WO2007030010A1

WO2007030010A1 - STEEL ARTICLE PROVIDED WITH A Zn-Fe-Co COATING AND METHOD TO DEPOSIT SUCH A COATING ON A STEEL ARTICLE

Info

Publication number: WO2007030010A1
Application number: PCT/NL2006/050202
Authority: WO
Inventors: Zeeshan Farooq Lodhi; Johannes Hendrik Wilhelmus De Wit; Herman Albert Terryn
Original assignee: Netherlands Institute For Metals Research
Priority date: 2005-09-05
Filing date: 2006-08-15
Publication date: 2007-03-15
Also published as: NL1029885C2

Abstract

Corrosion protection of steel by using Zn-Co-Fe alloy coatings. It is proposed to use a Zn-Co-Fe coating having a relatively high Co content. According to the invention at least 10 and more particular 10-40 % by wt. Co is used in the Zn-Fe-Co coating. Fe is used in quantity of 1-5 % by wt. and the remaining material is Zn. It has been found that such a coating is much more effective than conventional coatings so that the thickness thereof can be reduced with about 90%. This more than fully compensates for the high cost of Co. It has been found that the difference in electrochemical potential of such coating and the substrate is relatively small. This slows down rapid dissolution of Zn from the coating.

Description

Steel article provided with a Zn-Fe-Co coating and method to deposit such a coating on a steel article.

The invention relates to an article provided with a Zn-Fe-Co coating and more particular a steel article.

It is generally known in the art to provide steel articles with a corrosion protective coating. In the past cadmium has extensively been used as barrier coating. Applications for steel can be found anywhere such as in automobiles, aerospace, fasteners and so on. However, because of its toxic nature cadmium has gradely phased out. Because of that alternatives for cadmium are presently used. One of the alternatives is to use a sacrificial coating based on zinc. An example is a zinc coating to which iron and cobalt have been added. An example thereof can be found in US-4,488,942 A in which a zinc iron cobalt coating is disclosed including a brightener.

In the prior art coatings generally a relatively large amount of Fe is present in the coating and a minor quantity of cobalt.

The problem associated with such coatings is the relative restricted service life. Under corrosive conditions zinc will be sacrified and protects attack of the steel substrate. However, because of the considerable differences in standard electrode potential between zinc and iron there is a considerable driving force to dissolve zinc in the corroding medium. Because of that a relatively thick layer of zinc or one of its alloys has to be used to provide sufficient service life of the related article under the most unfavourable conditions.

Because of that layer thicknesses of 200 μm for the zinc based coating are not unusual. In view of the considerable thickness of the layer considerable amounts of Zn are necessary involving high cost. Furthermore the use of thick Zn layers might give environmental problems.

Attempts have been made to increase the quantity of Fe in Zn-Co-Fe coatings. However it revealed that appearance of such coating became reddish and although information was given to the users that this was due to the layer used and had nothing to do with the rust, such coatings were not accepted by the public using the related articles. m the past also tests have been undertaken to pacify steel articles by a Cr coating. However, there are also environmental objections against the use of chromium. The invention aims to provide a cadmium free coating for all kind of articles and more particular steel articles which can be provided with a relatively low thickness in a relatively simple way and gives sufficient corrosion protection under the usual conditions. According to the invention this is realized with an article provided with an Zn-Fe-

Co coating wherein said coating comprises at least 10 % by wt. Co.

More particular 10-40 % by wt. Co is present in the coating. It has been found that when a still higher % Co is used the standard electrode potential of the coating will be less negative than the standard electrode potential of usual Zn-Co-Fe coatings. Of course it should be observed that the potential should never be more positive than the potential of steel of the related article.

Because the difference in standard electrode potential is smaller under corrosive conditions the speed of dissolving of Zn in the coating will be lowered resulting in the possibility to use a much thinner layer. For example a 10 μm coating layer according to the subject invention is fully comparable with a 200 μm standard Zn-Co-Fe coating under usual corrosive conditions. This will result in reduced environmental objections and at the same time a relatively high price of Co is justified.

According to a further preferred embodiment of the invention a relatively low percentage Fe is used and more particular 1-5%. According to a further embodiment of the invention there is at least two times as much Co in the coating composition than Fe.

If required brighteners and other products improving the appearance of the coating can be used.

If the coating is provided electro galvanically, the hydrogen which results during this process might diffuse into the article. If this article is a steel article and more particular a high tensile steel, diffusing hydrogen could have an impact on the strength properties thereof and more particular the fatigue strength.

In such cases according to a further embodiment of the invention it is recommended to first provide a thin layer of nickel on the related substrate. The thickness of such layer is preferably between .5 and 1.0 μm. This can be realized in a separate bath. However, no stringent requirements are set to rinsing the article after the nickel layer has been deposited. It has been found that nickel in a Zn-Co-Fe bath has no negative effect. Because of the different standard electrode potentials if an electrochemical bath is used special measures should be taken that the desired composition is deposited from the bath. I.e. the Zn concentration in the bath should be lower than the desired Zn concentration in the later coating. It could be expected that much extra cobalt would be necessary in order to obtain the required layer. However, it has surprisingly been found that this effect is only relative. If for example an acidic chloride bath is used the following composition gives optimum results: ZnCl₂ = 70-170g/L, CoCl₂.6H₂O = 100-240g/L, FeCl₂= 5-15g/L, KCl = 150- 240g/L, H₃BO₃=30-50g/L, NH₄Cl=I 5-30g/L and 0.1-0.5g/l of Citric acid as Chelating agent The pH used for this electrolyte can be in the range of 1.0-5.0.. The current density between 1-5 A/d/m² is preferably used. In such acidic chloride bath citric acid might be added as chelating agent in order to prevent deposition of iron hydroxydes in the electrolyte. It is also possible to start from an acidic sulfate bath having the following composition: 7-40 g/L ZnSO₄7H₂O, 70-90 g/L CoSO₄7H₂O, 8-15 g/L FeSO₄7H₂O, 25 g/L H₃BO₃, 50 g/L )NH₄)₂SO₅ and 40/50 g-L NA₂SO₄. In this case the pH should be in the range of 1.0-5.0.

If a sulfate based bath is used a relatively low quantity of zinc should be used. With a chloride bath a high quantity of zinc can be present because of lower H₂ generation and a higher cathodic efficiency. The article is at the cathode side whereas the anode can be either be one of the above metals and/or a combination thereof or can be an inert material such as platinum. Example 1

An aqueous electrolyte is prepared suitable for depositing a Zinc-Cobalt-Iron alloy containing 160g/l of ZnCl₂, 235g/l of CoCl₂OH₂O and 15g/l OfFeCl₂. There is no brightener used for the coating but can be used, if required. The bath is adjusted to pH of 3.8 and a current density of 2 A/dm² is applied. The anodes are made of pure Cobalt and the temperature during electrodeposition is maintained at 27°C with a constant stirring at 300 rev/min. A ZnCoFe alloy of lustrous and semi bright appearance coating is obtained which upon analysis contains 71% Zn, 27%Co and 2%Fe. Example 2

An aqueous electrolyte is prepared suitable for depositing a Zinc-Cobalt-Iron alloy containing 160g/l of ZnCl₂, 235g/l of CoCl₂OH₂O and 15g/l OfFeCl₂. There is no brightener used for the coating but can be used, if required. The pH of the bath is adjusted to 3.8 and a current density of 2A/dm² is applied. The anodes are made of pure Cobalt and the temperature during electrodeposition is maintained at 25°C with a constant stirring at 300 rev/min. A semi bright ZnCoFe alloy coating is obtained which upon analysis contains 72.6 % Zn, 25.6%Co and 1.8%Fe. After the above further embodiments of the invention, being in combination with measures known in the prior art, are immediate obvious for the person skilled in the art and within the range of the appended claims.

Claims

I. Article provided with a Zn-Fe-Co coating, wherein said coating comprises at least 10% by wt. Co.

2. Article according to claim 1, wherein the wt.% Co is at least two times the wt.% Fe.

3. Article according to one of the preceding claims, comprising 1-5 % by wt. Fe.

4. Article according to one of the preceding claims, wherein said coating has a thickness less than 20 μm.

5. Article according to one of the preceding claims, wherein said coating has a thickness of about 10 μm.

6. Article according to one of the preceding claims, wherein between said Zn-Fe- Co coating and said article a nickel layer is provided.

7. Article according to claim 6, wherein said nickel layer has a thickness of 0.5- 1.5 μm.

8. Article according to one of the preceding claims, comprising a steel material.

9. Method of galvanically depositing a Zn-Fe-Co coating on a steel substrate, comprising the provision of an electrochemical acidic chloride bath containing 70-160 g/L ZnCl₂, 100-240 g/L CoCl₂OH₂O, 5-15 g/L FeCl₂H₂O, 180-240 g/L KCl, 30-50 g/L H₃BO₃, 15-3O gZL NH₄Cl.

10. Method according to claim 9, wherein the pH is between 1.5 and 5.0.

II. Method according to claim 9 or 10, wherein said bath comprises .1-.5 g/L of a citric acid.

12. Method of galvanically depositing a Zn-Fe-Co coating on a steel substrate comprising the provision of an acidic sulfate bath containing 7-40 g/L ZnSO₄7H₂0, 70- 90 g/L CoSO₄7H₂O, 8-15 g/L FeSO₄7H₂0, 25 g/L H₃BO₃, 50 g/L )NH₄)₂SO₅ and 40/5O g-L NA₂SO₄.

13. Method according to claim 12, wherein the pH of the electrolyte bath is in the range of 1.0-5.0.