US3653852A

US3653852A - Coated ferrous substrate

Info

Publication number: US3653852A
Application number: US866290A
Authority: US
Inventors: Bernard C Seiler
Original assignee: Bethlehem Steel Corp
Current assignee: Bethlehem Steel Corp
Priority date: 1969-10-14
Filing date: 1969-10-14
Publication date: 1972-04-04
Anticipated expiration: 1989-04-04

Abstract

A steel sheet has a composite coating thereon comprising a layer of chromium 5 to 100 microinches thick contiguous to the steel and a layer of aluminum 100 to 1,500 microinches thick contiguous to the layer of chromium. The chromium is deposited on the sheet while the steel is at a temperature within the range of 1,200* to 1,700* F.

Description

United States Patent Seiler 1 Apr. 4, 1972 [54] COATED FERROUS SUBSTRATE 2,957,782 10/1960 Boiler ..29/196.6 x

3,436,805 4/1969 Friske et al.. [72] Inventor Bernard Bethlehem 2,856,333 10/1958 Topelian ..29/197 [73] Assignee: Bethlehem Steel Corporation [22] Filed: Oct. 14 1969 Primary Examiner-L. Dewayne Rutledge ASSISIG"! Examiner-G. K. White [21] Appl. No.: 866,290 Attorney-Joseph J. OKeefe 52 us. on ..29/196.6, 29/1962, 29/197 ABSTRACT [51] .....B32b 15/18, B32b 15/20 A Steel sheet has a com posite coating thereon comprising a [58] Field of Search ..29/ 196.2, 196.6, 197 layer of chromium 5 to 100 microinches thick contiguous to 56 R f C" d the steel and a layer of aluminum 100 to 1,500 microinches 1 e erences e thick contiguous to the layer of chromium. The chromium is UNITED STATES PATENTS deposited on the sheet while the steel is at a temperature within the range of 1,200 to 1,700 F. 2,917,818 12/1959 Thomson ..29/l96.2 3,323,881 6/1967 Nelson et al. ..29/l96.6 X 4 Claims, 2 Drawing Figures Cf THICKNESS (MICROINCHES) Patented April 4, 1972 2 Sheets-Sheet l mmm OON

OOO. 000m 000m OOO 000m INVENTOR Bernard C. Sei/er Patented April 4, 1972 2 Sheets-Sheet 2 Fig. 2

0 O u M M o M W 2 .7 O E O N P Q o O 7 P 0%0 0 0 H w .1. n m U q o a O m 0 o O 8 O 0 6 m H 0 a U 0 W H 4 m M m O u M E M o O H A u 2 W 0 O O O o 0 O O O O 9 8 7 6 5 4 3 2 I ZQWOEIOU .rZwu mum TEMPERATURE (F) INVENTOR Bernard C. Sei/er COATED FERROUS SUBSTRATE BACKGROUND OF THE INVENTION This invention relates to a ferrous substrate having a coating thereon, and more particularly to a steel sheet having a composite coating of chromium and aluminum thereon.

Steel articles which are exposed to the atmosphere, e.g., sheds, buckets, garbage cans, etc., are generally coated with another metal in order to extend the service life of the article. In addition to protecting the steel from corrosion, it is essential for the coating to be ductile and to adhere satisfactorily to the steel base so that the article can be readily formed from the coated steel base.

In the past, steel sheets have been coated with metals such as aluminum to provide a coated sheet having resistance to atmospheric corrosion. While articles made from such coated sheets have a fairly long service life, it is desirable to provide a coated steel sheet having an even longer service life.

It is an object of this invention to provide a coated ferrous substrate having a service life longer than that of steel sheets coated solely with aluminum, said coated ferrous substrate having excellent ductility.

It is a further object to provide a method for producing such a coated ferrous substrate.

SUMMARY OF THE INVENTION 1 have discovered that the foregoing objects can be attained by heating a ferrous substrate to a temperature within the range of 1,200" to l,700 F. and depositing a layer of chromium on said substrate while said substrate is within said temperature range, said layer having a thickness of from 5 to 100 microinches. A layer of aluminum is then deposited on said layer of chromium.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the life of coated steel sheets in the ASTM B-l l7-62 salt fog test.

FIG. 2 is a graph showing the ductility of the coated steel sheet as a function of the temperature of the steel during the deposition of the chromium thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows the results of the ASTM B-l 17-62 salt fog test, said test being a general indication of the samples resistance to atmospheric corrosion. The abscissa is in terms of thickness of the chromium layer, in microinches, while the ordinate is in terms of hours exposure per mil of total coating thickness until failure. The aluminum varied in thickness from about 100 to 700 microinches. In all cases, both the chromium and the aluminum were deposited on sheet steel samples by vacuum vapor deposition. The chromium was deposited while the steel was above l,000 F while the aluminum was deposited while the steel was at ambient temperature. In order to avoid any confusion which might result from plotting the entire mass of data, the results for a particular composition range were averaged and plotted as a straight line extending over the composition range from which the data were obtained. Above each line is the number of samples averaged to obtain said line.

As is shown in FIG. 1, if the coating comprisesonly aluminum, i.e., if there is no layer of chromium intermediate the steel and the aluminum, the life in the salt fog test averaged about 1,500 hours per mil of coating thickness. However, a layer of chromium only 5 microinches thick between the steel and the aluminum resulted in a salt fog test life of about 4,000 hours per mil of total coating thickness, said life being more than double that for aluminum alone. The maximum life occurred at about 20 microinches of chromium, and was about 8,000 hours per mil of total coating thickness. Surprisingly, as the layer of chromium increased in thickness beyond 20 microinches, the life in the salt fog decreased. As can be seen, the life in the salt fog had fallen to about 1,000 hours per mil of total coating thickness at 200 microinches of chromium.

Between 5 and microinches of chromium, the salt fog life is consistently above 3,500 hours per mil of total coating thickness, and this range of chromium thickness, marked A" in FIG. 1, constitutes the preferred range of the invention.

While the coatings may be applied by any known methods, e.g., electrodeposition or hot dip, they are preferably applied by vacuum vapor deposition. In order to get satisfactory ductility, it was found necessary to deposit the chromium on the steel sheet while the steel was at a temperature of at least l,200 F. In Table I, below, the ductility of samples coated with chromium was tested by stripping the steel base from the coating and bending the coating over on itself. If no cracks occurred, the coating was considered ductile.

TABLE I Temperature of Steel FIG. 2 shows the ductility of steel sheets, coated with a layer of aluminum over a layer of chromium, as a function of the temperature of the steel sheet during the deposition of the chromium thereon. The abscissa is in terms of the temperature of the steel, immediately prior to the deposition of chromium thereon, while the ordinate is in terms of percentage corrosion in the modified blotter test. In this test, beer can ends (2.687 inch in diameter) are punched by conventional methods from the coated sheet to be tested. These ends are closed over can bodies with the coating on the outside. The cans are then placed on a wet blotter with the test coating thereagainst. The blotter is kept moist by maintaining both ends thereof in a reservoir of distilled water. A weight is placed on top of the cans to ensure intimate contact between the cam rim and the blotter. After 48 hours, the cans are removed and the length of the corrosion products from exposed steel along the rim of the can ends is measured and expressed as a percentage of the circumference of the can end.

The rim of the can end making contact with the blotter is deep drawn during the end punching operation and is subjected to a mild ironing and a 3T bend during the closing operation on the can body. Steel will be readily exposed if the coating is very brittle.

As is shown in FIG. 2, the coating is quite brittle at a chromium deposition temperature of 200 F., but as the temperature of the steel increases the degree of brittleness rapidly decreases until a low level is reached at about 1,200" P. This low level of brittleness is maintained at temperatures up to at 1,500" F. Higher temperatures were not investigated, inasmuch as strip handling becomes quite difficult at temperatures above l,500 F. However, it appears from an extrapolation of the curve that the ductility will remain satisfactory up to at least l,700 F.

In view of the foregoing, the ferrous substrate is heated to a temperature within the range of 1,200 to 1,700 F., and the chromium is deposited thereon, preferably by vacuum vapor deposition, while the substrate is within said range. The aluminum coating is then applied, also preferably by vacuum vapor deposition. Preferably, the aluminum is 100 to 1,500 microinches thick.

A preferred ferrous substrate for the subject coatings comprises a sheet steel consisting essentially of 0.03 to 0.15 weight percent carbon, 0.20 to 0.60 weight percent manganese, balance iron. By balance iron, I do not wish to exclude normal impurities such as 0.003 to 0.020 weight percent phosphorus and 0.010 to 0.050 weight percent sulfur. If

desired, the carbon in the steel may be stabilized by means of titanium.

EXAMPLE I As a first specific example of my invention, a titanium-stabilized steel strip 0.014 inch thick was vapor degreased in trichlorethylene. The steel consisted essentially of 0.038 weight percent carbon, 0.34 weight percent manganese, 0.43 weight percent titanium, balance iron. The strip was then cleaned cathodically in a hot Pennsalt solution. The cleaned strip was preheated to l,500 F. in a vacuum coating chamber evacuated to a pressure of about torr., and 8 microinches of chromium were deposited thereon by conventional vacuum vapor deposition techniques.

The coated strip was next re-wound in its original position and 164 microinches of aluminum were deposited, at ambient temperature, on top of the layer of chromium. Two test panels were cut from the strip. One of said panels was deformed by the Ericksen Cup Tester to the maximum cup depth without rupturing the steel. Both panels were then subjected to the above-referred-to salt fog test. Inasmuch as the strip was coated on only one side thereof, the backs and the edges of the panels were protected with electroplaters tape during said test.

The cupped panel had a life of 1,080 hours, or 6,280 hours per mi] of total coating. The other panel had a life of 890 hours, or 5,170 hours per mil of total coatingv EXAMPLE ll As a second specific example of my invention, a low carbon steel strip 0.0105 inch thick and consisting essentially of 0.07 weight percent carbon, 0.3 weight percent manganese, balance iron, was coated with 15 microinches of chromium at 1,200 F. by the same procedures outlined in Example I. A layer of aluminum 172 microinches thick was then vacuum vapor deposited on the layer of chromium. As in Example 1, two test panels were cut from the strip, one panel was cupped, and both were then tested in salt fog. The cupped panel had a life of 980 hours, or 5,700 hours per mil of coating. The other panel had a life of 1,240 hours, or 7,220 hours per mil of total coating.

1 claim:

1. An article comprising a ferrous substrate having a coating thereon, said coating comprising a layer of chromium 5 to microinches thick contiguous to said substrate and a layer of aluminum contiguous to said chromium, said article having a minimum life in the ASTM B-l 17-62 salt fog test of 3,500 hours per mil of total coating thickness.

2. An article as recited in claim 1, in which said aluminum is 100 to 1,500 microinches thick.

3. An article as recited in claim 1, in which said ferrous substrate is sheet steel.

4. An article as recited in claim 3, in which said steel consists essentially of 0.03 to 0.15 weight percent carbon, 0.20 to 0.60 weight percent manganese, balance iron.

Claims

2. An article as recited in claim 1, in which said aluminum is 100 to 1,500 microinches thick.
3. An article as recited in claim 1, in which said ferrous substrate is sheet steel.
4. An article as recited in claim 3, in which said steel consists essentially of 0.03 to 0.15 weight percent carbon, 0.20 to 0.60 weight percent manganese, balance iron.