GB2225591A

GB2225591A - Corrosion-resistant plating layers

Info

Publication number: GB2225591A
Application number: GB8923616A
Authority: GB
Inventors: Teruhisa Takahashi; Seiya Takahata
Original assignee: Usui Kokusai Sangyo Kaisha Ltd
Current assignee: Usui Kokusai Sangyo Kaisha Ltd
Priority date: 1988-10-29
Filing date: 1989-10-19
Publication date: 1990-06-06
Anticipated expiration: 2009-10-19
Also published as: DE3934111C2; JP2750710B2; JPH02120034A; GB2225591B; KR940003789B1; GB8923616D0; DE3934111A1; KR900006565A

Description

---2 STEEL PRODUCT WITH HEAT-RESISTANT, CORROSION-RESISTANT PLATING LAYERS

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to a steel product with heat-resistant, corrosion-resistant plating layers, and more particularly, to a steel product such as sheets, pipes, couplings, clamps, bolts, and nuts used for automobiles and various machines and equipment, said steel product having heat-resistant, corrosion-resistant plating layers on the surface thereof. 2. Description of the Prior Art:

It has been a common practice to form a zinc plating layer and then a chromate film on sheets, pipes, couplings, clamps, bolts, and nuts used for automobiles and various machines and equipment. However, it has been found that the zinc plating alone does not meet the requirement for these components which has become severer than before. For the improved corrosion resistance, the zinc plating has been superseded by Sn-Zn or ZnNi alloy plating or a combination of this alloy plating and Zn plating. In this connection, there is disclosed in Japanese Patent Laid-open No. 165387/1985 a corrosionresistant steel pipe with plating layers composed of an 1 9 electroplated Zn-Ni alloy layer (on the outside of the steel pipe), an electroplated zinc layer (on the Zn-Ni alloy layer), and a chromate film (on the zinc layer).

The above-mentioned plated steel products exhibit improved corrosion resistance in corrosive environments at normal temperature but they do not in high-temperature environments such as automotive engine rooms. SUMMARY OF THE INVENTION

The present invention was completed to solve the above-mentioned problem. Accordingly, it is an object of the present invention to provide a steel product with plating layers which exhibits not only high corrosion resistance but also good heat resistance'.

The present inventors carried out a series of researches which led to the finding that the object of the present invention is achieved if the plating layers are composed of a nickel plating layer of specific thickness (as the lower layer), a Zn-Ni alloy plating layer (as the intermediate layer), and a chromate f ilm. (as the top layer). The present invention was completed on the basis of this finding. The gist of the pregent invention resides in a steel product (or copper-plated steel product) with heat-resistant, corrosion-resistant platIng layers which are cam-posed of a 0.2-10 gm thick nickel 2 - 1 plating layer formed on said steel product, a Zn-Ni alloy plating layer formed on said nickel plating layer, and a chromate film formed on said Zn-Ni alloy plating layer. DETAILED DESCRIPTION OF THE INVENTION

The steel product pertaining to the present invention includes sheets, pipes, couplings, clamps, bolts, nuts, and the like. The steel product also includes a comparatively thin lap-welded steel tube (10 mm or below in outside diameter) with copper plating for welding. (Suc1-i a steel tube may be used for automotive hydraulic and fuel piping.) It has three plating layers. The lower layer is a 0.2-10 im thick nickel plating layer. With a thickness less than 0.2 pm, this layer does not cover the ground of a steel product completely, nor does it improve heat resistance and corrosion resistance so much. With a thickness in excess of 10 pn, this layer is liable to peel and crack during bending and hence does not improve corrosion resistance for its increased thickness. This nickel plating layer may be formed by electroplating using a Watts bath, which provides a plating layer having less stress. The intermediate layer is a Zn-Ni alloy plating layer, which may be formed by electroplating using a chloride bath or sulfate bath. The content of nickel varies depending on the bath composition and current density; bul. it should be 2 to 20%, preferably 12 to 1555%, for improved 1 corrosion resistance, bendability, and ease with which the chromate f ilm is f ormed thereon afterwards. The top layer is a chromate film, which may be formed from a chromate. solution or dichromate-sulfuric acid solution or a commercial chromate treating solution (e.g., ZN-80 YMU, a product of Ebara-Udylite Co., Ltd.).

The thus obtained steel product with plating layers exhibits good corrosion resistance even in hightemperature environments and on its bent parts, as demonstrated in Examples which follow.

EXAMPLES

The invention will be described in more detail with reference to the following examples. Example 1 A lap-welded steel tube, measuring 8 mm in diameter, 0.7 mm in wall thickness, and 380 mm long, was made of cold rolled carbon steel sheet designated as SPCC according to JIS G-3141, with the surface thereof coated with an about 3-= thick copper plating layer for welding. This tube underwent nickel plating in a Watts bath at a bath temperature of 52- 57C with a current density of 3 A/dM2. The plating thickness ranged from 0.5 pm to 10 pm. The plated steel tube further underwent Zn-Ni alloy plating for 6 minutes in a bath solution (pH 5.7) containing 100 g/L of ZnC12, 130 g/L of NiC12_ 6HO, and 200g/L of NH4 C " -.4- a t 1 a bath temperature of 34-36C with a current density of 3 A/dM2 Thus there was formed a 5-gm thick Zn-Ni alloy plating layer on the nickel plating layer. The Zn-Ni alloy plating layer was further coated with a chromate film by dipping in ZN-80 YMU (a product of Ebara-Udylite Co., Ltd.) at pH 2.0 and a bath temperature of 48-52C for 20 seconds. Thus there was obtained a steel tube having plating layers.

The plated tube, with one end bent through 180 around a mandrel 25 mm in radius, was examined for corrosion resistance by salt spray test according to JIS Z-2371. The time (in days) required for red rust to occur was measured. The plated tube was also examined for heat resistance and corrosion resistance by heating at 200C for 24 hours and then by salt spray test according to JIS Z-2371. The time (in days) required for red rust to occur was measured. Both tests were carried out using two samples each. The results are shown in Table 1. (Sample Nos. 1 to S.) Example 2 An electric welded steel tube of the same dimensions as in Exam.,ple 1 was made of carbon steel designated as STPG-38 according to JIS G-3454. The steel tube was plated and tested in the same manner as Example 1. The results are shown in Table 1. (Sample Nos. 6 tc 10.) Comparative Example 1 A lap-welded steel tube having plating layers and an electric welded steel tube having plating layers were produced in the same manner as in Examples 1 and 2, respec tively, except that the nickel plating was 0.1 gm. thick or 15 gm. thick. The steel tubes were tested in the same manner as in Example 1. The results are shown in Table 1. (Sample Nos. 11 to 14. ) Comparative Example 2 The same lap-welded steel tube as in Example 1 and the same electric welded steel tube as in Example 2 were provided with a Zn-Ni alloy plating layer and chromate film, without the formation of a nickel plating layer, in the same manner as in Example 1. The steel tubes were tested in the same manner as in Example 1. The results are shown in Table 1. (Sample Nos. 15 and 16.) Comparative Example 3, The same lap-welded steel-tube as in Example 1 and the same electric welded steel tube as in Example 2 were provided with a Zn-Ni alloy plating layer, a zinc plating layer, and a chromate film on top of the other. The Zn-Ni alloy plating layer was formed in the same manner as in Example 1. The zinc plating layer was formed using a bath containing 28 g/L of ZnO, 50 g/L of NaCN, and 80 g/L of ll 1 NaOH. The chromate film was formed using a treating solution containing 2 g/L OE Cr031 0.25 mL/L of H2S04, and 0.5 mL/L of HN03. The thus obtained steel tube with plating layers composed of a Zn-Ni alloy plating layer, Zn plating layer, and chromate film was tested in the same manner as in Example 1. The results are shown in Table 1. (Sample Nos. 17 and 18.) z - Table 1

Corrosion Heat and corro resistance sion resistance Example Sample Steel Thickness Thickness Thickness Sent Straight Bent Straight No. No. tube of nickel of Zn-Ni of zinc part part part pa plating alloy plating (gm) pia (gm) ting (gm) 1 1 A 0.5 5 - 84 105 63 105 136 63 105 1 2 A 1.0 5 136 84 105 84 105 63 136 1 3 A 2.5 5 - 125 84 84 125 125 75 125 1 4 A 5.o 5 - 156 >209 146 156 146 105 146 1 5 A 10.0 5 63 156 75 209 209 63 136 2 6 B 0.5 5 - 105 125 84 125 136 75 105 2 7 B 1.0 5 - 125 156 84 125 84 105 84 105 2 8 B 2.5 5 - 105 136 84 105 1.36 156 105 136 2 9 B 5.0 5 - 125 146 125 146 156 209 136 209 2 10 B 10.0 5 - 84 175 75 154 146 63 146 (1) 11 A 0.1 5 - 75 75 6 is so 75 10 25 (1) 12 A 15.0 5 - 40 146 42 156 42 156 30 125 (1) 13 B 0.1 1 5 - - 75 105 1 15 25 1 125 is 40 (1) 14 B 15.0 5 42 12 40 1.46 1 1 1 42 175 330 105 (2) 15 A 0 5 75 84 6 15 63 50 6 is (2) 16 B 0 5 - 84 105 10 25 63 105 is 30 (3) 17 A - 5 5 30 75 3 3 84 3 3 (3) 18 8 5 5 25 84 6 10 105 3 7 A: lao-welded steel tube, B: electric welded steel tube Time (in days) required for red rust to ocCur.

Parenthesized num.les ird',ca'e Comparative Examples.

1 The steel product of the present invention has plating layers f ormed on top of the other, the lower layer being a nickel plating layer having a specific thickness, the intermediate layer being a Zn-Ni alloy plating layer, and the top layer being a chromate film. Owing to the plating layers, it exhibits outstanding corrosion resistance and retains it even after heat treatment or on its bent parts. Therefore, it is suitable for use in high-temperature environments such as automotive engineroom.

Claims

1. A coating on metal, the coating comprising a layer of nickel 0.2 ums thick beneath a Zu-NI alloy layer bearing a chromate f ilm.

2. Coated metal according to claim 1 wherein the metal is steel optionally plated with copper, the nickel layer being formed on the steel or on the copper.

3. Coated metal with heat-resistant, corrosion-resistant plating layers which comprise a 0.2-10 um thick nickel plating layer f ormed on the metal, a Zn-Ni alloy plating layer formed on the Zn-ni alloy plating layer.

4. Coated metal according to any preceding claim wherein the nickel plating layer is formed by electroplating using a Watts bath.

5. Coated metal according to any preceding claim wherein the Zn-Ni alloy plating layer contains 2-20% of nickel.

6. Coated metal according to any preceding claim wherein the Zn-Ni alloy plating layer contains 12-15% of nickel.

7. Coated metal according to any preceding claim wherein the Zn-Ni alloy plating layer is f ormed by electroplating using a chloride bath or sulphate bath.

8. Coated metal according to any preceding claim wherein the chromate film is formed from a chromate solution or dichromate- sulphuric acid solution.

9. Coated metal according to any preceding claim wherein the chromate f ilm is f ormed f rom a chromate treating solution f or ZnNi alloy plating.

10. Coated metal, more particularly a steel or copper-plated steel product substantially as hereinbefore described with reference to the examples.

11. A coated steel or copper plated steel product according to any preceding claim.

Published 1990 at The Patent Office, State House, 66 71 High Holborn. Lond(DnWC1R4TP-F-urther copies maybe obtained from The Patent Office