CORROSION-RESISTANT DOUBLE-COATED STEEL MATERIAL
United States Patent 3857684
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.
US Patent References:
Coating of metallic refrigerator parts
Boone et al. - October 1936 - 2057762
Coated metallic sheet
Kramer - October 1941 - 2258327
Manufacture of rustproof electrolytic coatings for metal stock
Nachtman - September 1947 - 2428033
Alloy coated steel article
Virzi - January 1966 - 3231127
Method of coating tin over basis metals
Vaught - June 1967 - 3323938
Coating of metallic refrigerator parts
Boone et al. - October 1936 - 2057762
Coated metallic sheet
Kramer - October 1941 - 2258327
Manufacture of rustproof electrolytic coatings for metal stock
Nachtman - September 1947 - 2428033
Alloy coated steel article
Virzi - January 1966 - 3231127
Method of coating tin over basis metals
Vaught - June 1967 - 3323938
Application Number:
05/211087
Publication Date:
12/31/1974
Filing Date:
12/22/1971
View Patent Images:
Export Citation:
Assignee:
Usui Kokusai Sangyo Kabushiki Kaisha (Sunto-gun, Shizuoka Prefecture, JA)
Primary Class:
Other Classes:
428/935, 428/926, 428/659
International Classes:
C23C2/06; C23C2/08; C23C2/38; C23C2/04; C23C2/36; B32B15/18; B23P3/10
Field of Search:
29/196.5
Primary Examiner:
Stallard W.
Attorney, Agent or Firm:
Weiner, Irving M.
Claims:
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.
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.
Description:
This invention relates to a novel corrosion-resistant coated steel material obtained by improving corrosion-resistant coatings such as zinc and tin or tin-based alloys applied onto steel materials such as pipe, rod, plate, and wire.
Zinc-coated steel materials with chromate treatment have hitherto been in 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 may be applied to the surface of steel materials by means of melt-plating, electro-plating, or metalizing. The zinc coating is corroded by the liquid metal corrosion (abbreviated to LMC hereunder) when tin or a tin-based alloy is double-coated thereon by fusion bonding, but it is possible to allow the zinc coating to stay as the inner layer between the surface of steel material and the outer coating by lowering the temperature of 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 corrosion-resistant 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 tin-based 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.
1. Coating of zinc, tin, or a tin-based alloy alone on the surface of steel material does not impart such superior corrosion-resistance as that achieved by this invention.
2. Unexpected outstanding corrosion resistance is obtained by double-coating, by fusion bonding, tin or 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 above-mentioned 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 electro-plating 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 zinc-plating. The heat-treatment is carried out by heating the electro-plated double coatings in an atmosphere of non-oxidizing gas or reducing gas.
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 double-coated steel pipe prepared according to the invention.
Ex. 1, 2, 3, 4, and 5 in the tables correspond respectively to the examples 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 ordinary method, which is omitted in this specification.
Table 1 ____________________________________________________________ ______________ Results of Salt-Water Spray Test ____________________________________________________________ ______________ Sample Coating Time 48 hr 168 336 672 1176 ____________________________________________________________ ______________ Ex. 1 Double- Inner Fusion bonded Zn coated Coating thickness 2μ Outer Fusion bonded Sn not 1W 6W 7B 10R Coating thickness 6μ observed Total 8μ Comp. 1 Same as not 3W 8W 15B 15R Ex. 1 observed ____________________________________________________________ ______________
(Note)
1. The immersion time of tin bath in Ex.1 is 0.8 seconds and the immersion time of tin bath in comp. 1 is 1.2 seconds.
2. It is considered that the outer coating of Ex. 1 consists mostly of Sn-Zn alloy due to the great effect of LMC, as will be explained in the following Example 1.
3. The coating thickness indicates the average value measured at the both ends and center. This applies to the following tables.
4. The salt water spray tests were carried out in accordance with the following method.
The test pieces made in the example mentioned below were tightly plugged with a synthetic resin pieces 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 scaled 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 5% of the refined sodium chloride in distilled water was used at 35°C. The salt water at 35°C 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 3 . The amount of the sprayed salt water was 3.8 liters/24 hrs.
(d) The spraying device consisted of two nozzles. The spraying pressure was 1 kg./cm 2 .
5) The tests were recorded by the following marks.
W: fine white matters
B: black spots
R: red rust spots
Rr: streaky red rust spots
Numerals prefixed to these marks denote number of occurrence. "Many" denotes many occurrence, "All" denotes the entire surface area, and so on.
For instance, "All W" indicates a state in which the entire surface of coating is covered by fine white matters, and "All 1/2 RR" indicates a state in which a half of the surface is covered by streaky red rust spots. These matters appear in the order of W, B, R, and RR, but the only observation at the time of taking records is shown in the tables. For example, when B and R was observed at said time, only R is shown in the table.
Table 2 ____________________________________________________________ ______________ Results of Salt-Water Spray Test ____________________________________________________________ ______________ Sample Coating Time 48 hr 168 336 672 1176 ____________________________________________________________ ______________ Ex. 2 Double- Inner Fusion bonded Zn coated Coating thickness 7μ not not Outer Fusion bonded Sn-Pb* ob- ob- 2W 5W 9R Coating thickness 10μ served served Total 17μ Comp. 2 Same as 1W 5W 12W 17B 17R Ex. 2 ____________________________________________________________ ______________ * The component ratio of Sn-Pb is 80:20. (Note ) 1) The immersion time of alloy bath in Ex.2 is 0.5 seconds and the immersion time of alloy bath in Comp.2 is 1.5 seconds.
Table 3 ____________________________________________________________ ______________ Results of Salt-Water Spray Test ____________________________________________________________ ______________ Sample Coating Time 48 hr 168 336 672 1176 ____________________________________________________________ ______________ Ex. 3 Double- Inner Electro-plated Zn coated Coating thickness 2μ not not Outer Fusion bonded Sn-Pb* ob- ob- 5W 7B 15R Coating thickness 10μ served served Total 12μ Comp. 3 Same as 4W 11W 5B 3R 25R Ex. 3 ____________________________________________________________ ______________ * The component ratio of Sn-Pb is 80:20 (Note) 1) The immersion time of alloy bath in Ex.3 is 0.5 seconds and the immersion time of alloy bath in Comp.3 is 2 seconds.
Table 4 ____________________________________________________________ ______________ Results of Salt-Water Spray Test ____________________________________________________________ ______________ Sample Coating Time 48hr 168 336 672 1176 ____________________________________________________________ ______________ Ex. 4 Double- Inner Fusion bonded Zn coated Coating thickness 6μ not Outer Fusion bonded Sn-Cd* ob- 2W 6W 5B 4R Coating thickness 7μ served Total 13μ Comp. 4 Same as not 2W 7W 7B 5R Comp. 3 ob- served Comp. 5 Single- Fusion bonded Sn-Pb* All W 5R 6RR 17RR / coated Coating thickness 8μ ____________________________________________________________ ______________ * The component ratio of Sn-Cd is 70:30. (Note) 1) The immersion time o alloy bath in Ex.4 is 1 second and the immersion time of alloy bath in Comp.4 is 1.5 seconds.
Table 5 ____________________________________________________________ ______________ Results of Salt-Water Spray Test ____________________________________________________________ ______________ Sample Coating Time 48 hr 168 336 672 1176 ____________________________________________________________ ______________ Ex. 5 Double- Inner Electro-plated Zn coated Coating thickness 13μ Outer Electro-plated Sn not not ob- ob- 2W 4B 6R Coating thickness 8μ served served Total Fusion bonded Double Thickness 19μ Comp. 5 Same as not 1W 4W 8B 7R ob- Ex. 5 served ____________________________________________________________ ______________ (Note) 1) The time for passing through the electric 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. × 0.71 mm thick × 500 mm long × 5 pieces
b. Molten flux
ZnCl 2 70 wt%, NH 4 Cl 30 wt%, Temperature 350°C
c. Molten zinc bath; bath temperature 470°C
d. Molten tin bath; bath temperature 280°C
Preparation of double-coated steel pipe:
Five steel pipes with clean surface treated in a usual manner were immersed in the molten flux at 350°C for 15 seconds and then immediately immersed in the molten zinc bath at 470°C for 10 seconds to obtain the zinc coating with a thickness of about 12μ. These coated steel pipes were then immersed in the molten tin bath at 280°C for about 0.8 seconds to obtain double-coated mild steel pipes with about 8μ 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 2μ thick and the outer coating of Sn-Zn alloy is about 6μ. It is considered that the thickness of the inner coating was decreased to only 2μ because the zinc coating was partly lost due to the LMC action when the outer coating was fusion-bonded.
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 1 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 1.
EXAMPLE 2
Raw materials:
a. Fusion-bonded zinc-coated mild steel pipes, 5 pieces
Coating thickness: about μ8
Size is the same as that in Example 1.
b. Molten flux
Same as that used in Example 1.
c. Plating bath
Sn--Pb alloy with an 80:20 composition by weight.
Bath temperature: 240°C
Preparation of double-coated steel pipe:
Five zinc-coated mild steel pipes were immersed in the molten flux bath at 350°C for 15 seconds and then immediately immersed in the above-mentioned plating bath at 240°C for 0.5 seconds to obtain the coating with an average thickness of 17μ. 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 7μ thick and the outer coating fusion-bonded thereon is about 10μ. 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 electro-plated zinc-coated steel pipes with a coating thickness of 3μ 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. It was found that the inner zinc coating is about 2μ thick and the outer Sn--Pb (80:20) coating is about 10μ thick, the total coating being about 12μ 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 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. 3 in Table 3.
EXAMPLE 4
Raw materials:
a. Fusion-bonded zinc-coated mild steel pipes, 5 pieces
Coating thickness: about 6μ
Size is the same as that in Example 1.
b. Molten flux
Same as that used in Example 1.
c. Plating bath
Sn--Cd alloy with a 70:30 composition by weight
Bath temperature: 220°C
Preparation of double-coated steel pipe:
Five zinc-coated mild steel pipes were immersed in the molten flux bath at 350°C for 15 seconds and then immersed in the above-mentioned plating bath at 220°C 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 6μ thick and the outer Sn-Cd coating is about 7μ thick, the total coating being about 13μ 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 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. 4 in Table 4.
EXAMPLE 5
a. Steel material tested:
Mild steel pipes; 1 piece
10 mm O.D. × 0.71 mm thick × 10 m long
b. Preparation of double-coated steel pipe;
The sample pipe was electro-plated with zinc to a coating thickness of 13μ by the usual method. This pipe was further electro-plated with tin to a coating thickness of 8μ. Thus, a double-electro-coated pipe with a coating thickness of 21μ 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 double-coated steel pipe having a zinc inner coating and a tin outer coating fusion-bonded to the inner coating. The average coating thickness was 19μ. Which is about 2μ thiner than the average coating thickness before the heat treatment carried out at 245°C 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.
Zinc-coated steel materials with chromate treatment have hitherto been in 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 may be applied to the surface of steel materials by means of melt-plating, electro-plating, or metalizing. The zinc coating is corroded by the liquid metal corrosion (abbreviated to LMC hereunder) when tin or a tin-based alloy is double-coated thereon by fusion bonding, but it is possible to allow the zinc coating to stay as the inner layer between the surface of steel material and the outer coating by lowering the temperature of 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 corrosion-resistant 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 tin-based 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.
1. Coating of zinc, tin, or a tin-based alloy alone on the surface of steel material does not impart such superior corrosion-resistance as that achieved by this invention.
2. Unexpected outstanding corrosion resistance is obtained by double-coating, by fusion bonding, tin or 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 above-mentioned 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 electro-plating 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 zinc-plating. The heat-treatment is carried out by heating the electro-plated double coatings in an atmosphere of non-oxidizing gas or reducing gas.
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 double-coated steel pipe prepared according to the invention.
Ex. 1, 2, 3, 4, and 5 in the tables correspond respectively to the examples 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 ordinary method, which is omitted in this specification.
Table 1 ____________________________________________________________ ______________ Results of Salt-Water Spray Test ____________________________________________________________ ______________ Sample Coating Time 48 hr 168 336 672 1176 ____________________________________________________________ ______________ Ex. 1 Double- Inner Fusion bonded Zn coated Coating thickness 2μ Outer Fusion bonded Sn not 1W 6W 7B 10R Coating thickness 6μ observed Total 8μ Comp. 1 Same as not 3W 8W 15B 15R Ex. 1 observed ____________________________________________________________ ______________
(Note)
1. The immersion time of tin bath in Ex.1 is 0.8 seconds and the immersion time of tin bath in comp. 1 is 1.2 seconds.
2. It is considered that the outer coating of Ex. 1 consists mostly of Sn-Zn alloy due to the great effect of LMC, as will be explained in the following Example 1.
3. The coating thickness indicates the average value measured at the both ends and center. This applies to the following tables.
4. The salt water spray tests were carried out in accordance with the following method.
The test pieces made in the example mentioned below were tightly plugged with a synthetic resin pieces 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 scaled 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 5% of the refined sodium chloride in distilled water was used at 35°C. The salt water at 35°C 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 3 . The amount of the sprayed salt water was 3.8 liters/24 hrs.
(d) The spraying device consisted of two nozzles. The spraying pressure was 1 kg./cm 2 .
5) The tests were recorded by the following marks.
W: fine white matters
B: black spots
R: red rust spots
Rr: streaky red rust spots
Numerals prefixed to these marks denote number of occurrence. "Many" denotes many occurrence, "All" denotes the entire surface area, and so on.
For instance, "All W" indicates a state in which the entire surface of coating is covered by fine white matters, and "All 1/2 RR" indicates a state in which a half of the surface is covered by streaky red rust spots. These matters appear in the order of W, B, R, and RR, but the only observation at the time of taking records is shown in the tables. For example, when B and R was observed at said time, only R is shown in the table.
Table 2 ____________________________________________________________ ______________ Results of Salt-Water Spray Test ____________________________________________________________ ______________ Sample Coating Time 48 hr 168 336 672 1176 ____________________________________________________________ ______________ Ex. 2 Double- Inner Fusion bonded Zn coated Coating thickness 7μ not not Outer Fusion bonded Sn-Pb* ob- ob- 2W 5W 9R Coating thickness 10μ served served Total 17μ Comp. 2 Same as 1W 5W 12W 17B 17R Ex. 2 ____________________________________________________________ ______________ * The component ratio of Sn-Pb is 80:20. (Note ) 1) The immersion time of alloy bath in Ex.2 is 0.5 seconds and the immersion time of alloy bath in Comp.2 is 1.5 seconds.
Table 3 ____________________________________________________________ ______________ Results of Salt-Water Spray Test ____________________________________________________________ ______________ Sample Coating Time 48 hr 168 336 672 1176 ____________________________________________________________ ______________ Ex. 3 Double- Inner Electro-plated Zn coated Coating thickness 2μ not not Outer Fusion bonded Sn-Pb* ob- ob- 5W 7B 15R Coating thickness 10μ served served Total 12μ Comp. 3 Same as 4W 11W 5B 3R 25R Ex. 3 ____________________________________________________________ ______________ * The component ratio of Sn-Pb is 80:20 (Note) 1) The immersion time of alloy bath in Ex.3 is 0.5 seconds and the immersion time of alloy bath in Comp.3 is 2 seconds.
Table 4 ____________________________________________________________ ______________ Results of Salt-Water Spray Test ____________________________________________________________ ______________ Sample Coating Time 48hr 168 336 672 1176 ____________________________________________________________ ______________ Ex. 4 Double- Inner Fusion bonded Zn coated Coating thickness 6μ not Outer Fusion bonded Sn-Cd* ob- 2W 6W 5B 4R Coating thickness 7μ served Total 13μ Comp. 4 Same as not 2W 7W 7B 5R Comp. 3 ob- served Comp. 5 Single- Fusion bonded Sn-Pb* All W 5R 6RR 17RR / coated Coating thickness 8μ ____________________________________________________________ ______________ * The component ratio of Sn-Cd is 70:30. (Note) 1) The immersion time o alloy bath in Ex.4 is 1 second and the immersion time of alloy bath in Comp.4 is 1.5 seconds.
Table 5 ____________________________________________________________ ______________ Results of Salt-Water Spray Test ____________________________________________________________ ______________ Sample Coating Time 48 hr 168 336 672 1176 ____________________________________________________________ ______________ Ex. 5 Double- Inner Electro-plated Zn coated Coating thickness 13μ Outer Electro-plated Sn not not ob- ob- 2W 4B 6R Coating thickness 8μ served served Total Fusion bonded Double Thickness 19μ Comp. 5 Same as not 1W 4W 8B 7R ob- Ex. 5 served ____________________________________________________________ ______________ (Note) 1) The time for passing through the electric 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. × 0.71 mm thick × 500 mm long × 5 pieces
b. Molten flux
ZnCl 2 70 wt%, NH 4 Cl 30 wt%, Temperature 350°C
c. Molten zinc bath; bath temperature 470°C
d. Molten tin bath; bath temperature 280°C
Preparation of double-coated steel pipe:
Five steel pipes with clean surface treated in a usual manner were immersed in the molten flux at 350°C for 15 seconds and then immediately immersed in the molten zinc bath at 470°C for 10 seconds to obtain the zinc coating with a thickness of about 12μ. These coated steel pipes were then immersed in the molten tin bath at 280°C for about 0.8 seconds to obtain double-coated mild steel pipes with about 8μ 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 2μ thick and the outer coating of Sn-Zn alloy is about 6μ. It is considered that the thickness of the inner coating was decreased to only 2μ because the zinc coating was partly lost due to the LMC action when the outer coating was fusion-bonded.
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 1 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 1.
EXAMPLE 2
Raw materials:
a. Fusion-bonded zinc-coated mild steel pipes, 5 pieces
Coating thickness: about μ8
Size is the same as that in Example 1.
b. Molten flux
Same as that used in Example 1.
c. Plating bath
Sn--Pb alloy with an 80:20 composition by weight.
Bath temperature: 240°C
Preparation of double-coated steel pipe:
Five zinc-coated mild steel pipes were immersed in the molten flux bath at 350°C for 15 seconds and then immediately immersed in the above-mentioned plating bath at 240°C for 0.5 seconds to obtain the coating with an average thickness of 17μ. 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 7μ thick and the outer coating fusion-bonded thereon is about 10μ. 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 electro-plated zinc-coated steel pipes with a coating thickness of 3μ 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. It was found that the inner zinc coating is about 2μ thick and the outer Sn--Pb (80:20) coating is about 10μ thick, the total coating being about 12μ 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 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. 3 in Table 3.
EXAMPLE 4
Raw materials:
a. Fusion-bonded zinc-coated mild steel pipes, 5 pieces
Coating thickness: about 6μ
Size is the same as that in Example 1.
b. Molten flux
Same as that used in Example 1.
c. Plating bath
Sn--Cd alloy with a 70:30 composition by weight
Bath temperature: 220°C
Preparation of double-coated steel pipe:
Five zinc-coated mild steel pipes were immersed in the molten flux bath at 350°C for 15 seconds and then immersed in the above-mentioned plating bath at 220°C 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 6μ thick and the outer Sn-Cd coating is about 7μ thick, the total coating being about 13μ 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 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. 4 in Table 4.
EXAMPLE 5
a. Steel material tested:
Mild steel pipes; 1 piece
10 mm O.D. × 0.71 mm thick × 10 m long
b. Preparation of double-coated steel pipe;
The sample pipe was electro-plated with zinc to a coating thickness of 13μ by the usual method. This pipe was further electro-plated with tin to a coating thickness of 8μ. Thus, a double-electro-coated pipe with a coating thickness of 21μ 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 double-coated steel pipe having a zinc inner coating and a tin outer coating fusion-bonded to the inner coating. The average coating thickness was 19μ. Which is about 2μ thiner than the average coating thickness before the heat treatment carried out at 245°C 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.