GB2083378A

GB2083378A - Method for forming a resin- coated aluminum-plated steel member and member formed thereby

Info

Publication number: GB2083378A
Application number: GB8122681A
Authority: GB
Original assignee: Sumitomo Electric Industries Ltd
Current assignee: Sumitomo Electric Industries Ltd
Priority date: 1980-07-26
Filing date: 1981-07-23
Publication date: 1982-03-24
Also published as: GB2083378B; DE3129333A1; DE3129333C2; FR2487224A1; FR2487224B1; CA1154637A

Abstract

A method for forming a resin- coated aluminum-plated steel member, comprising forming an aluminum plating layer containing a total of at least about 1 wt%, based on the aluminum content, of one or both of Si and Zn, on a steel substrate, by (1) hot dipping said substrate to form an aluminum-plated layer having a thickness of at least about 10 mu m (2) cooling, (3) heating the plated steel again to a temperature of from about 150 DEG C to 600 DEG C, (4) cooling, (5) roughening the aluminum plating layer surface by electrochemical, chemical or mechanical means, and (6) forming a resin coating on the aluminum plating layer surface, and the aluminum-plated steel member produced thereby.

Description

SPECIFICATION Method for forming a resin-coated aluminum-plated steel member and member formed thereby This invention relates to a method for coating the surface of a steel element with a resin.

Resins such as fluorocarbon resins, silicone resins and polysulfone resins are plastics having a high degree of non-tackiness, heat resistance, and chemical resistance, and an increasing number of metal moldings are coated with these resins, most commonly fluorocarbon resins for use in household kitchen utensils and various industrial parts. By the term "fluorocarbon resin" is meant polymeric substances containing fluorine atoms in the molecules thereof, including both a homopolymer such as a polytetrafluoethylene, and a copolymer such as a tetrafluoroethylene/hexafluoropropylene copolymer. The metals to be coated with these resins are typically aluminum and stainless steel, but a metal getting increasing attention is non-stainless steel, which is less expensive than both aluminum and stainless steel, and which has higher strength and smaller heat distortion than aluminum.

However, the resins mentioned above have poor adhesion to metals. They are conventionally bonded to metals by the following procedures: (1) If aluminum is used, it is given microfine projections and recesses of the surface thereof by electro-chemical or chemical etching, and a resin coating is formed on the resulting rough surface giving an "anchor effect" to ensure firm bondage. This method is referred to as an "etching" process.

(2) An adhesive layer, called a "primer", is formed on the metal surface, and a resin layer is bonded to the metal through the primer. This method is referred to as a "primer" process.

(3) A resin coating containing a bonding aid is formed on the metal.

The etching process is better than the other two methods since it provides a firmer adhesion and permits the formation of a resin coating on a metal blank which then is shaped into a desired form.

As a result of extensive studies on the formation of a resin coating on an aluminum-plated steel element by the methods described above, it has been found that the aluminum should contain a total of at least 1 wt%, based on the aluminum content, of silicon and/or zinc and that steel element with an aluminum plating formed by hot dipping should be heated again after cooling. According to this finding, if the second heating step is omitted, the adhesion between the resin coating and the metal substrate is too weak to be used for practical applications.

Therefore, this invention constitutes a method for forming a resin-coated aluminum-plated steel member which comprises forming an aluminum plating layer containing a total of at least about 1 wt%, based on the aluminum content, of one or both of Si and Zn, on a steel (including steel alloy) substrate, by (1) hot dipping said substrate to form an aluminum plated layer having a thickness of at least 1 0 ym, (2) cooling, (3) heating the plated steel again to a temperature of from about 1 50 to 60Oac (4) cooling, (5) roughening the aluminum plating layer surface electrochemically, chemically, or mechanically, and (6) forming a resin coating on the aluminum plating layer surface.In another aspect, this invention constitutes a member made by the process of the invention.

The method of this invention can be used to form a coating of any resins that is used in general-purpose paint compositions. Great advantages are obtained by using a resin having a high degree of non-tackiness, and particularly great advantages are achieved by using a fluorocarbon resin.

The steel substrate is most generally plated with aluminum by hot dipping. To inhibit the formation of an Al-Fe alloy layer at the interface of the aluminum and iron, it is necessary to add a total of at least about 1 wt% of Si and/or Zn to the aluminum (based on the aluminum content). If the sum of the Si and Zn contents is less than about 1 wt%, the Fe-AI alloy layer grows rapidly at high temperature such as those employed during plating or formation of the resin coating, and only poor adhesion results between the resin coating and aluminum plated layer. Therefore, the aluminum with which the steel substrate is plated must contain a total of at least about 1 wt%, and preferably at least 3 wt%, of one or both of Si and Zn, on the basis of the aluminum content.It is preferred that the total amount of one or both of Si and Zn be not more than about 20% on the basis of the aluminum content.

The period of heating the Al-plated steel again after it has been cooled, e.g., to room temperature, varies with the temperature; heating for a few minutes is adequate if the temperature is as high as about 600"C, whereas heating for at least 1 hour is necessary if the temperature is between 1 50 C and 200 C. If the temperature exceeds 600"C, the Al-Fe alloy layer grows to such an extent that the corrosion resistance and the workability of the steel are reduced. If the temperature is less than 1 50 C, the heating of the Al-plated steel after cooling achieves substantially no effect and firm adhesion cannot be attained between the resin coating and an etched aluminum surface.

The step of heating the Al-plated steel substrate after cooling is essential to the process of this invention, and without this, an etched aluminum plating layer becomes so brittle that a resin coating cannot be formed on the plating layer without breaking the plating layer, and as a result no adhesion is attained between the resin coating and the aluminum plating layer.

The aluminum plating is then provided with a roughened surface by electrochemical etching, chemical etching, or mechanical graining. In electrochemical etching, an equeous solution of halide such as sodium chloride is electrolyzed with the plated steel used as an anode. By this procedure, microfine projections and recesses are formed on the aluminum surface. In chemical etching, aluminum is dissolved out in an aqueous solution of hydrochloric acid, etc., to form microfine projections on and recesses in the aluminum surface. In mechanical graining, the aluminum surface is roughened by mechanical means, such as sand blasting or liquid honing.

The "anchor effect" is not as great when the aluminum surface is treated by the mechanical graining, and, therefore, the aluminum plating layer can be coated with a resin only after forming a primer layer or by using a resin containing a bonding aid.

According to the method of this invention, the steel substrate must be plated with an aluminum layer is at least about 10 pm thick. If the aluminum plating layer is thinner than about 10 'lem, the iron layer will become exposed during the subsequent roughening step, and, as a result, not only is the corrosion resistance decreased, but also the aluminum plating layer achieves only poor adhesion to the resin coating. Therefore, the aluminum plating layer must have a thickness of at least about 10 ym, and preferably at least 1 5 ,um.

A resin coating can be formed on the roughened aluminum surface by spray-coating or flowcoating a resin dispersion or solution, forming an electrostatic coating of a resin powder, or by lamination of a resin film. The treated surface is then melted by heating it to a temperature higher than the melting point thereof, and cooled, e.g., to room temperature, to form a resin coating.

As described in the foregoing, this invention provides a method for forming a resin coating that is firmly bonded to an aluminum-plated steel member. The resulting steel element has a high degree of non-tackiness on the resin coated side, has high corrosion resistance due to the aluminum plating, and retains the high strength of the steel substrate. Hence, the element can be used not only for kitchen utensils but also for sliding parts and other industrial components.

The invention is now described in greater detail by reference to the following examples and comparative examples, which are given here for illustrative purposes only, and are not intended to limit the scope of the invention. In the following examples and comparative examples peeling tests were carried out according to the method prescribed in ASTM-D-903.

EXAMPLE 1 An aluminum plating layer containing 7 wt% of Si was formed on both sides of a steel plate by hot dipping using a bath of Al containing 7 wt% of Si kept at about 750"C, to give a thickness of 25 ,um. Four Al-plated samples were prepared by the same method. After cooling to room temperature, the samples were heated again under the conditions indicated in Table 1.

The aluminum surface of each sample was etched electrochemically in a 5% aqueous NaCI solution (15 coulomb/cm2) to form microfine projections on and recesses in the surface. An aqueous dispersion of tetrafluoroethylene was spread by flow coat method on the roughened aluminum surface, and after dewatering, the samples were baked in air at 380"C for 20 minutes to provide tetrafluoroethylene coated steel plates. The resin coating was 25 ym thick.The strength of adhesion between the resin coating and the aluminum plating was checked by a 180 peeling test and a crosscut peeling test (100 squares measuring 1 mm on each side were cut by a sharp blade in a checker-board pattern to a depth that reached the aluminum surface, an adhesive tape was pressed against the resin coating, and peeled off immediately to see if the crosscut section peeled off the aluminum surface). The results are shown in Table 1 below.

COMPARATIVE EXAMPLE 1 The procedure of Example 1 was repeated in Comparative Examples 1-1 and 1-2, except that the conditions of heating the Al-plated steel substrates after cooling to room temperature were varied as indicated in Table 1. In Comparative Example 1-3, no such heating was conducted. The results of the 180 peeling test and crosscut peeling test are shown in Table 1.

TABLE 1 Relation between Reheating after Al Plating and Adhesion Strength Strength Crosscut* against Peeling Run No. Reheating 180 Peeling Test (kg/2.5 cm) Example 1-1 550"C X 10 min 3.0 0/100 Example 1-2 400"C x 30 min 3.2 0/100 Example 1-3 300"C X 60 min 2.8 0/100 Example 1-4 170"C x 120 min 2.6 0/100 Comparative Example 1-1 630"C X 5 min 1.2 10/100 Comparative Example 1-2 120"C X 200 min 0.7 50/100 Comparative Example 1-3 No heating 0.1 100/100 The figures in the column of "crosscut peeling test" indicate how many of the 100 crosscut squares peeled off; 0/100 indicates no crosscut peeled off and 100/100 indicates all crosscuts peeled off.

Table 1 shows that the heating of an Al-plated steel substrate at a temperature of from about 150"C to 600"C after cooling to room temperature is essential to achieving firm adhesion between the resin coating and aluminum surface.

EXAMPLE 2 Aluminum platings containing Si and Zn in the amounts indicated in Table 2 were formed on a steel plate by hot dipping using a bath of Al containing 7 wt% of Si kept at about 750"C.

After cooling to room temperature, the samples were heated again at 400"C for 30 minutes.

The aluminum surface of each sample was etched chemically in 10% hydrochloric acid for 5 minutes to form microfine projections on and recesses in the surface. An aqueous dispersion of tetrafluoroethylene/hexafluoropropylene copolymer was spread by air spray method on the roughened aluminum surface to give a thickness of 25 ym, and . after dewatering, the samples were baked in air at 360"C for 20 minutes to provide steel plates coated with the tetrafluoroethylene/hexafluoropropylene copolymer. The adhesion between the resin coating and the aluminum plating layer of each sample was evaluated as in Example 1. The results are shown in Table 2.

COMPARATIVE EXAMPLE 2 The procedure of Example 2 was repreated except that the Si and Zn contents of the aluminum plating were varied as indicated in Table 2. The results of the 180 peeling test and crosscut peeling test as in Example 1 are shown in Table 2.

TABLE 2 Relation between Si and Zn Contents (wt%) and Adhesion Strength Strength Crosscut Si Zn against Peeling Run No. Content Content 180 Peeling Test (wt%) (wt%) (kg/2.5 cm) Example 2-1 7 < 0.2 3.1 0/100 Example 2-2 5 < 0.2 3.0 0/100 Example 2-3 3 < 0.2 2.7 0/100 Example 2-4 < 0.2 5.0 2.9 0/100 Example 2-5 3 4 - 3 0/100 Comparative Example 2-1 0.4 < 0.2 0.5 50/100 Comparative Example 2-2 < 0.2 < 0.2 0.5 70/100 As Table 2 shows, the aluminum plating layer wherein the sum of the Si and Zn contents weas less than 1 wt% did not provide firm adhesive of the resin coating even if the Al-Plated steel substrate was heated at a remperature between 1 50 C and 600"C after cooling.

EXAMPLE 3 Aluminum platings containing 5 wt% of Si were formed on a steel plate by hot dipping using a bath of Al containing 7 wt% of Si kept at about 750"C to give the thicknesses indicated in Table 3. After cooling to room temperature, the Al-plated steel plates were heated at 450"C for 30 minutes. The aluminum surface of each sample was etched electrochemically in a 3% aqueous solution of potassium chloride (13 coulomb/cm2) to form microfine projections on and recesses in the surface. A coating of PFA resin (tetrafluoroethylene/perfluoroalkoxyethylene copolymer) powder was formed by air spray method on the roughened aluminum surface by electrostatic coating to give a thickness of 30 pm. The steel plates were baked in air at 380"C for 30 minutes to provide PFA-coated steel plates.The strength of adhesion between the resin coating and the aluminum plating was evaluated by crosscut peel test as in Example 1. The corrosion resistance of each sampie was evaluated by salt spray test in compliance with the method described in JIS Z 2371. The results are shonw in Table 3.

COMPARATIVE EXAMPLE 3 The procedure of Example 3 was repeated except that the thickness of the aluminum plating was varied as indicated in Table 3. The results of crosscut peel test and salt spray are indicated in Table 3.

TABLE 3 Plating Layer Thickness Crosscut Corrosion" Run No. (one side) Peel Test Resistance Wm) Comparative Example 3-1 5 10/100 x Comparative Example 3-2 8 5/100 t Example 3-1 12 0/100 o Example 3-2 17 0/100 o Example 3-3 25 0/100 o * The corrosion resistance was evaluated by the severity of corrosion that developed in a salt spray test (JIS-Z-2371) continued for 100 hours; o: no corrosion, G slight corrosion, X heavy corrosion As shown by Table 3, the method of this invention requires an aluminum coating having a thickness of at least 10 ,liy.

EXAMPLE 4 An aluminum plating layer containing 7 wt% of Si was formed on one side of a steel plate by hot dipping using a bath of Al containing 7 wt% of Si kept at about 750"C to form a layer having a thickness of 25 pm. Four aluminum-plated steel plate samples were prepared by the same method. After cooling to room temperature, the samples were heated again under the conditions indicated in Table 4. The aluminum surface of each sample was grained by sand blasting. An aqueous dispersion of tetrafluoroethylene primer was spread on the grained aluminum surface, and, after dewatering, an aqueous dispersion of tetrafluoroethylene was spread by air spray method of the primer coating and baked in air at 380"C for 20 minutes to provide tetrafluoroethylene coated steel plates. The resin coating (including the primer coating) was 35 ym thick. The strength of adhesion between the resin coating and the aluminum surface was checked as in Example 1. The results are indicated in Table 4 below.

COMPARATIVE EXAMPLE 4 The procedure of Example 4 was repeated in Comparative Examples 4-1 and 4-2 except that the conditions of heating the Al-plated steel substrates after cooling was varied as indicated in Table 4. In Comparative Example 4-3, no such heating was conducted. The results of the 180 peeling test and crosscut peeling test as in Example 1 are shown in Table 4.

TABLE 4 Relation between Reheating after Al Plating and Adhesion Strength Strength Crosscut against Peeling Run No. Reheating 180 Peeling Test (kg/2.5 cm) Example 4-1 550"C X 10 min 2.5 0/100 Example 4-2 400"C X 30 min 2.7 0/100 Example 4-3 300"C X 60 min 2.4 0/100 Example 4-4 170"C X 120 min 2.2 0/100 Comparative Example 4-1 630"C X 5 min 1.0 10/100 Comparative Example 4-2 120"C X 200 min 0.5 50/100 Comparative Example 4-3 No heating 0.1 100/100 Table 4 shows that the heating of an Al-plated steel substrates at a temperature between 150"C and 600"C after cooling is also essential to achieving firm adhesion between the resin coating and the aluminum plating grained by mechanical means.

EXAMPLE 5 Aluminum plating layers containing Si and Zn in the amounts indicated in Table 2 were formed on a steel plate which was then heated at 400"C for 30 minutes. The aluminum surface of each sample was grained by liquid honing. A coating of modified tetrafluoroethylene resin ("Toughcoat" of Daikin Kogyo Co., Ltd.) was formed by air spray method on the grained surface in a thickness of 20 cm and baked in air at 350"C for 20 minutes. The strength of the adhesion between the resin coating and the aluminum plating layer was evaluated as in Example 1. The results are shown in Table 5 below.

COMPARATIVE EXAMPLE 5 The procedure of Example 5 was repeated except that the Si and Zn contents of the aluminum plating were varied as in Table 5. The results of the 180 peeling test and crosscut peeling test as in Example 1 are shown in Table 5.

TABLE 5 Relation between Si and Zn Contents (wt%) and AdhesionJStrength Strength Crosscut Si Zn against Peeling Run No. Content Content 180" Peeling Test (kg/2.5 cm) Example 5-1 7 < 0.2 2.7 0/100 Example 5-2 5 < 0.2 2.7 0/100 Example 5-3 3 < 0.2 2.4 0/100 Example 5-4 < 0.2 5.0 2.6 0/100 Example 5-5 3 4 2.5 0/100 Comparative Example 5-1 0.4 < 0.2 0.4 50/100 Comparative Example 5-2 < 0.2 < 0.2 0.3 70/100 As Table 5 shows, the aluminum plating layer wherein the sum of the Si and Zn contents was less than 1 wt% did not provide firm adhesion to the resin coating even if the Al-plated steel substrate was heated at a temperature between 150"C and 600"C after cooling.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims

1. A method for forming a resin-coated aluminum-plated steel member, comprising forming an aluminum plating layer containing a total of at least about 1 wt%, based on the aluminum content of one or both of Si and Zn, on a steel substrate, by (1) hot dipping said substrate to form an aluminum-plated layer having a thickness of at least about 10 'lem, (2) cooling, (3) heating the plated steel again to a temperature of from about 1 50'C to 600'C, (4) cooling, (5) roughening the aluminum plating layer surface by electrochemical, chemical or mechanical means, and (6) forming a resin coating on the aluminum plating layer surface.

2. A method according to Claim 1, wherein the resin is a fluorocarbon resin.

3. A method according to Claim 1 or 2, wherein the aluminum plating layer surface is roughened by electrochemical or chamical means.

4. A method according to Claim 1, 2 or 3, wherein the aluminum plating layer contains a total of at least 3 wt% of one or both of Si and Zn.

5. A method according to Claim 1, 2, 3 or 4 wherein the aluminum plating layer has a thickness of at least 1 5 sum.

6. A resin-coated aluminum-plated steel member comprising a steel substrate, an aluminum plating layer thereon containing a total of at least about 1 wt%, plating layer the aluminum content, of one or both of Si and Zn, and a resin coating on the aluminum plating layer surface, wherein said member is formed by a method comprising (1) hot dipping said substrate to form an aluminum plated layer haing a thickness of at least about 10 cm, (2) cooling, (3) heating the plated steel again to a temperature of from about 1 50'C to 600 C, (4) cooling, (5) roughening the aluminum plating layer surface by electrochemical, chemical, or mechanical means, and (6) forming a resin coating on the aluminum plating layer surface.

7. A resin-coated aluminum-plated steel member according to Claim 6, wherein the resin is a fluorocarbon resin.

8. A resin-coated aluminum-plated steel member according to Claim 6 or 7, wherein the aluminum plating layer surface is roughened by electrochemical or chemical means.

9. A resin-coated aluminum-plated steel member according to Claim 6, 7 or 8, wherein the aluminum plating layer contains a total of at least 3 wt% of one or both of Si and Zn.

10. A resin-coated aluminum-plated steel member according to Claim 6, 7, 8 or 9, wherein the aluminum plating layer has a thickness of at least 15 cm.

A method for forming a resin-coated aluminum-plated steel member substantially as hereinbefore described in any one of Examples 1 to 5.

1 2. A resin-coated aluminum-plated steel member substantially as hereinbefore described in any one of Examples 1 to 5.