GB2148324A

GB2148324A - Plated steel products

Info

Publication number: GB2148324A
Application number: GB08410432A
Authority: GB
Inventors: David Walter Bird
Original assignee: British Steel Corp
Current assignee: British Steel Corp
Priority date: 1983-04-26
Filing date: 1984-04-24
Publication date: 1985-05-30
Also published as: GB8410432D0; GB2148324B; GB8311320D0

Abstract

A method of manufacturing tin-electroplated steel sheet comprises electroplating both sides of a steel sheet with tin in a quantity up to 3gms/m2 to provide a tin layer, and applying a layer of zinc to one or both tin layers in a quantity of up to 2gms/m2. The plated steel sheet has less tin than conventional tinplate and may be used in the manufacture of corrosion resistant cans or containers for food products.

Description

SPECIFICATION Improvements in or relating to plated steel products.

This invention relates to steep products.

More especially the invention relates to tin-electroplated steel sheet.

Such sheet is commonly known as "Tinplate" and is used for example in the manufacture of cans or containers for food products.

A requirement of such tin plate is that it should be resistant to a known extent to corrosion or rusting from atmospheric conditions under normal operational circumstances. In other words a generally accepted corrosion resistance level is required of the tinplate.

One problem however with such tin plate is the continuing increase in price of tin making the production of tinplate having the desired thickness of tin coating for required corrosion resistance increasingly expensive to produce.

It is an object of the present invention to overcome or at least substantially reduce this problem.

According to one aspect of the invention there is provided a method of manufacturing tin-electroplated steel sheet comprising electroplating both sides of a steel of a steel sheet with tin in a quantity up to 3gms/m3 to provide a tin layer, and applying a layer zinc to one or both tin layers in a quantity of up to 2gms/m2.

According to another aspect of the present invention, there is provided a tin-electroplated steel sheet having a layer of electroplated tin applied on both sides thereof, said layers being of a quantity of up to 3gms/m2, and said sheet having a layer of zinc applied to one or both of the said tin layers of a quantity up to 2gms/m2.

The steel sheet is preferably a cold-rolled low carbon steel sheet.

The zinc layer is preferably applied to one side only of the steel sheet, the side to which it is applied being that side which in the manufacture of tin cans, for example, is outwards facing.

The invention has particular, although not exclusive, application to the steel sheet for tin cans where in some applications it is not common practice to lacquer the surface forming the outside of the can. A simple reduction in the tin coating on the outside would, in the absence of the invention require such a lacquer coating and thereby increase production costs.

A coating on the outer surface, in accordance with the invention, of tin and zinc has very satisfactory atmospheric corrosion resistance properties, and a coating on the inner surface, in accordance with the invention, of tin alone is equally satisfactory for internal corrosion resistance properties if, as is common, an internal lacquer coating is provided.

In other tin can applications on the other hand (for example for non-aqueous products) where a lacquer outer coating but no inner lacquer coating is commonly provided, a coating on the inner surface in accordance with the invention, of tin and zinc, and a coating on the outer surface in accordance with the invention of tin alone, have similarly satisfactory corrosition resistance properties.

The electroplated tin layer is preferably in the quantity range of 0.5 to 2gms/m2. In one particular embodiment the layer quantity is 1.4gms/m2.

The zinc layer is preferably in the quantity range of 0.1 to 1.2gms/m2, and may for example be in the range from 0.4 to 1.1 gms/m2.

The.zinc and/or tin layers would normally be chemically treated in a solution of sodium dichromate or chromic acid to give a total surface chromium value of between 1 and 30mg/m2.

We have found that by means of the invention it is possible to achieve with the tin and zinc coating herein specified, a corrosion resistance equivalent to a considerably thicker layer of tin such as would be regarded normally as necessary for corrosion resistance purposes.

In order that the invention may be more readily understood two examples thereof will now be described.

Example 1 A cold rolled low carbon steel sheet was subjected in a normal manner to electrolytic degreasing, rinsing, pickling, and re-rinsing operations. The steel sheet was then electroplated with 1.4gms/m2 of tin, rinsed and then electroplated with zinc.

The zinc plating bath included 200gms per litre of zinc sulphate 1 .5gms per litre of sulphuric acid. Its ph value was 2.3 and the temperature 40"C. The bath was operated at a current density of 16 amps/dm2.

Operational time was between 0.2 to 10 seconds. Samples of the coated steel were subjected to zinc plating for three different sets of time, namely 0.5 seconds, 1 second and 2 seconds, to provide an electrolytically plated layer of approximately 0.3 gms/m2 (low level), 0.5gms/m2 (medium level) and 1.1 gm/m2 (high level) respectively. Each sheet sample was then rinsed and dried.

Example 2 Steel sheet was prepared and coated with tin followed by zinc as proposed in the previous example, but the final zinc coated sheet samples were flow melted by electric resistance heating above the melting point oftin.

Tests on examples produced: Sheet specimens produced by the above two examples were stamped into can ends and exposed to the atmosphere in such a way that dew could form on them although they were protected from direct rainfall.

They were assessed for atmospheric rusting after 35 days on a scale in which zero represents complete rusting and 10 represents no rusting. Other specimens from Examples 1 and 2 were again stamped into can ends and were then processed in steam for 1 hr at 115 C.

These were then exposed to the atmosphere in such a way that dew could form on them although they were protected from direct rainfall. These again were assessed for rusting after 35 days.

The results of the above referenced testing are set out in the following table. it is to be noted that by way of comparison a number of electroplated tin coated steel speciments (without zinc coating) and one specimen with zinc coating only were also subjected to the same rust testing for a period of 35 days.

ATMOSPHERIC CORROSION TESTING ( 35 days Description Tin Coating Non Steam Steam Weight Processed Processed Controls (fl.melted) Laboratory 2.8gm/2 o 1 made tinplate, Laboratory made end 5.0 gum/2 5 4 Commercial 2.8 gm/2 Not tested 5 tinplate, Laboratory 4.9 gm/2 Not tested 7 made can end Commercial 5.6 gm/2 Not tested 8 can end Inflowed (Example 1) Sn + low 1.4gm/2 6 8 level zinc Sn + medium 1.4gm/2 7 5 level zinc Sun + high 1.4gm/2 10 5 level zinc Medium 1 0 level zinc only Flowed (Example 2) Sn + low 1.4gm/2 5 7 level zinc Sn + medium 1.4gm/2 8 10 level zinc Sun + high 1.4gm/2 10 1 level zinc As will be seen most clearly from the above table a relatively thin coating of tin with medium or high thickness coatings of zinc were at least as good as commercially available electroplated tin coated steel sheet in terms of rust resistance. In particular it is to be noted that a thin coating of tin with a medium thickness level coating of zinc, subjected to flow brightening was significantly better than commercially available tinplate when subjected to steam processing.

We have found that based on contact resistance measurements the weldability of the zinc / tin coated steel should be as good as normally available commercial tinplate.

Because of the significant cost differential between tin and zinc, the invention provides real economic benefit in the production of tinplate. Indeed because, in our preferred embodiment, a thin layer of tin is applied to both sides of the sheet and a layer of zinc to one side only, very considerable economic saving can be achieved whilst still providing satisfactory corrosion resistance for tin cans for example both on the outside through the double coating, and to the inner side through the thin tin layer, if lacquered.

Claims

1. A method of manufacturing tin-electroplated steel sheet comprising electroplating both sides of a steel sheet with tin in a quantity up to 3gms/m2 to provide a tin layer, and applying a layer of zinc to one or both tin layers in a quantity of up to 2gms/m2.

2. A method according to claim .1. in which the zinc and/or tin layers are chemically treated to give a surface chromium value of 1 to 30 mgms/m2.

3. A method according to claim 1 or 2 in which the electroplated tin layer is in the quantity range of 0.5 to 2 gms/m2.

4. A method according to claim 3 in which the electroplated tin layer quantity is approximately 1 .4gms/m2.

5. A method according to any one of the preceeding claims in which the zinc layer is in the quantity range of 0.1 to 1.2 gms/m2.

6. A method according to claim 5 in which the zinc layer quantity is approximately in the range 0.4 to 1.1 gms/m2.

7. A method according to any one of the preceeding claims in which a layer of zinc is applied to one layer only of tin.

8. A method of manufacturing tin-electroplated steel sheet substantially as hereinbefore described in examples 1 or 2 hereof.

9. A tin-electroplated steel sheet having a layer of electroplated tin applied on both sides thereof, said layers being of a quantity of up to 3gms/m2, and said sheet having a layer of zinc applied to one or both of the said tin layers of a quantity up to 2gms/m2.

10. A sheet according to claim 9 in which the zine and/ortin layers are chemically treated to give a surface chromium value of 1 to 30 mgms/m2.

11. A sheet according to claim 9 or 10 in which the electroplated tin layer is in the quantity range of 0.5 to 2gms/m2.

12. A sheet according to claim 11 in which the electroplated tin layer quantity is approximately 1 .4gms/m2.

13. A sheet according to any one of claims 9 to 12 in which the zinc layer is in the quantity range of 0.1 to 1.2 gms/m2.

14. A sheet according to claim 13 in which the zinc layer quantity is approximately in the range 0.4 to 1.1 gms/m2.

15. A sheet according to any one of claims 9 to 14 in which a layer of zinc is applied to one layer only of tin.

16. A tin-electroplated steel sheet substantially as hereinbefore described in examples 1 or 2 hereof.

17. A tin can fabricated from tin-electroplated steel sheet according to any one of claims 9 to 16.