EP0111223B1 - Process for phosphatizing metallic surfaces, and solutions for use therein - Google Patents

Abstract

A process for phosphating metal surfaces, particularly iron, steel and zinc-plated steel, by treating them with aqueous, acidic zinc phosphate baths at only moderately elevated temperatures in the range of from 22 DEG to 38 DEG C. The baths used have 2 to 6 g/l of Zn; 4 to 23 g/l of PO43-; a free acid content of 0.05 to 0.4 points; and a pH-value of 3.0 to 4.0. The ratio of Zn to PO43- is preferably in the range from 1:2 to 1:11. The baths can advantageously contain nickel-(II)-ions, with a nickel content not exceeding a Zn:Ni ratio of 1:0.5. This process gives iron-containing zinc phosphate layers having an iron content of from 5 to 20% by weight and is particularly useful for pretreatment for subsequent cathodic electro-dip-lacquering.

Description

The invention relates to a process for the phosphating of iron, steel and galvanized steel with aqueous, acidic baths which contain zinc phosphate and, if desired, customary activating and / or layering-improving additives. The new process is particularly suitable as a pretreatment of the metal surfaces for a subsequent cathodic electrodeposition.

A number of characteristic bath types and associated process conditions are known today for phosphating metal surfaces. The classic zinc phosphate baths work in the comparatively high temperature range of 50 - 60 ° C and form essentially iron-free zinc phosphate layers on the metal surface. In a later development, with relatively low-zinc and phosphate-rich baths - also at comparatively higher temperatures - the incorporation of iron into the deposited zinc phosphate layers and thus the formation of particularly desirable stable zinc phosphate layers is made possible. DE-A-22 32 057 describes aqueous acidic phosphating solutions with a weight ratio of Zn: P0 ₄ = 1: (12-110) for the surface treatment of metals. The thin and uniform phosphate coatings are particularly suitable as a basis for the subsequent electrocoating. For example, nitrite ions and / or aromatic nitro compounds are known as accelerators for such phosphating baths - compare DE-A-30 04 927.

In contrast, EP-A-0 056 881 wants to achieve an improvement by working with chlorate-containing zinc phosphate solutions in the temperature range from 30-60 ° C., the 0.5-1.5 g / 1 Zn, 0.4-1, 3 g / I Ni, 10 - 25 g / IP ₂ 0 ₅ and 0.8 - 5 g / 1 C10 ₃ , to which no nitrite is added and in which the weight ratio of Zn: Ni to a value between 1: (0 , 5 - 1.5), Zn: P ₂ 0 ₅ to a value between 1: (8 - 85) and free P ₂ O ₅ : total P ₂ 0 ₅ to a value between (0.005 (at approx. 30 ° C) - 0.06 (at approx. 60 ° C)): 1 is set. Compliance with the concentration ratio between Zn and P ₂ 0 ₅ should be of crucial importance for the quality of the phosphate layers produced by this method. With a conversion factor P ₂ O ₅ → P0 _{4 of} 1.338, the lower limit of this ratio (Zn / P ₂ 0 ₅ = 1/8) is 10.7 parts by weight P0 ₄ per 1 part by weight Zn.

All of these proposals of the prior art work with comparatively high total acid contents, or in other words: with a not inconsiderable consumption of chemicals per unit volume of the aqueous treatment solution. Reducing this need for chemicals could significantly improve the profitability of such phosphating processes. For example, the loss of material due to drag-out can be significantly reduced. Overall, the chemical consumption is reduced.

Furthermore, EP-A-0 036 689 describes a method for applying phosphate coatings on iron, zinc and / or aluminum, which can serve as pretreatment of the metal parts before a cathodic electrodeposition. The phosphating solutions disclosed here contain zinc, manganese, phosphate and accelerator, the manganese content being 5 to 33% by weight of the zinc content and a combination of chlorate and nitrobenzenesulfonate being used as the accelerator. The content of these solutions in total acid is preferably 20 points and in free acid 0.8 to 1.0 points. The application temperature of the solutions is preferably 25 to 50 ° C, in particular 25 to 35 ° C.

FR-A-23 89 683 also describes a means for applying phosphate coatings on metal surfaces made of iron, zinc or aluminum. These phosphating solutions contain zinc ions, phosphate ions. Manganese ions - the manganese content being 5 to 50% by weight of the zinc content - an oxidizing agent and boron fluoride. The total acid score of the solutions is preferably greater than 20 and the ratio of total acid to free acid is preferably 50: 1 to 100: 1. The application temperature of such solutions is equal to or less than 43 ° C. and is preferably in the range from 21 to 43 ° C. The metal parts phosphated in this way can also be coated with an electric lacquer.

The subject of FR-A-13 42 472 is a method for applying phosphate layers on surfaces made of iron, steel, zinc or their alloys by means of solutions which are at least as large as zinc, alkali metal, phosphate, nitrate, nitrate ions of phosphate ions. Here, the metal surfaces are treated at about 20 to 45 ° C, preferably at 35 to 40 ° C, with a solution in which - if a ratio of P0 ₄ : Zn: N0 ₃ = 1: (0.8 to 1, 7): (1.5 to 3.2) and a total acid score of about 8 to 14 points - to prevent the occurrence of free acid, a nitrite content of about 0.1 to 1 g / is maintained. The slight formation of free acid in the solution when metal parts pass through is thus completely compensated for by the addition of nitrite. The phosphate coatings formed on the metal surfaces in this way are suitable as a lacquer primer for commercially available lacquers and to facilitate cold working.

The layer thickness of these phosphate coatings can be increased by adding a small amount of Citric acid and / or tartaric acid or their salts are adjusted or regulated to the phosphating solutions. Such an addition can increase the free acid content in the solutions to 0.4 points.

The invention is based on the object of proposing acidic, aqueous zinc phosphate-containing baths within the scope of the knowledge known per se for phosphating metal surfaces, which baths work in particular with a significantly lower total acid content. At the same time, however, the production of high-quality zinc phosphate layers should be ensured, which are characterized in particular by comparatively high iron contents and are accordingly very particularly suitable for subsequent cathodic electrocoating. At the same time, the invention aims to enable a type of process that can be operated effectively at very low temperatures.

The solution of the task according to the invention is based on the finding that the combination of certain wheel parameters enables the effective reduction in the total acid content and thus the desired reduction in the chemical requirement, but that at the same time these baths at temperatures below 40 ° C effectively contain the desired iron Zinc phosphate layers can be deposited.

• a) 2 bis 4 g/I Zn
• b) 4 bis 13 g/I P0₄ ^3-
• c) Gewichts-Verhältnis Zn: P0₄ ^3- = 1: (2 bis 4)
• d) Gehalt an freier Säure: 0,1 bis 0,2 Punkte
• e) Gehalt an Gesamtsäure: 9 bis 15 Punkte
• f) pH-Wert: 3,5 bis 4,0

The invention accordingly relates in a first embodiment to a process for phosphating metal surfaces made of iron, steel and galvanized steel, for pretreatment for a subsequent cathodic electrocoating, iron-containing zinc phosphate layers having an iron content of 5 to 20% by weight on iron and steel be formed by treating the surfaces with aqueous, acidic zinc phosphate baths at only moderately elevated temperatures, which is characterized in that one works in the temperature range from 28 to 38 ° C with baths that are free of manganese and meet the following conditions: .

• a) 2 to 4 g / l Zn
• b) 4 to 13 g / I P0 ₄ ^3-
• c) Weight ratio Zn: P0 ₄ ^3- = 1: (2 to 4)
• d) Free acid content: 0.1 to 0.2 points
• e) Total acidity: 9 to 15 points
• f) pH: 3.5 to 4.0

In a further embodiment, the invention further relates to an aqueous, acidic phosphating bath for the above-mentioned application, the composition of which is also characterized by the features mentioned above and which additionally contains customary activating and / or layer-improving additives.

The determination of the free acid content according to the number of points as well as the number of points for total acid, which will be discussed later, is carried out according to the recognized methods relevant here (compare, for example, "The Phosphating of Metals", Leuze-Verlag / Saulgau, 1974, pp. 274-277 ):

The free acid score corresponds to the consumption of ml of n / 10 NaOH in the titration of 10 ml bath solution until the change in the first H ₃ P0 ₄ stage (indicator methyl orange or bromophenol blue).

Punktzah) of total acid corresponding to the consumption of ml of n / 10 NaOH when titrating 10 ml bath solution against phenolphthalein as an indicator.

The process according to the invention thus uses comparatively high levels of zinc in the bath solution, in particular in comparison with the cited prior art documents, but at the same time only limited amounts of phosphate ions - and ultimately only limited amounts of total acid - are used. It works with baths in which the weight ratio Zn / P0 ₄ ^{3- is} in the range of 1: 2 to 1: 4.

With regard to the P0 ₄ ^3- content of the bath, this means in absolute terms that it is possible to work with comparatively low concentrations of the P0 ₄ ^3- value. This P0 ₄ ^3- content is in the range of 4 to a maximum of 13 g / 1 bath solution. According to the invention, it is particularly expedient to work with a P0 ₄ ³ content in the range of about 4-8 g / 1 bath solution.

The zinc content in the phosphating bath is in the range of 2-4 g / l bath solution. The free acid content is in the range of 0.1 - 0.2 points. The pH range for these phosphating baths of the invention is in the range 3.5 - 4.0. This type of bath can be used effectively in the temperature range of 28 - 38 ° C.

It is crucial for the method according to the invention and the results obtained therewith that the combination of parameters selected according to the invention enables the formation of zinc phosphate layers with a high iron content on iron and steel. According to the invention, such zinc phosphate layers with iron contents in the range of about 5-20% by weight are created on iron and steel. This results in the phosphating layers in the process according to the invention which - presumably because of their high phosphophyllite content - have the desired high stability when subsequently used in cathodic electrocoating.

Within the scope of the objective according to the invention, phosphating baths are used, the total acid content of which does not exceed numerically limited values in the range of 15 points. Phosphating baths with a total acid content in the range of 9-15 points are suitable for the process according to the invention.

Otherwise, the auxiliary components or constituents customary in baths of this type can also be used in the phosphating solutions according to the invention. It is of particular importance here, however, that the common use of manganese to this day is not necessary. This is an important advantage of the method according to the invention over other characteristic baths of the prior art, in particular over the so-called low-zinc baths, which operate at a comparatively higher temperature.

Customary activating additives include components such as chlorate, nitrate, nitrite, hydrogen peroxide, aromatic nitro compounds, simple and / or complex fluorides and / or organic and / or inorganic complexing agents. The following applies here in detail:

The use of chlorate is generally recommended. The chlorate content is expediently in the range from approximately 0.1 to 30 g / l bath solution, preferably in the range from approximately 1.5 to 10 g / l bath solution. Any nitrate ions used are expediently in the concentration range of 1 - 10 g / l bath solution. If nitrite ions are to be used in the bathroom, the range of 0.01 - 1 g / 1 bath solution is particularly suitable. Hydrogen peroxide can be used in the same area. Aromatic nitro compounds, especially 3-nitrobenzenesulfonic acid or its salts, but also other representatives of this class of substances, e.g. B. nitroresorcinol or nitrobenzoic acid are known accelerators in phosphating baths. Compounds of this type are expediently used in amounts of 0.01-2 g / l bath solution.

In a manner known per se, the layer formation on the metal surfaces can be improved by adding simple and / or complex fluorides. The fluoride ion content is expediently in the range of 0.01-2 g / l bath solution. In addition, or instead, the complex fluoride can be, for example, the SiF ₆ ^{2 ion} , with concentration ^ranges of about ^0.01-2 g / l bath solution being preferred here too.

The solutions can also contain complexing agents known per se of organic or inorganic origin. Suitable organic complexing agents are, for example, tartaric acid or tartrate, hydroxyethylenediamino-triacetic acid or its salts, gluconic acid or its salts and / or citric acid or its salts. Inorganic complexing agents are polyphosphates, for example tripolyphosphate or hexamethaphosphate. Complexing agents of this type are usually present in the bath in quantities of 0.01-5 g / l.

In addition to zinc, the treatment bath can contain further metal cations, in particular divalent metal cations. The use of nickel (II) in the phosphating bath may be preferred. In the preferred embodiments of the invention, however, the nickel content is limited in relation to the zinc and at most reaches this zinc content. However, the weight ratio Zn / Ni of 1: 0.5 is preferably not exceeded. Preferred nickel contents within the framework of the teaching according to the invention can be in the range of approximately 0.01-1 g / 1 bath solution.

It may be expedient - but is by no means necessary - to subject the metal surfaces to be treated to an activation known per se before the phosphating baths according to the invention are used. Appropriate activating agents based on titanium phosphate are suitable, for example.

Zinc phosphate layers with a high iron content designed according to the invention are suitable for all types of use of phosphate layers known to date, but they have particular advantages for subsequent cathodic electrocoating. They show a high resistance of the paint film against paint infiltration under corrosive stress as well as high and satisfactory values of the paint adhesion to the metallic surface. The method according to the invention is thus used, for example, in the phosphating of car bodies.

A specific embodiment of the invention is illustrated in the following example.

example

A concentrate was made from

This concentrate was diluted to a solution with 0.34% Zn, 0.85% P0 ₄ , 0.34% N0 ₃ , 0.27% CIO ₃ , 0.007% SiF _s , 0.007% F and 0.014% 3-nitrobenzenesulfonic acid to give a total acid score of 14.4. The free acid was reduced to a pH in the range 3.5-4 by adding sodium hydroxide.

Steel parts were sprayed for 2 minutes at 40 ° C with an alkaline cleaning solution and then rinsed with water. The parts were then sprayed for 1 minute and 2 minutes phosphated while diving at 32 ° C with the working solution described above. The parts were then rinsed with water, rinsed with distilled water and dried by blowing with compressed air. The parts were then coated with a cathodic electrocoating material and dried for 20 minutes by heating at 185 ° C.

The dry film thickness of the paint was 18 µm. The parts were then provided with individual cuts and subjected to the salt spray test DIN 50021 for 240 hours. The evaluation according to DIN 53167 showed an infiltration of <0.1 mm. It follows that the proposed method of working gives a good coating despite the low treatment temperature.

Claims (4)

1. A process for phosphating metal surfaces of iron, steel and galvanized steel in preparation for subseguent cathodic electrodeposition, iron-containing zinc phosphate coatings having an iron content of from 5 to 20% by weight being formed on iron and steel, by treatment of the surfaces with aqueous, acidic zinc phosphate baths at only moderately elevated temperatures, characterized in that the process is carried out at temperatures of from 28 to 38° C using baths which are free from manganese and which meet the following conditions:

a) 2 to 4 g/I Zn

b) 4 to 13 g/I P04

c) ratio by weight Zn: PO₄ ^-3 = 1: (2-4)

d) free acid content: 0,1 to 0,2 points

e) total acid content: 9 to 15 points

f) pH-value: 3,5 to 4,0.

2. A process as claimed in Claim 1, characterized in that the baths used additionally contain nickel(II) ions, the nickel content not exceeding the ratio by weight Zn:Ni = 1:0.5

3. A process as claimed in Claims 1 and 2, characterized in that the baths used contain standard activating additives and/or additives which improve formation of the coating, such as chlorate, nitrate, nitrite, hydrogen peroxide, aromatic nitro compounds, simple and/or complex fluorides and/or complexing agents.

4. An aqueous acidic treatment bath for phosphating metal surfaces or iron, steel and galvanized steel in preparation for subsequent cathodic electrodeposition, iron-containing zinc phosphate coatings having an iron content of from 5 to 20% by weight being formed, characterized in that the bath is free from manganese and in that its composition meets the following conditions:

a) 2 to 4 g/I Zn

_b) ₄ to 13 g/l P04

c) ratio by weight Zn: P04 = 1: (2-4)

d) free acid content: 0,1 to 0,2 points

e) total acid content: 9 to 15 points

f) pH-value: 3,5 to 4,0

and additionally containing standard activating additives and/or additives which improve formation of the coating.