US3788877A - Method for producing adherent,ductile zinc coating on ferrous substrates by vacuum deposition - Google Patents

Abstract

A ZN COATING OF UP TO SEVEN MILS THICKNESS IS DEPOSITED ON A CONVENTIONALLY CLEANED, FERROUS SUBSTRATE BY CONDENSATION OF ZN VAPORS IN VACUO. TO MAKE THE COATING ADHERENT, IT IS PASSED OUT OF THE VACUUM CHAMBER AND RAPIDLY HEATED TO A TEMPERATURE OF 750*F.-1000*F. AND THEN IMMEDIATELY QUENCHED. THE ELASPED TIME FROM BEGINNING OF HEATING TO THE CONCLUSION OF THE QUENCHING MUST BE NO GREATER THAN ABOUT 25 SECONDS.

Description

United States Patent 'Oflice 3,788,877 Patented Jan. 29, 1974 3,788,877 METHOD FOR PRODUCING ADHERENT, DUCTILE ZINC COATING ON FERROUS SUBSTRATES BY VACUUM DEPOSITION Lawrence E. Helwig, Hampton Township, Allegheny County, and Michael V. Murray, Monroeville, Pa., assignors to United States Steel Corporation No Drawing. Filed July 20, 1972, Ser. No. 273,573 Int. Cl. C23c 13/02 US. Cl. '11762 5 Claims ABSTRACT OF THE DISCLOSURE A Zn coating of up to seven mils thickness is deposited on a conventionally cleaned, ferrous substrate by condensation of Zn vapors in vacuo. To make the coating adherent, it is passed out of the vacuum chamber and rapidly heated to a temperature of 750 F.1000 F. and then immediately quenched. The elapsed time from beginning of heating to the conclusion of the quenching must be no greater than about 25 seconds.

The invention is directed to a method for the vapor deposition of Zn coatings in vacuo and is specifically directed to a method for significantly improving the adherence of such vapor deposited coatings to ferrous substrates.

Zn coatings have been applied to ferrous strands (sheet, strip, wire, rod, etc.) by hot-dipping electroplating, cementation, hot spraying and by vacuum deposition. Each of these techniques is accompanied by certain advantages and disadvantages. Cementation is a slow process, not well adapted for high speed continuous application and produces an iron-zinc alloy coating which lacks ductility. Wire spraying does not yield uniform coatings. Electroplating is adapted to continuous application of uniform, ductile coatings. However, the process is very slow and therefore is only utilized, commercially, for very thin films. Therefore, only hot-dipping and vacuum deposition are commercially practicable for the continuous, high speed application of thick zinc coatings. For hot dip coatings to be adherent, the molten zinc bath must contain a small amount of aluminum, which has the disadvantage of making the coating susceptible to staining or white rusting. Additionally, if a smooth surface is required, the hot-dipped spangled surface can only be minimized by subsequent treatments such as temper-rolling, annealing or steam treatment. Even when such subsequent treatments are employed, coating smoothness and ductility are almost impossible to achieve for coatings thicker than about two mils, partly because of the brittle iron-zinc alloy layer that forms at the zinc-iron interface.

Thick, smooth, pure Zn coatings, without a brittle alloy layer have been obtained by vacuum deposition techniques. However, a number of rather expensive expedients have been required to make the coating adherent; since, without such expedients, the Zn so deposited can be readily removed with transparent, adhesive tape. In one such process (US. Pat. 3,326,177) for providing an adherent coating, the ferrous substrate must be extensively cleaned, i.e. by glow-discharge in hydrogen, immediately before condensation of the Zn vapors. In another (US. Pat. 3,278,331), a very thin Zn coating is deposited, which is then heated in vacuum to alloy with the substrate. Subsequently, the substrate is cooled in vacuum and only then coated with the remainder of the Zn to achieve the desired thickness. In yet another process, a bonding layer, e.g. iron, is deposited at 350 F.; a thin Zn layer is then deposited at about the same temperature, and the strip is then cooled prior to deposition of the remainder of Zn. Even when such expensive expedients are employed, it is often the case that coatings thicker than about one mil crack and lose adherence in a simple handkerchief bend test.

It has now been found, that thick, adherent Zn coatings may be deposited on ferrous substrates which have been cleaned in conventional manner, i.e., merely to remove surface grease and oils. The Zn is deposited in vacuo at a pressure below about 4 10- mm. Hg. No. heating of the substrate is required and it is preferable that the substrate temperature be below 200 F. At this point in the process, the deposited coating is not adherent, and care must be taken to prevent abrasion by the exit seals. If the substrate surface is roughened prior to deposition, then some slight abrasion can be tolerated. After coating is effected, the substrate is removed from the vacuum chamber and is rapidly heated to a temperature of 750 to 1000 F. (depending on the atmosphere employed) and immediately quenched to below 200 F. The rapid heating and quenching is critical, so that the whole process occurs in a period of no more than about 25 seconds. No special atmosphere is required for the heating step. The only requisite being that the pressure is sufficient to prevent the re-evaporation of the Zn. Thus, for example, air, neutral or reducing gases, at atmospheric pressure may be utilized.

Accordingly, it is an object of this invention to provide an economical method for the vacuum deposition of Zn coatings.

It is a further object of this invention to provide a thick vacuum deposited Zn coating exhibiting a superior combination of ductility and adherence.

It is another object of this invention to provide a method for obtaining thick, spangle free coatings.

These and other objects and advantages of this invention will become more apparent in the course of reading the following detailed description, taken in conjunction With the appended claims. The following specific exam ples are offered to illustrate the importance of the subsequent heat treatment of this invention and the parameters relevant thereto.

Sheet and wire speciments were coated by evaporating reagent grade Zn shot from a radiant-heated steel tray. All the sheet specimens and some of the Wire specimens were solvent cleaned with toluene and alcohol prior to coating. Those wire specimens, which were not solvent cleaned, were alkaline cleaned and then pickled in citric acid prior to coating. Specimens were at room temperature (except for those indicated otherwise) at the start of Zn condensation and were only heated by the latent heat given up by the Zn vapors as they condensed. Coatings of varying thickness were achieved at a variety of deposition rates, in periods of from about 0.5 to 3 minutes. Sheet specimens were coated at pressures of 1 10- mm. Hg; wire specimens at pressures of from 1 l0- to 2 10- mm. Hg. The coated specimens were removed from the bell-jar vacuum coating chamber and post-heated (in the cases indicated) by resistance methods to various temperatures and then quenched in water. The temperature of each speciment during heating and cooling was monitored by a thermocouple attached thereto. Adherence was measured by a 180-degree fold of the TABLE I Substrate temp., F.

Coating thickness Prior to End of (mils) deposition deposition Adhesion 80 None. 150 300 Do. 250 450 Do. 350 530 Do. 450 630 None (40% r e-evaporated). 550 670 None (50% re-evaporated).

At least one prior art reference had suggested that the adherence of vacuum deposited coatings could be improved by pre-heating of the substrate. It may be seen, however, that at least with respect to Zn, such preheating is ineffective in improving coating adherence.

In the examples reported in Table II, specimens were post-heated at varying heating rates. All speciments were quenched with water to 100 F. in less than two seconds. The total elapsed time from beginning of heating to the end of the quench is reported.

ess, since such pressures may be obtained with mechanical pumps. However, to insure the adherence and appearance of the coating, it is preferred to operate at chamber pressures below about 1 10 mm. Hg.

While a number of prior art processes have been mainly concerned with the formation of a critical, intermediate iron-zinc alloy layer, the instant method is apparently dependent on a totally different mechanism. In a particular experiment, two samples were identically vacuum-coated with Zn layers about 5 mils in thickness. Both specimens were post-heated to 800 F. and quenched in water. However, therelapsed time for one sample was 12 seconds and for the other 27 seconds. The thickness (about 0.5 mil) of the intermediate iron-zinc alloy layer which formed, was exactly the same for both samples. However, as shown above, the coating of the 12-second sample was adherent while that of the 27-second sample exhibited poor adherence. It therefore appears that when longer elapsed times are employed, the loss in adherence is due to the formation of voids at the interface as a result of the coalescence of vacancies within the coating. This conclusion is borne out by the fact that even normally adherent, hot-dipped or electrodeposited Zn coatings lost adhesion when heated at 720 F. for an elapsed time in excess of two minutes. The critical time-temperature relation for vacuum-deposited Zn coatings is much more limited however, since such coatings have more defects (vacancies, faults, etc.) than do hot-dipped or electroplated coatings. It therefore appears that the condensation of voids not only occurs during the post-heating of TABLE II Coating Maximum Elapsed thickness substrate time (mils) temp., F. (seconds) Adhesion and remarks 0.7 500 6.0 Not adherent.

0.7 680 8.0 Poor.

0.7 750 7.2 Excellent.

1.3 800 9 Excellent.

2. 6 800 27 Poor, easily peeled from bend.

2.6 800 Ngnectlherent, fell off when 3.2 800 42 Very poor, very easily peeled om ben 2.7 800 54 Nonadherent, fell off when bent.

The critical effect of achieving a temperature of at least about 750, as well as that of elased time, may readily be seen.

The effect of elapsed time, as distinguished from heat-up rate is shown in the examples below.

TABLE III Time to Time to cool from Coating heat to 800 F. to thickness 800 F. 100 F.

(mils) (sec.) (sec.) Adherence 0. 6 6 2 Excellent. 0. 5 7 1 120 Flaked ofi. 0. 8 95 2 Do.

* Air cooled, rather than quenching in water.

Itis therefore seen, that even when rapid heating is employed (Sample No. 2), an adherent coating 'Wll not result unless the specimen is rapidly cooled so that the total, elapsed period is below the requisite time.

Additional experiments showed that even when elapsed times of less than 15 seconds were achieved, an adherent coating would not result unless chamber pressure was maintained at below 4 l0- mm. Hg. Such a limitation will not Seriously 8.5991 the economics of the instant procthe specimen, but also during the deposition of the Zn. Thus, a certain amount of post heating is essential in order to annneal out the so-formed voids, but that excess post heating is counter productive.

The process of this invention may therefore be carried out in the following manner. The ferrous article to be coated is cleaned, employing methods conventionally employed by the art, such as in the electroplating of various metals, e.g. Sn, Zn, Cr. The cleaned article is passed to a vacuum chamber maintained at a pressure below about 1X10 mm. Hg. While the article may be preheated, such a procedure is not required. For commercial applications (e.g. high speed plating) it is desirable to deposit the Zn at very rapid rates. During such rapid deposition, the temperature of the substrate will be substantially raised by the heat of condensation. Since the temperature of the substrate should not be permitted to reach the point at which the coating would re-evaporate, it is preferred that the strip enter the chamber at about room temperature. However, as long as the substrate is in the vapor beam (directly over the crucible), temperatures of about 600 F. (depending on chamber pressure) can be tolerated. When the desired coating thickness is achieved, the article is removed from the chamber and heated in air to a temperature of about 750 to 900 F. and immedi' ately quenched in water to a temperature below about 200 F.; the elapsed time from initiation of the heating to the consumption of the quench being no more than about 25 seconds. The upper limit of 900 F. is determined principally by considerations of appearance and corrosion protection, rather than by adherence of the coating. Vacuum-deposited Zn coatings on wire specimens, heated in air to above 900 F. and quenched to 200 F. in an elapsed time of seven seconds exhibited excellent adherence. However, the Zn coating exhibited excessive oxidation. That is excessive oxidation is meant to define the production of the resultant yellow oxide which was not only unattractive, but offered less protection as a sacrificial coating. If neutral or reducing atmospheres are employed (thus adding to the expense of the process) for the post-heating step, then temperatures somewhat higher than 900 F. may be employed. In any case, the temperature should be below 1000 F. to prevent the coating from burning otf.

Although the annealing of the voids effected by the instant post-treatment is evidently a dilfusion process, it has surprisingly been found that the permissible elapsed time of the instant post-heating procedure, appears to be substantially the same for the entire range of temperatures employed. Thus, a period of up to about 25 seconds is permissible for maximum substrate temperatures of 900 F. as well as for substrate temperatures of 750 F. However, to insure that the critical time is not exceeded, it is preferable to achieve an elapsed time of less than about 18 seconds.

We claim:

1. In the method for the coating of a cleaned ferrous substrate with a corrosion resistance layer of Zn, which comprises:

passing the substrate into a chamber maintained at a pressure no greater than about 4 10- mm. Hg and depositing Zn to a thickness of up to about 7 mils on at least one surface of said substrate, by the condensation of vapors issuing from a source of Zn maintained Within said chamber; the improvement which comprises thereafter, passing said substrate out of said chamber and heating said substrate to a temperature of from 750 F. to 1000 F. in an atmosphere and at a pressure sufiicient to prevent both excessive oxidation and the vaporization of the Zn coating; and thereafter cooling said susbtrate to a temperature below 200 F., said heating and cooling being sufficiently rapid so that the elapsed time from the beginning of said heating to the conclusion of said cooling is not greater than about 25 seconds. 2. The method of claim 1, wherein said substrate is at a temperature below 200 F. prior to the condensation of said Zn vapors.

3. The method of claim 2, wherein said elapsed time is less than about 18 seconds.

4. The method of claim 3, wherein said chamber pressure is below about 1 10- mm. Hg.

5. The method of claim 4, wherein said heating is conducted in air at about atmospheric pressure, at a temperature of from 750 F. to about 900 F.

References Cited UNITED STATES PATENTS 3,674,445 7/1972 Wlodek 117-107 3,700,485 10/1972 Rubin 117--107 RALPH S. KENDALL, Primary Examiner I. W. MASSIE, Assistant Examiner US. Cl. X.R.

I W. MASSIE, Assistant Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3,788,877 Dated January 29, 1978 Patent No.

Inventor(s) Lawrence E. Helwig et a1.

ppears in the above-identified patent It is certified that error a hereby corrected as shown below:

and that said Letters Patent are Column 5, line 3, "consumption" should read consummation Column 6, line LL, Claim 1, beginning of the line;

cancel "the improvement which comprises".

Signedand 'sealed this 18th day 51" June-"197E.

(SEAL) Attest: v I EDWARD M.FLETCHEB,JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents 7 FORM po-msouo-ss) ussoMwDc wand,

GOVERNMENT PRINTING OFFICE ll, O ''S6-3 l4,