EP1019564A1

EP1019564A1 - Method for phosphatizing a steel strip

Info

Publication number: EP1019564A1
Application number: EP98949988A
Authority: EP
Inventors: Jörg Riesop; Reinhard Seidel
Original assignee: Henkel AG and Co KGaA
Current assignee: Henkel AG and Co KGaA
Priority date: 1997-09-17
Filing date: 1998-09-05
Publication date: 2000-07-19
Also published as: CA2303877A1; WO1999014397A1; DE19740953A1

Abstract

The invention concerns a method for phosphatizing a steel strip or a zinc-coated or galvanized steel strip by depositing an alloy on one or two surfaces, by spraying or immersion, for 2 to 20 seconds, with an acid phosphatizing solution, containing zinc, magnesium and manganese, at a temperature of the order of 50 to 70 DEG C. Said method is characterised in that the phosphatizing solution does not contain nitrate ions and that it contains: between 1 and 4 g/l of zinc ions, between 1.2 and 4 g/l of manganese ions, between 1 and 4 g/l of magnesium ions, between 10 and 30 g/l of phosphate ions, between 0.1 and 3 g/l of hydroxylamine in free, ionic or combined form. The acid content of said phosphatizing solution ranges between 0.4 and 4 points and its total acid content is of the order of 15 to 45 points.

Description

"Process for phosphating steel strip"

The invention relates to a method for phosphating steel strip or steel strip galvanized on one or both sides or alloy galvanized by spraying or immersion treatment, which takes place for a period of time in the range of approximately 2 to approximately 20 seconds, depending on the strip speed.

Processes for phosphating surfaces made of iron, steel, zinc and their alloys as well as aluminum and its alloys have long been state of the art. The phosphating of the surfaces mentioned serves to increase the adhesive strength of paint layers and to improve corrosion protection. The phosphating is carried out by immersing the metal surfaces in the phosphating solutions or by spraying the metal surfaces with the phosphating solutions. Combined methods are also known. Shaped metal parts such as automobile bodies can be phosphated, but also metal strips in high-speed conveyor systems. The present invention is concerned with such a band phosphating. Belt phosphating differs from partial phosphating in that, because of the high belt speeds, the phosphating, i.e. H. the growth of a closed metal phosphate layer must take place within a short period of time, for example about 2 to about 20 seconds.

Processes for phosphating metal strips, in particular electrolytically galvanized or hot-dip galvanized steel strips, are known in the prior art. For example, WO 91/02829 describes a method for Phosphating of electrolytically and / or hot-dip galvanized steel strip through short-term treatment with acid phosphating solutions, which, in addition to zinc and phosphate ions, contain manganese and nickel cations as well as anions containing oxygen

Contain acids with accelerator effect. The latter term is to be understood in particular as nitrate ions. DE-A-35 37 108 also describes a process for the phosphating of electrolytically galvanized steel strips

Treatment with acid phosphating solutions, which in addition to zinc, manganese and

Phosphate ions other metal cations such as nickel ions and / or

Anions of oxygen-containing acids with accelerating action, in particular

Nitrate ions included. The contents of zinc cations are in the relatively low range of 0.1 to 0.8 g / 1.

DE-A-39 20 296 describes a process for the production of zinc phosphate coatings containing manganese and magnesium on steel, zinc, aluminum and / or their alloys by spraying, spray-immersion and / or immersion with an aqueous solution which is 0.2 to 1.0 g / 1 zinc ions, 0.2 to 2.0 g / 1 manganese ions, 0.5 to 2 g / 1 magnesium ions, 10 to 20 g / 1 phosphate ions, 0.2 to 10 g / 1 nitrate ions and up to Contains 1.0 g / 1 fluoride ions. This phosphating solution also necessarily contains as accelerator: 0.02 to 0.2 g / 1 nitrite ion and / or 0.4 to 1 g / 1 chlorate ion and / or 0.2 to 1 g / 1 of an organic oxidizing agent. However, the nitrate ions necessary in this process can lead to a loss of quality of the phosphate layer on galvanized steel surfaces due to speck formation. Here, "specks" are understood to be whitish corrosion points on the metal surface which show a crater-like appearance in microscopic images. From the phosphating times of more than 2 minutes mentioned in the exemplary embodiments, it can be seen that this is not a process for band phosphating. German patent application 196 39 596 attempts to provide a phosphating process which, on the one hand, solves the problem of speck formation and, on the other hand, enables even galvanized steel strips or the non-galvanized side of steel strips galvanized on one side in the short phosphating times customary in strip systems with a closed crystalline phosphate layer Mistake. According to this document, this object is achieved by a method for phosphating steel strip or steel strip galvanized on one or both sides or alloy galvanized by spraying or dipping treatment for a period of time in the range from 2 to 15 seconds with an acidic, zinc and manganese-containing phosphating solution with a Temperature in the range from 40 to 70 ° C, characterized in that the phosphating solution contains 1 to 4 g / 1 zinc ions,

0.8 to 3.5 g / 1 manganese ions

10 to 30 g / 1 phosphate ions

Contains 0.1 to 3 g / 1 hydroxylamine in free, ionic or bound form and not more than 1 g / 1 nitrate ion, a free acid content in the range from 0.4 to 4 points and a total acid content in the range of 12 up to 50 points.

The crystalline zinc phosphate layers produced by this process already very well meet the demands placed on zinc phosphating today. This applies in particular when this method is used in an embodiment in which either 0.8 to 3.5 g / 1 nickel ions or 0.002 to approximately 0.2 g / 1 copper ions are present at the same time. If you work without these cations, you have to accept disadvantages in terms of paint adhesion and corrosion protection. The use of nickel ions in phosphating baths is increasingly undesirable these days for ecological and toxicological reasons. For the phosphating of parts, such as the phosphating of automobile bodies with phosphating times in the range of more than about 2 minutes, phosphating processes have recently been used in which copper ions in the above-mentioned lower concentration range are used instead of the nickel ions. Even for high-speed conveyor systems with their short phosphating times in the range from about 2 to about 20 seconds, intensive searches are being carried out for processes which do not use the toxicologically and ecologically disadvantageous nickel ions. However, replacing the nickel ions with low concentrations of copper ions is not a solution for conveyor systems, since with the large area to be phosphated per unit of time and the small bath volume compared to part phosphating, the copper content would have to be determined and readjusted at intervals of less than 1 minute. No suitable measurement and control technology is available for this.

The object of the invention is therefore to provide a phosphating process for high-speed conveyor systems with phosphating times in the range from about 2 to about 20 seconds, which on the one hand works nickel-free and on the other hand leads to phosphate layers, the quality of which does not protect those of nickel-containing phosphate layers in terms of corrosion protection and paint adhesion is inferior.

This object is achieved by a method for phosphating steel strip or steel strip galvanized on one or both sides or galvanized with alloys by spraying or dipping treatment for a period of time in the range from 2 to 20 seconds with an acidic, zinc, magnesium and manganese-containing phosphating solution with a temperature in the range of 50 to 70 ° C, thereby characterized in that the phosphating solution is free of nitrate ions and that it

1 to 4 g / 1 zinc ions,

1.2 to 4 g / 1 manganese ions

1 to 4 g / 1 magnesium ions

10 to 30 g / 1 phosphate ions

Contains 0.1 to 3 g / 1 hydroxylamine in free, ionic or bound form, a free acid content in the range from 0.4 to 4 points and one

Total acid content in the range of 15 to 45 points.

The steel strips can be galvanized or hot-dip galvanized or alloy-galvanized. “Galvanized alloy” is understood to mean that the steel surface has been coated with an alloy which, in addition to zinc, contains other metals such as iron, nickel or aluminum. An alloy galvanizing with a zinc-iron alloy can take place, for example, by tempering a galvanized steel strip , which causes a diffusion of iron atoms into the zinc layer and vice versa. The layer thicknesses of the galvanizing layers are usually in the range from about 5 to about 20 μm.

The terms "free acid" and "total acid" are generally known in the field of phosphating. They are determined by titrating the acid bath sample with 0.1 normal sodium hydroxide solution and measuring its consumption. The consumption in ml is given as a score. In this document, the number of free acids means the consumption in ml of 0.1 normal sodium hydroxide solution in order to titrate 10 ml of bath solution, which has been diluted to 50 ml with deionized water, up to a pH of 4.0 . Similarly, the total acid score indicates consumption in ml up to a pH of 8.2. Preferably the free acid is set to the range 1.5 to 2.8 points, the total acid to the range 25 to 35 points.

Hydroxylamine can be used as a free base, as a hydroxylamine-releasing compound such as hydroxylamine complexes and ketoximes or aldoximes or in the form of hydroxylammonium salts. If free hydroxylamine is added to the phosphating bath or a phosphating bath concentrate, it will largely exist as a hydroxylammonium cation due to the acidic nature of these solutions. When used as a hydroxylammonium salt, the sulfates and the phosphates are particularly suitable. In the case of the phosphates, the acid salts are preferred due to the better solubility. A combination of free hydroxyamine and hydroxylammonium sulfate can advantageously be used to take into account economic aspects on the one hand and on the other hand to not burden the phosphating baths with too much sulfate ions. Hydroxylamine or its compounds are added to the phosphating solution in amounts such that the calculated concentration of the free hydroxylamine is between about 0.1 to about 3 g / 1, preferably between about 0.15 and about 1 g / 1.

For the indication of the phosphate concentration, the total phosphorus content of the phosphating bath is considered to be present in the form of phosphate ions POz ^ -. Accordingly, the known fact that the pH values of the phosphating baths in the range from about 2.0 to about 3.6, which are in the acidic region, only a very small part of the phosphate is actually in the form is ignored when calculating or determining the concentration the triple negatively charged anions are present. At these pH values it is rather to be expected that the phosphate is present primarily as a simply negatively charged dihydrogenphosphate anion, together with undisociated phosphoric acid and with smaller amounts of double negatively charged hydrogen phosphate anions.

No nickel ions are added to the phosphating solution. Ideally, this solution is therefore completely free of nickel ions. However, if the phosphating system consists of nickel-containing stainless steel or if zinc-coated steel strips are phosphated, the zinc layer of which also contains nickel, it cannot be ruled out that nickel ions will get into the solution in small concentrations due to the attack of the phosphating solution. However, the phosphating solution should not contain more than about 0.05 g / 1 nickel ion and in particular not more than about 0.015 g / 1 nickel ion.

In addition to the layer-forming cations mentioned, the phosphating solutions contain alkali metal and / or ammonium cations in order to adjust the value of the free acid to the desired range.

The presence of fluoride ions in the phosphating solution is generally not necessary for the phosphating of non-galvanized steel. However, the phosphating of hot-dip galvanized steel strips is facilitated by fluoride ions, and the presence of fluoride ions can also be advantageous for the phosphating of electrolytically galvanized steel strips for a uniform layer formation. Accordingly, a further preferred embodiment of the invention consists in using phosphating solutions which contain up to about 0.8 g / 1 fluoride in free or complex-bound form. For example, for the phosphating of electrolytically galvanized steel strip, the preferred fluoride contents are in the range from 0.0 to about 0.5 g / 1, in particular in the range from about 0.1 to about 0.2 g / 1. The phosphating solutions are generally prepared in the manner known to the person skilled in the art. For example, phosphate is introduced into the phosphating solutions in the form of phosphoric acid. The cations are added in the form of acid-soluble compounds such as, for example, the carbonates, the oxides or the hydroxides of phosphoric acid, so that this is partially neutralized. The further neutralization to the desired pH range is preferably carried out by adding sodium hydroxide or sodium carbonate. Suitable sources of free fluoride anions are, for example, sodium or potassium fluoride. For example, tetrafluoroborate or hexafluorosilicate can be used as complex fluorides.

In a further aspect, the invention relates to the use of the phosphating method described above for the production of phosphate layers on both sides with a mass per unit area in the range from approximately 0.4 to approximately 2.0 g / m 2 on steel strip or on steel strip galvanized or alloy-galvanized on one or both sides . Phosphate layers with a mass per unit area in the range from approximately 0.9 to approximately 1.8 g / m 2 are preferably produced. The mass per unit area (“layer weight”) can, as is known to the person skilled in the art, be determined by weighing a phosphated sample sheet, detaching the phosphate layer in 5% chromic acid solution and weighing the sample sheet back. This method is described, for example, in DIN 50942. For the production of phosphate layers with the desired mass per unit area, phosphating solutions are preferably used, the free acid content of which is in the range from approximately 1.5 to approximately 2.8 points and the total acid content is in the range of approximately 25 to approximately 35 points Range from about 50 to about 70 ° C. and in particular in the range from about 55 to about 65 ° C. Preferred treatment times are in the range from about 5 to about 10 seconds. The phosphating solution is preferably sprayed onto the metal surface and rinsed off with water after the desired treatment time.

The metal surface must be completely water wettable before applying the phosphating solution. This is usually the case in continuously operating conveyor systems. However, if the belt surface is oiled, this oil must be removed by a suitable cleaner before phosphating. The procedures for this are common in the art. Before phosphating, activation is usually carried out using activation agents known in the art. Solutions or suspensions are usually used which contain titanium phosphates and sodium phosphates. The activation is followed by the use of the phosphating process according to the invention, which is advantageously followed by a passivating rinse. An intermediate rinse with water usually takes place between phosphating and passivating rinsing. Treatment baths containing chromic acid are widely used for passivating rinsing. For reasons of work and environmental protection and for disposal reasons, however, there is a tendency to replace these chromium-containing passivation baths with chromium-free treatment baths. Purely inorganic bath solutions, in particular based on hexafluorozirconates, or also organic-reactive bath solutions, for example based on substituted poly (vinylphenols), are known for this. Rinse solutions which contain 0.001 to 10 g / l of one or more of the following cations can also be used: lithium ions, copper ions, silver ions and / or bismuth ions.

The metal tapes according to the invention can be phosphated directly with an organic coating. However, you can also in the initially unpainted state after cutting, shaping and joining to components such as Automobile bodies or household appliances are put together. The associated forming processes are facilitated by the phosphate layer. If the finished components have a low corrosive stress, such as in household appliances, the devices assembled from the pre-phosphated metal can be painted directly. For higher corrosion protection requirements, such as those made in automobile construction, it is advantageous to have a phosphating treatment again after assembling the bodies.

The invention is explained in more detail by the following exemplary embodiments (Table 1). Phosphating processes according to DE-A-39 20 296 were used as comparative examples. Cleaned sample sheets were activated with an activating agent containing titanium phosphate and phosphated in the spray. After an intermediate rinse, they were treated with a post-passivation solution based on chromate and, after rinsing with fully deionized water, painted gray with BASF KTL FT 85-7042. Results of corrosion protection tests in accordance with VDA Alternating Climate Test 621-415 over 10 rounds, combined with stone chip testing in accordance with VW standard P 1210 (K = 10: worst value, K = 1: best value) are shown in Table 2. The paint infiltration was measured on a scratch over half the scratch width.

Table 1: Phosphating parameters and layer weights

^l) ZE: galvanized steel on both sides; Z: hot-dip galvanized steel on both sides

Table 2:

Corrosion test results VDA alternating climate test 621-415, 10 rounds, with stone chips according to VW P 1210

Claims

claims

1. Method for phosphating steel strip or steel strip galvanized on one or both sides or galvanized with alloys by spraying or immersion treatment for a period in the range from 2 to 20 seconds with an acidic, zinc, magnesium and manganese-containing phosphating solution with a temperature in the range of 50 to 70 ° C, characterized in that the phosphating solution is free of nitrate ions and that

1 to 4 g / 1 zinc ions,

1.2 to 4 g / 1 manganese ions

1 to 4 g / 1 magnesium ions

10 to 30 g / 1 phosphate ions

Contains 0.1 to 3 g / 1 hydroxylamine in free, ionic or bound form, has a free acid content in the range from 0.4 to 4 points and a total acid content in the range from 15 to 45 points.

2. The method according to claim 1, characterized in that the phosphating solution contains no more than 0.05g / l nickel ions.

3. The method according to one or both of claims 1 and 2, characterized in that the phosphating solution contains up to 0.8 g / 1 fluoride in free or complex-bound form.

4. The method according to one or more of claims 1 to 3, characterized in that the phosphating solution contains 0.15 to 1 g / 1 hydroxylamine in free, ionic or bound form.

5. The method according to one or more of claims 1 to 4, characterized in that the phosphating solution has a free acid content in the range from 1.5 to 2.8 points and a total acid content in the range from 25 to 35 points.

6. The method according to one or more of claims 1 to 5 for the production on both sides of phosphate layers with a mass per unit area in the range of 0.4 to 2.0 g / m ² on steel strip or on one or both sides galvanized or alloy galvanized steel strip.