EP0658636B1 - Corrosion protecting process for steel sheet, plated on one side with a Zn-Ni allog - Google Patents

Abstract

Corrosion inhibitor for a metallic material such as steel, of the type consisting of an aqueous solution containing an alkali metal nitrite, characterised in that the said solution contains 0.01 to 0.3 mol/l of an alkali metal nitrite, 0.01 to 0.3 mol/l of an alkali metal phosphate and phosphoric acid in the proportion of a quantity sufficient to maintain the pH of the said solution, measured at ambient temperature, between 6.9 and 7.2.

Description

The invention relates to the field of inhibitors of corrosion with which the surface of metals, in particular of steel, prior to their storage.

Different solutions containing inhibitors of corrosion are used by steelmakers to coat, together with oils, the surface of the steel sheets after their rolling and before their packaging in coils and their shipment to the customer who ordered them. These corrosion inhibitors have the function of passivating the product surface (case of passivating inhibitors, based, for example nitrites) by forming a protective film therein, or to form precipitating compounds on the surface of the product and forming a protective film (case of inhibitors not passivants, based, for example, on phosphates), or cause neutralization or alkalization of the medium near the surface and adsorption (case of inhibitors organic amines). These inhibitors can also serve as lubricants during operations subsequent work hardening of the sheet.

This protection of sheets by such inhibitors is particularly necessary in the case of steel sheets which have been coated by electrodeposition on only one of their faces. On the one hand, during the various stages linked to this processing, the sheet is in contact with different liquids which can be aggressive for the uncoated side if they do not are not subsequently completely eliminated: the solutions degreasing and brightening, electrolyte, fluids rinsing ... Also the atmosphere of the line is charged with electrolyte salts and acid. All the conditions are therefore met for require treatment of the uncoated side of the sheet by a corrosion inhibitor, in order to avoid its pitting during reel storage.

Inhibitors containing both sodium nitrite NaNO ₂ and azoles have been shown to be effective in protecting steel sheets. However, they cannot be used on sheets of which only one side is coated and comprises, for example, a deposit of a zinc-nickel alloy, since the contact during winding between this coating and the inhibitor deposited on the bare face of the neighboring coil causes the appearance of spots on the zinc-nickel layer. However, they do not risk generating nitrosamines during a possible subsequent heating of the sheet, unlike amines. Since nitrosamines are known for their carcinogenic properties, such a possibility of formation should be avoided.

DE-A-4 228 470 describes a process for the phosphating of steel sheets coated on one of its faces with a Zn-Ni alloy. The article "Electrochemical Investigation of Passivating Systems" published in The Internation Water Conference, 35 ^th Annual Meeting, October 1974 describes on page 172 a steel passivation solution containing 0.25% NaH ₂ PO ₄ , 0.25% Na ₂ HPO ₄ and 0.5% NaNO ₂ .

The object of the invention is to propose a method of corrosion protection that can be used in the case of steel sheets coated on only one of their faces, without leading to the deterioration of the coated face and to the generation of toxic compounds during heating ulterior.

To this end, the subject of the invention is a method of protecting against corrosion of a steel sheet which has been previously coated on one of its faces with a Zn-Ni alloy, characterized in that that is deposited on at least one face of said sheet when it is running a layer of an inhibitor consisting of an aqueous solution containing 0.01 to 0.3 mol / l of an alkali metal nitrite, 0, 01 to 0.3 mol / l of a tertiary orthophosphate of an alkali metal, and of orthophosphoric acid in an amount sufficient to maintain the pH of said solution, measured at room temperature, between 6.9 and 7.2, said layer containing 2 to 6 mg / m ^{2 of} said alkali metal calculated as sodium, and in that one then proceeds to the drying of said layer, then to an oiling and to a winding of said sheet.

As will be understood, the invention consists in jointly add a phosphate of an alkali metal and phosphoric acid with known nitrite inhibitors sodium, in well-defined proportions, such so that we get a neutral, non-aggressive solution for the face coated with zinc-nickel single-sided sheets, and apply the solution obtained on at least one side of said moving sheets.

The inventors have found that the appearance of spots on the zinc-nickel coating of the coated face of the sheets whose bare face is protected by sodium nitrite inhibitors was due to the pronounced basicity and the absence of a buffer medium. of these inhibitors. Their pH is around 9 to 11, which causes alkaline corrosion of the coating by the formation of zincates. The idea underlying the invention is that it is possible to avoid this alkaline corrosion by buffering the inhibitor at a neutral pH, compatible with the passivation of iron. This can be achieved by adding a buffer salt to the sodium nitrite. A mixture of sodium orthophosphate Na ₃ PO ₄ and orthophosphoric acid H ₃ PO ₄ is particularly indicated (sodium can be replaced by another alkali metal such as potassium). Indeed, nitrite ions and orthophosphate ions have synergistic effects in their anti-corrosion action, the former acting as neutralizing inhibitors and the latter as passivating inhibitors.

The fact of providing a buffer medium accompanying the corrosion inhibitor agrees well with the execution of an acid rinse prior to coating the sheet, and with the possible presence of K ⁺ , Zn ²⁺ and Ni ²⁺ in the atmosphere of the plating and winding hall. For this, it is imperative that the buffer impose a pH not less than 6.9, measured at room temperature (therefore on the inhibitor put in aqueous solution and not yet applied to the sheet).

The corrosion inhibitor used in the invention is an aqueous solution of an alkali metal nitrite at a concentration of 0.01 to 0.3 mol / l, and of an alkaline tertiary orthophosphate at a concentration of 0.01 at 0.3 mol / l, and also containing phosphoric acid at a concentration such that it imposes on the solution a pH of between 6.9 and 7.2 measured at room temperature. It is also desirable that the molar concentrations of alkaline ions of nitrite and alkaline ions of phosphate do not differ by more than 15%. For example, very good results have been obtained with a solution containing 6 g / l of sodium nitrite, 10 g / l of sodium orthophosphate Na ₃ PO ₄ , 12H ₂ O, the amount of orthophosphoric acid to adjust the pH to 7, and a non-ionic wetting agent.

We tested its effectiveness by applying to a sheet non-oiled corrosive solution of chloride ions, oiling the metal and subjecting it to cycles involving successively 8 hours of exposure to a humid atmosphere and 16 hours of exposure to a dry atmosphere. After 68 hours of such treatment, we observe half as many pitting on the sheet when it has been coated with a solution of this inhibitor before applying the corrosive solution.

Optionally, we can add to this solution to inhibit a nonionic or anionic wetting agent, such as than a phosphoric ester. We thus overcome the bad wettability of the zinc-nickel layer, without use amine-based products in the inhibitor. Together with the wetting agent, it is desirable add an anti-foam. The quantities added can be for example, from 1 to 5 ml / l of wetting agent and from 0.2 to 1 ml / l of anti-foam.

• électrozingage de la tôle sur une de ses faces par un alliage zinc-nickel ;
• rinçage acide, puis rinçage à l'eau déminéralisée de la tôle ;
• application de la solution d'inhibiteur ;
• séchage de la tôle, entraínant l'évaporation de l'eau ;
• huilage de la tôle ;
• bobinage de la tôle, et stockage ou expédition de la bobine.

The application of the inhibitor is in accordance with the following scheme:

• electrogalvanizing of the sheet on one of its faces by a zinc-nickel alloy;
• acid rinsing, then rinsing with demineralized water from the sheet;
• application of the inhibitor solution;
• drying of the sheet, causing the evaporation of water;
• oiling of the sheet;
• winding of the sheet, and storage or shipping of the reel.

Different modes of application of the inhibitor can be envisaged, for example using dewatering rollers between which the sheet runs, and the lower parts of which soak in tanks containing the solution containing the inhibitor. These drain rollers must apply, for example, 1 to 2 ml of solution per m ² of sheet (on each face concerned by the treatment), corresponding to an amount of sodium of 2 to 6 mg / m ² . It is understood that one can, as desired, coat the two faces of the sheet or only the bare face.

Claims (5)

1. Process of anticorrosion protection of a sheet of steel which has been precoated on one of its faces with a Zn-Ni alloy, characterized in that, while the sheet is travelling through, a layer of an inhibitor is deposited on at least one face of the said sheet, this inhibitor consisting of an aqueous solution containing 0.01 to 0.3 mol/l of an alkali metal nitrite, 0.01 to 0.3 mol/l of a tertiary orthophosphate of an alkali metal, and orthophosphoric acid in an amount which is sufficient to keep the pH of the said solution, measured at ambient temperature, between 6.9 and 7.2, the said layer containing 2 to 6 mg/m² of the said alkali metal calculated as sodium, and in that drying of the said layer is then carried out, followed by oiling and winding of the said sheet.
2. Process according to Claim 1, characterized in that the molar concentrations of alkali metal ions of the nitrite and of the tertiary orthophosphate do not differ by more than 15 %.
3. Process according to Claim 1 or 2, characterized in that the said solution also contains a wetting agent.
4. Process according to one of Claims 1 to 3, characterized in that the said wetting agent is a nonionic or anionic wetting agent.
5. Process according to one of Claims 1 to 4, characterized in that the said aqueous solution also contains a foam-suppressing agent.