GB2182352A

GB2182352A - Surface treatment of iron and steel products

Info

Publication number: GB2182352A
Application number: GB08624168A
Authority: GB
Inventors: Yoshio Nagae; Koji Hetsugi; Yoshihiko Sawazaki
Original assignee: Nihon Parkerizing Co Ltd
Current assignee: Nihon Parkerizing Co Ltd
Priority date: 1985-10-08
Filing date: 1986-10-08
Publication date: 1987-05-13
Also published as: FR2588276A1; DE3632335A1; CA1265392A; IT1197361B; GB2182352B; GB8624168D0; IT8621921A0; IT8621921A1; JPS6283477A

Abstract

Fine particles of zinc or zinc coated metals are shot-blasted onto the surface of iron or steel products, especially large structures, to form a zinc coating, a phosphate coating is formed on the surface and the treated surface is painted.

Description

SPECIFICATION Surface Treatment for Iron and Steel Products This invention relates to a method of surface treatment of iron and steel products by shot-blasting and phosphate treatment.

It is known to pre-treat iron and steel surfaces prior to painting by providing a phosphate coating in order to improve the corrosion resistance of the painted surface. Conventional processes, however, provide insufficient corrosion resistance, especially for large and expensive constructions such as TV towers, which require very long term corrosion prevention.

It is known to improve the corrosion resistance of these products by plating zinc onto the surfaces before phosphate treatment. In such processes the metal surface often carries rust or mill scale and it is necessary to pickle the surface to activate it before zinc plating. In addition the pickled surface must be rinsed. There are several disadvantages with the process. It is a muiti-stage process and thus time consuming and expensive in apparatus costs. Waste water treatment for plating is necessary. Where large parts are treated a large plating bath is necessary which is expensive in equipment costs. The control of the plating bath is complicated.

In a new process according to the invention an iron or steel surface is shot-blasted with particles having a zinc or zinc alloy surface, then treated with a phosphating solution containing nickel ion and subsequently provided with a coating or paint.

By the process of the invention it is possible to form a zinc or zinc alloy coating on the iron or steel surface whilst simultaneously removing rust and oxide films such as mill scale from the surface by the collision of the particles with the surface. The process does not suffer the above-mentioned problems with the conventional zinc-plating process.

Although the process is of particular value where the iron or steel surface carries rust or oxide film such as mill scale, it is also of value where the surface is free of such deposits. The phosphating solution containing nickel provides a surface coating which is particularly suitable as a base for the application of paint, having extremely good corrosion resistance properties.

The process is particularly suitable for the treatment of hot-rolled steel sheet and sections.

The particles used for the shot-blasting generally have a diameter of 0.2 to 1.1 mm. In general it is desirable for them to be as hard as possible, especially to promote the removal of oxide film. The particles can consist of zinc or zinc alloy but it is often preferred to use particles having a metal core coated by zinc or zinc alloy. Preferred particles are prepared by coating zinc onto ferrous fine particles followed by heating so that the zinc coating is alloyed with iron in the ferrous particles. Suitable particles of this type are available from KK Sanpou under the trade name Z-lron Alloyed Shot (hardness'350 to 450).

The weight of the coating of zinc and zinc alloy that is formed on the metal surface as a result of the shot-blasting is generally in the range 0.1 to 50 g/m2, often 5 to 20 g/m2.

The weight of the coating can be varied by varying the duration of the shot-blasting or by varying the force of blasting.

The amount of shot-blasting is generally such as to give a spraying thickness in the range 1 1 50 mm.

The spraying thickness is the quantity of discharged shot per unit of area of surface blasted divided by the apparent specific gravity of shot.

As phosphating solution, the conventional acidic phosphating solution for zinc surface can be applied.

For example, the solution described in Japanese Patent Publication Sho 42 (1967)-12130 can be applied.

Nickel ion is an essential component of the phosphating solution for suppressing dissolution of zinc or zinc alloyed film formed on the metallic surface during phosphating. 0.05--10 gn of nickel ion can be added. If the added amount is less than 0.05 g/l, the effect is not fully shown, whereas if it exceeds 10 g/l, it is disadvantageous because of cost, and it is preferred to be 0.1-5 g/l. Generally, acidic phosphating solution consists of 5--100 g/l of phosphate ion, 0.3--7 g/l of zinc ion, 1-30 g/l of nitrate ion, 0.1-5 g/l of fluoride ion, 0-10 g/l of organic acid and nickel ion.Nickel ion can be added as nickel nitrate, nickel carbonate, etc.

This phosphate treatment is normally conducted by immersion or spraying for 5 seconds to 10 minutes at a temperature from room temperature to 80"C.

In order to accelerate the phosphate conversion reaction, and to suppress dissolution or zinc or zinc alloy coating during phosphating, it is preferable to pretreat the zinc or zinc alloy coated surface with a surface conditioning aqueous solution which contains colloidal titanium. Conventional solutions containing colloidal titanium can be used. Suitable solutions may comprise 1-50 ppm of titanium ion, 2003000 ppm of phosphate ion and 50--600 ppm of pyrophosphate ion and have a pH of 8.S9.5.

Generally, after the phosphate coating has been formed the treated metal is water rinsed and dried.

After phosphate treatment, in order to improve corrosion resistance, sealing may be effected by the conventionally known aqueous solution of chromic acid, aqueous dispersion of resin or aqueous solution of resin, aqueous solution of tannic acid, aqueous dispersion of colloidal silica, etc. The treated surface may be rinsed and dried. Finally the treated surface may be provided with a coat of paint of conventional type by conventional means.

In the process the fine particles physically remove any oxide film from the surface and concurrently will firmly adhere to or penetrate into the material surface and will form thereon a metallic zinc or zinc alloy film.

Since this film is an aggregate formed by collision of fine particles, there are pin holes so that the zinc or zinc alloy coated surface is inferior in corrosion resistance to the conventional zinc plated surface. However, by the phosphate treatment, a phosphate coating is formed on the zinc or zinc alloy coated surface to fill the pin holes with phosphate crystals.

The treated surface has excellent corrosion resistance and is a suitable paint base. The optional surface conditioning with a solution containing colloidal titanium and the presence of nickel ion in the phosphating solution controls the etching reaction for the zinc or zinc alloy film formed on the surface with the phosphating solution, thereby preventing enlargement of the holes in the zinc or zinc alloy coated surface.

The following examples illustrate the invention.

Test Method Paint Adhesion Test 1) Cross Cut-Erichsen Test 100 checkers of 1 mm2 are scribed on the measuring area of the paint coated surface. The scribe must reach the base metal through the paint film. The scribed surface is subjected to 5 mm height cup forming by Erichsen Cupping Tester.

Then cellophane tape is applied to cover the checkers and rapidly pulled-off. The adhesion rating was evaluated by counting number of squares which remain on the test surface.

Standard for evaluation is as shown in the table below.

Rating No. of checkers 95 or more 5 remaining on the 80--94 4 test surface 60--79 3 40--59 2 39 or less 1 2) Impact Test Impact test is carried out with a Du Pont impact tester, placing the test panel with the paint coated side up, under the conditions at 112" q) of impact bar, 1 kg loading and 50 cm height. A cellophane tape is applied to the impacted surface and rapidly pulled off. After testing, the loss of the paint film at the impacted surface is evaluated.

The evaluation standard forthe paint adhesiveness is as follows:

Loss of Paint Film | Rating No.

No failure 5 Failure 5% or less 4 6%-25% 3 " 26%-50% 2 " 51% or more 1 Consequently, in both tests the higher the rating, the better is adhesion.

1. Salt Spraying Test For Painted Surface.

For evaluating blister creepage from the crosshatch scribe formed as in the cross-cut Erichson test, i.e.

which is made on the surface of the paint coated sheet by a knife blade and reaches the base metal through the paint film, the test panels are subjected to salt spraying test for 120 hours in accordance with xIIS-Z-2371. The plates are then water rinsed and dried, and then a cellophane tape is applied to cover the scribed line and rapidly pulled-off.

Evaluation standard is as follows:

Unscribed Portion Scribed Portion Rating No. No failure No failure 5 No failure Blister width from the scribe (both side) Within 1 mm 4 No failure Within 2 mm 3 Blister occurs Larger blister 2 in small spots width with loss of paint Blister occurs in Larger blister 1 larger size like width with loss patches of paint

2. For Unpainted Surface After salt spraying test is conducted for 24 hours, the white rust occurrence on the unpainted surface is evaluated.

Evaluation standard is shown in the table below:

Rusting Area Rating No.

White rust occurrence 05% 5 White rust occurrence S15% 4 White rust occurrence 1 S35% 3 White rust occurrence 3S50% 2 White and red rust More than 51% 1 occurrence EXAMPLE 1 The fine particles of zinc having a diameter of 0.2-0.3 mm were shot-blasted for 10 minutes onto a rusted SPCC-D steel panel of 0.8x100x150 mm to remove rust and to form a metallic zinc film of 15 g/m2.

Next, the panel treated above was subjected to a surface conditioning and a phosphate treatment, then it was water rinsed and subsequently treated with a chromate solution containing trivalent chromium ion (10 g/l of Parcolen* 62, manufactured by Nihon Parkerizing Co., Ltd). And then it was squeezed with a roll and dried, thereby a phosphate coating having coating weight of 1.4 g/m2 was obtained. The treating conditions were as shown in Table below.

Surface Conditioning

Treating solution Aqueous solution containing colloidal titanium (1 g/l of Parcolene* Z, produced by Nihon Parkerizing Co., Ltd.) Treating conditions Spray, 50 C, 5 seconds *Trade marks Phosphate Treatment

Treating solution Zn2+:2.5 gil P04-.10 gil NO,:3 gil Ni:2.0 g/l F-::0.2 g/l Treating conditions Spray 70"C, 7 seconds The phosphated steel panel was coated with melamine-alkyd paint (Amirack* No. 1 White, manufactured by Kansai Paint Co., Ltd.) using a bar coater. After setting for 20 minutes it is baked for 25 minutes in the circulating hot air oven at 140do and atmospheric pressure to obtain a painted panel with a film thickness of 30+20 micron. Paint adhesion test and salt spraying test were carried out with reference to this paint coated panel. The results are shown in Table 1.

*Trade mark EXAMPLE 2 Conducted under the same conditions as Example 1, except that the surface conditioning with colloidal titanium containing solution was omitted.

The phosphate coating weight was 1.9 g/m2. The results are shown in Table 1.

EXAMPLE 3 Conducted under the same conditions as Example 1, except that the fine particles of zinc-iron alloy coated metal (Product name: Z-lron Alloyed Shot, manufactured by K. K. Saupou) was used as the shot material instead of zinc fine particles.

The phosphate coating weight was 1.1 g/m2. The results are shown in Table 1.

COMPARATIVE EXAMPLE 1 An electrically zinc plated (15 g/m2) steel panel of0,8x100x150 mm was subjected to phosphate treatment with the same phosphate solution as that used in Example 1, and coated with the melamine-alkyd paint the same as that used in Example 1. After setting for 20 minutes, it was baked for 25 minutes at 140"C and dried. The paint coated steel panel was subjected to the same evaluation tests as Example 1. The results as shown in Table 1.

COMPARATIVE EXAMPLE 2 Conducted under the same conditions as Example 2 except that nickel ion was removed from the phosphating solution. The results are shown in Table 1.

COMPARATIVE EXAMPLE 3 Conducted under the same conditions as Example 1 except that fine iron particles having a particle diameter of 0.2-0.3 mm were used instead of the fine zinc particles having a particle diameter of 0.2-0.3 mm. The results are shown in Table 1.

TABLE 1

Item Paint Adhesion Test Salt Spraying Test Unpainted Painted Cross cut Example Ericksen Impact 24 Hours 120 Hours Example 1 5 5 5 4 Example 2 4 4 5 4 Example 3 5 5 5 5 Comparative Ex. 1 4 4 4 3 Comparative Ex. 2 3 3 4 3 Comparative Ex. 3 4 2 1 1

Claims

1. A process in which an iron or steel surface is shot-blasted with particles having a zinc or zinc alloy surface, then treated with a phosphating solution containing nickel ion and subsequently provided with a coating of paint.

2. A process according to claim 1 in which the surface is the surface of a hot-rolled steel sheet or section.

3. A process according to claim 1 or claim 2 in which after the shot-blasing and before treatment with the phosphating solution the surface is treated with an aqueous solution containing colloidal titanium.

4. A process according to any preceding claim in which the particles are metal particles having diameters in the range 0.2 to 1.1 mm, formed of zinc or a zinc alloy or formed of metal coated with zinc or zinc alloy.

5. A process according to claim 4 in which the particles are of iron coated with zinc or zinc alloy.

6. A process according to any preceding claim in which the phosphating solution contains nickel in an amount in the range 0.05 to 10 g/l, phosphate ions in an amount in the range 5 to 100 g/l and zinc ions in an amount in the range 0.3 to 7 g/l.

7. A process according to claim 6 in which the phosphating solution comprises fluoride ions in an amount in the range 0.1 to 5 g/l, nitrate ions in an amount in the range 1 to 30 g/l and organic acid in an amount in the range 0 to 10 g/l.

8. A process according to any preceding claim in which the phosphate solution is sprayed onto the surface for a period in the range 5 seconds to 10 minutes and at a temperature in the range room temperature to 80"C.