Method and flux for hot galvanization

Abstract

The invention relates to the technical field of galvanization of iron-based or iron-containing components, especially steel-based or steel-containing components (steel components), preferably for the automotive or motor vehicle industry, but also for other industrial fields of application (for example for the construction industry, the field of general mechanical engineering, the electrical engineering industry etc.), by means of hot galvanization (hot clip galvanization). More particularly, the invention relates to a method of hot galvanization (hot dip galvanization) and to a plant for this purpose, and additionally to a flux and flux bath usable in this connection and to the respective uses thereof, and additionally also to the products obtainable by the method and/or in the plant (i.e. hot galvanized iron or steel components).

Claims

1. A method for hot-dip galvanization of an iron or steel component, wherein the method comprises the following method steps in the order listed below: (a) degreasing treatment of the iron or steel component; then (b) optionally, rinsing of the iron or steel component which has been previously degreased in method step (a); then (c) pickling treatment of the iron or steel component which has been previously degreased in method step (a) and optionally rinsed in method step (b); then (d) optionally, rinsing of the iron or steel component which has been previously pickled in method step (c); then (e) flux treatment of the iron or steel component which has been previously pickled in method step (c) and optionally rinsed in method step (d), by means of a flux composition comprised in a flux bath, wherein the flux bath comprises a liquid phase comprising an alcohol-water mixture, with the liquid phase of the flux bath comprising the flux composition, wherein the alcohol of the alcohol-water mixture of the flux bath is a water-miscible or a water-soluble alcohol and is selected from the group of C.sub.1-C.sub.6 alcohols and mixtures thereof, and wherein the flux composition comprises as ingredients: (i) zinc chloride in amounts in the range of from 70 to 82 wt. %, (ii) ammonium chloride in amounts in the range of from 12 to 20 wt. %, (iii) at least one of an alkali metal or alkaline earth metal salt in amounts in the range of from 4 to 10 wt. %, and (iv) at least one of an aluminum salt or silver salt in amounts sufficient to precipitate low levels of organic impurities soluble in the alcohol-water mixture, wherein the amounts sufficient range from 5.Math.10.sup.−5 to 5.Math.10.sup.−3 wt. %, wherein all amounts are based on the composition and are to be selected as to result in a total of 100 wt. %, and wherein the flux composition is free from any further transition metals and heavy metals; then (f) optionally, drying treatment of the iron or steel component which has been previously subjected to the flux treatment in method step (e); then (g) hot-dip galvanization of the iron or steel component which has been previously subjected to the flux treatment in method step (e) and optionally dried in method step (f), in a galvanizing bath comprising an aluminum-containing zinc melt, wherein the aluminum-containing zinc melt comprises an amount of aluminum in the range of from 0.1 to 8 wt. %.

2. The method as claimed in claim 1, wherein the flux bath is adjusted to an acidic pH value.

3. The method as claimed in claim 1, wherein the flux bath is adjusted to a pH value range from 0 to 6.9.

4. The method as claimed in claim 1, wherein the flux bath is adjusted to a pH value range from 1.5 to 5.

5. The method as claimed in claim 1, wherein the flux bath comprises the alcohol-water mixture in a weight-based alcohol-water ratio in the range of from 0.5:9.5 to 99:1, based on the alcohol-water mixture.

6. The method as claimed in claim 1, wherein the flux bath comprises the alcohol-water mixture in a weight-based alcohol-water ratio in the range of from 10:90 to 30:70, based on the alcohol-water mixture.

7. The method as claimed in claim 1, wherein the alcohol of the alcohol-water mixture of the flux bath is selected from the group of C.sub.1-C.sub.4 alcohols and mixtures thereof.

8. The method as claimed in claim 1, wherein the aluminum salt is aluminum chloride AlCl.sub.3.

9. The method as claimed in claim 1, wherein the silver salt is silver chloride AgCl.

10. The method as claimed in claim 1, wherein the flux composition comprises, as alkali metal or alkaline earth metal salt of component (iii), an alkali metal or alkaline earth metal chloride.

11. The method as claimed in claim 1, wherein the flux composition comprises, as alkali metal or alkaline earth metal salt of component (iii), at least two alkali metal or alkaline earth metal salts different from one another.

12. A method for hot-dip galvanization of an iron or steel component, wherein the method comprises the following method steps in the order listed below: (a) degreasing treatment of the iron or steel component; then (b) optionally, rinsing of the iron or steel component which has been previously degreased in method step (a); then (c) pickling treatment of the iron or steel component which has been previously degreased in method step (a) and optionally rinsed in method step (b); then (d) optionally, rinsing of the iron or steel component which has been previously pickled in method step (c); then (e) flux treatment of the iron or steel component which has been previously pickled in method step (c) and optionally rinsed in method step (d), by means of a flux composition comprised in a flux bath, wherein the flux bath comprises a liquid phase comprising an alcohol-water mixture, with the liquid phase of the flux bath comprising the flux composition, wherein the alcohol of the alcohol-water mixture of the flux bath is a water-miscible or a water-soluble alcohol and is selected from the group of C.sub.1-C.sub.6 alcohols and mixtures thereof, and wherein the flux composition consists of the following ingredients: (i) zinc chloride in amounts in the range of from 70 to 82 wt. %, (ii) ammonium chloride in amounts in the range of from 12 to 20 wt. %, iii) sodium chloride and potassium chloride in amounts in the range of from 4 to 10 wt. %, and (iv) at least one of an aluminum salt or silver salt in amounts in the range of from 5.Math.10.sup.−5 to 5.Math.10.sup.−3 wt. %, wherein all amounts are based on the composition and are to be selected such as to result in a total of 100 wt. %, and wherein the flux composition is free from any further transition metals and heavy metals; then (f) optionally, drying treatment of the iron or steel component which has been previously subjected to the flux treatment in method step (e); then (g) hot-dip galvanization of the iron or steel component which has been previously subjected to the flux treatment in method step (e) and optionally dried in method step (f), in a galvanizing bath comprising an aluminum-containing zinc melt, wherein the aluminum-containing zinc melt comprises an amount of aluminum in the range of from 0.1 to 8 wt. %.

Description

(1) Further features, advantages and possible applications of the present invention are apparent from the description hereinafter of exemplary embodiments on the basis of drawings, and from the drawings themselves. Here, all features described and/or depicted, on their own or in any desired combination, constitute the subject matter of the present invention, irrespective of their subsumption in the claims and their dependency references.

(2) In these drawings:

(3) FIG. shows a schematic method sequence of the individual stages or method steps of the method of the invention according to one particular embodiment of the present invention;

(4) FIG. 2 shows a schematic representation of a system of the invention according to one particular embodiment of the present invention.

(5) In the flow diagram of the method shown in FIG. 1, the successive method stages or method steps a) to i) are shown schematically, with method steps b), d), f), h), and i), especially method steps h) and i), being optional.

(6) In accordance with the diagram shown in FIG. 1, the method sequence is as follows, the method of the invention successively comprising the below-specified steps in this order: degreasing (step a)), rinsing (step b), optional), pickling (step c)), rinsing (step d), optional), flux bath treatment (step e)), drying (step f), optional), hot dip galvanizing (step g)), cooling (step h), optional), and afterworking or aftertreating (step i), optional).

(7) For further details concerning the method sequence according to the invention, reference may be made to the general observations above concerning the method of the invention.

(8) FIG. 2 shows, schematically, the system according to the present invention, with the individual facilities (A) to (I), with facilities (B), (D), (F), (H) and (I), more particularly facilities (H) and (I), being optional.

(9) According to the diagram of the system of the invention shown in FIG. 2, this system comprises, in the order listed below, the following facilities: degreasing facility (A), optionally rinsing facility (B), pickling facility (C), optionally rinsing facility (D), flux treatment facility (E), optionally drying facility (F), hot dip galvanizing facility (G), optionally cooling facility (H), and optionally afterworking or aftertreating facility (I).

(10) For further details relating to the system of the invention, reference may be to the general observations above concerning the system according to the present invention.

(11) Further configurations, modifications and variations of the present invention are readily recognizable and realizable for the skilled person reading the description, without that person departing from the scope of the present invention.

(12) The present invention is illustrated with the exemplary embodiments below, which, however, are in no way intended to limit the present invention, but which instead merely illustrate the exemplary and nonlimiting modes of implementation and configuration.

EXEMPLARY EMBODIMENTS

(13) General Protocol for Implementation (Inventive)

(14) Various hot dip galvanizing cycles are carried out with specimen sheets of type S235 (2 mm thickness, 100 mm×100 mm width) according to the method sequence of the invention as per FIG. 1 and with the system of the invention as per FIG. 2. The flux composition and the zinc bath alloys are varied in each case according to the details below.

(15) The hot dip galvanizing process carried out in each case encompasses the following method steps in the order listed below (the system employed in accordance with the invention is designed accordingly); (a) alkaline degreasing treatment in a degreasing bath (15 minutes, 70° C., degreasing bath composition as per example 1 of EP 1 352 100 B1), (b) twofold rinsing in two successive rinsing baths with water, (c) acidic pickling treatment (40 minutes, 30° C., pickling bath composition as per example 1 of EP 1 352 100 B1), (d) twofold rinsing in two successive rinsing baths with water, (e) flux treatment in flux bath according to specifications below (3 minutes, 60° C., dip treatment), (f) drying treatment (hot air stream 260° C., 30 seconds), (g) hot dip galvanizing with an aluminum-containing or aluminum-alloyed zinc melt (“Zn/Al melt”) in a galvanizing bath according to specifications below (50 seconds' dip treatment of the preheated and fluxed sheet in the galvanizing bath, 450° C.), (i) air cooling of the hot dip galvanized sheet removed from the galvanizing bath.

Example Series 1 (Inventive)

(16) Various specimen sheets are subjected to hot dip galvanization as described above, including corresponding pretreatment steps as described above. The specification of the flux composition used and of the flux bath used is as follows:

(17) Flux Composition:

(18) 78.995 wt % ZnCl.sub.2, 13 wt % NH.sub.4Cl, 6 wt % NaCl, 2 wt % KCl, 0.005 wt % (50 ppm) AlCl.sub.3

(19) Flux Bath:

(20) Flux amount/concentration (total salt content): 550 g/l

(21) Ammonia solution (5%): 10 ml per liter of flux bath to adjust (raise) the pH

(22) pH: 3.5 (without ammonia solution: 3.2)

(23) wetting agent (nonionic surfactant): 0.3%

(24) Variation of the Alcohol Fraction in the Flux Bath

(25) a) 0% propanol (100% water)

(26) b) 5% propanol (40 g propanol, balance to 1000 ml made up with water)

(27) c) 20% propanol (160 g propanol, balance to 1000 ml made up with water)

(28) d) 71.8% propanol (574.4 g propanol, balance to 1000 ml made up with water)

(29) e) 100% propanol

(30) Galvanizing Bath

(31) 100 ppm aluminum, 0.05 wt % bismuth, 0.3 wt % tin, 0.04 wt % nickel, balance zinc (i.e., ad 100 wt %)

(32) Results ad a) By being immersed into the flux solution, the sheet is fully covered with salts. After the drying step, the surface of the component is still completely damp. A very largely homogeneous zinc layer is formed, but with minimal flaws. ad b) By being immersed into the flux solution, the sheet is fully covered with salts. After the drying step, the surface of the component has already slightly dried. For monitoring, the sheets are weighed after pickling and after drying. In comparison to variant a), it is found that the film of flux weight 2.5% less, attributable to a lower residual moisture content as a result of more rapid drying. After galvanization, a homogeneous zinc layer is formed, without any flaws. ad c) By being immersed into the flux solution, the sheet is fully covered with salts. After the drying step, the surface of the component is very largely dry. In a comparison of the weights of the film of the flux with variant a), an 11.5% weight reduction is found. After galvanization, a homogeneous zinc layer is formed, without any flaws. ad d) By being immersed into the flux solution, the sheet is fully covered with salts. After the drying step, the surface of the component is completely dry. In a comparison of the weights of the film of the flux with variant a), a 15% reduction is found. After galvanization, a homogeneous zinc layer is formed, without any flaws. ad e) The flux salts form a sediment which cannot be dissolved. Accordingly, when the sheet is immersed into the flux, there is no efficient wetting of the steel surface with flux salts. On subsequent galvanizing, there is no reaction between zinc alloy and steel; in other words, galvanizability is not efficient.

(33) General Findings

(34) Under the same drying conditions (i.e., equal drying times and drying temperatures), the use of alcohol in the flux bath, even with small quantitative fractions and also up to high qualitative fractions, results in more rapid drying of the film of flux and to a better quality of galvanization. The result of this is that better drying leads to a better quality of galvanization.

(35) In corrosion tests as well (salt spray test or salt spray mist test according to DIN EN ISO 9227:2012), the hot dip galvanized sheets pretreated with the alcohol-containing flux exhibit significantly longer service lives (a service life improvement of up to 40%) relative to hot dip galvanized sheets pretreated with the otherwise identical flux (but without any alcohol fraction, i.e., purely aqueous),

Example Series 2 to 5 (Inventive)

(36) Example series 1 is repeated, but with a different composition of the galvanizing bath.

(37) Galvanizing Bath for Example Series 2

(38) 500 ppm aluminum, 0.05 wt % bismuth, 0.3 wt % tin, 0.04 wt % nickel, balance zinc (i.e., ad 100 wt %)

(39) Galvanizing Bath for Example Series 3

(40) 1000 ppm aluminum, 50 ppm silicon, balance zinc (i.e., ad 100 wt %)

(41) Galvanizing Bath for Example Series 4

(42) 5.42 wt % aluminum, balance zinc (i.e., ad 100 wt %)

(43) Galvanizing Bath for Example Series 5

(44) Aluminum 4.51 wt %, balance zinc (i.e., ad 100 wt %)

(45) Results

(46) Results analogous to those for example series 1 are obtained, and specifically in the case of example series 4 and 5, the resulting surfaces also show significant optical improvement, in other words being particularly glossy.

Example Series 6 to 10 (Inventive)

(47) Example series 1 to 5 are repeated, but with a differing flux composition (use of 0.005 wt % or 50 ppm of AgCl instead of AlCl.sub.3).

(48) Results

(49) Results analogous to those of example series 1 to 5 are obtained.

Example Series 11 to 15 (Inventive)

(50) Example series 1 to 5 are repeated, but with a differing flux composition (use of a combination of 0.0025 wt % or 25 ppm of AgCl and 0.0025 wt % or 25 ppm of AlCl.sub.3 instead of AlCl.sub.3 alone).

(51) Results

(52) Results analogous to those of example series 1 to 5 are obtained.

Example Series 16 to 30 (Comparative)

(53) Example series 1 to 15 are repeated, but with a differing flux composition (complete omission of AlCl.sub.3 and AgCl).

(54) Results

(55) In the case of the alcohol contents a) to d), in each case after galvanization, the results are highly inhomogeneous zinc layers with a significant number of flaws and distinctly visible defect structures.

(56) In the case of the alcohol contents of e), here again there is no galvanizability at all, because the flux salts form an insoluble sediment.

(57) General Recipes for Fluxes (Inventive)

(58) Given below is general recipe information for typical flux compositions and flux baths of the invention, with optimization depending on the composition of the zinc/aluminum melt.

(59) Flux Composition

(60) ZnCl.sub.2 56 to 85% for Al=4.2 to 6.2%: typically 77 to 82% for Al up to 1000 ppm: typically 56 to 62%

(61) NH.sub.4Cl 10 to 44% for Al=4.2 to 6.2%: typically 10 to 15% for Al up to 1000 ppm: typically 38 to 44%

(62) NaCl >0 to 6% for Al=4.2 to 6.2%: typically 5 to 7% for Al up to 1000 ppm: typically >0 to 1%

(63) KCl >0 to 6% for Al=4.2 to 6.2%: typically 1 to 3% for Al up to 1000 ppm: typically >0 to 0.5%

(64) AgCl/AlCl.sub.3 0.5 to 500 ppm

(65) All percentages (wt %) above are based on the salt solids content (dry weight).

(66) Flux Bath

(67) Salt content (flux composition) in total 200 to 700 g/l, typically 450 to 550 g/l

(68) pH in the range from 2.5 to 5 for Al=4.2 to 6.2%: typically 2.5 to 3.5 for Al up to 1000 ppm: typically 4 to 5%

(69) sufficient amount of inorganic acid and ammonia solution to adjust the required pH (fine adjustment with ammonia solution)

(70) Flux temperature in the range from 15 to 80° C. for Al=4.2 to 6.2%: typically 50 to 70° C. for Al up to 1000 ppm: typically 35 to 60° C.

(71) Wetting agent content 0.2 to 5%

(72) Solution with a propanol and/or ethanol fraction of 0.2 to 72% for Al=4.2 to 6.2%: typically 5 to 20% for Al up to 1000 ppm: typically 5 to 20%