US3857684A - Corrosion-resistant double-coated steel material - Google Patents

Abstract

Zinc-coated steel materials in general use which have undergone the chromate treatment for improved corrosion resistance suffer from a disadvantage that the corrosion-resistant coating is susceptible to flexure and cracks or peels off at flexures, with the result that the protective function of chromate is lost. The improved corrosion-resistant double-coated steel material to which the present invention relates is obtained by coating in a fusion-bonded state tin or tin-based alloy onto a zinc coating applied to the surface of steel material or by applying a zinc coating on the surface of steel material and further applying in a fusion-bonded state a coating of tin or a tin-based alloy. Thus obtained double-coating free from defects which chromate has, increases in mechanical strength and improves corrosion resistance to an unexpected extent.

Description

1451 Dec. 31, 1974 CORROSION-RESISTANT DOUBLE-COATED STEEL MATERIAL [75] Inventor: Naomi Kubu, Nagoya, Japan [73] Assignee: Usui Kokusai Sangyo Kabushiki Kaisha, Sumo-gun, Shizuoka Prefecture, Japan 221 Filed: Dec. 22, 1971 21 Appl. 190.; 211,087

[30] Foreign Application Priority Data May 10, 1971 .lapan.... 46-30959 [52] U.S. C1. 29/1965 [51] Int. Cl 823p 3/10, B32b 15/18 [58] Field of Search. 29/1965 2 [56] References Cited UNITED STATES PATENTS 933,612 9/1909 Allen 29/1965 2,057,762 10/1936 Boone et 31.... 29/1965 2,258,327 10/1941 Kramer...., 29/1965 2,428,033 9/1947 Nachtman 204/37 3,231,127 1/1966 Virzi 29/1965 3,323,938 6/1967 Vaught 29/1965 Primary Examiner-W. Stallard Attorney, Agent, or Firm-Irving M. Weiner 5 7] ABSTRACT Zinc-coated steel materials in general use which have undergone the chromate treatment for improved corrosion resistance sufier from a disadvantage that the corrosion-resistant coating is susceptible to flexure and cracks or peels off at flexures, with the result that the protective function of chromate is lost.

The improved corrosion-resistant double-coated steel material to which the present invention relates is obtained by coating in a fusion-bonded state tin or tin-based alloy onto a zinc coating applied to the surface of steel material or by applying a zinc coating on the surface of steel material and further applying in a fusion-bonded state a coating of tin or a tin-based alloy. Thus obtained double-coating free from defects which chromate has, increases in mechanical strength and improves corrosion resistance to an unexpected extent.

3 Claims, No Drawings CORROSION-RESISTANT DOUBLE-COATED STEEL MATERIAL This invention relates to a novel corrosion-resistant coated steel material obtained by improving corrosionresistant coatings such as zinc and tin or tin-based alloys applied onto steel materials such as pipe, rod, plate, and wire. v

Zinc-coated steel, materials with chromate treatment have hitherto beenvin general use for their outstanding corrosion resistance, but this corrosion-resistant coating is susceptible to flexure and cracks or peels off at flexures during bending,.with the result that the protective function of chromate is lost.

This invention provides a coated steel material which is endurable to bending and exhibits the extremely outstanding corrosion resistance without chromate treatment. This invention is more particularly concerned with corrosion-resistant double coated steel materials prepared by applying a zinc coating and subsequently applying in a fusion-bonded state a coating of tin or a tin-based alloy (simply called an outer coating hereunder) onto said zinc coating. This invention has been made based on our finding that the above-mentioned double-coating which has never occurred to anybody in the past exhibits the outstanding corrosion-resistance.

In embodiments of this invention the zinc coating means of melt-plating, electro-plating, or metalizing.

The zinc coating is corroded by the liquid metal corrosion (abbreviated to L'MC hereunder) when tin or a tin based alloy is double-coated thereon by fusion bonding, but it is possible to allow the 'zinc coatingto stay as the inner layer between the surface of steel material a tin-based alloy onto the zinc coating provided on the surface of steel material.

3. When the double-coated steel material is subjected to the salt-water spray test, white spots appear before it gets rusty.

The corrosion resistance of the corrosion-resistant double-coated steel material with which this invention is concerned is improved by providing the abovementioned outer coating, in a fusion-bonded state, onto the above-mentioned inner coating. The outer coating is formed usually by allowing the steel material provided with the inner coating to soak in or pass through a bath for the outer coating. Where the uniformity of coating thickness is required,'the inner and outer coatings are formed in two layers by means of electroplating and subsequently heat-treating thus obtained electro-plated double coatings. In this case the outer coating of tin-plating is chosen for the inner coating of Samples of double-coated steel pipe prepared according to the invention and samples of single-coated steel pipe prepared by coating zinc, tin, or a tin-based alloy alone, were subjected to the salt-water spray test. The test results shown in the following tables indicate the outstanding corrosion resistance of the doublecoated steel pipe prepared according to the invention.

Ex. 1, 2, 3, 4, and 5 in the tables correspond respectively to theexamples described below and denote respective samples obtained in the examples. Also, Comp. 1, 2, 3, 4, and 5 in the tables denote samples of single-coated steel pipes subjected to the salt-water test. These comparison samples were coated by an orand the outer coating by lowering the temperature of dinary method, which is omitted in this specification.

Table 1 Results of Salt-Water Spray Test Sample Coating Time Ex. 1 Doublelnner Fusion bonded Zn coated Coating thickness 2 Outer Fusion bonded Sn not lW 6W 7B 10R Coating thickness 6 observed Total 8p. Comp. l Same as not 3W SW 158 15R observed Ex. l

tin or tin-based alloy bath, increasing the speed of steel material to pass through the bath, or shortening the immersion time.

The corrosion resistance achieved by the corrosionresistant double-coated steel material with which this invention is concerned is surprisingly outstanding and can not be expected from the corrosion resistance of steel materials simply coated with zinc, tin, or a tinbased alloy. It is considered from the following facts that such corrosion resistance is obtained because pin holes on the outer coating are clogged by zinc carbonate and zinc hydroxide.

l. Coating of zinc, tin, or a tin-based alloy alone on the surface of steel material does not impart such supe- The test pieces made in the example mentioned rior corrosion-resistance asthat achieved by this inven- 6 5 below were tightly plugged with a synthetic resin pieces tion. v 2. Unexpected outstanding corrosion resistance is' obtained by double-coating, by fusion bonding, tin or at each end, were defatted on-the surface and were hung at an angle of inclination of 30 degrees at intervals of 10 mm. in a sealed chamber which was filled with the below-mentioned salt water as-a spray. The

variation with the lapse of time of the rust produced on.

the surface of each test'pieces in the atmosphere of this' salt water spray was observed and reconded.

Summary of the used salt water and apparatus:

(a) Purity: Refined sodium chloride of purity of more than 99.5%.

(b) Salt water prepared by dissolving of the refined sodium chloride in distilled water was used at 35C. The salt water at 35C was of a specific gravity of 1.020 and pH of 6.9.

(c) The sealed chamber was of a capacity of 0.34 m. The amount of the sprayed salt water was 3.8 liters/24 hrs.

RR: Streaky red rust spots 5 SO on.

For instance, All W indicates a state in which the entire surface of coating is covered by fine white matters, and All [/2 RR indicates a state in which a half of the surface is' covered by streaky red rust spots. These matters appear in the order Table 2 Results of Salt-Water Spray Test Sample Coating Time 48 hr I68 336 672 l 176 Ex. 2 Doublelnner Fusion bonded Zn coated Coating thickness 7p. not not Outer Fusion bonded Sn-Pb* obob- 2W 5W 9R Coating thickness 10p. served served Total 17p. Comp. 2 Same as lW 5W 12W 17B 17R The componentratio o l Sn-Pb is 80:20.

(d) The spraying device consisted of two nozzles. (Note 1) The immersion time of alloy bath in Ex.2 is 0.5 seconds and the immersion time of alloy bath in The spraying pressure was 1 kg./cm 5) The tests were recorded by the following marks.

Comp.2 is 1.5 seconds.

Table 3 Results of Salt-Water Spray Test Sample Coating Time 48 hr 168 336 672 H76 Ex. 3 Double- Inner Electro-plated Zn coated Coating thickness 2p. not not Outer Fusion bonded Sn-Pb* obob- SW 78 lSR Coating thickness 10p. served served Total 12p. Comp. 3 Same as 4W llW 5B 3R 25R The component ratio of Sn-Ph is 80:

(Note) 1) The immersion'time of alloy bath in Ex.3 is

50 Comp.3 is 2 seconds.

Table 4 Results of Salt-Water Spray Test 0.5 seconds and the immersion time of alloy bath in Sample Coating Time 48hr I68 336 672 H76 Ex. 4 Doublelnner Fusion bonded Zn coated Coating thickness 6 not Outer Fusion bonded Sn-Cd* ob- 2w 6W 5B 4R Coating thickness 7 served Total 13 Comp. 4 Same as not 2W 7W 78 SR Comp. 3 ob served Comp. 5 Single- Fusion bonded Sn-Pb All W 5R 6RR l7RR coated Coating thickness 8p.

' The component ratio of Sn-Cd is :30.

Table 5 Size is the same as that in Example I. b. Molten flux Same as that used in Example 1.

Results of Salt-Water Spray Test Sample Coating Time Ex. 5 Doublelnner Electro-plated Zn coated Coating thickness 13p.

Outer Elcctroplated Sn not not obob- 2W 48 OR Coating thickness 8p. served served Total Fusion bonded Double Thickness 19p. Comp. 5 Same as not 1W 4W 8B 7R ob- Ex. 5 served (Note) 1) The time for passing through the electric 20 c. Plating bath furnace in Ex.5 is 0.25 seconds and the time for passing through the electric furnace in Comp.5 is 1.3 seconds.

EXAMPLE 1 Raw materials:

a. Mild steel pipe 6.35 mm O.D. X 0.71 mm thick X 500 mm long X 5 pieces b. Molten flux ZnCl; 70 wt%, Nl-I Cl wt%, Temperature 350C c. Molten zinc bath; bath temperature 470C d. Molten tin bath; bath temperature 280C Preparation of double-coated steel pipe:

Five steel pipes with clean surface treated in a usual manner were immersed in the molten flux at 350C for 15 seconds and then immediately immersed in the molten zinc bath at 470C for 10 seconds to obtain the zinc coating with athickness of about 12p. These coated steel pipes were then immersed in the molten tin bath at 280C for about 0.8 seconds to obtain double-coated mild steel pipes with about So thick coating. From among these five pipes were selected three samples with uniform coating, one for the inspection of coating thickness and two for the corrosion resistance test, one bent to Z shape and another unbent.

The inspection for coating thickness revealed that the inner coating of zinc is about 2p thick and the outer coating of Sn-Zn alloy is about 6p. It is considered that the thickness of the inner coating was decreased to only 2p. because the zinc coating was partly lost due to the LMC action when the outer coating was fusionbonded.

The corrosion resistance test by the salt-water spray test gave no difference in the occurrence of rust between the two samples tested. In the case of chromate pipes, craking and peeling occur at flexures, but in the case of these samples such trouble did not occur. The test result in the column Ex. 1 in Table l is an average value of two samples. It is seen that the corrosion resistance of this example is unexpectedly outstanding as compared with the sample of Comp. 1 in Table l.

EXAMPLE 2 SnPb alloy with an :20 composition by weight.

Bath temperature: 240C Preparation of double-coated steel pipe:

Five zinc-coated mild steel pipes were immersed in the molten flux bath at 350C for 15 seconds and then immediately immersed in the above-mentioned plating bath at 240C for 0.5 seconds to obtain the coating with an average thickness of 17p. From among these five pipes were selected three samples with uniform coating, one for the inspection of coating thickness and two for the corrosion resistance test, one bent to Z shape and another unbent.

The inspection for coating thickness revealed that the inner coating of zinc is about'7p. thick and the outer coating fusion-bonded thereon is about 10p. Unlike Example 1, thinning of the inner zinc coating is not observed in this example. This was achieved by selecting a proper composition of an alloy for the outer coating, which allows one to lower the bath temperature, and by shortening the immersion time so that the zinc coating is not affected by the LMC.

The corrosion resistance test by the salt-water spray test was carried out for two sample pipes in the same manner as in Example 1. Results are shown in the column of Ex. 2 in Table 2. As in the case of Example 1, there was no tendency that rust occurs particularly at flexures.

It is seen that the corrosion resistance achieved in this example is unexpectedly outstanding as compared with that of the sample of Comp. 2 in Table 2.

EXAMPLE 3 Five double-coated steel pipes were prepared in the same manner as in Example 2, except that electroplated zinc-coated steel pipes with a coating thickness of 3p. were used instead of fusion-bonded zinc-coated steel pipes. As in the case of Example 2, two pipes were subjected to the corrosion resistance test and one pipe was subjected to the inspection of coating thickness. [t was found that the inner zinc coating is about ZIL thick and the outer Sn-Pb (80:20) coating is about 10p. thick, the total coating being about 12p thick.

The corrosion resistance test by the salt-water spray test was carried out with regard to two sample pipes in the same manner as in Example 1. Results are shown in the column of Ex.3 in Table 3. As in the case of Example l, there was no tendency that rust occurs particularly at flexures.

It is seen that the corrosion-resistance achieved in this example is unexpectedly outstanding as compared with that of the sample of Comp. 3 in Table 3.

EXAMPLE 4 Raw materials:

a. Fusion-bonded zinc-coated mild steel pipes, 5

pieces Coating thickness: about 6 1.

Size is the same as that in Example 1.

b. Molten flux Same as that used in Example 1.

c. Plating bath 7 Sn-Cd alloy with a 70:30 composition by weight Bath temperature: 220C Preparation of double-coated steel pipe:

Five zinc-coated mild steel pipes were immersed in the molten flux bath at 350C for seconds and then immersed in the above-mentioned plating bath at 220C for 1 second. As in the case of Example 1, two pipes were subjected to the corrosion resistance test and one pipe was subjected to the inspection of coating thickness. It was found that the inner zinc coating is about 6p. thick and the outer Sn-Cd coating is about 7p. thick, the total coating being about 13 thick.

The corrosion resistance test by the salt-water spray test was carried out with regard to two sample pipes in the same manner as in Example 1. Results are shown in the column of Ex.4 in Table 4. As in the case of Example I, there was no tendency that rust occurs particularly at flexures.

It is seen that the corrosion resistance achieved in this example is unexpectedly outstanding as compared with that of the sample of Comp. 4 in Table 4.

EXAMPLE 5 a. Steel material tested:

Mild steel pipes; 1 piece 10 mm O.D. X 0.71 mm thick X10 m long b. Preparation of double-coated steel pipe;

The sample pipe was electro-plated with zinc to a coating thickness of 13p. by the usual method. This pipe was further electro-plated with tin to a coating thickness of 8p. Thus, a double-electro-coated pipe with a coating thickness of 21g was obtained. This pipe was then allowed to pass through an electric furnace having an atmosphere of ammonia decomposition gas, in order to obtain a doublecoated steel pipe having a zinc inner coating and a tin outer coating fusion-bonded to the inner coating. The average coating thickness was 19;/.. Which is about 2p. thiner than the average coating thickness before the heat treatment carried out at 245C and at a pass speed of 0.25 m/sec.

Then, this double-coated steel pipe was cut to 20 samples 500 mm long. Two samples were selected at random for the corrosion resistance test; one was bent to Z shape and another was not bent.

The corrosion resistance test by the salt-water spray test was carried out for the two samples in the same manner as in Example 1. Results are shown in the column of Ex. 5 in Table 5. It is seen that the corrosion resistance achieved in this example is unexpectedly outstanding as compared with that of the sample of Comp. 5.

What is claimed is:

1. In steel materials having corrosion-resistant double-coating consisting of inner and outer two layers, an improved corrosion-resistant double-coated steel material characterized in that said inner layer is a zinc coating and said outer layer is a tin-predominant coating formed in a fusion-bonded state on said inner layer.

2. An improved corrosion-resistant double-coated steel material in accordance with claim 1 in which the tin-predominant outer coating consists of tin alone.

3. An improved corrosion-resistant double-coated steel material in accordance with claim 1 in which the tin-predominant outer coating consists of a tin-based alloy.

UIi'ttiD s'lATEs PA.'IEI-"I OFFICE 1 CERTIFICATE OF CORRECTION PATENTNO. 3,857,684

DATED December 31, 1974 INVENTOMX) Naomi KUBU It is certttiect that arm: appears in the absve-tdentitted patent and that smd Letters Patent are hereby lortected as snot-m below: Title ag correct the inventor 5 name to read --I ubo- (both occurrences) Title page, in the Abstract, line 14, change "Thus" to -The thus--7 before "free" insert -is--; line 15, insert a comma after "strength" Column 2, line 16 insert a comma after "electroplating": before "thus" insert --the--: line 65, change "pieces" to --piece-7 line 66 insert a comma after "surface" 7 line 68, change "scaled" to sealed--. Column 3, line 3, change "pieces" to ---piece; line 4, correct the spelling of --recorded-7 line ll, after "and" insert --a---.' Column 4, line 4, change "rence" to --rences-7 line 11, change "the only" to only the--. Column 5, line 42, after "pipes" insert ---there-; line 56 correct the spelling of -cracking-; line 58, change "column" to --row of--: line 60 change "unexpectedly" to -so-; line 61, change "compared with" to -can never be expected from the result of the corrosion resistance test on-. Column 6, line 29, after "pipes" insert --there--7 lines 45-46, change "column" to --row--; line 67 change "column" to -row--. Column 7, line 30, change "column" to --row--. Column 8, lines 22-23, change "column" to -row.

Table 1, after "outer" change "Fusion bonded Sn" to --Fusion bonded Sn-Zn--.

Table 4, After "Comp. 4" change "Same as Comp. 3" to -Same as EX. 4--7 delete the following last two lines:

"Comp.5 Single- Fusion bonded Sn-Pb* All W 5R 6RR l7RR coated Coathing thickness 8 Signed and sealed this 10th day of June 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks

Claims (3)

1. IN STEEL MATERIALS HAVING CORROSION-RESISTANT DOUBLE COATING CONSISTING OF INNER AND OUTER TWO LAYERS, AN IMPROVED CORROSION-RESISTANT DOUBLE-COATED STEEL MATERIAL CHARACTERIZED IN THAT SAID INNER LAYER IS A ZINC COATING AND SAID OUTER LAYER IS A TIN-PREDOMINANT COATING FORMED IN A FUSION-BONDED STATE ON SAID INNER LAYER.

2. An improved corrosion-resistant double-coated steel material in accordance with claim 1 in which the tin-predominant outer coating consists of tin alone.

3. An improved corrosion-resistant double-coated steel material in accordance with claim 1 in which the tin-predominant outer coating consists of a tin-based alloy.