than 5.0 percent by weight, the coating becomes less lustrous and uniform, presenting more roughness all over the surface of the coating layer. When no arsenic is added to the lead,,the coating process requires two to six times as long as that conducted with the lead bath containing arsenic, and it is also to be noted that the coating itself is less lustrous and lacks uniformity.

Example 2 A lead-base alloy containing arsenic was applied to several kinds of metal plates under the conditions (a) (e) as stated'in the following and the results obtained 6 from said coating are given in Table 2. Coating conditions:

(a) Lead bath quantity (h) Lead base alloy used 0.05% by weight of arsenic was added to lead and was melted.

(c) Bath temperature 350C. i 5C.

(d) Specimen Several kinds of metal plates. deoiled and pickled. size 300 X 300 X 0.4 mm. (e) Flux Commercially obtainable for hot-dip lead coating use.

(Principal ingredient:

zinc chloride).

Table 2 Hotdipping Number of Coating Specimen Time Luster Pinholes Uniformity sec.

Steel plate l0 Metallic luster Nil Excellent Stainless l5 do. do. Good steel plate lS-S) do. (l3 Cr) 15 do. do. do. Copper plate It) do. do. Excellent Brass plate l5 do. do. do. (60 Cu It is apparent from Table 2 that the lead-base alloy which was used in the aforementioned example can be applied very satisfactorily to ferrous metals such as steel plates and stainless steel plates as well as cuprous metals such as copper plates and brass plates.

When the lead-base alloy containing arsenic is used in hot-dip coating according to the present invention, not only is a lead coating surface with excellent coating uniformity obtainable in the direct lead coating of both ferrous and ,cuprous metals, but also many other advantages are obtained including the formation of no pinholes. Thus, a dramatic improvement is attained by simply adding a small amount of arsenic to lead to provide an excellent lead coating at a very low cost. Incidentally, the arsenic-lead alloy related in the foregoing example can be readily made by adding arsenic to molten lead and stirring the mixture.

EXAMPLE 3 A lead-base alloy was applied to steel plates to form a coating thereon under the conditions (a) (e) as mentioned below and the results are shown in Table 3. Coating conditions:

(a) Lead bath quantity 100 kg (b) Bath temperature 350C. i C.

Finished steel plates, deoiled and pickled, size 300 X 300 X 0.4 mm.

(c) Specimen increases. It is also to be noted that when more than 5 percent by weight of tin is contained, the coating becomes less uniform and the roughness on the surface of the coating increases. When the lead base alloy bath d Fl Commerciall obtainable l one f l lead 5 prepared according to this Example 18 applied to copg t s l d I per plates and stainless steel plates to coat them, the ref, f sults obtained with these plates are substantially the (c) Immersion time seconds same as those shown in the foregoing example.

Table 3 Bath Components Immersed for 150 hrs. As (71) Sn (72) Pb (72) in a 57: NaCl solution Luster Number of Pinholes I00 Many pinholes No luster (yielding white matter) 0.0l 99.99 Nil Metallic luster 3 97 do. do. 5 95 do. do. 3 0.] 96.9 do. Silvery white reflective gloss 3 I 96 do. do. 3 2 i 94.5 do. do. 3 3 94.0 One pinhole do. 3 4 93 Three pinholes do.

EXAMPLE 4 75 EXAMPLE 5 The lead-base alloys respectively having the same bath components as in Example 3 where applied to steel strip coils, l X 0.5 mm. having a length of Lead-base alloys were coated on steel plates under the conditions (a) (e) as mentioned in the following and the results obtained from said operations are Table 4 Immersed for l hrs. in a 571 NaCl Bath Components Hardness solution Luster (wt '71) (Vickers') (number of pinholes) As Sh Pb I00 (19 Many pinholes No luster (yielded white matter) 0.0] 99.99 7l No pinhole Metallic luster 3 97 90 do. do. 5 95 99 do. do. 0.3 0.] 99.6 I00 do. do. 3 l 96 [08 do. Silvery white luster (line crystal) 3 5 92 l 15 do. do. 3 l0 87 I24 do. Silvery white avcnturine (fine crystal) meters. to apply a continuous coating under the condi- 50 shown 11'] Table 4. Coating conditions:

tions that the bath quantity is 700 kg and that the strip speed is approximately 10 m/sec. The thickness of the coating was controlled by means of rollers equipped on the surface of the bath.

In the case of the arsenic-lead bath wherein no tin is contained. the thickness of the coating ranged from 8-l0p.; however, in the case where the alloy bath contained arsenic and more than 0.1 percent by weight of tin as in this example, the thickness of the film was less than 5;; and no pinholes were observed at all.

As it is apparent from the foregoing Examples 3 and 4, it has been made possible to form a continuous coating on ferrous metals by the application of an arsenictin-lead alloy in accordance with the present invention as well as to improve the luster on the coating surface. As for corrosion resistance, however, it is to be noted that there is a tendency for the coating to become somewhat less corrosion resistant as the amount of tin Finished steel plates, deoiled and pickled.

size 300 X 300 X 0.4 mm. Commercially obtainable one for hot-dip lead coating use.

(Principal ingredient: zinc chloride).

15 seconds (a) Lead bath quantity (b) Bath temperature (0) Specimen (d) Flux (e) lmmersing time 9. of antimony increases until itfinally exceeds percent by weight, the hardness also increases; the adhesion of the coating to the metal. beneath decreases and becomes apt to peel off. Therefore from the practical viewpoint of industrial technology, it has also been noted that the content of antimony may be regardedas best when it ranges between 0.1 and 10 percent by weight. Furthermore, with regard. to the amount of arsenic, it has also been noted that when more than 5v percent by weight of arsenic is used, the surface of the coating becomes less uniform and more rough. Incidentally, when As-Sb-Pb alloys prepared as in this Example were coated on copper plates and stainless steel plates, the results obtained from these coating operations were substantially the same as those obtained in this Example. lf coating layers, dim and lusterless in appearance, are intended to be formed by the use of As-Sb-Pb alloys prepared according to this Example, the desired effect can be obtained by increasing the amount of antimony to be contained therein.

EXAMPLE 6 (a) Lead bath quantity (b) Bath temperature 100 kg 350C. i 5C.

(c) Specimen Finished steel plates, deoiled and pickled, size 300 X 300 X 0.04 mm. (d) Flux Commercially obtainable one for hotdip lead coating use. (Principal ingredient: zinc chloride).

(e) lmmersing time l5 seconds.

Sb-Sn-Pb alloys, alloy No. 17 whose percentage of tin content is high cannot fulfill the main object of this invention.

Incidentally, when alloys (Nos. 5-16) prepared according to this Example were coated on copper plates and stainless steel plates, the results obtained from them were the same as those obtained in this Example.

The plates used in the foregoing examples can be replaced by individual formed articled such as steel and brass bolts and fixtures, drums and tanks, and the like. Likewise steel or brass wires or tubes can be coated in place of the strip steel used. The plates, strip, wire or tubes can be used directly or subsequently formed and shaped for use.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for hot-dip coating lead-base alloy onto a surface of an article consisting substantially of a metal selected from the group consisting of iron, iron-base alloys, copper and copper-base alloys, which comprises dipping said article into a bath of molten lead-base alloy having atemperature in the range of from about 330 to 390C, said alloy being selected from the group consisting of the following (a), (b), (c) and (d) alloys, in order to thereby form a coating layer on a surface of said article:

a. an alloy consisting of 0.01 5 percent by weight of arsenic, the balance being lead;

b. an alloy consisting of 0.01 5 percent by weight of arsenicand 0.l 3.0 percent by weight of tin, the balance being lead;

0. an alloy consisting of 0.01 5.0 percent by weight of arsenic and 0.1 10 percent by weight of antimony, the balance being lead; and

Table 5 Alloy/Bath Components Hardness Immersed for hrs No. As Sb Sn Pb (Vickers) (Number of Pinholes) Luster l 100 69 Many pinholes (yield- Lusterless ing white matter) 2 0.0] 99.99 71 No pinhole Metallic luster 3 3 97 do. do.

4 5 99 do. do.

5 3 0.l 0.1 96.8 [00 do. do.

6 3 0.l 1 95.9 I00 do. do.

7 3 0.] 2.5 94.4 99 do. do.

8 3 l 0.1 95.9 107 do. Silvery white luster 9 3 l l 95 106 do. do. I0 3 l 2.5 93.5 do. do. ll 3 5 0.l 9L9 do. do. 12 3 5 l 91 l 13 do. do. l3 3 5 2. 89.5 l 12 do. do. 14 3 10 0.1 86.9 124 do. Silvery white aventurine l5 3 l0 1 86 122 do. do. l6 3 l0 2.5 84.5 lZl do. do. 17 3 5 10 82 1 l0 4 pinholes, (red rust) Silvery white luster It will be noted from Table 5 that the hardness of the coatings formed by the use of alloy No. 2 comprising 0.01 percent by weight of arsenic and the balance of lead is somewhat lower than that of the coatings formed by the use of the respective alloys (nos. 5-17) but has no pinholes thereupon. It is recognized that the coating phases formed by the use of As-Sb-Sn-Pb alloys 65 have no pinholes and have a hardness whose value ranges between 100 and 124 and also excellent corrosion resistance. It is also noticed, however, that of As- 5. The method according to claim 1 wherein the arti- 7. A method according to claim 1, in which said article is steel and the lead-base alloy is (c). cle is dipped into the bath for a period of from about 6. The method according to claim 1 wherein the arti- 4 to about 20 seconds.

cle is steel and the lead-base alloy is (d).

Claims

2. The method according to claim 1 wherein the lead-base alloy is (a).
3. The method according to claim 1 wherein the article is steel and the lead-base alloy is (a).
4. The method according to claim 1 wherein the article is steel and the lead-base alloy is (b).
5. The method according to claim 1 wherein the article is steel and the lead-base alloy is (c).
6. The method according to claim 1 wherein the article is steel and the lead-base alloy is (d).
7. A method according to claim 1, in which said article is dipped into the bath for a period of from about 4 to about 20 seconds.