GB1558779A - Process for coating zinc or zinc alloy surfaces - Google Patents

Description

(54) PROCESS FOR COATING ZINC OR ZINC ALLOY SURFACES (71) We, PYRENE CHEMICAL SER- VICES LIMITED, a British Company, of Ridgeway, Iver, Buckinghamshire, SLO 9JJ, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a process for forming a protective coating on zinc or zinc alloy metal surfaces.

Zinc is widely used to make steel corrosion resistant, by the hot galvanized or electrolytic plating method. However, in atmospheres of high humidity, zinc-based metal surfaces develop white stains, giving rise to a decrease in corrosion resistance and in the adhesion of subsequently applied paints. This disadvantage has generally been countered by chromating processes employing mainly chromic or bichromic acid or a salt thereof. Such chromating processes are inexpensive and give good corrosion resistance, but they have toxic effects on humans and their use involves dangerous problems of environmental pollution. Accordingly, the standards for controlling the use of chromic acids and chromates have become increasingly severe.Further, problems such as, for example, process controls for chromating, chromating waste water treatment and secondary contamination by chromated articles have arisen.

Another method of treating metal surfaces is to use tannic acid, as disclosed, for example, in Japanese Patent Kokai Nos. 47224/74 and 8317/73. However, the corrosion resistance of metallic zinc surfaces achieved in these processes is generally unsatisfactory. In addition, when used as a base for paints, the resistance against paint scratches is insufficient and the paint films tend to blister in atmospheres of high humidity.

A further method of treatment of metal surfaces employs silica or silicates and has been disclosed in Japanese Patent Publications Nos.

19686/69 and 5130/70. However, the chromate-free coatings obtained by such processes give insufficient corrosion resistance and have often been used as electrically insulating coatings, as disclosed in Japanese Patent Publications Nos. 7849/68 and 13562/69.

According to the present invention, a process for forming a protective coating on the surface of a zine or zinc alloy workpiece comprises contacting the surface with an aqueous solution or suspension containing from 0.01 to 20% by weight of a water-dispersible com- ponent selected from silica, silica sol and alumina sol and from 0.1 to 20% by weight of tannin or tannic acid, in which the weight ratio of the water-dispersible component to the tannin or tannic acid is from 100:1 to 1:100.

The tvpes of metal workpieces which can be treated by the process of the invention are of zinc or zinc alloy and include galvanized steel workpieces.

The tannin or tannic acid component used in this invention may be a hydrolysable tannin, a condensed tannin or a partially decomposed derivative thereof. Suitable sources of this component include depside tannin, gallotannin, Chinese tannic acid, Turkey tannic acid, hamameli-tannic acid, tannic acid from Acer Ginnala, cheblinic acid, sumach tannin, Chinese gallotannin, ellagitannin, catechine, catechine-tannic acid and quebrachotannin.

The most preferred concentration of tannin or tannic acid depends on the source but is from 0.1 to 20%, and usually from 1 to 5% bv weight. At a concentration of less than 0.1 / by weight, the effect of this component become insignificant and, at a concentration of higher than 20% by weight, the coating process becomes difficult to operate and is uneconomic.

Forms of silica which can be used in this invention include Aerosil (available from Nihon Aerosil Co. Ltd., Japan) and Siloid (available from Fuji Devison Co. Ltd., Japan).

(CC Aerosil" is a registered Trade Mark).

Silica gel obtained by removing sodium ions from an aqueous solution of sodium silicate through an ion-exchange column, or commerciallv available silica sol containing from 20 to 30% of SiO2, e.g. Snowtex (available from Nissan Chemicals Co. Ltd., Japan), can be used in the invention, as can silica sol obtained either by dispersing silica gel in an alkali solution or by neutralising sodium silicate with an appropriate acid and isolating the resulting sol (which is thus free from sodium ions) or by dispersing solid SiO2 such as Aerosil in, for example, an aqueous medium. In every silica sol, the SiO2 should be dispersed homo geneously in the colloidal state in the aqueous medium.Lithium oxide may be included with this component, as disclosed in Japanese Pat ent Publications Nos. 19686/69 and 5130/70, to improve the corrosion resistance of the coated product. Commercially available pro ducts of this type include Lithium Silicate (the trade name of a product available from Nissan Chemicals Co. Ltd., Japan).

As an example of an alumina sol which can be used in this invention, commercially avail able alumina sol dispersed in water e.g. (as available from Nissan Chemicals Co. Ltd., Japan) is convenient for ease of handling.

This product contains more than 10% Awl203 and is stabilised at a pH of from 2.5 to 6 with an inorganic or organic acid. The preferred amount of the water-dispersible component in the invention is from 1 to 5% by weight of the aqueous composition.

The relative weights of tannin or tannic acid and the water-dispersible component in the composition must be from 100:1 to 1:100.

If the amount of tannin or tannic acid is too high then blistering tends to occur in the paints applied to the coated surfaces, when tested by the corrosion test. If the amount of the silica and/or alumina component is too high then the coating tends to be brittle.

Since the stable pH range for the aqueous composition depends on the components used, an inorganic acid such as phosphoric, nitric, sulphuric, hydrofluoric or hydrochloric add, or a salt thereof, an organic acid such as oxalic, citric, malic, maleic, phthalic, acetic, lactic, tartaric, chloroacetic acid or acrylic aid, or a salt thereof, or an alkaline compound such as sodium hydroxide, potassium hydroxide or lithium hydroxide or an amine such as ammonia, ethylamine, diethylamine, triethyl amine or ethanolamine may be used for pH adjustment However, since the residual content of water soluble salts in the coating facilitates the ad sorption of moisture from the air and may enhance corrosion, the amount of any such salts should be made as small as possible and preferably excess soluble salts are removed, if possible, by rinsing with water. The pH of the compositions used in the invention is usually from 2 to 6 and preferably from 3.5 to 5.5.

The aqueous solution or suspension of this invention may include a water-soluble, wateremulsifiable or water-dispersible organic polymeric resin to improve the adhesion of the coating, to provide flexibility thereto and to improve its corrosion resistance. Suitable resins are disclosed in our copending Application No. 52050/76 (Serial No. 1,556,869).

The coating composition may be applied to the metal surface by any suitable application method, e.g. by immersion, roll coating, flooding, spraying, electrostatic coating or brush- ing.

The following Examples 1 and 2 illustrate the invention.

Example 1.

A hot galvanized steel plate (100 X 300 X 0.3 mm in size), which had been treated with chromic acid, was polished three times by the wet Scotch process to remove the chromic acid which adhered to the surface.

A coating composition was prepared by dispersing and dissolving 30 g of Chinese gallotannin (available from Fuji Chemical Industries Co. Ltd., Japan, under the trade name Tannic Acid AL) and 30 g of powderv silica (available from Nihon Aerosil Co. Ltd., Japan, under the trade name Aerosil 380) in about 500 ml of deionized water, and by making the dispersion up to 1000 g with pure water. The pH- of the resultant suspension was about 3.5.

The cleaned galvanized steel plate was coated with the suspension by the bar coating process (No. 6 bar) and dried in a hot air circulating oven at 120 C for one minute to form a protective coating. The treated plate was then subjected to the salt spray test accord inr to TIS-Z-2371, and white stains developed only after about 200 to 240 hours.

Comparative Example 1.

A hot galvanized steel plate similar to that used in Example 1 was polished three times by the wet Scotch process and the cleaned galvanized steel plate, now free from chromic acid, was employed as the test plate.

Comparative Example 2.

A cleaned galvanized steel plate, treated as in Comparative Example 1, was coated with an aqueous solution containing 1% bv weight chromic acid in a manner similar to Example 1, and dried.

Comparative Example 3.

A cleaned galvanized steel plate. treated as in Comparative Example 1. was coated with an aaueous solution prepared bv dissolving 30 g of Chinese gallotannin in about 500 ml of deionized water and made un to 1000 g bv the addition of deionized water (in a manner similar to Example 1), and dried.

Comparative Example 4.

A cleaned galvanized steel plate, treated as in Comparative Example 1, was coated with an aqueous suspension prepared by dispersing 30 g of the powdery silica as used in Example 1 in about 500 ml of deionized water, using a high speed agitator, and dried.

The test plates prepared in Example 1 and in each of Comparative Examples 1, 2, 3 and 4 were coated with a thermosetting alkyd paint of the type used for coating the back surfaces of coloured galvanized steel plates by the bar coating process (No. 12 bar), and dried and baked in a hot air circulating oven at 2800 C for 50 seconds to form coatings having a thickness of 6 microns. The coated plates were cut linearly through the painted layer to the base metal using a razor blade and were then subjected to the salt spray test according to JIS-Z-2371, for 240 hours. Cello- phane (registered Trade Mark) tapes (available from the Sumitomo Three M. Co., Japan, under the trade name Scotch Tape) were stuck on the coated plates and the tapes were peeled rapidly.

On the test plate prepared in Example 1, the extent of corrosion which developed from the edge of the cut was less than 3 mm wide on both sides, and no blisters were observed on the smooth paint film surface. When the film surface was scratched with a copper coin, it was difficult to produce scratches which reached to the galvanized layer.

When the test plates prepared in Comparative Examples 1, 2, 3 and 4 were painted and subjected to the above tests, the results were inferior to those recorded for the painted plate of Example 1. These results are shown in the following Table.

Corrosion resistance of coated Corrosion resistance test plate in of uncoated salt spray tests Scratch teste plate in I resistance salt spray test Width to copper Example (hours) (mm) Blisters coin 1 1 200 - 240 3 None Good Comparative 1 2 10 Overall Inferior Comparative 2 120 - 240 3 Slight Inferior Comparative 3 48 - 72 10 Overall Inferior Comparative 4 24 10 Overall Good Example 2.

Example 1 was repeated but using 100 g of alumina sol (available from Nissan Chemi cals Co. Ltd., Japan, under the trade name of Alumina Sol 100) instead of the powdery silica component. When the corrosion test of Example 1 was carried out, white stains de veloped only after 200 to 240 hours.

WHAT WE CLAIM IS: 1. A process for forming a protective coating on the surface of a zinc or zinc alloy workpiece comprising contacting the surface with an aqueous solution or suspension containing from 0.01 to 20% by weight of a water-dispersible component selected from silica, silica sol and alumina sol and from 0.1 to 20% by weight of tannin or tannic acid, in which the weight ratio of the water-dispersible component to the tannin or tannic acid is from 100:1- to 1:100.

2. A process according to claim 1 in which the aqueous solution or dispersion addiuonally includes lithium oxide.

3. A process according to claim 1 or claim 2 in which the aqueous solution or suspension additionally contains a water-soluble, wateremulsifiable or water-dispersible organic polymeric resin.

4. A process according to any preceding claim in which the aqueous solution or dispersion contains 1 to 5% by weight of the water-dispersible component.

5. A process according to any preceding claim in which the aqueous solution or suspension contains from 1 to 5% by weight of tannin or tannic acid.

6. A process according to any preceding claim in which the pH of the aqueous solution or suspension is from 2 to 6.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. the addition of deionized water (in a manner similar to Example 1), and dried. Comparative Example 4. A cleaned galvanized steel plate, treated as in Comparative Example 1, was coated with an aqueous suspension prepared by dispersing 30 g of the powdery silica as used in Example

1 in about 500 ml of deionized water, using a high speed agitator, and dried.

The test plates prepared in Example 1 and in each of Comparative Examples 1, 2, 3 and 4 were coated with a thermosetting alkyd paint of the type used for coating the back surfaces of coloured galvanized steel plates by the bar coating process (No. 12 bar), and dried and baked in a hot air circulating oven at 2800 C for 50 seconds to form coatings having a thickness of 6 microns. The coated plates were cut linearly through the painted layer to the base metal using a razor blade and were then subjected to the salt spray test according to JIS-Z-2371, for 240 hours. Cello- phane (registered Trade Mark) tapes (available from the Sumitomo Three M. Co., Japan, under the trade name Scotch Tape) were stuck on the coated plates and the tapes were peeled rapidly.

On the test plate prepared in Example 1, the extent of corrosion which developed from the edge of the cut was less than 3 mm wide on both sides, and no blisters were observed on the smooth paint film surface. When the film surface was scratched with a copper coin, it was difficult to produce scratches which reached to the galvanized layer.

When the test plates prepared in Comparative Examples 1, 2, 3 and 4 were painted and subjected to the above tests, the results were inferior to those recorded for the painted plate of Example 1. These results are shown in the following Table.

Corrosion resistance of coated Corrosion resistance test plate in of uncoated salt spray tests Scratch teste plate in I resistance salt spray test Width to copper Example (hours) (mm) Blisters coin 1 1 200 - 240 3 None Good Comparative 1 2 10 Overall Inferior Comparative 2 120 - 240 3 Slight Inferior Comparative 3 48 - 72 10 Overall Inferior Comparative 4 24 10 Overall Good Example 2.

Example 1 was repeated but using 100 g of alumina sol (available from Nissan Chemi cals Co. Ltd., Japan, under the trade name of Alumina Sol 100) instead of the powdery silica component. When the corrosion test of Example 1 was carried out, white stains de veloped only after 200 to 240 hours.

WHAT WE CLAIM IS: 1. A process for forming a protective coating on the surface of a zinc or zinc alloy workpiece comprising contacting the surface with an aqueous solution or suspension containing from 0.01 to 20% by weight of a water-dispersible component selected from silica, silica sol and alumina sol and from 0.1 to 20% by weight of tannin or tannic acid, in which the weight ratio of the water-dispersible component to the tannin or tannic acid is from 100:1- to 1:100.

2. A process according to claim 1 in which the aqueous solution or dispersion addiuonally includes lithium oxide.

3. A process according to claim 1 or claim 2 in which the aqueous solution or suspension additionally contains a water-soluble, wateremulsifiable or water-dispersible organic polymeric resin.

4. A process according to any preceding claim in which the aqueous solution or dispersion contains 1 to 5% by weight of the water-dispersible component.

5. A process according to any preceding claim in which the aqueous solution or suspension contains from 1 to 5% by weight of tannin or tannic acid.

6. A process according to any preceding claim in which the pH of the aqueous solution or suspension is from 2 to 6.

7. A process according to claim 1 substanti

ally as herein described with reference to Example 1 or Example 2.

8. A zinc or zinc alloy workpiece when coated bv a process according to any preceding claim.