MANUFACTURING PROCESS FOR NONCONTINUOUS GALVANIZATION WITH ZINC-ALUMINUM ALLOYS OVER METALLIC MANUFACTURED PRODUCTS

Abstract: A process is disclosed for non-continuous galvanization of a metal object comprising pre-coating the surface with a protective metal by an electroless process with a uniform, continuous, thin coating of the metal, and then dipping the pre-coated metal into a hot Zn-Al alloy bath and controlling the temperature and dipping time such that the pre-coating substantially completely reacts with Al in the bath to form an interface compound layer, thereby causing the ZnAl alloy bath to react with the surface of the metal and form an adherent layer and produce a continuous galvanized coating of good morphology.

Description: MANUFACTURING PROCESS FOR NONCONTINUOUS GALVANIZATION WITH ZINC- ALUMINUM ALLOYS OVER METALLIC MANUFACTURED PRODUCTS SPECIFICATION BACKGROUND OF INVENTION Field of Invention The present invention is directed to a pre-coating for use in galvanizing procedures with zinc-aluminum alloys for metallic manufactured products. More precisely, it refers to the noncontinuous coating of metallic manufactured products of any size or shape in which electroless pre-coating with a metal to protect the part precedes dipping into a molten bath of zinc-aluminum alloy. State of the Art Currently, it is possible to improve oxidation and corrosion resistance, of parts particularly steel parts, through galvanization with metals such as zinc, cadmium and aluminum, or their alloys. The zinc-aluminum galvanizing process in particular imparts superior resistance to adverse weather conditions and greater mechanical performance. Generally, metallic galvanizing may occur in either a molten metal bath or an electrolytic bath. Both baths may be either continuous or noncontinuous. At present, noncontinuous processes are mainly applicable to metallic products of a limited size and dimension, such as screws, bolts, etc.. However, there is a trend towards coating metallic products of indefinite dimension, such as metallic strips, bars and wires, using a continuous galvanizing process. The strips, bars or wires are then transformed into the desired final products by means such as cutting and pressing the strips. This method has several disadvantages. For example, the desired final products have cut edges without any protective coating and are therefore exposed to attacks from environmental agents. Due to the need for quality products in the market, these disadvantages are becoming more relevant than the advantages of galvanizing with the continuous process. Interest has been growing in a noncontinuous process for galvanizing metallic parts such as girders, brackets and metals for the automotive, shipbuilding and appliance industries. Noncontinuous zinc-aluminum galvanization presents numerous advantages because it imparts superior resistance to hot oxidation and the attack of many aggressive media. However, in practice, it has been impossible to obtain good results from hot zinc-aluminum alloy noncontinuous galvanization because it is more expensive and less practical to apply surface preparation techniques, such as high temperature treatment with hydrogen, in noncontinuous versus continuous processes. In addition, classical zinc chloride and ammonium fluxes lose their effectiveness as soon as the aluminum content in the bath is over 0.01%, a problem for noncontinuous processes. As a result, bad surface preparation prevents the melted alloy from galvanizing, so the final product displays black stains on the surface and contains areas with no coating at all. These problems persist even though numerous efforts have been made to develop an effective industrial process for noncontinuous zinc- aluminum galvanizing. In one such effort, described in the Proceedings of the Intentional Galvanizing Conference, Rome, 5-10 June 1988, the relevant process uses a superficial conditioning system consisting of a 50% boiling caustic soda wash, followed by a wash, a 50% hydrochloric acid picking, wash, drying, flux either in ammonium chloride and zinc chloride (ratio 3:1) or in ammonium chloride, chrysolite and ammonium fluoride (ratio 5:3:1) and final wash. The products are then immediately dipped in a 600 - 650°C Al-55% Zn-43.5% Si-1.5% bath. Good results are obtained for carbon steel products and malleable gray pig iron products, but the pretreatment is complex and expensive and the galvanizing process uneconomical because of the large amount of dross produced. Another process, developed in Japan (Proceedings of the International Galvanizing Conference, Rome, 5 -10 June 1988), consists of a pretreatment that includes electrochemical cleaning, wash, treatment in a special flux and then dipping into a zinc-5% aluminum-sodium alloy at 460°C. This process is normally used only for wires and small nuts and bolts. Another attempt, in the laboratory stage, has been made in Belgium (Corrosion, vol. 47, Number 7, pages 536-541, 1992). Using a zinc-5 wt% aluminum alloy, the treatment consists of ultrasonic cleaning in trichloroethylene for 1 minute, then dipping into 60°C alkali for 3 minutes, then a wash and then a 50°C pickling for 3 minutes in 15% hydrochloric acid containing 1% thiourea. Afterwards the products are washed and fluxed in a solution of zinc chloride in alcohol at 60 °C for 3 minutes and then dried at 120° C for 10 minutes before dipping into the molten alloy. In Japan (Proceedings of the 1st Asian-Pacific General Galvanizing Conference, pages 149-157, Taipei, Taiwan, September 15 - 18, 1992), a process has been proposed consisting of pickling in 12% hydrochloric acid followed by dipping in a zinc and ammonium chloride flux also containing stannic chloride or bismuth. The coating alloy includes zinc and 4.9% aluminum. In this case, the positive results obtained are attributed to the use of salts of stannum or bismuth in the flux. Large-scale research has also been done on a zinc-5% aluminum alloy in Taiwan (Proceedings of the 1st Asian-Pacific General Galvanizing Conference, pages 158-166, Taipei, Taiwan, September 15-18, 1992). In this case the flux includes zinc chloride, 15-20%) ammonium, 5-10% alcohol, and 0.05-0.1%) non ionic surface-active agent. The parts to be treated are dipped at 65 °C for 30-120 sec. The coating baths also contain rare earth elements, i.e., 0.02% La and 0.02-0.04%) Ce, and are kept at 450-520 °C. In Sheet Material Industries, (Feb. 1956, pages 87-98), a process is described where a 5 cm wide strip, after regular pickling, is protected from oxidation by glycerol or a thin layer of copper before being dipped into a coating bath. When the strip enters the bath, the glycerol burns away or the copper melts in the bath, leaving in either case a clean surface to which the aluminum sticks. All of these methods are inconvenient and impractical in industrial applications. More specifically, these methods are either expensive and complex, not easily industrialized, dedicated to a specific coating composition, i.e., alloys with a relevant content of aluminum (greater than 50% > ) or to an alloy bath with a low content of aluminum (about 5%), or to pure aluminum, or they are dedicated to continuous processes and are therefore not practically transferable to noncontinuous applications. In particular, the technique of plating a thin zinc-aluminum alloy layer over the intended part before hot-dipping into the final coating alloy during continuous galvanization does not seem easily transferable to non-continuous applications. Traditional pretreatment plays a critical role in galvanizing procedures. It eliminates the final residues that may be left on the surface of the parts to be coated after pickling and helps protect the surface from those residues while dipping into the molten alloy bath. In the bath, the pretreatment flux reacts, releasing volatile compounds and creating a reducing atmosphere that protects the surface of the parts from oxidation. The volatile compounds are then quickly eliminated without any further problems. However, even in galvanization baths with very low aluminum content, as used in some examples above, the flux reacts with the aluminum to produce stable compounds, in particular oxides, that cannot be eliminated and prevent uniform galvanization of the part to be coated, causing widespread defects. This problem does not arise if the part is coated with a flash of extremely thin, highly reactive metallic coating, thus avoiding use of the pretreatment flux. The flash coating most likely functions by protecting the metal surface before dipping and then promptly being replaced by the galvanization coating without interfering with the adherence of the galvanization alloy. This type of treatment is commonly used to protect manufactured products made of nickel or a nickel alloy with aluminum galvanization, or to protect manufactured products of aluminum with nickel galvanization (Trans. Met. So. of S, vol. 242, page 1695, Aug. 1968) The major draw back of the process as currently employed is that the coating must be layered by thermal diffusion at Temp. >1100°C for many hours. In summary, currently there exists no process that allows noncontinuous galvanization with zinc-aluminum based alloys over metallic bodies, in particular steel-made bodies, in a simple way easily applicable to industry. Description of the Invention The present invention is directed to a metal electroless pre-coating treatment for the surface of a part to be galvanized using a hot zinc-aluminum alloy coating. Preferably, the pre-coating metal should be chosen among the group including nickel, copper, cobalt, and tin. The pre-coating must have a weight range of 1 to 35 g/m 2 , or, more preferably, a range of 5 to 25 g/m 2 . The treatment process includes the usual stages of cleaning, pickling and washing followed by the addition of a thin, light, metallic protective pre-coating. After a new wash and drying, the part is dipped into the molten bath of zinc-aluminum alloy. To create the electroless metallic protective pre-coating, a bath with sodium hypophosphite as a reductant may be used. This bath should be stabilized with 1 - 4 ppm of Pb, with the temperature between 80 and 90 °C and the pH fixed between 4.5 and 6 for Ni and with the temperature between 20 and 30 °C and the pH between 12 and 13 for Cu. The plating time ranges between a few seconds (i.e. 10 seconds for ripped steel bars using Ni) up to a few minutes (i.e. 600 sec. for steel sheets using Cu). The P percentage of the pre- coating should be between 8 and 12 wt%>. Either Ni or Cu are preferred as a flash pre- coating for use with zinc-aluminum alloys containing between 0.1 and 99.1 wt%> Al. Zinc-aluminum alloys containing between 0.1 and 25 wt%, and preferably about 5% aluminum are preferred. In such cases the copper flash pre-coating should be between 0.3 and 3 μm thick. When used, the nickel flash pre-coating should be between 0.1 and 4 μm thick. Ni pre-coating occurs in strongly reducing conditions on a steel surface with no oxides, the oxides having been previously removed by the HC1 pickling. The metallic layer protects the steel surface from oxidation which primarily occurs during immersion into the molten metal. In the bath, the Ni reacts with the Al of the Zn-Al alloy to form an interface compound layer (Ni 3 Al 3 -NiAl 3 or NiAl). Once the Al has completely transformed the Ni into the above compounds, the reaction between Al, Zn and Fe starts and the so-called adherence layer forms and a coating with good- morphology ensues. If, on the contrary, during the immersion, Ni is not transformed entirely by the reaction with Al, for instance because of a too short immersion time, or a too-thick Ni layer, or because the bath temperature is incorrect, then the Al-Zn-Fe reaction will not start and a good coating will not form. A similar reaction occurs when Cu, Co, or Sn is used as the pre-coating metal. Therefore, the amount of Ni, Cu, Co or Sn deposited does influence the final result. In general, coating quality is a function of the reaction time in the electroless pre-coating solution, of the solution temperature, of steel surface reactivity, and, finally, of the hot-dip coating parameters. The Al content of the final coating does affect the dipping time and the molten bath temperature, which must be at least 50°C above the alloy melting point. However, the compositions of both the Ni and Cu pre-coating flashes are fixed and related to the Al content of the final coating only in determining which of the two flashes is preferable. The exact parameters necessary to obtain a good coating vary with the composition of the steel or other metal to be coated, the metal used for pre-coating and the Al content of the hot-dip alloy. However, the thickness of the pre-coating when Ni is used is given by the equation: Thickness = ((4.4 X 10 "3) X t) + (6 X 10 "4) , where Thickness = μm and t = seconds. (This equation does not apply when the sample undergoes pre-reduction with hypophosphite.) Cu coating occurs at a rate of 0.03-0.04 μm/min. The best mode parameters for coating steel sheets or ripped steel bars (see Example 1 for detailed descriptions of the steel) with a Ni pre-coating followed by a Zn-5%>-Al-0.1%> mischmetal hot-bath coating are given is Table 4. The parameters for coating Type 1 steel (see Table 1 ) with a Ni pre-coating followed by a Al-55 wt%, Zn-43.5 wt%, Si-1.5 wt% hot bath are given in Table 5. The best mode parameters for coating sheet steel (see Example 1) with a Cu pre-coating followed by a Zn-5%-Al-0.1%> mischmetal hot-bath are given in Table 7. The above invention will be described in further detail in the following examples which do not restrict the broad purposes and uses of the invention itself. EXAMPLES From ripped bars for reinforced concrete of 12 mm 0 (diameter) of various known compositions (see Table 1), some samples 7 cm in length were made. Other samples of latten 7 X 12 were made from a FeP0 4 steel sheet by pressing. All . the samples were cleaned and then pickled in 1 : 1 HC1 inhibited with hexamethylenteramine (3 g/1). After rinsing, the pre-coating was performed in a laboratory reactor. Finally, the samples were rinsed with water and dried in a hot air draft, then galvanized in a hot molten alloy bath. The operative conditions of the galvanization phases are shown in Tables 2, 3, 4 and 6. The quality of the coating has been ranked and judged according to the following empirical scale. Votes reflect the lease favorable judgment. (*) Most samples were completely coated. When coverage was not complete the judgment was really poor. EXAMPLE 1 Table 1 - Composition of the steel of the ripped bars (remainder Fe) Table 2 - Electroless Ni-P solution composition Table 3 - Electroless Ni-P flash followed by hot-dip into Zn-5%>A1-0.1%> mischmetal * The layer's thickness varies with time according to: Thickness - ((4.4 X 10 "3) X t) + (6 X 10 "4) , where Thickness = μm and t = seconds. (This equation does apply when the sample undergoes pre-reduction with hypophosphite.) Table 4 presents the best mode for Zn-5%A1-0.1% mischmetal coatings: (*) Parameters must be chosen within the given interval taking care to combine low Ni thickness with low hot-dipping temperature, and vice-versa. EXAMPLE 2 Table 5 - Electroless Ni-P flash followed by hot-dip into Al-55 wt% Zn-43.5 wt% Si- 1.5 wt%> using Type I Steel. EXAMPLE 3 For Cu flash pre-coating, a solution whose composition is given in Table 5, below, was used. The deposition procedures were the same as those described for Ni. The thickness of the Cu flash was calculated by measuring the weight gain (10 min. deposition corresponds to 0.3 - 0.4 μm). Table 6 - Cu Deposition Table 7 - Electroless Cu flash followed by hot-dip into Zn - 5% Al- 0.1%) mischmetal - Steel Sheet FePO Λ .

Claims: CLAIMS 1. A process for non-continuous galvanization of a metal object comprising: preparing the surface of the metal object so as to remove residues; pre-coating the surface with a protective metal by an electroless process so that a uniform, continuous, thin coating of the pre-coating metal is obtained sufficient to protect the surface from oxidation prior to dipping into the galvanization bath, and yet sufficiently thin that the pre-coating can substantially completely react with Al in a molten Zn-Al alloy bath; dipping the pre-coated metal into a hot Zn-Al alloy bath and controlling the temperature and dipping time such that the pre-coating substantially completely reacts with Al in the bath to form an interface compound layer, thereby causing the ZnAI alloy bath to react with the surface of the metal and form an adherent layer and produce a continuous galvanized coating of good morphology. 2. The process according to claim 1, wherein the protective metal is at least one of the group consisting of Ni, Cu, Co and Sn. 3. The process according to claim 1, wherein the preparation of the surface comprises washing, pickling with HC1, then rinsing. 4. The process according to claim 2, wherein the pre-coating has a weight range of 1 to 35 g/m 2 . 5. The process according to claim 2, wherein the pre-coating metal includes Ni. 6. The process according to claim 5, wherein the pre-coating solution comprises 1- 4 ppm Pb. 7. The process according to claim 5, wherein the pre-coating solution has a pH of 4.5-6. 8. The process according to claim 5, wherein the pre-coating solution comprises sodium hypophosphite as a reductant. 9. The process according to claim 5, wherein the Ni-containing pre-coating layer is between 0.1 and 4 μm thick. 10. The process according to claim 6, wherein the Zn-Al alloy comprises between 0.1 and 25 wt%> Al. 11. The process according to claim 10, wherein the Zn-Al alloy comprises about 5% Al. 12. The process according to claim 11 , wherein the metal to be galvanized is exposed to the pre-coating solution for 30-60 seconds. 13. The process according to claim 11 , wherein the pre-coating solution is at a temperature between 85 °C and 90 °C. 14. The process according to claim 11, wherein the metal to be galvanized is dipped into the Zn-Al alloy for 240-300 seconds. 15. The process according to claim 11 , where in the Zn-Al alloy is at a temperature between 450 °C and 550 °C. 16. The process according to claim 5, wherein the Zn-Al alloy comprises between 25 and 99.9 wt%> Al. 17. The process according to claim 16, wherein the metal to be galvanized is exposed to the pre-coating solution for 240-300 seconds. 18. The process according to claim 16, wherein the pre-coating solution is between 85 °C and 90 °C. 19. The process according to claim 16, wherein the metal to be galvanized is dipped into the Zn-al alloy for 120-300 seconds. 20. The process according to claim 16, wherein the Zn-Al alloy is at a temperature between 620 °C and 640 °C. 21. The process according to claim 1 , wherein the pre-coating metal includes Cu. 22. The process according to claim 21 , wherein the pre-coating solution comprises between 1-3 ppm Pb. 23. The process according to claim 21 , wherein the pre-coating solution has a pH between 12-13. 24. The process according to claim 21 , wherein the Cu-containing pre-coating is between 0.3 and 3 μm thick. 25. The process according to claim 21 , wherein the Zn-Al alloy comprises between 0.1 and 25 wt%> Al. 26. The process according to claim 25, wherein the Zn-Al alloy comprises about 5%> Al. 27. The process according to claim 25, wherein the metal to be galvanized is exposed to the pre-coating solution for about 600 seconds. 28. The process according to claim 25, wherein the pre-coating is at a temperature of 25 °C. 29. The process according to claim 25, wherein the metal to be galvanized is dipped in the Zn-Al alloy for 30-60 seconds. 30. The process according to claim 25, wherein the Zn-Al alloy is at a temperature of 460 °C.

Inventors: BREVAGLIERI BRUNO, CECCHINI MICHELE, MEMMI MASSIMO, NATALI STEFANO, BREVAGLIERI, BRUNO, CECCHINI, MICHELE, MEMMI, MASSIMO, NATALI, STEFANO
Applicants: INT LEAD ZINC RES, INTERNATIONAL LEAD ZINC RESEARCH ORGANIZATION, INC.
Application year: 1999
Main classification: B05D1
IPC codes: C23C2, C23C18