DE2654580A1 - METHOD OF APPLYING PROTECTIVE COATING TO ZINC OR ZINC ALLOYS - Google Patents

Description

METALLGESELLSCHAFT Frankfurt / M., November 29th, 1976

Aktiengesellschaft λ DrBr / LWü

Prov. No. 7992 M.

Process for applying a protective coating to zinc or zinc alloys

Surfaces of zinc as well as of metallic materials that Coated with zinc, for example hot-dip galvanized or electrolytically galvanized steel, suffer under humid conditions a formation of white spots. The presence of such spots, also known as "white rust", on the Surface deteriorates the appearance of the objects, deteriorates and lowers corrosion resistance noticeable adhesion of paints applied to surfaces. Effective methods of preventing the Formation of white spots are different chromating processes, in which mainly solutions of chromic acid, Chromic acid or its salts are used ΐ / earth. The chromating process are not expensive and provide good corrosion resistance, but have the disadvantage that the compounds of hexavalent chromium have toxic effects can exercise and are therefore subject to increasingly stricter regulations for environmental reasons. Procedural control problems and wastewater treatment have resulted from this. There is therefore a desire to have a cover available, which provides a comparable protection achieved with chromating processes, but without applying it Application of chromium VI compounds is being worked on.

The method according to the invention for applying a protective coating on zinc or zinc alloys is characterized by

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ORIGINAL

that the surfaces are treated with an aqueous solution or suspension containing 0.1 to 20% by weight of tannin or tannic acid and 0.01 to 20% by weight of silica and / or silica sol and / or aluminum oxide sol, calculated as oxide _s.

As tannin or tannic acid can hydrolyzable or condensed tannins or partially ^ degradation products thereof are used, such as Depsidtannin, tannic acid, Chinese tannic, Turkish tannic acid, Hamamelitanninsäure, tannic Acer ginnala, chebulinic, sumac tannin, Chinese tannic acid, ellagitannins, catechol, Catechintanninsäure , Quebracho tannin. If the concentration is less than 0.1% by weight, the effectiveness of the tannin becomes insufficient. A concentration of more than 20% by weight is disadvantageous for reasons of machinability and economy.

Commercially available solid SiOp, such as those available under the trademarks "Aerosil" from Nihon Aerosil Co., Ltd., or "Siloid" from Fuji Devison Co., Ltd., can be used as the silica. A solution obtained by removing sodium ions from a sodium silicate solution by means of an ion exchanger can be used as the silica sol or gel, or commercially available silica sol containing SiO ₂ in an amount of about 20 to 30%, such as that under the trademark " Snowtex "product available from Nissan Chemicals Co., Ltd.. In any case, the SiO ₂ should be homogeneously dispersed in the colloidal state in the aqueous medium. It is also possible to use an aqueous solution or dispersion of silica or silica sol to which lithium oxide is added in a known manner, such as, for example, the product available from Nissan Chemicals Co., Ltd. under the trademark "Lithium Silicate". _{Commercially available Al 2} O - * - Sol, dispersed in water, can be used as the aluminum oxide sol. For example, the by Nissan Chemicals Co., Ltd., commercially available product which contains Al ₂ O, in an amount of over 10% and is stabilized at a pH value of 2 · ^Ν, 5 to 6 with an inorganic or organic acid, convenient to use.

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The composition of the aqueous solution or suspension depends to a certain extent on the method used to form the Protective coating, e.g. by dipping, roller application, flooding, spraying, electrostatic application, brushing on. The above However, concentrations must be observed. It has proven to be useful if the weight ratio of tannin or Tannic acid to silica and / or silica sol and / or aluminum oxide sol is in the range from 100: 1 to 1: 100. If tannin or Tannic acid is present in a higher amount, there is a risk of blistering after painting. When the oxides are in a higher amounts are present, there is a risk that the coating formed will become brittle.

Because the stable pH ranges depend on the types of tannin or tannic acid used and the silica or oxide brine used Depending on, it may be useful to adjust the pH of the aqueous solution or suspension. For this purpose, an inorganic or organic acid or its salts are used, such as phosphoric acid, sulfuric acid, hydrofluoric acid, oxalic acid, citric acid, Maleic acid, phthalic acid, acetic acid. It can also use an alkaline compound, such as alkali hydroxide or amine, for pH adjustment to be used. However, since a residual content of water-soluble salts in the coating adversely affects the Corrosion resistance, such salts should be limited to a minimum amount.

The aqueous solution or suspension can also be a water-soluble, emulsifiable or dispersible organic polymeric resin can be added to improve the adhesive strength of the coating on the metal surface or the coating to make it smoother.

The protective coatings applied to surfaces of zinc or its alloys in accordance with the procedure according to the invention result excellent corrosion resistance and an excellent base for paints with high adhesive strength.

In the following examples, the advantages of the invention are shown in more detail.

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example 1

Hot-dip galvanized steel sheets (100 × 300 × 0.3 mm) that had been temporarily treated with chromic acid were processed three times by the wet grinding brush method in order to remove adhering chromic acid from the surface. By dispersing and dissolving 30 g of Chinese gallotannin (commercial product of Fuji Chemical Industries Co. Ltd. "Tannic Acid AL") and 30 g of silica powder (commercial product of Nihon Aerosil Co. Ltd. "Aerosil 380") in about 500 ml of deionized Water and making up to 1000 g with pure water, a treatment suspension was prepared. The pH of the suspension was about 3.5. The purified zinc-plated steel sheets were coated with the suspension by the Naßfilmziehverfahren (bar coating; bar # 6.) And dried to form the protective coating 1 minute at 120 ⁰ C in an oven with circulating hot air.

Some of the panels were then subjected to the standard salt spray test (JIS-Z - 2371) subject. It took about 200 to 240 hours for white spots to appear. The coating was therefore excellent Corrosion resistance.

In another part of the sheet was thermoset alkyd varnish (a varnish for coating the rear surface of colored galvanized steel sheets) applied by the Naßfilmziehverfahren (coater No. 12), getocknet and baked 50 sec. At 280 ⁰ C in an oven with circulating hot air, The coatings were 6 microns thick. The lacquered panels were then cut linearly through the lacquer layer down to the metal base using a razor blade edge and then subjected to the standard salt spray test (JIS-Z-2371) for 240 hours. The coated surfaces were then taped with cellophane tape and the tape quickly peeled off. The extent of the corrosion that occurred was less than 3 mm on either side of the cutting line. No bubbles were found on the smooth surface of the paint film. . %

When the paint surface was scratched with a copper coin, scratches reaching the zinc layer were difficult to achieve. 7Q9826 / 1076 " ⁵ "

For comparison, further identical, hot-dip galvanized steel sheets, as indicated above, were threefold using the wet grinding brush method cleaned and used as test panels:

In the comparison series a) the purified, chromium-r acid-free galvanized steel sheets used as such;

In the comparative series b) the cleaned sheets were treated with an aqueous solution containing 1% by weight of chromic acid in the corresponding ¥ ice, as stated above, coated and dried;

In the comparative series c) the cleaned sheets with a dissolving of 30 g of Chinese gallotannin in about 500 ml deionized water and topping up with deionized water to 1000 g of the prepared solution in a corresponding manner, as indicated above, coated and dried;

in the comparative series d) the cleaned sheets were treated with a by dispersing 30 g of silica powder ("Aerosil 380") in about 500 ml of deionized water by means of a high speed stirrer prepared aqueous suspension, as indicated above, coated and dried.

Some of the sheets of the comparative series a) to d) were subjected to the salt spray test without painting, like the sheets treated according to the invention subject. Another part of the sheets was coated with the thermosetting alkyd varnish as indicated above and then tested.

The results obtained from the tests are shown below Table compiled:

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'' Corrosion resistance Corrosion test, scratch resistance durability of the un- lacquered sheets painted in the coin test sheets 240 hours (copper coin) in the salt spray test salt spray test

(Hours) Corrosive bladder formation

(mm)

According to the invention

treated Blecte 200 - 240

no

Well

Comparative series a)

more than 10 bubbles all over the surface.

small amount

Comparative series b) 120 - 240

low blistering

small amount

Comparative series c) 48-72

more than blow 10 on the entire Surface more than blow 10 on the entire Surface

small amount

Comparative series d)

24

Well

From the test results it can be seen that the inventive Treatment leads to significant benefits.

Example 2

Hot-dip galvanized steel sheets, which had been cleaned in a corresponding manner as in Example 1, were treated with a suspension, by dissolving 30 g of Chinese gallotannin in about 500 ml of deionized water, then dispersing 100 g of aluminum oxide sol (Commercial product of Nissan Chemicals Co. Ltd. "Alumina Sol 100")

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and made up to 1000 g with deionized water was coated and dried in the same manner as in Example 1. When carrying out the specified in Example 1 Corrosion tests took 200 to 240 hours for white spots to appear.

- 8 - Claims

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Claims (2)

2654530 claims 1. A method for applying a protective coating to zinc or zinc alloys, characterized in that the surfaces with an aqueous solution or suspension containing 0.1 to 20% by weight of tannic acid or tannic acid and 0.01 to 20% by weight of silica and / or Silica sol and / or alumina oil _? calculated as an oxide that contains. 2. The method according to claim 1, characterized in that 'in the suspension the weight ratio of tannin or Tannic acid to silica and / or silica sol and / or aluminum oxide sol is in the range from 100: 1 to 1: 100. 709-826 / 1076