WO1992018662A1 - Galvanizing method and zinc alloy for use therein - Google Patents

Abstract

A method for hot-dip galvanizing a series of individual steel objects. When all the objects to be galvanized contain at most 0.4 % of silicon, a zinc bath containing at least 0.005 % of silicon is used. When some of the objects contain over 0.04 % of silicon, a zinc bath containing at least 0.005 % of silicon as well as either 0.02-0.06 % of aluminium and 0.002-0.1 % of Ca, or 0.05-0.12 % of nickel, is used. A zinc-based alloy for compensating the galvanizing bath's consumption is also provided. The method may be used for coating steel.

Description

 GALVANIZATION PROCESS AND ZINC ALLOY THAT CAN BE USED IN THIS PROCESS

The present invention relates to a method of galvanizing by dipping a series of individual steel objects, according to which a zinc bath containing silicon is used at a concentration which may go as far as saturation.

Such a process is known from document DE-A-3734203. This document essentially concerns the dip galvanizing of steel objects containing more than 0.02% (by weight) of silicon. According to this document, the galvanization of such a steel poses serious problems when using the conventional galvanizing process, that is to say when using a bath consisting essentially of zinc. In fact, zinc coatings are obtained in this case, which are both too thick and too fragile and which, in addition, appear grayed out. This is due to the fact that the layer made of iron-zinc alloys, which forms on the surface of the steel in contact with a conventional galvanizing bath, increases linearly as a function of time throughout the duration of the immersion, when the steel contains more than 0.02% silicon. This is not the case with steels containing less silicon, the growth rate here being proportional to the root of the immersion time.

Still according to this document, it has been observed that the linear growth of the layer of iron-zinc alloys on steels with more than 0.02% of silicon is due to the disturbance of the crystal lattice of the alloys constituting the layer, by silicon having diffused steel in the layer, this disturbance facilitating the diffusion of iron from the steel through the layer.

Consequently, it is proposed in this document to counter the diffusion of the silicon from the steel into the layer by increasing the activity of the silicon within the bath. For this purpose, a zinc bath containing silicon, preferably saturated with silicon, is used, not only when all the objects to be galvanized have a silicon content of more than 0.02%, but also when at least one objects to be galvanized with such a silicon content, that is to say when it is not excluded that among the objects to be galvanized there is <* na whose silicon content does not exceed 0.02%, because it has been found that galvanizing in such a bath, steels whose silicon content does not exceed 0.02%, leads to very good results which are quite comparable to those obtained with these types. ^* _ in a classic bath.

The Applicant has found that this known method has the drawback of insufficiently meeting the current needs of galvanizers.

REPLACEMENT SHEET The object of the present invention is to provide a process as defined above, which avoids this drawback.

To this end, according to the invention (a) or else the said series is constituted so that all the objects are made of a steel containing at most 0.04% silidum and the bath contains in this first case at least 0.005 % silidum; (b) or the said series is constituted so that at least one of the objects is in an ader containing more than 0.04% of silidum and the bath contains in this second case at least 0.005% of silidum as well as either 0, 02-0.06% aluminum and 0.002-0.1% calcium, i.e. 0.05-0.12% nickel.

Indeed, with regard to the dip galvanization of aders containing at most 0.04% silidum, that is to say aders free of or with a low silidum content, the applicant has found, on the contrary of what is suggested by the document DE-A-3734203 predté, that the addition of silidum to a bath of galvanization dassique, that is to say a bath made up essentially of zinc, leads to the obtaining of coatings , which are considerably thinner than "dassic" coatings, that is to say the coatings which are obtained with aders containing at most 0.02% silidum in a dassic galvanizing bath. Thus, for example, the coating formed on an adder without silidum immersed for 5 minutes in a dassic bath at 450 ° C. has a "dassic" thickness of 66 / μm, while one obtains on the same ader and in the same conditions a coating with a thickness of only 37 µm after adding 0.03% silidum to the bath. Needless to say, zinc consumption decreases to the same extent as the thickness of the coating. The reduction in thickness is already substantial when 0.005% silidum is added to the bath.

It should be noted id that a "dassique" thickness, that is to say a thickness of about 70 μm, has become less and less necessary in many fields of technology. For example, automakers would settle for many applications with a coating thickness of only about 40 µm. Part (a) of the process of the present invention is therefore particularly advantageous when it is necessary to galvanize a series of ader objects with or without a low silidum content (up to 0.04%) and we want to obtain a thickness of coating lower than the base thickness, more particularly a lower thickness

REPLACEMENT SHEET With regard to the dip galvanization of aders containing more than 0.04% of silidum, the Applicant has found, also contrary to what is suggested in the document DE-A-3734203 predté, q. _. it is not enough to add silidum to the zinc bath to adequately solve the problem of the excessive growth of coatings, but that for this purpose it is necessary to add, in addition to 0.005% or more of silidum, ie 0.02-0 , 06% aluminum and 0.002-0.1% calάum, i.e. 0.05-0.12% nickel. The minimum contents of aluminum (0.02%) and nickel (0.05%) are required to obtain a substantial reduction in the thickness of the coating. The minimum silidum content (0.005%) is necessary on the one hand to reinforce the inhibitory effect of aluminum and nickel on the growth of the coating, in particular when the ader to be galvanized contains up to 0.20% of silidum, and secondly to avoid the formation in the bath of nickel-iron mattes. The minimum calrium content (0.002%) is necessary to obtain a coating which is free or practically free from defects in the continuity of the coating. B has proven difficult to avoid these defects, even in the presence of caldum, when the aluminum content is greater than 0.06%. A nickel content greater than 0.12% leads to the formation of interfering Ni-Zn compounds. The caldum added above 0.1% plays no role. In view of the above, it is obvious that the galvanizing bath according to part (b) of the process of the present invention is particularly advantageous, not only when one has to galvanize a series of ader objects which all have a silidum content of more than 0.04%, but also when a mixed series must be galvanized consisting partly of free or low silidum objects (up to 0.04%) and partially of objects with higher content silidum, and therefore, when a series of ader objects whose composition is unknown is to be galvanized, which is generally the case in custom galvanizing.

Note that it is already known from document CN-A-85109366 to use a galvanizing bath consisting of 0.1% silidum, 0.2% aluminum, 0.1% nickel, 0.1% tin, the rest being zinc. It must be a bath saturated with silidum, because the solubility of silidum in zinc at 450 ° C, that is to say the temperature normally used in galvanizing, is only about 0 . 03%. It is not specified what type of ader this bath is intended for. Nor is it specified whether the bath is intended for continuous galvanizing or for galvanizing individual objects. In any event, the Applicant has found that such a bath is in no way suitable for galvanizing individual objects, this bath certainly leading to multiple defects in the continuity of the coating, whatever the content of silidum of the objects to be galvanized.

. REPLACEMENT SHEET The Applicant has also found that the presence in the bath of at least 0.005% of silidum prevents the formation of bottom mattes (iron-zinc mattes), whatever the silidum content of the ader to be galvanized, which makes the process of the invention even more attractive.

It is desirable that the bath contains in both cases at least 0.01% of silidum, on the one hand to obtain a considerable reduction in the thickness of coating, more particularly on aders at 0-0.20% silidum, and secondly to avoid the formation in the bath of bottom mattes. It is also desirable that the bath contains in both cases lead at a concentration which can go as far as saturation, for example 0.1 to 1.2, this in order to reduce the surface tension of the bath. It is further desirable that the bath contains in the first case 0.001-0.015% aluminum and / or 0.002-0.1% caldum, ced to protect the donkey against oxidation; otherwise a yellowish film forms on the surface of the bath, which dirty galvanized objects. For the same reason it is advantageous that the nickel bath used in the second case also contains 0.001-0.015% aluminum and / or 0.002-0.1% caldum.

The preferred caldum content is 0.005-0.05%. The preferential zinc content is at least 98%.

Very good results are obtained when the bath contains, in addition to zinc, only the elements mentioned at the mentioned contents and unavoidable impurities.

It is obvious that the composition of the bath will change during the operations, the consumption rates, by oxidation and other reactions, of the components, zinc and additives, at the temperature of use (normally close to 450 ° C) and in the presence of flux (ZnC-2 and NH4CI) being different, and practically all the higher as the metal is oxidizable. Additive deficits as a result of oxidation relate mainly to silidum and caldum as well as aluminum when the caldum is absent.

However, the Applicant has found that it is possible to maintain the composition of the bath during the galvanizing operations by compensating for the consumption of bath by adding a zinc-based alloy containing to the bath.

(a) in the first case 0.1-1.5% of silidum, 0-0.8% of aluminum, 0-1% of caldum and 0-1, 2% of lead; and

(b) in the second case 0, 1-1, 5% silidum and 0-1, 2% lead as well as 0.1-0.8% aluminum and 0.02-1% caldum, i.e. 0.05-0.12% nickel, 0-0.8% aluminum and 0-1% caldum.

REPLACEMENT SHEET The zinc-based alloy to be used, preferably in the form of ingots, may contain, depending on the case, either 0.1-1.5% of Si; or 0.1-1.5% of Si and 0.01-0.8% of A_; or 0.1-1.5% of Si and 0.1-1.2% ofPb; or 0.1-1.5% of S and 0.02-1% of Ca; or 0.1-1.5% of S; , 0.01-0.8% Al and 0.02-1% Ca; or 0.1-1.5% of S:, 0.01-0.8% of Al and 0.1-1.2% ofPb; or 0.1-1.5% of S:, 0.02-1% of Ca and 0.1-1.2% ofPb; or 0.1-1.5% of Si, 0.01-0.8% of Al, 0.02-1% of CaetO, M, 2% of Pb; or 0.1-1.5% of Si, 0.1-0.8% of Al and 0.02-1% of Ca; or 0.1-1.5% of S, 0.1-0.8% of Al, 0.02-1% of Ca and 0.1-1.2% of Pb; or 0.1-1.5% of S; and 0.05-0.12% Ni; or 0.1-1.5% of Si, 0.05-0.12% of Ni and 0.1-1.2% ofPb; or 0.1-1.5% of S, 0.05-0.12% of Ni and 0.01-0.8% of Al; or 0.1-1.5% of Si, 0.05-0.12% of Ni, 0.01-0.8% of Al and 0.1-1.2% ofPb; or 0.1-1.5% of Si, 0.05-0.12% of Ni and 0.02-1% of Ca; or 0.1-1.5% of S: 0.05-0.12% of Ni, 0.02-1% of Ca and 0.1-1.2% ofPb; or 0.1-1.5% of Si 0.05-0.12% of Ni, 0.01-0.8% of Al and 0.02-1% of Ca; or 0.1-1.5% of S, 0.05-0.12% of _ "_, 0.01-0.8% of Al, 0.02-1% of Ca and 0.1- 1.2% of Pb.

Excellent results are obtained when the alloy contains, in addition to the zmc, only the elements mentioned with the mentioned contents and unavoidable impurities. It is obvious that the equivalent of zmc can be substituted for an equivalent in the form of at least one master alloy and of zinc or in the form of at least one master alloy and of a less alloy. loaded with additives as the alloy to be replaced.

Thus, for example, one could use as equivalent of 100 kg of zinc alloy with 1% Si and 0.1% Al (first case): i.e. 10 kg of mother alloy with 10% Si and 1% Al (prepared by powder metallurgy) and 90 kg of Zn, i.e. 10 kg of mother alloy to 10% Si (ex powder metallurgy), 1 kg of alloy- mother with 10% Al and 89 kg of Zn, i.e. 1 kg of alloy-mother with 10% of Al and 99 kg of alloy with 1.01% of Si.

The zinc-based alloy defined above can be used for other applications than that described above.

REPLACEMENT SHEET A variant of the process of the invention consists in constituting said series in the first case so that all the objects are in an ader containing at most 0.02% silidum (instead of 0.04%), and in the second case so that at least one of the objects is in an ader containing more than 0.02% of silidum (instead of 0.04%).

In the above all percentages are by weight.

EXAMPLE 1

This example concerns the galvanization of an ader having the following composition, in% by weight: 0.050 C, 0.28 Mn, 0.012 Si, 0.009 S, 0.014 P, 0.020 Al, 0.020 Ni, 0.020 Cr and 0.025 Cu.

In a first test a zinc bath is used and in a second test a zinc bath with 0.029% Si. In both cases the temperature of the bath is 450 ° C and the immersion time 5 minutes. In the first test a coating with a thickness of 66 µm is obtained and in the second test a coating with a thickness of only 39 µm.

EXAMPLE 2

This example concerns the galvanization of an ader having the following composition, in% by weight: 0.144 C, 0.920 Mn, 0.092 Si, 0.010 S, 0.014 P, 0.048 Al, 0.020 Ni, 0.020

Cr, 0.025 Cu.

In a first test a zinc bath with 0.029% Si is used, in a second test a zinc bath with 0.10% Ni and in a third test a zinc bath with

0.10% Ni and 0.029% Si. In the three tests the bath temperature is

450 ° C and the immersion time of 5 minutes.

In the first test, a thickness of more than 200 μm is obtained, in the second test a thickness of 69 μm and in the third test a thickness of only

REPLACEMENT SHEET

Claims

1) A process of galvanizing by dipping a series of individual ader objects, according to which a zmc bath containing silidum is used at a concentration which can go as far as saturation, characterized in that (a) or else the said series is constituted so that all the objects are in an ader containing at most 0.04% of silidum and the zmc bath contains in this first case at least 0.005% of silidum; (b) or else the said series is constituted so that at least one of the objects is in an ader containing more than 0.04% of silidum and the zinc bath contains in this second case at least 0.005% of silidum as well as either 0.02-0.06% aluminum and 0.002-0% caldum, ie 0.05-0.12% nickel. 2) Method according to claim 1, characterized in that the bath contains in both cases at least 0.01% of silidum. 3) Process according to claim 1 or 2, characterized in that the bam in both cases contains lead at a concentration which can go as far as saturation. 4) Method according to claim 3, characterized in that the bath contains 0.1-1.2% lead. 5) Method according to any one of claims 1-4, characterized in that the bath contains in the first case 0.001-0.015% aluminum. 6) Method according to any one of claims 1-5, characterized in that the bath contains in the first case 0.002-0.1% of caldum. 7) Method according to any one of claims 1-4, characterized in that the bath contains in the second case 0.02-0.06% aluminum and 0.005-0.05% caldum. 8) Method according to any one of claims 1-4, characterized in that the bam contains in the second case 0.05-0.12% of nickel as well as 0.001-0.015% of aluminum and / or 0.002 -0.1% caldum. REPLACEMENT SHEET 9) A method according to any one of claims 1-8, characterized in that the bath contains, in addition to zinc, only the elements mentioned in the contents mentioned¬ born and inevitable impurities. 10) Method according to any one of claims 1-9, characterized in that the composition of the bath is maintained during the galvanizing process by compensating for the consumption of bath by adding to the bath either an alloy zinc based containing (a) in the first case 0.1-1.5% of silidum, 0-0.8% of aluminum, 0-1% of caldum and 0-1, 2% of lead; and (b) in the second case 0.1-1.5% of silidum and 0-1.2% of lead as well as 0.1-0.8% of aluminum and 0.02-1% of caldum, or 0.05-0.12% nickel, 0-0.8% aluminum and 0-1% caldum. either of an equivalent of said zinc-based alloy in the form of at least one master alloy and of pure zinc or in the form of at least one mother alloy and of a zinc-based alloy less charged with 'additives than the above-mentioned alloy. 11) A method according to claim 10, characterized in that a zinc-based alloy is used which contains, in addition to zinc, only the elements mentioned at the mentioned contents and unavoidable impurities. 12) Zinc-based alloy ingot, in particular for use in the process according to claim 10 or 11, characterized in that it contains either 0.1-1.5% deSi; or 0.1-1.5% Si and 0.01-0.8% Al; or 0.1-1.5% of Si and 0.02-1% of Ca; or 0.1-1.5% of Si, 0.01-0.8% of Al and O, 02-1% of Ca; or 0.1-1.5% Si and 0.1-1.2% Pb; or 0.1-1.5% of Si, 0.01-0.8% of AletO, l-1.2% of Pb; or 0.1, 1.5% of Si, 0.02-1% of Ca and 0.1-1.2% of Pb; or 0.1-1.5% Si, 0.01-0.8% Al, 0.02-1% Ca and 0.1-1.2% Pb; or 0.1-1.5% Si, 0.1-0.8% AIetO, 02-1% Ca; or 0.1-1.5% of Si, 0.1-0.8% of Al, 0.02-1% of Ca and 0.1-1.2% ofPb; or 0.1-1.5% of Si and 0.05-0.12% of Ni; or 0.1, 1.5% of Si, 0.05-0.12% of Ni and 0.1-1.2% of Pb; or 0.1-1.5% Si, 0.05-0.12% Ni and 0.01-0.8% Al; or 0.1, 1.5% of Si, 0.05-0.12% of Ni, 0.01-0.8% of Al and 0.1-1.2% of Pb. REPLACEMENT SHEET or 0.11.5% of Si, 0.05-0.12% of Ni and 0.02-1% of Ca; or 0.1, 5% of Si, 0.05-0.12% of Ni, 0.02-1% of Ca and 0.1-1.2% of Pb; or 0.1, 5% of Si, 0.05-0.12% of Ni, 0.01-0.8% of Al and O, 02-1% of Ca; or 0.1-1.5% of Si, 0.05-0.12% of Ni, 0.01-0.8% of Al, 0.02-1% of Ca and 0.1-1, 2% Pb. 13) Ingot according to claim 12, characterized in that it contains, in addition to zinc, only the elements mentioned at the levels mentioned and impurities inévi¬ tables. 14) Variant of the method according to claims 1-11, characterized in that, instead of constituting said series in the first case so that all the objects are in an ader containing at most 0.04% silidum, it is constitutes in such a way that all the objects are in an ader containing at most 0.02% of silidum, and, instead of constituting said series in the second case so that at least one of the objects is in an ader containing more than 0.04% silidum, it is constituted so that at least one of the objects is in an ader containing more than 0.02% silidum. REPLACEMENT SHEET