DE69024774T2 - Process for the chemical conversion of metal objects, bath used therefor and concentrate for producing the bath - Google Patents

Abstract

Chemical conversion process characterised in that it comprises the use, at the time of the actual phosphatising stage, of a zinc conversion bath containing phosphate and nitrate ions as well as ferrous or ferric ions and from approximately 21 to approximately 100 g/l of at least one organic chelating agent.

Description

The invention relates to a chemical conversion process for metal substrates, in particular based on iron or galvanized steel, with a view to increasing their resistance to corrosion, their deformability or both at the same time.

It also relates to a chemical conversion bath suitable for use in that process and to a concentrate for preparing the bath.

It is known to carry out the chemical transformation of metal substrates by the classical processes of crystalline phosphating which lead to the deposition of a layer of insoluble phosphates on the metal surface.

The known processes by crystalline phosphating generally use acidic baths containing at least the following components:

- Orthophosphoric acid H₃PO₄

- a primary phosphate ion H₂PO₄-

- one or more metals selected from zinc or iron, manganese, nickel, copper, cobalt, calcium,

- at least one accelerator comprising an oxidizing agent such as chlorate, nitrate ions, nitrite or sodium metanitrobenzenesulfonate.

These treatments can be carried out on the substrates to be treated by spraying or dipping.

A treatment process includes work steps, which can include:

- one or more degreasing steps,

- one or more rinsing steps,

- if necessary, one or more pickling steps, followed by rinsing,

- if necessary, a surface conditioning step,

- the actual phosphating step,

- one or more rinsing steps,

- if necessary, a passivation step, generally in a chromate environment and a rinsing step,

- if necessary, a drying or heat treatment step.

The substrates treated in this way can be used as received or can be subsequently coated with a lubricant, protective oil or paint.

The type and composition of the resulting layer of insoluble phosphates depends essentially on:

- the type of substrate,

- the composition of the phosphating bath,

- the method of application of the product (spraying or immersion)

- from the pretreatment steps.

The state of the art describes various improvements for the chemical conversion baths, which aim to increase their performance.

Thus, according to patent FR-A-1 362 202, an improvement in the corrosion resistance of the phosphate layers is obtained by adding a small amount (up to 2% and preferably comprising between 0.5 and 1.5%) of organic complexing agents; resistance performances to salt mist ASTM B-117 of less than or equal to 25 hours are reported in this patent.

According to patent US-A-3 116 178 it is possible to add 2 to 50 g/l of aliphatic polyhydroxymonocarboxylic acid and gluconic acid to a phosphating bath.

According to patent FR-A-1 585 660, the coating weight can be increased by adding an organic polyacid to a phosphating bath accelerated by chlorate and containing almost no iron ions.

Finally, according to patent FR-A-2 531 457, the combination of a complexing agent and a soluble polyphosphate makes it possible to obtain coatings in which the corrosion resistance is considerably improved, mentioning resistance to salt mist which can reach 310 hours with respect to simply phosphated pieces.

The French patent in question indicates that an excess of polyphosphates can prevent the formation of the crystalline layer and suggests maturing the baths in question by pH development before use.

Since the maturation in question does not take place instantaneously, the regeneration steps for the bath are necessarily not continuous, which is a disadvantage since most industrial baths are continuously regenerated.

The invention aims primarily to remedy this disadvantage and to provide a chemical conversion bath which can be continuously regenerated with equivalent performance to that of the process described in patent FR-A-2 531 457.

Consequently, the chemical conversion process is characterized in that it comprises, at the time of the actual phosphating step, the use of a zinc conversion bath without chlorate ions and at a temperature of 70 to 99°C, containing 0.5 to 25 g/l of zinc ions, 5 to 40 g/l of phosphate ions, 2 to 40 g/l of nitrate ions and 0.1 to 15 g/l of iron ions and 21 to 100 g/l of at least one organic complexing agent comprising a polycarboxylic acid selected from the group comprising oxalic, malic, glutamic, tartaric, aspartic, malonic and citric acid or one of their salts.

The salts of the polycarboxylic acids mentioned are selected from among those of alkali metals, alkaline earth metals, ammonium, iron, zinc, manganese, molybdenum, cobalt and nickel.

Zinc ion, present in an amount of 0.5 to 25 g/l, can be introduced in any suitable way and in particular in the form of its salts with nitric, phosphoric, carboxylic, gluconic, citric acid or in the form of its oxide. It can be combined with all the metals currently used in a phosphating bath, such as manganese, nickel, cobalt, calcium, magnesium.

The iron ions, present in an amount of 0.1 to 15 g/l, can be introduced by dissolving iron in a solid state, such as iron splinters, solid pieces or steel scrap, or by means of iron salts with sulphuric, hydrochloric, hydrofluoric, phosphoric or nitric acid; they can be ferrous ion, ferric ion or a mixture of the two.

The phosphate ions on the one hand and the nitrate ions on the other hand are each present in the bath in an amount that can reach 40 g/l.

The bath used according to the invention can also contain hydrofluoric, silicofluoric or fluoroboric acid or several of them in order to regulate the balance between the pickling of the support by the acidic solution and the growth of the crystalline layer.

The bath used according to the invention is prepared by dilution, starting from a concentrate which is characterized in that it

- up to 500 g/l polycarboxylic acid, especially citrate,

- up to 200 g/l phosphate ions,

- up to 125 g/l zinc ions,

- contains up to 200 g/l nitrate ions.

The coatings produced by using the process according to the invention provide, in the sense of the test according to the ASTM B-117 standard, a resistance to salt mist comparable to that obtained according to patent FR-A-2 531 457, while allowing continuous regeneration.

The substrates treated by using the method according to the invention can be stored or used without subsequent protection, but it is also possible to paint these substrates in order to further increase their corrosion resistance; they can optionally be cold-formed after a possible lubricant application step.

According to an advantageous embodiment of the inventive method, the bath used contains

- 5 to 20 g/l phosphate ions,

- 21 to 60 g/l citrate,

- 2 to 40 g/l nitrate ion,

- 2 to 10 g/l zinc ion,

- 0.1 to 15 g/l iron.

According to a variation, it contains 0.02 to 3 g/l of nickel, cobalt or copper.

The chemical conversion bath used according to the invention is applied by spraying onto the metal substrate to be treated or by immersing the substrate in the bath, with immersion being preferred.

The chemical conversion process according to the invention comprises a complex of treatment steps, i.e.:

- one or more degreasing steps,

- one or more rinsing steps,

- if necessary, one or more activation or pickling steps

- a rinsing step,

- the actual chemical conversion step by using the bath according to the invention,

- a rinsing step,

- a drying step.

Due to the excellent resistance of the phosphate layers obtained by using the bath according to the invention to dry corrosion, the process turns out to be simplified by omitting the steps of chromate passivation and protection, subsequent to the chemical conversion of phosphating, and required in the processes of the known prior art.

The invention will be well understood thanks to the following non-limiting examples in which the advantageous embodiments of the bath used according to the invention are indicated.

EXAMPLE 1 (comparative example)

A conversion bath is prepared according to the known state of the art, which contains the following ions in addition to water:

Zn++ : 6 g/l

PO₄ total : 16.7 g/l

NO₃- : 10 g/l

Fe total : 1 g/l

Ni++ : 0.2 g/l

Bring its temperature to 95ºC; adjust the free acidity to 8.0 points in the working zone with concentrated sodium hydroxide solution (the free acidity is determined by titrating 10 ml of the cold solution with a N/10 sodium hydroxide solution to pH = 3.6).

Cold rolled steel plates with 0.02% carbon of grade ZES, currently used by the automotive industry, with dimensions of 150 mm x 140 mm, are subjected to the following treatment program:

Step 1: alkaline degreasing in aqueous solution based on degreasing agents sold by the applicant under the brand names RIDOLINE 1550 CF/2 (at 1.5% vol/vol) and RIDOSOL 550 CF (1.5% vol/vol). Temperature: 60ºC. Treatment time: 3 minutes.

Step 2: Rinse with cold running water for 2 minutes.

Step 3: Nitrate pickling Mordant: 60% nitric acid in aqueous solution (10% vol/vol). Temperature: 20ºC. Treatment time: 10 seconds.

Step 4: Rinse with cold running water for 1 minute.

Step 5: Actual conversion at 95ºC for 30 minutes.

Step 6: Rinse with cold running water for 10 seconds.

Step 7: Heat treatment for drying at a temperature of approximately 95ºC for 10 minutes.

The panels thus produced are subjected to a salt mist test at 5% (according to ASTM B-117).

After one hour of exposure, a very homogeneous rusting of 50% is achieved.

EXAMPLE 2 (comparative example)

A conversion bath is prepared according to the known state of the art by adding a quantity of 10 g/l of citric acid to the bath according to Example 1. The free acidity of the bath is adjusted to 15.0 points. The same procedure as above is carried out with the same cold-rolled steel plates. After 8 hours of exposure, a very homogeneous rusting of 50% is obtained.

EXAMPLE 3

A conversion bath for use in the process according to the invention is prepared by adding a quantity of 30 g/l of citric acid to the bath according to Example 1. The free acidity of the bath is adjusted to 18/19 points. Analogous cold-rolled steel plates and after 150 hours of exposure, receives a rusting of 10% in places. TABLE 1 Example No. Citric acid content of the bath (g/l) Salt mist test Rust formation immediately Haze after 100 h Rust to 50% less than 1 h Some rust spots after 150 h

EXAMPLE 4

The aim of this example is to demonstrate the possibility of applying the process to different industrial workpieces. The treatment is carried out on workpieces made of cement steel HRC 55 (with a carbon content of 0.55%) and mild steel XC 10 (with 0.1% carbon).

The following three compositions are produced:

Composition A:

56.7 g of water, 20 g of citric acid and 22.3 g of 75% phosphoric acid are mixed. 1 g of metallic iron is dissolved in this mixture.

Composition B:

46.7 g of water, 30 g of citric acid and 22.3 g of 75% phosphoric acid are mixed. 1 g of metallic iron is dissolved in this mixture.

Composition C:

7.5 g of zinc oxide and 1 g of nickel nitrate with 20% nickel are dissolved in a mixture containing 74.8 g of water and 16.7 g of 60% nitric acid.

The order of treatment is the same as that described in Example 1 with the exception of step 3, the nitrate treatment, which is replaced by an aqueous phosphate pickling carried out for 2 minutes at a temperature of 60ºC in an aqueous solution containing 7.75% vol/vol of 75% phosphoric acid.

The baths used at the time of the actual conversion step are composed as follows:

. Bath 1 : - 800 ml water

- 100 ml Composition A

- 100 ml Composition C

. Bath 2 : - 800 ml water

- 100 ml Composition B

- 100 ml Composition C

Two series of steel pieces of grade XC 10 and HRC 55, respectively, subjected to the treatment sequence of Example 1, using bath 1 for the XC 10 steel pieces and bath 2 for the HRC 55 steel pieces, are subjected to the salt spray test according to ASTM B-117 and the time after which red rust appears is determined.

The results are shown in Table 2. Table II Steel piece of the type Treatment according to Example 1 with Bath 1 (20 g/l citric acid Bath 2 (30 g/l citric acid Rust formation after 400 h Rust formation after 10 h No rust formation after 700 h Rust formation after 40 h

It is found that the use of a concentration of more than 20 g/l of citrate ion in the conversion bath gives a notable gain in terms of corrosion resistance, even in a difficult-to-treat steel such as cement steel HRC 55.

EXAMPLE 5

An aqueous bath is prepared containing (in weight/volume):

- 25 g/l citric acid

- 10 g/l NO₃ ion

- 16.7 g/l PO₄ ion

- 1 g/l Fe++ ion

- 6 g/l Zn++ ion

- 1 g/l Mn++ ion.

The bath is heated to 95ºC and the free acidity is adjusted to a value of 20 points using sodium hydroxide solution.

Cold rolled steel sheets of type ZES are treated according to an identical sequence to that described in Example 1 (with the conversion step using the indicated bath), except for steps 3 and 4 which are omitted. After treatment, the sheets are tested in a salt spray test (according to ASTM B-117) and no rust development is observed after 400 h of exposure.

Claims (2)

1. Chemical conversion process, characterized in that it comprises, at the time of the actual phosphating step, the use of a zinc conversion bath without chlorine ions and at a temperature of 70 to 99°C, containing 0.5 to 25 g/l of zinc ions, 5 to 40 g/l of phosphate ions, 2 to 40 g/l of nitrate ions and 0.1 to 15 g/l of iron ions and 21 to 100 g/l of at least one organic complexing agent comprising a polycarboxylic acid selected from the group comprising oxalic, malic, glutamic, tartaric, aspartic, malonic and citric acid or one of their salts. 2. Process according to claim 1, characterized in that the bath used comprises: - 5 to 20 g/l phosphate ions, - 21 to 60 g/l citrate ions, - 2 to 40 g/l nitrate ions, - 2 to 10 g/l zinc ions, - 0.1 to 15 g/l iron ions.