EP0155547A1 - Process for the zinc-calcium phosphatizing of metal surfaces at a low treating temperature - Google Patents

Abstract

The invention relates to a process for phosphatizing metal surfaces with an acidic phosphatizing solution containing zinc ions, calcium ions, phosphate ions and accelerators and, optionally, other additives, characterized in that, after degreasing and without activation, the metal surfaces are brought into contact at 30 DEG to 65 DEG C. with solutions which contain Zn2+, Ca2+, PO43- and, as accelerators, nitrate and/or nitrite and/or chlorate ions and/or an organic nitro compound and/or H2O2 or an inorganic peroxide and which have a pH-value of from 2.2 to 3.8, a ratio of free acid to toal acid of from 1:10 to 1:60 and a ratio by weight of (Ca2++Zn2+) to PO43- of from 1:>8 to 1:40, and to the use of this process for pretreating the metal surfaces for painting by electrodeposition, particularly by cathodic electrodeposition.

Description

The invention relates to a method for phosphating metal surfaces, in particular surfaces made of iron, steel, zinc and / or aluminum by means of a zinc, calcium, phosphate and accelerator, and, if appropriate, further additives containing phosphating solution and the use of this method for pretreating the metal surfaces for the Electrocoating, in particular for cathodic electrocoating.

The protection of metallic surfaces, in particular the protection of iron and steel surfaces, by phosphate-containing coatings has been known for a long time. The so-called "non-layer-forming phosphating", ie the use of alkali and / or ammonium orthophosphate solutions for producing iron phosphate layers in which the iron ion comes from the metallic surface to be coated, and Distinguished the so-called "layer-forming phosphating", in which zinc phosphate layers or zinc calcium phosphate layers are formed on metal surfaces using zinc or zinc / calcium phosphate solutions.

Such phosphate layers not only improve the corrosion protection of the metal surfaces, but also increase the adhesion for paints to be applied to the surface. In certain cases, they can also help to improve the properties of metal sheets during cold forming and when using deep-drawing processes.

Zinc-calcium-phosphate solutions are increasingly being used especially for the phosphating of metal surfaces that are later to be coated with electrocoating materials. Experiences about the composition of the bathroom solutions on the one hand and basic knowledge about the structure of phosphating layers on the other (A. Neuhaus and M. Gebhardt, Werkstoffe und Korrosion, 567 (1966)) have given the teaching that the construction of uniform and closed phosphating layers is not only of depends on the composition of the bath solutions, but also on the pretreatment of the raw sheets, the activation before the phosphating step, the choice of a suitable accelerator and other process parameters.

DE-OS 15 21 818 discloses aqueous phosphating solutions which contain zinc, calcium, nickel, phosphate, nitrite and nitrate ions as essential constituents and which permit the phosphating of galvanized iron metal surfaces at elevated temperatures. A disadvantage of the disclosed phosphatie Solution, however, is that the temperature of the phosphating step must be relatively high in order to be able to achieve an economically justifiable, short application time for the solutions: With total immersion times of 1 to 20 seconds, the temperatures are between 66 and 116 ° C. The solutions used contain very high levels of zinc and calcium ions with a low phosphate ion content; the weight ratio of the sum of zinc and calcium ions to phosphate ions is in the range from 1: 3.5 to 1: 7.1.

Aqueous solutions for phosphating processes which contain zinc, calcium, phosphate and optionally also nickel ions and additionally H ₂ 0 ₂ as accelerators are described in BE-PS 81 12 20. However, the application temperatures of such solutions are also relatively high. In relation to the phosphate content, the content of zinc and calcium ions is very high.

DE-AS 22 32 067 describes processes for phosphating metal surfaces by means of acidic, zinc-phosphate solutions containing oxidizing agents, in which a comparatively low zinc content contrasts with a significantly higher phosphate content and which can contain further divalent metal ions, for example also Ca ²⁺ ions and DE-OS 31 18 375 described. With the method according to DE-AS 22 32 067, high-quality phosphate layers can be formed with fresh application solutions; After passing through a higher number of sheets, however, the corrosion protection deteriorates due to irregular phosphating layers, in some cases no more useful protective layers are formed.

Another disadvantage of most of the processes described so far is that the quality of the heavy metal phosphate layers formed in the phosphating step is very different from that of the degreasing agent treatment of the metal surfaces and also depends on the activation. In particular, the activation step is of great importance insofar as it forms the basis for the adhesion of the later phosphate layers and thus significantly influences the quality of the phosphate coatings that form. The desired formation of thin, fine-grained, crystalline phosphate coatings is only possible after sufficient activation by means of suitable activation agents, for example activation solutions containing phosphate. This results in particular in the difficulty of avoiding speck formation, which adversely affects the quality of the phosphating layer.

Surprisingly, it has now been found that thin, fine-grained, crystalline zinc-calcium-phosphate layers of high homogeneity are obtained even at a low treatment temperature if metal surfaces are treated with acidic aqueous solutions, the zinc, calcium and phosphate ions, and one or more accelerators contain, whereby a narrow range of pH, a certain acid ratio, as well as a predetermined weight ratio of the sum of calcium and zinc ions to phosphate ions are to be observed.

The invention therefore relates to a process for phosphating metal surfaces, in particular surfaces made of iron, steel, zinc and / or aluminum by means of an acidic, zinc, calcium, phosphate and accelerator, and optionally further additives containing phosphating solution, which is characterized in that the surfaces after degreasing cleaning without activation step in the temperature range from 30 to 65 ° C in contact with solutions that are more than 0.5 to 1.5 g. 1 ^-1 Ca ²⁺ , 0.5 to 1.5 g. 1 ^-1 Zn, 10 to 50 g. 1 ^-1 PO ₄ ^3- , as well as Be accelerator 0.5 to 30 g. 1 NO ₃ ^- and / or 0.01 to 0.6 g. 1 ^-1 NO ₂ ^- and / or 0.2 to 10 g. 1 ^-1 ClO ₃ ^- and / or 0.1 to 2 g. 1 ^{-1 of} an organic nitro compound and / or 0.01 to 0.5 g. 1 H ₂ 0 ₂ or an inorganic peroxide and a pH of 2.2 to 3.8, a ratio of free acid to total acid from 1:10 to 1:60, and a weight ratio (Ca ²⁺ + Zn ²⁺ ): PO ₄ ^3- from 1:> 8 to 1:40.

The invention also relates to the use of this method for pretreating the metal surfaces for electrocoating, in particular for cathodic electrocoating

The method according to the invention is particularly suitable for the phosphating of metal surfaces made of iron, steel and zinc. However, surfaces made of aluminum must also be coated with zinc-calcium layers in accordance with the method presented.

As could be found, the treated metal surfaces are coated with a layer of dizinc calcium phosphate dihydrate (Scholzit). Although zinc and calcium ions are incorporated into the 'phosphating layer in a molar ratio of 2: 1, it is essential for the composition of the bath solutions according to the process of the invention that they contain zinc and calcium ions in a weight ratio of 1: 0.5 to 1: 1.5, preferably in a ratio of 1: 1.

For this purpose, suitable water-soluble zinc or calcium salts or solutions are added to the solutions, so that the content of Zn2 + 0.5 to 1.5 g. 1 ^-1 phosphating solution and the content of Ca ²⁺ more than 0.5 to 1.5 g. 1 ^-1 phosphating solution. In particular ZnO, phosphoric acid and Ca (NO ₃ ) ₂ . 4H ₂ 0 used as starting compounds.

The proportion of PO ₄ ^3- ions which can be adjusted by phosphoric acid in the phosphating solutions according to the invention is considerably higher: it is in the range from 10 to ₅₀ g. 1 ^-1 .

The stated amounts of the active ions forming the main constituents of the phosphating solution according to the invention are therefore characterized in that the weight ratio of the sum of calcium and zinc to phosphate is always in the range from 1: greater than 8 to 1:40. This ensures that homogeneous scholzite layers are formed on all treated metal surfaces; the formation of e.g. No tertiary zinc phosphate tetrahydrate (Hopeit) or decy iron phosphate tetrahydrate (phosphophyllite), which together with scholzite would lead to a less homogeneous and poorly adhering protective layer, is observed.

Another essential process parameter can be seen in the fact that the molar ratio of free acid to total acid (acid ratio) is to be set to values of 1:10 to 1:60. This means in particular that a relatively low value for the concentration of free acid is particularly important for the formation of good scholzite layers.

As an accelerating oxidizing agent, the phosphating bath solutions for the process according to the invention are nitrate ions in an amount of 0.5 to 30 g. 1 ^-1 , nitrite ions in an amount of 0.01 to 0.6 g. 1 ^-1 , chlorate ions in an amount of 0.2 to 10 g. 1-1, organic nitro compounds in one menu from 0.1 to 2 g. 1 ^-1 and / or inorganic peroxides or H ₂ 0 ₂ in an amount of 0.01 to 0.5 g. 1 bath solution added.

Are to be phosphated by the novel process of aluminum surfaces so the bath solutions may be simple and / or complex fluorides are added in an amount of 0.01 to 2 g ^-1 .1 to even the smallest amounts of aluminum, consisting of the metal surface in the bath occur and could impair its effectiveness by complexing with fluoride ions.

The phosphating solutions with which metal surfaces are phosphated in accordance with the method according to the invention can also contain further metal ions, for example Ni2 +. Their content is in the range from 0.01 to 1.5 g. 1 ^-1 bath solution.

The treatment of the iron, steel, zinc and / or aluminum surfaces according to the present method can be carried out by spraying, dipping or flooding; however, they are also combined processes, e.g. Spray diving applicable with equally good success. The times during which the phosphating solutions are in contact with the metal surfaces are between 60 and 240 seconds. for spraying processes between 60 and 180 seconds and for diving 90 to 240 seconds. However, considerably shorter treatment times are also possible.

According to the invention, the metal surfaces are treated with the phosphating solutions in the temperature range from 30 to 65 ° C. Temperatures between 48 and 57 ° C are preferably used.

One of the main advantages of the method according to the invention is that the formation of the scholzite layers on the metal surfaces takes place completely independently of what type of cleaning has preceded the phosphating step. When using the method according to the invention there is complete freedom with regard to the choice of degreasing and cleaning agents.

Another advantage can be seen in the fact that particularly thin, fine-grained crystalline phosphate layers are obtained even without the use of activating agents, as was customary in previously known processes. This not only saves at least one process step prior to phosphating, but also the necessary raw materials, such as Titanium phosphates, which are used as activating agents.

According to the method of the present invention, excellent protective layers with a layer thickness between 0.5 and 5 μm are obtained, which are outstandingly suitable as a basis for electrocoating materials, in particular for cathodic electrocoating materials, as are increasingly used in the automotive industry. However, the phosphating layers obtained according to the invention are also suitable as a basis for other organic surface protective layers.

The invention is illustrated by the examples below.

The phosphating solutions according to the invention were prepared by processes known per se by adding the desired components, in particular zinc oxide, Concentrates, salts and solutions containing phosphoric acid and calcium nitrate tetrahydrate combined and diluted with water to the concentrations according to the invention.

example 1

• 1,0 gl^-1 Ca²⁺
• 1,2 gl^-1 Zn²⁺
• 29,5 gl^-1 PO₄ ^3-
• ₁,₀ gl^-1 ClO₃
• ₃,2 gl^-1 NO₃ ^-
• ₀,1 gl^-1 N0₂

enthielt.A phosphating solution was prepared that

• 1.0 gl ^-1 Ca ²⁺
• 1.2 gl ^-1 Zn ²⁺
• 29.5 gl ^-1 PO ₄ ^3-
• _{1, 0} gl ^-1 ClO ₃
• ₃ , 2 gl ^-1 NO ₃ ^-
• _0, 1 gl ^-1 N0 ₂

contained.

The phosphating solution had the following characteristics: pH: approx. 3.1
Acid ratio: approx. 1:19

Steel sheets which had previously been dipped with an alkaline cleaning solution at 50 ° C. for 3 minutes and rinsed with water were treated with the aforementioned phosphating solution for 4 minutes at 55 ° C. in immersion. It was then rinsed with water and with distilled water and dried.

The phosphate layers produced were finely crystalline and closed.

The sheets were then coated with a cathodic electrocoating material and dried for 20 minutes by heating at 185 ° C. The dry film thickness of the lacquer was 18 µm.

The sheets were then provided with a single cut in accordance with DIN 53167 and subjected to the salt spray test in accordance with DIN 50021 for a period of 480 hours. The evaluation according to DIN 53167 showed an infiltration of <0.1 mm.

The example illustrates that the process according to the invention produces good phosphate layers.

Example 2

• 0,6 g_l ^-1 Ca²⁺
• ^0,7 gl^-1 Zn²⁺
• 22,6 gl^-1 PO₄ ^3-
• 0,3 gl^-1 Ni²⁺
• ₂,₇ gl^-1 NO₃ ^-
• 0.5, gl^-1 F
• 0,1 gl^-1NO₂ ^-

enthielt.A phosphating solution was prepared that

• 0.6 g _l ^-1 Ca ²⁺
• ^0.7 gl ^-1 Zn ²⁺
• 22.6 gl ^-1 PO ₄ ^3-
• 0.3 gl ^-1 Ni ²⁺
• _{2, 7} gl ^-1 NO ₃ ^-
• 0.5, gl ^-1 F
• 0.1 gl ^-1 NO ₂ ^-

contained.

The phosphating solution had the following characteristics: pH: approx. 3.3
Acid ratio: approx. 1:39

Steel sheets which had previously been spray-cleaned with an alkaline cleaning solution at 45 ° C. for 60 seconds were sprayed with the aforementioned phosphating solution for 90 seconds at 48 ° C. It was then rinsed with water and with distilled water and dried by compressed air.

The phosphate layers produced were finely crystalline and closed.

The sheets were then coated with a cathodic electrocoating material and dried for 20 minutes by heating at 185 ° C. The dry film thickness of the lacquer was 18 µm.

The sheets were then provided with a single cut in accordance with DIN 53167 and subjected to the salt spray test in accordance with DIN 50021 for a period of 480 hours. The evaluation according to DIN 53167 showed an infiltration of <0.1 mm.

The example illustrates that the process according to the invention produces good phosphate layers.

Example 3

• 1,3 gl^-1 Ca2+
• 1,3 gl^-1 Zn²⁺
• 21,2 g_l ¹ PO₄ ^3-
• 1,0 gl^-1 Ni²⁺
• ₂,₂ gl^-1 ClO₃ ^-
• 0,6 gl^-1 Natriumnitrobenzolsulfonat

enthielt.A phosphating solution was prepared that

• 1.3 gl ^-1 Ca2 +
• 1.3 gl ^-1 Zn ²⁺
• 21.2 g _l ¹ PO ₄ ^3-
• 1.0 gl ^-1 Ni ²⁺
• ₂ , ₂ gl ^-1 ClO ₃ ^-
• 0.6 gl ^-1 sodium nitrobenzenesulfonate

contained.

• pH-Wert: ca. 2,9

Säureverhältnis: ca.1 : 18.6

The phosphating solution had the following characteristics:

• pH: approx.2.9

Acid ratio: approx. 1: 18.6

Electrolytically galvanized steel sheets, which had previously been cleaned with an alkaline cleaning solution at 50 ° C. for 3 minutes by immersion treatment and rinsed with water, were treated with the aforementioned phosphating solution at 57 ° C. for 3 minutes in immersion. It was then rinsed with water and with distilled water and dried by compressed air.

The phosphate layers produced were finely crystalline and closed.

The sheets were then coated with a cathodic electrocoating material and dried for 20 minutes by heating at 185 ° C.

The dry film thickness of the lacquer was 18 µm.

The sheets were then provided with a single cut in accordance with DIN 53167 and subjected to the salt spray test in accordance with DIN 50021 for a period of 480 hours. The evaluation according to DIN 53167 showed an infiltration of <0.1 mm.

The example illustrates that the process according to the invention produces good phosphate layers.

Example 4

• 1,0 gl^-1 Ca²⁺
• 1,4 gl^-1 Zn²⁺
• 26,6 gl^-1 PO₄ ^3-
• ₃,2 gl^-1 NO₃ ^-
• ₀,1 gl^-1 NO₂ ^-

enthielt.A phosphating solution was prepared that

• 1.0 gl ^-1 Ca ²⁺
• 1.4 gl ^-1 Zn ²⁺
• 26.6 gl ^-1 PO ₄ ^3-
• ₃ , 2 gl ^-1 NO ₃ ^-
• _0, 1 gl ^-1 NO ₂ ^-

contained.

• pH-Wert ca. 3,6

Säureverhältnis: ca 1 : 48

The phosphating solution had the following characteristics:

• pH approx.3.6

Acid ratio: approx. 1:48

Steel sheets which had previously been spray-treated with an alkaline cleaning solution at 45 ° C. for 60 seconds were sprayed with the aforementioned phosphating solution for 120 seconds at 35 ° C. It was then rinsed with water and with distilled water and dried by compressed air.

The phosphate layers produced were finely crystalline and closed.

The sheets were then coated with a cathodic electrocoating material and dried for 20 minutes by heating at 185 ° C. The dry film thickness of the lacquer was 18 µm.

The sheets were then provided with a single cut in accordance with DIN 53167 and subjected to the salt spray test in accordance with DIN 50021 for a period of 480 hours. The evaluation according to DIN 53167 showed an infiltration of 0.2 mm.

The example illustrates that the process according to the invention produces good phosphate layers.

Comparative example

According to DE-OS 15 21 818, p. 12, solution B was prepared.

Steel sheets which had previously been cleaned with an alkaline cleaning solution at 72 ° C. for 30 seconds were sprayed with the aforementioned phosphating solution for 60 seconds at 66 ° C. It was then rinsed with water and with distilled water and dried by compressed air. The phosphate layers produced were roughly crystalline and not completely closed.

The sheets were then coated with a cathodic electrocoating material and dried by heating at 185 ° C. for 20 minutes. The dry film thickness of the lacquer was 18 µm.

Then the sheets were provided with a single cut according to DIN 53167 and the salt spray test in accordance with. Subject to DIN 50021 for a period of 480 h. The evaluation according to DIN 53167 showed an infiltration of 4 to 6 mm

The comparative example illustrates that, in contrast to the examples according to the invention, significantly reduced corrosion protection is achieved.

Claims (7)

1. A process for phosphating metal surfaces, in particular surfaces made of iron, steel, zinc and / or aluminum, using an acidic, zinc, calcium, phosphate and accelerator, and optionally further additives, such as nickel or fluoride, containing a phosphating solution, characterized in that that one brings the surface after degreasing cleaning without activation step in the temperature range from 30 to 65 ° C in contact with solutions that a) more than 0.5 to ₁ g; _5. 1 ^-1 Ca ²⁺ ; b) 0.5 to 1.5 g. 1 ^-1 Zn ²⁺ , c) 10 to 50 g. 1 ^-1 PO ₄ ^3- ,
as well as an accelerator d) 0.5 to 30 g. 1 ^-1 NO ₃ ^- and / or 0.01 to 0.6 g. 1 ¹ N0 ² and / or 0.2 to 10 g. 1 ^-1 ClO ₃ ^- and / or 0.1 to 2 g. 1 ^{-1 of} an organic nitro compound and / or 0.01 to 0.5 g. 1 ^{-1 of} an inorganic peroxide or ^H ₂ ⁰ ₂
contain and a pH of 2.2 to 3.8, a ratio of free acid to total acid from 1:10 to 1:60, and a weight ratio (Ca ²⁺ + Zn ²⁺ ): PO ₄ ^3- of 1 :> 8 to 1:40. 2. The method according to claim 1, characterized in that the metal surfaces are brought into contact with phosphating solutions, the Zn ²⁺ and Ca ^{2 +} ions in a weight ratio of 1: 0.5 to 1: 1.5, preferably in Ratio 1: 1 included. 3. Process according to Claims 1 and 2, characterized in that the metal surfaces are brought into contact with phosphating solutions, the Ni ²⁺ in an amount of 0.01 to 1.5 g. 1 ^-1 included. 4. Process according to Claims 1 to 3, characterized in that the metal surfaces are brought into contact with phosphating solutions which contain simple and / or complex fluorides in an amount of 0.01 to 2 gl ^-1 . 5. Process according to Claims 1 to 4, characterized in that the metal surfaces are brought into contact with the phosphating solutions at 48 to 57 ° C. 6. Process according to Claims 1 to 5, characterized in that the metal surfaces are treated in immersion, spraying, flooding or by combined processes. 7. Application of the method according to claims 1 to 6 for the pretreatment of metal surfaces, in particular of iron, steel, zinc and / or aluminum for electrocoating, in particular for cathodic electrocoating.