WO1997035041A1

WO1997035041A1 - A fluxless hot dip galvanizing process

Info

Publication number: WO1997035041A1
Application number: PCT/EP1997/001174
Authority: WO
Inventors: Michael Gilles
Original assignee: Union Miniere NV SA
Current assignee: Umicore NV SA
Priority date: 1996-03-20
Filing date: 1997-03-07
Publication date: 1997-09-25
Anticipated expiration: 1998-09-20
Also published as: ZA972338B; ID16367A; AU2096597A

Abstract

The article to be galvanized is coated with a thin metal layer e.g. of copper in an aqueous medium, rinsed with water, rinsed with a water-miscible organic liquid and then dipped into a zinc bath. The process is particularly useful for depositing Zn-Al coatings.

Description

A fluxless hot dip galvanizing process

The present invention relates to a process of fluxless hot dip galvanizing steel articles comprising the steps of (a) applying a thin metal layer to the articles in an aqueous medium,

(b) rinsing the thus coated articles with water, whereby obtaining coated articles to which adheres a film of rinsing water,

(c) removing the film of rinsing water from the articles, and

(d) dipping the articles, from which the film of rinsing water has been removed, into a zinc or zinc alloy bath.

Such a process is disclosed in GB-A-2 099 857. In this known process step (c) consists in subjecting the articles to a drying operation in a drying station.

The aim of the present invention is to provide a process such as defined hereinbefore, which avoids the drying operation in a drying station of the known process.

According to the invention, step (c) consists in rinsing the articles with a water-miscible organic liquid and any intentional drying treatment of the articles is omitted between the end of step (c) and the beginning of step (d), which means that during their transport from the outlet of the station, wherein step (c) is carried out, to the surface of the galvanizing bath, the articles are only exposed to the temperatures that inevitably prevail in a hot dip galvanizing plant.

So, the articles will enter the galvanizing bath in the dry state, when the time required for the said transport is relatively long and/or when the organic liquid is relatively volatile; they will enter the galvanizing bath, while still covered by a film of the organic liquid, when the said time is relatively short and/or when the organic liquid has a relatively low volatility.

The process of the present invention is based on the discovery that excellent results are achieved in step (d), even if the articles entering the galvanizing bath are still covered by a film of the organic liquid.

Just like in the process according to the abovementioned GB-A-2 099 857, in the process of the present invention:

- it may be useful that the articles are degreased, pickled and rinsed with water before being subjected to step (a); - in principle, all metals which protect the article surface against oxidation in such a manner that a reaction with the galvanazing bath can take place on the entire article surface are suitable for coating the article with the thin metal layer; suitable metals are, for example, aluminium, lead, cadmium, copper, nickel, cobalt, bismuth, zinc, tin, indium and also alloys of these metals, the thin metal layer can be applied, for example, by electrochemical deposition, cementation or chemically reductive (electroless) deposition, the thin metal layer can be deposited simultaneously with a pickling and/or degreasing operation, - the thickness of the thin metal layer on the articles can be very small; preferably it should be below 1 μm, it is not necessary for this protective thin metal layer to be free of pores

The organic liquid may consist of at least one of methanol (CH₃OH), ethanol (C₂H₅OH), 1 -propanol (CH₃-CH2-CH₂OH), 2-propanol (CH₃-CHOH-CH₃), tert-butanol ((CH₃)₃COH), allylalcohol (CH₂--CH-CH₂OH), 1 ,2-ethanedιol (CH₂OH-CH₂OH), glycerol (CH₂OH-CHOH-CH₂OH) and acetone (CH₃-CO-CH₃).

The process of the present invention has been found to be particularly well adapted to deposit zinc coatings with an aluminium content of at least 0 4 wt%, especially zinc coatings with an Al content of at least 1 %, such as Supergalva (3-7 % Al, 0-3 % Mg, 0-0.1 % Na, rest Zn),

Galfan (5% Al, 0 03-0.10% Mischmetall, rest Zn) and Galvalume (55 % Al, 1.6 % Si, rest Zn) The deposition of such coating has always formed a problem, for which up to now no satisfactory solution was available.

The process of the invention can be performed batchwise on individual steel articles or continuously on articles such as steel wire or sheet.

Example

A steel test piece of 100 x 100 x 4 mm is subjected to the following operations.

(1) degreasing for 30 minutes at 60 °C in a bath containing 50 g/l of NaOH and 50 g/l of Na₂C0₃;

(2) rinsing in a water bath; (3) pickling for 4 minutes at 60 °C in a bath containing 200 g/l of HCI with added inhibitor and detergent,

(4) rinsing in a water bath,

®

(5) coppering for 1 minute at 40 °C in a bath containing 30 g/l of CUPROBOND 15 and 50 g/l of

® H₂S0₄, CUPROBOND 15 being a product that consists essentially of CuS0₄ 7H₂0 and that is manufactured and commercialized by CHEMETALL GmbH, Reuterweg 14, D-6000

Frankfurt am Main 1 ;

(6) rinsing in a water bath, (7) rinsing in an ethanol bath (20 °C); and

(8) dipping the specimen, to which still adheres a film of ethanol, into a Zn-5 % Al bath at 450 °C; keeping it immersed for 5 minutes and then removing it from the bath.

The so treated test piece is covered by a smooth and well adhering Zn-Al coating.

Claims

1. A process of fluxless hot dip galvanizing steel articles comprising the steps of

(a) applying a thin metal layer to the articles in an aqueous medium, (b) rinsing the thus coated articles with water, whereby obtaining coated articles to which adheres a film of rinsing water,

(c) removing the film of rinsing water from the articles, and

(d) dipping the articles, from which the film of rinsing water has been removed, into a zinc or zinc alloy bath, characterized in that step (c) consists in rinsing the articles with a water-miscible organic liquid and in that any intentional drying treatment of the articles is omitted between the end op step (c) and the beginning of step (d).

2. A process according to claim 1 , characterized in that step (d) is carried out while the articles are either already in the dry state or still covered by a film of the organic liquid.

3. A process according to claim 1 or 2, characterized in that the articles are degreased, pickled and rinsed with water before being subjected to step (a).

4. A process according to claim 1 , 2 or 3, characterized in that the thin metal layer consists of aluminium, antimony, lead, cadmium, copper, nickel, cobalt, bismuth, zinc, tin, indium or of an alloy of these metals.

5. A process according to claim 4, characterized in that the thin metal layer is produced by dipping the articles in an acid solution of a copper salt.

6. A process according to any of the claims 1-5, characterized in that the organic liquid consists of at least one of methanol, ethanol, 1 -propanol, 2-propanol, tert-butanol, allylalcohol, 1 ,2-ethanediol, glycerol and acetone.

7. A process according to claim 6, characterized in that step (c) consists in dipping the articles in ethanol.

8. A process according to any of the claims 1-7, characterized in that the zinc alloy melt contains at least 0.4 wt% of aluminium, preferably at least 1 % of Al.

9. A process according to claim 8, characterized in that the zinc alloy melt is composed essentially of zinc containing either 3-7 wt% Al, 0-3 wt% Mg and 0-0,1 wt% Na, or 5 wt% Al and 0.03-0.10 wt% Mischmetall, or 55 wt% Al and 1.6 wt% Si.

10. A process according to any of the claims 1-9, characterized in that said process is performed batchwise on individual steel articles, or continuously on articles such as steel wire or sheet.