DE1951776A1 - Zinc and zinc iron alloy treatment with - dichromate nitrate and tripolyphosphateion containing baths - Google Patents

Abstract

Zinc and zinc iron alloy treatment with dichromate nitrate and tripolyphosphate ion-containing baths. Bright, corrosion-resistant surfaces are obtained by heating Zn or Zn-Fe alloy surfaces, particularly applied by hot-galvanising, with an aqs. solution containing Cr2O72-, NO3-, P3O105- or organic acid ions and surface active cpds. at pH 0.3-1.5. Pref. bath contains 36-290 g/l Cr2O72-, 4.1-41 g/l NO3, 0.7-21 g/l P3O105- and 1.3-26.6 g/l K+ ions. CrO3 is pref. used as source for Cr2O72- and K2Cr2O7 as source for K+ and Cr2O72-. Pref. acids are formic, oxalic, tartaric, citric, propionic or acetic acid and/or their Na or K salts. Articles are pref. dipped in bath for 10-180 (30-60) sec. at 10-50 (30) degrees C.

Description

Description of the patent application pertaining to: Process for chemical Treatment of surfaces made of zinc or zinc-iron alloys Zinc or zinc-iron alloys become bichromate ions with an aqueous solution1 (Cr2O7--)) nitrate ions (NO3), triphophate ions (PDO1o), organic acid ions and contains surfactants1 treated at pH 0.3-1.5, thereby reducing the surfaces become bright and corrosion-resistant.

The invention relates to a method for the chemical treatment of Surfaces made of zinc or zinc-iron alloys, especially those made through Hot-dip galvanizing has been made to produce bare and corrosion-resistant surfaces leads.

It has long been known that many uncoated, are abraded or scraped off areas.

These defects in the zinc coating reduce the corrosion resistance or the shine of the zinc surface.

It has previously been suggested to address these defects by treating the zinc surfaces with a concentrated aqueous chromate solution containing a small amount of sulfuric acid, Contains nitric acid, hydrochloric acid or hydrofluoric acid. At this Usually only zinc surfaces can be made bare and corrosion-resistant will. The zinc-sisen alloy surfaces that lie under the zinc surfaces and mechanical cracks or scratches on the zinc surface or defects in When the zinc coating comes to the surface, it becomes dark and a great difference in gloss and appearance between zinc and zinc-iron alloy surfaces.

Attempts have also been made to make zinc surfaces on which there are surfaces made of zinc-iron alloys, with an aqueous solution containing phosphate ions corresponding to 0.5-2% by weight of phosphoric acid and either chromate ions corresponding to 0.5 - 3 GefyO chromic acid or nitrite ions corresponding to 0.1 - 0.4 Geno sodium nitrite contains, to be treated at pH 1.3 - 3.8. However, this treatment cannot be complete deliver satisfactory results, this solution shows, for example, only a low gloss finish Effect and not only the zinc-iron alloy surfaces but also the zinc surfaces, turn pale yellow and easily corrode.

if you place them continuously in a 5-digit sodium chloride for 10 days Solution pops up.

The present invention aims to address the problems mentioned above and disadvantages are overcome by a process by which both the zinc and the zinc-iron surfaces are made corrosion-resistant at the same time and obtained by the smooth and shiny surfaces and the. in hot-dip galvanizing resulting defective surfaces, uncovered areas, abrasions, abrasion points, Contains burn marks and rough spots, uniform be made, without spots and streaks. In addition, the zinc or zinc-iron alloy surfaces get good adhesiveness to paints, andthe resulting gloss of zinc or Zinc-iron alloy surfaces are very durable.

According to the invention, zinc or zinc-iron alloy surfaces are used with an aqueous solution containing 36 - 290 g / l bichromate ions (Cr207), accordingly approx. 49 - 395 g / l potassium dichromate or 33.3 - 268.5 1 auricular trioxide, 4.1 - 41 g / l nitrate ions (NO3), corresponding to 4.2 to 42 g / l nitric acid, 0.7 - 21 g / l tripolyphosphate ions (r3o10-corresponding approximately 1 - 30 g / l sodium tripolyphosphate, organic acid ions and contains surfactants, 10-180 sec at pH 0.3-1.5 and 10 - 50 ° C and advantageously 30 to 60 seconds at around 25 - 35 C and pH 0.5 treated to 1.3. The surfaces are then rinsed with water and by heating or air-dried.

If the pH value of the aqueous solution is lower than 0.3, can there is no good gloss on the zinc-iron alloy surfaces and above from pH 1.5 the zinc and zinc-iron alloy surfaces become thin with a Chromate film coated. After prolonged treatment, if the acidity of the aqueous solution through consumption .-- The constituents will decrease and the solution will be many different Contains impurities or fremae elements from the treated items, can restore bichromate ions, nitrate ions or other necessary components can be added to the aqueous solution to obtain the specified composition and the Maintain pH range. Many different impurities or foreign elements, that have come into the solution from the treated objects have a low level Influence on the effectiveness of the treatment.

cations have no influence on the effect of the treatment, ~ potassium ions However, (K +) in an amount of 1.3 - 26.6 g / l reduce the amount of zinc that iii the aqueous solution dissolves to less than half the amount contained in Absence of potassium ions is resolved.

Dipping or spraying methods can be used for the treatment and all objects that have zinc or zinc-iron alloy surfaces, such as galvanized iron sheet, items made from zinc-based alloys or Zinc objects can be treated successfully.

Normal commercially available products can be used as reagents.

In the preferred embodiment of the invention, the aqueous Solutions prepared by adding 30 - 200 g / l chromium trioxide, 5 - 50 ml / l 60% nitric acid, 1 - 30 g / l sodium tripolyphosphate, 5 - 100 g / l potassium dichromate, a small amount of salts of organic acids and surfactants in water there.

Chromic anhydride or chromium trioxide can be a source of en bichromation can be used, as well as various salts such as potassium dichromate, sodium dichromate and ammonium dichromate. Potassium dichromate is the preferred source for the dichromate and the potassium ions used.

Tripolyphosphate only has the shiny effect when it is in Presence of bichromate and titrations is used, although it almost does not work, when used in the absence of either bichromate or nitrate ions.

Many organic acids or salts, such as formic acid, oxalic acid, tartaric acid, citric acid, propionic acid, acetic acid and their potassium or sodium salts can be used. Gs surfactants can Polyoxyethylene alkyl ether, polyoxyethylene alkylphenol, Polyoxyethylene fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene alkyl amine or i> olyoxyäthylenamid are used and they contribute to the solubility of zinc, and to smooth the zinc or zinc alloy surfaces.

The advantages of this invention are numerous.

Zinc surfaces and zinc-iron surfaces are evenly bright made and show no difference in appearance and the treated zinc or Zinc-iron surfaces have a good resistance to corrosion.

The invention is illustrated in more detail by the following examples.

Example 1 Hot-dip galvanized steel tubes 16 mm in diameter and 300 mm mm length, which had some scratches and imperfections in the coating, became waterier with Show sodium hydroxide solution (NaOH) for 30 sec at 700C degreased and then with a aqueous 3% phosphoric acid solution (H3PO4) for 5 sec at 700. The steel tubes were after this pretreatment in a bath of an aqueous solution containing the had the following composition, immersed at 30 ° C. for 30 seconds.

Composition of the bath: CrO3 70 g K2Cr207 40 g pH 0.7 HN03 (60%) 20 ml Na5P3O10 10 g Na acetate 2 g surfactant 0.05 g water 1000 ml To After the treatment, the steel tubes were rinsed with water and dried. A beautiful metallic luster could be found on both the zinc and zinc-iron alloy surfaces and the defects of the surface became almost invisible.

Example 2 Example 1 was repeated with the exception that a Bath made from an aqueous solution with the following composition for 60 seconds at 30 ° C was used.

Composition of the bath: CrO3 50 g K2Cr207 30 g pH 0.9 HN03 (60in) 15 ml Na5P3010 7g Na acetate 1 g surfactant 0.05g (polyoxyethylene alkylphenol) Water 1000 ml A beautiful metallic luster could be obtained just as in Example 1 will.

Example 3 Example 1 was for defect-free galvanized iron sheet repeated. The corrosion resistance was increased extraordinarily and a nicer one metallic luster could be obtained.

Corrosion test 1 Hot-dip galvanized steel tubes 16 mm in diameter and 300mm Length were at 300C in a 50 ° aqueous solution immersed in sodium chloride. An untreated tube was removed within a day corroded and a tube treated according to Example 1 survived the treatment For 30 days and on the 31st day, 3 points of pitting corrosion were discovered, but the gloss of the surface was almost retained.

A tube made with the usual procedure as described below which was treated was first applied to the part of the zinc-iron alloy surfaces Corroded within 10 days and after 15 days the corrosion spread over the entire surface.

The usual procedure: The object was left at room temperature for 20 sec immersed in a first bath having the following composition.

CrO3 200 g HNO3 (60%) 40 ml HF (5% o) 30 ml H2S ° 4 (98%) 10 ml water 920 ml Then it was immersed in a second bath at room temperature for 20 seconds, which contained 50 g / l CrO3.

Corrosion test 2 A 5C> strength aqueous sodium chloride solution was used sprayed onto hot-dip galvanized steel tubes.

This occurred in a treated according to Example 1 Steel tube after 120 hours of pitting corrosion on and on an untreated steel pipe after 10 hours.

Corrosion Test 3 A galvanized iron sheet was subjected to an accelerated weathering test subject. After 700 hours of continuous exposure, the gloss was the Surface still available.

-Patent claims-

Claims (8)

Claims 1. A method for the chemical treatment of surfaces made of zinc or zinc-iron alloys, thereby indicating that one the surfaces with an aqueous solution the bichromate ions (Cr2O7 :), nitrate ions (NO, -), tripolyphosphate ions (P3010 ''), organic acid ions and surface-active ions Substances contains treated at pH 0.3 - 1.5. 2. The method according to claim 1, characterized in that g e k e n n -z e i c h n e t, that you get 36 - 290 g / l bichromate ions (Cr207), 4.1 - 41 g / l nitrate ions (NO)), 0.7 - 21 g / l tripolyphosphate ion (PDO1o) used. 3. The method according to claim 1 or 2, characterized in that g e -k e n n z e i c Note that 1.3 - 26.6 g / l potassium ions (K +) are used. ~ 4. Procedure according to Claim 1 to 3, characterized in that g e -. No indication that one can use chromium trioxide as a source for the Bichromate ions and potassium dichromate both as a source of the potassium ions as well used for the bichromations. 5. The method according to claim 1 to 4, characterized in that g e -k e n n z e i c h n e t that objects with zinc surfaces can be processed for 10 - 180 sec at 10 to 500C in the aqueous solution is immersed. 6. The method according to claim 1 to 5, characterized in that g e -k e n n z e i c h N e t that the objects with zinc surfaces can be placed at about 300C for 30 - 60 seconds immersed. 7. The method according to claim 1 to 6, characterized in that g e -k e n n æ e i c h n e t that the source of the organic acid ions is formic acid, oxalic acid, Tartaric acid, citric acid, propionic acid, acetic acid and / or their sodium or Potassium salts used. 8. The method according to claim 1 to 7, characterized in that g e -k e n n z e i c h n e t that non-ionic surfactants are used.