EP0754779B1 - Process for treating steel sheet surfaces which are partially coated by zinc or a zinc alloy - Google Patents

Abstract

A process for the surface treatment of partially zinc coated (one side) steel plate, is claimed. The plate is treated with an aq. inhibiting soln. to deposit a homogeneous film on the surface before final oil protection. The soln. contains an alkali salt of a 6-12 C linear aliphatic monocarboxylic acid, a hydrocarbyl triazole or diazole and a non-ionic surfactant, the pH of the soln. being maintained < 7.

Description

The invention relates to temporary protection against corrosion of sheets. steel partially coated with zinc or zinc alloy, in particular only on one side.

After electrogalvanizing, the steel sheets generally undergo a anti-corrosion surface treatment.

In the case of partial coating, especially on one side, the same treatment must be effective on both sides at the same time.

This anti-corrosion treatment must also be effective on sheets wound, during their storage period.

If it develops, corrosion usually manifests as stains on the coating and / or pitting on the bare face.

• les bains d'électrozingage , notamment les bains chlorures ;
• les traitements qui entourent l'électrozingage, notamment le dégraissage, l'avivage et le rinçage ;
• l'atmosphère de la ligne d'électrozingage, qui peut être elle-même très corrosive.

Among the possible causes of corrosion are:

• electro-zinc baths, in particular chlorides baths;
• the treatments surrounding electrogalvanizing, in particular degreasing, brightening and rinsing;
• the atmosphere of the electrogalvanizing line, which can itself be very corrosive.

The winding of the sheets brings bare parts and parts into contact coated with sheet metal, which increases the risk of corrosion.

The anti-corrosion treatment can consist of oiling the sheets at the outlet electrogalvanizing.

But the corrosion protection provided by a simple oiling is insufficient.

To improve the effectiveness of the protection, before oiling but always after electrogalvanizing, it is known to treat the coated strips using a corrosion inhibiting solution.

This inhibiting solution, besides that it must be adapted both to the corrosion prevention of steel and that of zinc or zinc alloy, must also to prevent corrosion from bare part contacts - coated parts, which are inevitably caused by winding the sheet.

This solution is generally applied on both sides of the strip, by coating or spraying.

The application by coating, for example by an ink roller, ensures generally a uniform spread of the inhibitor solution which is suitable for a uniform surface treatment.

The method of applying the solution is therefore suitable for obtaining a homogeneous film on the surface of the sheet to be protected.

To improve the spreading of the solution on the strip, we introduce generally in the inhibiting solution a wetting agent.

• les solutions à base de nitrosamines risquent de se décomposer en produits carcinogènes si l'on réchauffe ultérieurement la tôle.
• les solutions contenant du nitrite de sodium et des azoles peuvent être efficaces sur les parties nues de la tôle mais ont tendance à tacher les parties revêtues de la tôle, notamment si le revêtement est en alliage de zinc-nickel et, a fortiori, en zinc pur.
• les solutions contenant du nitrite et du phosphate de sodium, à pH de environ 7, engendrent des effluents coûteux à traiter ou à rejeter.

The aqueous corrosion-inhibiting solutions used hitherto before oiling for the treatment of sheets have drawbacks:

• solutions based on nitrosamines may decompose into carcinogenic products if the sheet is subsequently reheated.
• solutions containing sodium nitrite and azoles can be effective on the bare parts of the sheet but tend to stain the coated parts of the sheet, especially if the coating is made of zinc-nickel alloy and, a fortiori, zinc pure.
• solutions containing nitrite and sodium phosphate, at a pH of around 7, generate effluents which are expensive to treat or reject.

The use of nitrites can also have drawbacks in subsequent stages of sheet metal transformation: for example, during a degreasing, there is a risk of polluting the degreasing baths.

Furthermore, we know, in particular from document EP-A-0 308 037, aqueous corrosion-inhibiting solutions containing an alkaline acid salt aliphatic carboxylic having a linear chain of 6 to 12 atoms of carbon, an alkali borate and a hydrocarbyl triazole.

These solutions are used in closed cooling circuits, that is to say in an application entirely different from a processing of partially coated sheet protection, in which the surface of the sheet with a corrosion inhibiting solution and then oiled.

• la question du tachage n'est pas critique,
• la question de la compatibilité, voire même de la synergie, des composants de la solution inhibitrice avec une huile ne se pose pas,
• les risques de corrosion concernent essentiellement la corrosion en milieu liquide, et non pas essentiellement la corrosion aérienne comme dans l'invention.

Indeed, in the field of cooling circuits:

• the stain issue is not critical,
• the question of the compatibility, or even of the synergy, of the components of the inhibiting solution with an oil does not arise,
• the risks of corrosion relate essentially to corrosion in a liquid medium, and not essentially to aerial corrosion as in the invention.

The invention aims to provide better temporary protection against corrosion to a steel sheet partially coated with zinc or an alloy zinc by avoiding the drawbacks already mentioned, in particular the risks of staining and the difficulties of spreading oil.

The subject of the invention is a method of surface treatment of a sheet. of steel partially coated, in particular on one side only, with a layer of zinc or zinc alloy, in which the surface of said sheet is treated with a aqueous corrosion inhibiting solution to deposit a film homogeneous on said surface then oil is applied to said surface, characterized in that said aqueous solution contains an alkaline acid salt aliphatic monocarboxylic having a linear chain of 6 to 12 atoms carbon, a hydrocarbyl triazole or diazole and a nonionic surfactant and in that the pH of said solution is maintained at a value less than 7.

The inhibitory solution used in the process according to the invention, to provided that the pH is strictly lower than 7, for example 6.5, avoids the appearance of traces of corrosion on bare parts (pitting) or coated (stains) of the sheet thus treated then oiled, even after prolonged storage in a reel.

An advantage of this inhibiting solution according to the invention is that it is less sensitive to the vagaries of application, i.e. it remains effective even if the application conditions (for example: sheet spinning, conditions atmospheric) fluctuate or are poorly controlled: it is therefore particularly easy to apply on the sheet.

The surfactant of the inhibitory solution, of nonionic type, not only improves the spread of the inhibitor solution on the sheet, but also the compatibility of the components of this inhibiting solution with the oil applied later; improving this compatibility further strengthens the effectiveness of the corrosion protection provided by the treatment of surface according to the invention, including after winding of the sheet ("inter-turn" corrosion).

Preferably, to obtain good corrosion resistance while avoiding the risk of staining, the ratio between the molar concentration of salt alkaline monocarboxylic acid in inhibitory solution and concentration molar of hydrocarbyl triazole or diazole in this solution, is between 0.4 and 10.

To apply the inhibiting solution to the surface to be treated, we adapt preferably treatment with the inhibitor solution so that the film deposited during this treatment has a surface density less than 200 mg / m2, this surface density being measured on the dried film.

A relatively low deposit density indeed allows, during oiling subsequent treatment according to the invention, to obtain a uniform spread and homogeneous oil favorable to the effectiveness of corrosion protection.

It has in fact been found that the alkaline salts of monocarboxylic acid aliphatic inhibitor solution could cause problems with wettability and therefore spreading of the oil when the deposit density of inhibitory solution is too high.

Thanks to the alkaline character of the monocarboxylic acid salt of the solution inhibitory - that is, the cation of the salt is chosen from among the alkalis -, conventional surface analysis methods that are easy to implement can be used to quickly control the deposit density or amount of product deposited on the surface of the sheet, and restore it if necessary at the level necessary and sufficient.

• ledit hydrocarbyl triazole ou diazole est choisi parmi le 1-2-3 triazole, le 1-2-4 triazole, le 1-H benzotriazole (1,2, 3), le 5-alkyl 1-H benzotriazole (1, 2, 3), le benzimidazole (ou 1-3 benzodiazole).
• ledit acide monocarboxylique est l'acide heptanoïque,
• ledit agent tensio-actif non-ionique est de la famille des alcools polyéthoxylés.

The invention may also have one or more of the following characteristics:

• said hydrocarbyl triazole or diazole is chosen from 1-2-3 triazole, 1-2-4 triazole, 1-H benzotriazole (1,2, 3), 5-alkyl 1-H benzotriazole (1, 2, 3), benzimidazole (or 1-3 benzodiazole).
• said monocarboxylic acid is heptanoic acid,
• said nonionic surfactant is from the family of polyethoxylated alcohols.

Particularly in the case of alkaline salt of heptanoic acid and tolyltriazole, the inhibiting solution according to the invention provides, with protection, better protection than other aqueous solutions of art anterior and the two essential components, namely the alkaline acid salt heptanoic and tolyltriazole, have a synergy in function inhibitory.

• lorsque le revêtement de la tôle est en alliage de zinc-nickel, la concentration en tolyltriazole dans ladite solution inhibitrice est comprise entre 0,5 et 5 g/l.
• lorsque le revêtement de la tôle est en zinc pur, la concentration en tolyltriazole dans ladite solution inhibitrice est supérieure à 1,5 g/l et inférieure ou égale à 5 g/l.

The proportion of alkaline salt of heptanoic acid and of tolyltriazole in the inhibiting solution can be advantageously adapted to the nature of the partial coating of the sheet to be protected:

• when the coating of the sheet is made of zinc-nickel alloy, the concentration of tolyltriazole in said inhibiting solution is between 0.5 and 5 g / l.
• when the coating of the sheet is made of pure zinc, the concentration of tolyltriazole in said inhibiting solution is greater than 1.5 g / l and less than or equal to 5 g / l.

Finally, when the alkaline salt of aliphatic monocarboxylic acid is a sodium heptanoate, the treatment with said aqueous solution is carried out preferably so that the amount of sodium contained in said film deposited on the surface of the sheet is between 2 and 8.5 mg / m2.

The following examples illustrate the invention:

Example 1 :

The purpose of this example is to illustrate corrosion protection provided by the inhibitory solution used in the process according to the invention.

For this purpose, the electric current is measured as a function of the time between a bare steel electrode and a zinc coated steel electrode immersed in an inhibitory solution to be tested.

The solutions to be tested are prepared by diluting inhibitors of corrosion in water called "ASTM"; neither basic nor acidic agents are added to these solutions, which all have a so-called "natural" pH.

"ASTM" water contains 165 mg / l of sodium chloride, 138 mg / l of sodium bicarbonate and 148 mg / sodium sulfate.

The measured electric current decreases as a function of time and then becomes approximately constant after a certain time, said time of passivation, or "Pass time." measured in kiloseconds (k.s.).

This approximately constant level of current is called "plateau" electric.

The table below presents the results provided by different solutions: Solution Pass time - ks Level - µA / cm2 "ASTM" water (corrosion reference) 6.00 40.00 Totyltriazole (TTA) -1 g / l 0.03 20.00 Sodium heptanoate (NaC7) - 6 g / l 0.07 7.00 TTA at 0.5 g / l and NaC7 at 6 g / l - pH # 6.5 <0.01 0.03

It can therefore be seen that the solution according to the invention therefore provides a better protection than other prior art solutions and that both essential components of the inhibitory solution used in the process according to the invention, tolyltriazole and sodium heptanoate, exhibit a synergy.

Example 2 :

The purpose of this example is to illustrate that a sheet of steel, coated with a zinc-nickel layer on one side and treated with the inhibitor solution used in the process according to the invention, does not risk getting stained on the coated side, even if the coated side is brought into contact with the bare side (reproduction of "inter-turn" corrosion conditions).

• on prépare les solutions suivantes :
• S0 : eau distillée (pour essai de référence)
• S1 : solution dans l'eau distillée de nitrite et de phosphate de sodium, selon l'art antérieur.

The following test makes it possible to assess the risk of staining following treatment with an inhibiting solution to be tested:

• the following solutions are prepared:
• S0: distilled water (for reference test)
• S1: solution in distilled water of nitrite and sodium phosphate, according to the prior art.

This solution S1 is obtained from the following main compounds: Na3PO4, 12 H2O, H3PO4 and NaNO2, so that it contains 2.25 g / l Na + ions and 2.5 g / l NO2- ions and has a pH of about 6.5.

The solution also contains a wetting agent called commercially S4478 from the company SURFAZUR.

S2: several solutions according to the invention, in distilled water, containing 6 g / l of sodium heptanoate (i.e. 0.04 mole / l), 1 g / l of non-wetting agent ionic and different proportions of totyltriazole (hereinafter referred to as TTA): S2a: 0.5 g / l (i.e. 0.0044 mole / l) - S2b: 1 g / l - S2c: 3 g / l - S2d: 5 g / l, all these solutions having a so-called "natural" pH of less than 7.

• on asperge de solution à tester la face revêtue de plusieurs échantillons de tôle d'acier, préalablement dégraissés.
• on réalise ensuite des paquets par empilement d'échantillons face revêtue contre face nue, appelés FR/FN.

As non-ionic wetting agent, the product commercially known as MAGNUSPRAY AD from the company HENKEL is used; this product belongs to the family of polyethoxylated alcohols.

• the surface coated with several samples of steel sheet, previously degreased, is sprayed with the solution to be tested.
• packets are then produced by stacking samples face coated against bare face, called FR / FN.

• on stocke les paquets pendant trois jours à l'air libre.
• enfin, on défait les paquets pour évaluer l'aspect de surface des faces revêtues.

Each package is tightened to the same tightening force; these conditions reproduce the conditions for covering coated sheets on one side in a coil, or the conditions for inter-turn corrosion.

• the packages are stored for three days in the open air.
• finally, the packages are undone to assess the surface appearance of the coated faces.

The surface appearance is distributed on a scale of three dimensions: +++ for strong staining, ++ for medium staining, + for a surface appearance without a stain.

Depending on the treatment solution used, observation of the surface of the samples gives the following results: Solution S0 S1 S2a S2b S2c S2d FR / FN sides +++ + + + + +

The solutions according to the invention are suitable for preventing stains on a zinc-nickel coating, as well as a solution of nitrite and sodium phosphate, according to the prior art, as soon as the concentration of tolyltriazole in the solution is at least equal to 0.5 g / l (case S2a), or that the NaC7 / TTA molar ratio is approximately less than 10.

The solutions according to the invention generate less costly effluents to treat only the solution S1 of the prior art.

Example 3 :

The purpose of this example is to demonstrate that the processing of a steel sheet coated on one side with the solution according to the invention protects the bare face sheet steel against the risk of corrosion, especially pitting.

Indeed, the bare faces of steels coated on the other side with a zinc-nickel coatings are particularly difficult to protect due to the simultaneous presence on this bare face of traces of chlorides and nickel, which increase the reactivity of the surface.

• 1 - on asperge d'une solution corrosive de chlorure des échantillons de tôle d'acier revêtus sur une seule face d'une couche d'alliage zinc-nickel, directement issus d'une ligne industrielle. On choisit une solution corrosive de chlorure connue en elle-même représentative de l'électrolyte utilisé couramment pour réaliser des dépôts électrolytiques industriels, contenant notamment des composés ZnCl2 et KCI.
• 2 - on prépare des solutions inhibitrices comme dans l'exemple 2:
• S1 : solution selon l'art antérieur identique à la solution de l'exemple 2 (nitrite + phosphate).
• S3 : solution selon l'invention : 1 g/l d'agent mouillant, 1,5 g/l de tolyltriazole (soit 0,013 mole/l) et des proportions différentes d'heptanoate de sodium (NaC7) : S3a : NaC7 à 3 g/l - S3b : NaC7 à 6 g/l - S3c : NaC7 à 12 g/l (0,08 mole/l).
L'agent mouillant est le même que dans l'exemple 2.
• 3- on applique ensuite la solution à tester sur les échantillons. Pour appliquer la solution sur les échantillons, on peut procéder par aspersion ou par enduction ; grâce en général à l'utilisation d'un rouleau encreur au contact de la tôle, l'application par enduction assure une répartition plus homogène de la solution sur la surface à traiter que l'application par aspersion.
• 4 - on huile enfin les échantillons, pour déposer une pellicule uniforme de environ 1 g/m2.

The following test makes it possible to evaluate the risk of corrosion after application of an inhibiting solution to be tested:

• 1 - samples of steel sheet coated on one side with a layer of zinc-nickel alloy, directly from an industrial line, are sprayed with a corrosive chloride solution. We choose a corrosive chloride solution known in itself representative of the electrolyte commonly used to make industrial electrolytic deposits, containing in particular ZnCl2 and KCI compounds.
• 2 - inhibiting solutions are prepared as in Example 2:
• S1: solution according to the prior art identical to the solution of Example 2 (nitrite + phosphate).
• S3: solution according to the invention: 1 g / l of wetting agent, 1.5 g / l of tolyltriazole (i.e. 0.013 mole / l) and different proportions of sodium heptanoate (NaC7): S3a: NaC7 to 3 g / l - S3b: NaC7 at 6 g / l - S3c: NaC7 at 12 g / l (0.08 mole / l).
The wetting agent is the same as in Example 2.
• 3- the solution to be tested is then applied to the samples. To apply the solution to the samples, one can proceed by spraying or coating; thanks in general to the use of an ink roller in contact with the sheet, the application by coating ensures a more homogeneous distribution of the solution on the surface to be treated than the application by spraying.
• 4 - finally oil the samples, to deposit a uniform film of about 1 g / m2.

• 5 - on place ensuite les échantillons dans une chambre d'essais climatiques pour leur faire subir plusieurs fois le cycle suivant :
  • 8 h à 20°C en phase humide (humidité relative : 95%).
  • 16 h à 20°C en phase sèche (humidité relative : 30%).
• 6 - enfin, pendant le déroulement de ces cycles, on évalue l'aspect de surface des faces nues des échantillons, et son évolution en fonction du temps.

A conventional oil for temporary protection against corrosion is used, here for example the oil commercially known as 4107S from the company FUCHS.

• 5 - the samples are then placed in a climatic test chamber to make them undergo the following cycle several times:
  • 8 h at 20 ° C in the wet phase (relative humidity: 95%).
  • 4 p.m. at 20 ° C in the dry phase (relative humidity: 30%).
• 6 - finally, during the course of these cycles, the surface appearance of the bare faces of the samples is evaluated, and its evolution as a function of time.

The surface appearance is distributed on a scale of eight dimensions: 0 for absence of corrosion ... up to 8 for very strong corrosion.

We plan a reference test without performing step 3, that is to say in oiling the surface directly without prior treatment; the abbreviation "ref .: oil "means this test.

The table below illustrates the evolution of the surface appearance as a function the time (0 to 90 h) spent by the samples in the test chamber climatic conditions.

By applying the inhibiting solutions to be tested by spraying, the following rating of corrosion of the bare faces is obtained after the test according to the above procedure: Time 0 h 3 p.m. 25 hrs 45 hrs 65 h 90 h ref. oil 0 1.0 3.5 4.5 5.0 5.0 S1 0 0.5 3.5 4.0 4.5 4.5 S3 (a to c) 0 0 2.5 3.0 3.5 4.0

The solutions according to the invention would therefore provide, on the bare parts of a sheet partially coated with zinc-nickel, an improvement of the order of 25% corrosion resistance compared to a conventional solution containing nitrites and phosphates.

Corrosion resistance is roughly the same for all solutions S3 (a to c), i.e. for heptanoate concentrations of sodium between 3 and 12 g / l or for NaC7 / TTA molar ratios between 1.5 and 6.

By applying the same inhibiting solutions by coating, the following rating of corrosion of the bare faces is obtained after the test according to the above procedure: Time 0 h 3 p.m. 25 hrs 45 hrs 65 h 90 h ref. oil 0 2.2 3.2 3.8 4.4 S1 0 0.4 0.8 1.2 1.5 S3 (a to c) 0 0 0 0 0 0

When applied by coating, the solutions according to the invention, therefore provide a degree of improvement in corrosion resistance still superior, compared to inhibitor solutions of the prior art; in this example, the degree of improvement is indeed much greater than 25%.

The sodium deposited on the surface of the sheet was analyzed in the two application modes - spraying and coating; the table below indicates the amount of sodium per unit area (in mg / m2), which reflects the amount of sodium heptanoate deposited for the solutions according to the invention (the concentrations are in parentheses). application by S3a (3 g / l) S3b (6 g / l) S3c (12 g / l) aspersion 2.0 mg / m2 4.5 mg / m2 8.5 mg / m2 coating 2.0 mg / m2 3.5 mg / m2 5.0 mg / m2

Thus, for a sodium deposit of between 2 and 8.5 mg / m2 corresponding to a deposit of sodium heptanoate between 13 and 56 mg / m2, an improvement in corrosion resistance is obtained.

In the case of an inhibitory solution in which the molar ratio NaC7 / TTA would be worth 0.4, the total amount of inhibitor products deposited at the the sheet surface could therefore reach a maximum of 200 mg / m2.

It can be seen that the method of application by coating leads to a lower deposit density but still better resistance to corrosion than the spray application method.

Too high a deposit density can affect the proper spreading of the oil and therefore resistance to corrosion.

It has also been found that the wetting agent contained in the solutions inhibitory - cf. step 2 - not only contributed to uniform spreading of the corrosion treatment solution (step 3) but also at a uniform spreading of the oil layer (step 4).

In other tests not described here, solutions have been formulated inhibitors with other wetting agents, which do not belong to the family polyethoxylated alcohols, in particular fluorinated nonionic wetting agents marketed by ATOCHEM.

It has been found that these wetting agents are less suitable for exercising the double function of spreading the solution and spreading the oil layer and as the sheet, treated with the solutions containing these wetting agents and then oiled, was less resistant to corrosion.

Example 4 :

The purpose of this example is to demonstrate that a steel sheet coated with a layer of pure zinc on one side and treated with the solution according to the invention is not likely to stain on the coated face, unlike other solutions of the prior art.

Steel sheet coated with a layer of "pure zinc" means a sheet classic galvanized steel.

The same types of tests are carried out as in Example 2, but on samples of galvanized steel sheets on one side.

As in Example 2, the surface appearance of the coated faces is distributed on a scale of three ratings: +++ for heavy staining, ++ for a medium stain, + for a spotless surface appearance.

We use the same solutions as for Example 2, with the difference close that the S2b solution is replaced by the S'2b solution which contains 1.5 g / l instead of 1 g / l of tolyltriazole.

Observation of the surface of the samples gives the following results: Solution S0 S1 S2a S'2b S2c S2d FR / FN sides +++ ++ +++ ++ + +

The solutions according to the invention therefore provide protection against more effective staining than the solution S1 of the prior art, since the Tolyltriazole concentration in the solution is greater than 1.5 g / l.

Example 5 :

This example has the same purpose as Example 3, applied to the case of sheet metal steel coated with pure zinc on one side.

• huile A : dénommée commercialement 6130 de la Société QUAKER.
• huile B : mélange d'une huile minérale dénommé "Sn" de la Société SHELL et d'un additif anti-corrosion dénommé "Q" de la Société QUAKER.

The same tests are carried out as in Example 3, here applying the same inhibiting solutions only by spraying, but using two different types of temporary protection oil:

• oil A: commercially known as 6130 from the company QUAKER.
• oil B: mixture of a mineral oil called "Sn" from the company SHELL and an anti-corrosion additive called "Q" from the company QUAKER.

The surface appearance of the samples is distributed on a scale of eight ratings: 0 for absence of corrosion ... up to 8 for very corrosion strong.

Two reference tests are planned by oiling the surface directly without pre-treatment, designated by "oil ref. A" and "oil ref. B".

The table below illustrates the evolution of the surface appearance as a function the time (0 to 90 h) spent by the samples in the test chamber climatic conditions.

By applying oil A, the following corrosion rating of the bare faces is obtained after testing according to the above procedure: Time 0 h 3 p.m. 25 hrs 40 hrs 70 h oil ref A 0 3.2 4.5 5.0 6.5 S1 0 2.7 3.2 3.5 4.5 S3a 0 1.7 2.0 2.5 3.5 S3 (b and c) 0 1.2 1.7 2.2 3.0

Thus as in Example 3 (spraying case), we improve by more than 25% corrosion resistance on the bare face compared to a solution classic containing nitrites and phosphates.

Corrosion resistance increases slightly here with the concentration sodium heptanoate.

By applying oil B, the following rating of corrosion of bare faces is obtained after test according to the above: Time 0 h 3 p.m. 25 hrs 40 hrs 70 h 90 h ref. oil B 0 0.5 0.7 1.2 2.2 2.7 S1 0 0 0.5 0.5 1.0 1.5 S3a 0 0 0.2 0.5 1.0 1.7 S3 (b and c) 0 0 0.5 0.7 1.5 2.0

In the case of oil B, the corrosion resistance is approximately identical, whether using the S1 solution of the prior art or the S3 solutions according to the invention here, beyond a concentration of 3 g / l of heptanoate sodium in the solution, corrosion resistance is maintained approximately at same level.

Example 6 :

The purpose of this example is to illustrate the importance of the pH value of the corrosion inhibiting solution that is used when using the method according to the invention.

Indeed, all the solutions according to the invention of the previous examples are used at their "natural" pH, ie a pH below 7.

An inhibitor solution S6 / 0 containing 3 g / l of heptanoate is prepared. sodium and 5 g / l of tolyltriazole and the same wetting agent as in solutions S2 of example 2.

Without other additives, the inhibiting solution obtained S6 / 0 has a pH "natural" of about 6.5.

By addition of ammonia to this solution S6 / 0, one then prepares a S6 / 1 solution at pH = 7, then an S6 / 2 solution at pH = 10.

The solutions S6 / 0, S6 / 1 and S6 / 2 are then tested according to the same procedure as in example 2.

Depending on the treatment solution used, observation of the surface of the samples gives the following results: Solution S6 / 0 S6 / 1 S6 / 2 FR / FN sides + ++ ++

Inhibitory solutions containing an alkaline acid salt aliphatic monocarboxylic having a linear chain of 6 to 12 atoms carbon, a hydrocarbyl triazole or diazole and a nonionic surfactant, are therefore suitable for implementing the method according to the invention at provided that the pH is strictly lower than 7.

Claims (8)

1. Process for the surface treatment of a steel sheet partially coated, especially on only one side, with a layer of zinc or of zinc alloy, in which the surface of the said sheet is treated using a corrosion-inhibiting aqueous solution so as to deposit a homogeneous film on the said surface and then oil is applied to the said surface, characterized in that the said aqueous solution contains an alkaline salt of an aliphatic monocarboxylic acid having a linear chain of 6 to 12 carbon atoms, a triazole or diazole hydrocarbon and a non-ionic surfactant and in that the pH of the said solution is maintained at a value of less than 7.
2. Process according to Claim 1, characterized in that the ratio between the molar concentration of the said alkaline salt of a monocarboxylic acid in the said solution and the molar concentration of triazole or diazole hydrocarbon in the said solution is between 0.4 and 10.
3. Process according to any one of the preceding claims, characterized in that the treatment using the said aqueous solution is carried out so that the said deposited film has a surface density of less than 200 mg/m², the said surface density being measured on the said film when it is dried.
4. Process according to any one of the preceding claims, characterized in that the said triazole or diazole hydrocarbon is chosen from 1,2,3-triazole, 1,2,4-triazole, 1H-benzotriazole (1,2,3), 5-alkyl-1H-benzotriazole (1,2,3) and benzimidazole (or 1,3-benzodiazole).
5. Process according to any one of the preceding claims, characterized in that:

   the said monocarboxylic acid is heptanoic acid;

   the said triazole or diazole hydrocarbon is tolyltriazole (5-methyl-1H-benzotriazole);

   the said non-ionic surfactant is of the family of polyethoxylated alcohols.
6. Process according to Claim 5, characterized in that, when the said coating is made of a zinc/nickel alloy, the concentration of tolyltriazole in the said inhibiting solution is between 0.5 and 5 g/l.
7. Process according to Claim 5, characterized in that, when the said coating is made of pure zinc, the concentration of tolyltriazole in the said inhibiting solution is greater than 1.5 g/l and less than or equal to 5 g/l.
8. Process according to any one of the preceding claims, characterized in that, when the said alkaline salt of an aliphatic monocarboxylic acid is a sodium heptanoate, the treatment using the said aqueous solution is carried out so that the amount of sodium contained in the said film deposited on the surface of the sheet is between 2 and 8.5 mg/m².