EP0433118B1 - Chemical metal substrate conversion process, bath used therefor and concentrate for preparing 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 subject of the invention is a process for the chemical conversion of metallic substrates, in particular based on iron or zinc-plated steel, with a view to increasing their resistance to corrosion, their ability to deformation or simultaneously both.

It also relates to a chemical conversion bath suitable for being used in this process, as well as a concentrate for the preparation of the bath.

It is known to carry out the chemical conversion of metallic substrates by conventional crystalline phosphating treatments which lead to the deposition, on the surface of the metal, of a layer of insoluble phosphates.

• l'acide orthophosphorique H₃PO₄
• un ion phosphate primaire H₂PO₄ ^-
• un ou plusieurs métaux choisis parmi le zinc ou le fer, le manganèse, le nickel, le cuivre, le cobalt, le calcium,
• au moins un accélérateur constitué par un agent oxydant tel que les ions chlorate, nitrate, le nitrite ou le métanitrobenzène sulfonate de sodium.

The known crystalline phosphating treatments generally use acid baths which contain at least the following constituents:

• orthophosphoric acid H₃PO₄
• a primary phosphate ion H₂PO ₄ ^-
• one or more metals chosen from zinc or iron, manganese, nickel, copper, cobalt, calcium,
• at least one accelerator consisting of an oxidizing agent such as sodium chlorate, nitrate, nitrite or metanitrobenzene sulfonate.

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

• une ou plusieurs étapes de dégraissage,
• une ou plusieurs étapes de rinçage,
• éventuellement, une ou plusieurs étapes de décapage, puis de rinçage,
• éventuellement, une étape de conditionnement de surface,
• l'étape de phosphatation proprement dite,
• une ou plusieurs étapes de rinçage,
• éventuellement une étape de passivation, généralement en milieu chromique et une étape de rinçage,
• éventuellement, une étape de séchage ou d'étuvage.

They are part of a sequence of operational steps which may include:

• one or more degreasing steps,
• one or more rinsing steps,
• possibly one or more stages of pickling, then rinsing,
• optionally, a surface conditioning step,
• the actual phosphating step,
• one or more rinsing steps,
• possibly a passivation step, generally in a chromic medium and a rinsing step,
• optionally, a drying or baking step.

The substrates thus treated can be used as they are or else receive a subsequent application of lubricant, protective oil or paint.

• de la nature du substrat,
• de la composition du bain de phosphatation,
• du mode d'application du produit (aspersion ou immersion),
• des étapes de prétraitement.

The nature and composition of the insoluble phosphate layer obtained essentially depends on:

• the nature of the substrate,
• the composition of the phosphating bath,
• the method of application of the product (sprinkling or immersion),
• pretreatment steps.

The prior art describes various improvements to chemical conversion baths aimed at improving their performance.

Thus, according to patent FR-A-1 362 202, an improvement in the corrosion resistance of the phosphating layers is obtained by adding a small amount (up to 2%) and preferably between 0.5 and 1 , 5%) organic complexing agents; ASTM B-117 salt spray performance less than or equal to 25 hours is reported in this patent.

According to US Pat. No. 3,116,178, it is possible for a phosphating bath to contain from 2 to 50 g / l of aliphatic polyhydroxymonocarboxylic acid, such as gluconic acid.

According to patent FR-A-1,585,660, the layer weight can be increased by adding an organic polyacid in an accelerated chlorate phosphating bath and practically free of ferrous ions.

Finally, according to patent FR-A-2 531 457, the combination of a chelating agent and a soluble polyphosphate makes it possible to obtain layers whose resistance to corrosion is appreciably improved, resistance to salt spray being able to reach 310 hours being mentioned in connection with phosphate-only parts.

The French patent in question recalls that an excess of polyphosphates can prevent the formation of the crystalline layer and proposes to cure the baths claimed by evolution of the pH before implementation.

Since the ripening in question is not instantaneous, the bath regeneration operations are necessarily discontinuous, which constitutes a drawback, most industrial baths being regenerated continuously.

The object of the invention is, above all, to remedy this drawback and to provide a chemical conversion bath which, while providing performance equivalent to that of the process described in patent FR-A-2,531,457, can be regenerated by continued.

Consequently, the chemical conversion process is characterized in that it comprises the implementation, at the time of the phosphating step proper, of a zinc conversion bath free of chlorate ions and of a temperature from 70 to 99 ° C, containing from 0.5 to 25 g / l of zinc ion, from 5 to 40 g / l of phosphate ions, from 2 to 40 g / l of nitrate ions as well as from 0 , 1 to 15 g / l of iron ions and 21 to 100 g / l of at least one organic chelating agent consisting of a polycarboxylic acid chosen from the group comprising oxalic, malic, glutamic, tartaric, aspartic, malonic acids and citric acid, or a salt thereof.

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

The 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, carbonic, gluconic, citric acids. , or in the form of its oxide. It can be combined with all the metals commonly used in a phosphating bath such as, for example, manganese, nickel, cobalt, calcium, magnesium.

The iron ions, present in a proportion of 0.1 to 15 g / l, can be introduced either by dissolving iron in the solid state, such as for example iron wool, massive pieces or scrap steel parts, either thanks to iron salts with sulfuric, hydrochloric, hydrofluoric, phosphoric or nitric acids; it can be the ferrous ion, the 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 a proportion of up to 40 g / l.

The bath used in accordance with the invention may also contain hydrofluoric, fluosilicic or fluoboric acids or more of them to regulate the balance between the pickling of the support by the acid solution and the growth of the crystalline layer.

• jusqu'à 500 g/l d'acide polycarbonylique, notamment d'ion citrate
• jusqu'à 200 g/l d'ions phosphate
• jusqu'à 125 g/l d'ions zinc
• jusqu'à 200 g/l d'ion nitrate.

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

• up to 500 g / l polycarbonylic acid, especially citrate ion
• up to 200 g / l of phosphate ions
• up to 125 g / l zinc ions
• up to 200 g / l of nitrate ion.

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

The substrates treated by implementing the process according to the invention can be stored or used without further protection, but it is also possible to paint these substrates in order to further increase their resistance to corrosion; they can possibly be deformed cold after a possible operation of applying a lubricant.

• de 5 à 20 g/l d'ions phosphates
• de 21 à 60 g/l d'ion citrique
• de 2 à 40 g/l d'ion nitrate
• de 2 à 10 g/l d'ion zinc
• de 0,1 à 15 g/l de fer.

According to an advantageous embodiment of the process according to the invention, the bath used contains:

• from 5 to 20 g / l of phosphate ions
• from 21 to 60 g / l of citric ion
• from 2 to 40 g / l of nitrate ion
• from 2 to 10 g / l of zinc ion
• 0.1 to 15 g / l of iron.

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

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

• une ou plusieurs étapes de dégraissage,
• une ou plusieurs étapes de rinçage,
• éventuellement, une ou plusieurs étapes d'activation ou de décapage,
• une étape de rinçage,
• l'étape de conversion chimique proprement dite par mise en oeuvre du bain conforme à l'invention,
• une étape de rinçage,
• une étape de séchage.

The chemical conversion process according to the invention comprises a set of treatment steps, namely:

• one or more degreasing steps,
• one or more rinsing steps,
• possibly one or more stages of activation or pickling,
• a rinsing step,
• the chemical conversion stage proper by implementing the bath according to the invention,
• a rinsing step,
• a drying step.

Due to the exceptional resistance to dry corrosion of the phosphating layers obtained with the bath used in accordance with the invention, the process is simplified by eliminating the chromic passivation and protection steps, subsequent to the chemical phosphating conversion. and necessary in the processes of the prior art.

The invention will be better understood thanks to the nonlimiting examples which follow and in which are indicated advantageous embodiments of the bath used in accordance with the invention.

EXAMPLE 1 (comparative)

A conversion bath is constituted according to the prior art containing, in addition to water, the following ions: Zn⁺⁺: 6 g / l Total PO₄: 16.7 g / l NO ₃ ^- : 10 g / l Total Fe: 1 g / l Ni⁺⁺: 0.2 g / l

Its temperature is brought to 95 ° C; the free acidity is brought into the operating zone at 8.0 points with concentrated soda (the free acidity is determined by titrating 10 ml of cold solution with an N / 10 soda solution until pH = 3, 6).

Etape 1:

Dégraissage alcalin en solution aqueuse à base des dégraissants commercialisés par la Société Demanderesse sous les marques RIDOLINE 1550 CF/2 (à 1,5% v/v) et RIDOSOL 550 CF (1,5% v/v).
Température: 60°C.
Temps de traitement: 3 minutes.

Etape 2:

Rinçage froid à l'eau courante pendant 2 minutes.

Etape 3:

Décapage nitrique
Décapant: acide nitrique à 60% en solution aqueuse (10% v/v).
Température: 20°C.
Temps de traitement: 10 secondes.

Etape 4:

Rinçage froid à l'eau courante pendant 1 minute.

Etape 5:

Conversion proprement dite à 95°C pendant 30 minutes.

Etape 6:

Rinçage à l'eau froide courante pendant 10 secondes.

Etape 7:

Séchage à l'étuve à une température d'environ 95°C pendant 10 minutes.

Cold rolled steel plates with 0.02% carbon of ZES quality commonly used in the automotive industry, dimensions 150 mm x 140 mm, are subjected to the following treatment range:

Step 1 :

Alkaline degreasing in aqueous solution based on degreasers sold by the Applicant Company under the brands RIDOLINE 1550 CF / 2 (at 1.5% v / v) and RIDOSOL 550 CF (1.5% v / v).
Temperature: 60 ° C.
Processing time: 3 minutes.

Step 2 :

Cold rinse with running water for 2 minutes.

Step 3 :

Nitric pickling
Stripper: 60% nitric acid in aqueous solution (10% v / v).
Temperature: 20 ° C.
Processing time: 10 seconds.

Step 4 :

Cold rinse with running water for 1 minute.

Step 5:

Conversion proper at 95 ° C for 30 minutes.

Step 6 :

Rinse with running cold water for 10 seconds.

Step 7 :

Dry in an oven at a temperature of around 95 ° C for 10 minutes.

The plates thus prepared are subjected to 5% salt spray test (according to ASTM B-117).

After 1 hour of exposure, there is a very homogeneous 50% rustling.

EXAMPLE 2 (comparative)

Preparing a conversion bath according to the prior art by adding to the bath according to Example 1 an amount of 10 g / l of citric acid. The free acidity of the bath is brought to 15.0 points. The same treatment is carried out as above on the same cold-rolled steel plates. Obtained, after 8 hours of exposure, a homogeneous scalding of 50%.

EXAMPLE 3

A conversion bath is prepared for implementation in the process according to the invention by adding to the bath according to Example 1 an amount of 30 g / l of citric acid. The free acidity of the new bath is brought to 18/19 points. The cold-rolled steel plates are treated in the same way as in Example 1 and a 10% rusting is obtained locally after 150 hours of exposure. TABLE I Example No. Citric acid content of the bath (g / l) Salt spray test Beginning of rust Rust at 50% 1 0 immediate less than 1 hour 2 10 5 a.m. 8 a.m. 3 30 sailing after 100 h some rust spots after 150 h

EXAMPLE 4

The purpose of this example is to show the possibility of applying the process to various industrial parts. Parts of HRC 55 case hardened steel (0.55% carbon at heart) and mild steel XC 10 (0.1% carbon) are treated.

The following three compositions are prepared:

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

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

The treatment sequence is the same as that described in Example 1, with the exception of step 3, the nitric pickling being replaced by an aqueous phosphoric pickling carried out in an aqueous solution containing 7.75% v / v d '75% phosphoric acid, for 2 minutes at a temperature of 60 ° C.

. Bain 1 :

- 800 ml d'eau
- 100 ml de composition A
- 100 ml de composition C

. Bain 2 :

- 800 ml d'eau
- 100 ml de composition B
- 100 ml de composition C.

The baths used at the time of the conversion step proper are constituted as follows:

. Bath 1:

- 800 ml of water
- 100 ml of composition A
- 100 ml of composition C

. Bath 2:

- 800 ml of water
- 100 ml of composition B
- 100 ml of composition C.

Two series of steel parts, of quality XC 10 and HRC 55 respectively, having been subjected to the treatment sequence according to Example 1 with the use of bath 1 for steel parts XC 10 and bath 2 for parts made of HRC 55 steel, are subjected to the salt spray test according to standard ASTM B-117 and the time after which the red rust appears.

The results are reported in Table II. TABLE II Type steel parts Treatment according to Example 1 with bath 1 (20 g / l citric acid) bath 2 (30 g / l citric acid) XC 10 start of rust after 400 h no rust after 700 h HRC 55 rust starts after 10 a.m. rust begins after 40 h

It is found that the fact of using in the conversion bath a concentration greater than 20 g / l of citrate ion brings a significant gain in terms of corrosion resistance, even on a steel which is difficult to treat like a case hardened steel. HRC 55.

EXAMPLE 5

• 25 g/l d'acide citrique
• 10 g/l d'ion NO₃
• 16,7 g/l d'ion PO₄
• 1 g/l d'ion Fe⁺⁺
• 6 g/l d'ion Zn⁺⁺
• 1 g/l d'ion Mn⁺⁺.

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

• 25 g / l citric acid
• 10 g / l NO₃ ion
• 16.7 g / l PO₄ ion
• 1 g / l of Fe⁺⁺ ion
• 6 g / l of Zn⁺⁺ ion
• 1 g / l of Mn⁺⁺ ion.

The bath is heated to 95 ° C. and the free acidity is brought to a value of 20 points with sodium hydroxide.

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

Claims (2)

1. Process for the chemical conversion characterized by the fact that it comprises the use, at the moment of the phosphatization step proper, of a zinc conversion bath free of chlorate ions and having a temperature from 70 to 99°c, the said bath containing from 0.5 to 25 g/l of zinc ion, from 5 to 40 g/l of phosphate ions, from 2 to 40 g/l of nitrate ions as well as from 0.1 to 15 g/l of iron ions and from 21 to 100 g/l of at least one organic chelating agent consisting of a polycarboxylic acid selected from the group comprising oxalic acid, malic acid, glutamic acid, tartaric acid, aspartic acid, malonic acid and citric acid, or one of its salts.
2. Process according to claim 1, characterized by the fact that the bath used comprises:

   - from 5 to 20 g/l of phosphate ions

   - from 21 to 60 g/l of citric ions

   - from 2 to 40 g/l of nitrate ions

   - from 2 to 10 g/l of zinc ions

   - from 0,1 to 15 g/l of iron ions.