Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/46—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing oxalates
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/31504—Composite [nonstructural laminate]
  • Y10T428/31678—Of metal

Definitions

• the invention relates to a method for depositing a zinc oxalate-based layer on the zinc-based coating, excluding zinc-iron alloys, zinc-plated metal sheets or strips, and the sheets or strips obtained by this process.
• Oxalation is a surface conversion treatment applied for a long time on metallic surfaces, such as steel, zinc or aluminum, and intended to form on the surface a deposit based on oxalate whose properties of pre - lubrication facilitate cold forming.
• the present invention relates specifically to the treatment of galvanized surfaces, in particular those of steel sheets or strips known as “carbon”; “carbon steel” is understood to mean a steel whose proportion of addition or alloying elements is much lower than that found in stainless steels.
• the surface is coated with a thin film of oil (of the QUAKER6130 type for example) to provide it with temporary protection against corrosion, so that the sheet metal thus treated can be stored for a few weeks before being shaped later.
• a thin film of oil of the QUAKER6130 type for example
• the oxalation treatment of the zinc-coated surfaces therefore replaces the conventional pre-phosphating treatment, and has the advantage of being without harmful consequences for the subsequent assembly and painting operations performed by customers, because it is completely eliminated during the degreasing operation which precedes phosphating.
• patent FR 1 066 186 (SOCIETE CONTINENTALE PARKER) describes a process for treating metals such as steel or zinc in a bath of an aqueous solution comprising:
• the document US 2 809 138 (HOECHST) relates to a process for treating metallic surfaces, such as stainless steel or zinc, with an aqueous solution comprising: - 0.5 to 200 g / l of oxalic acid, ie 5.10 -3 to 2.2 mole / l,
• a mixed layer of 66% ferric oxalate and 33% zinc oxalate is thus formed on the galvanized surface. % which is not suitable for improving the forming of the treated product.
• a corrosion agent such as a base
• ferric hydroxide is obtained. according to the following reaction:
• ferric hydroxide has an appearance of red rust which will be unacceptable to the customer.
• the two stages involved in an oxalation treatment of galvanized sheet are: 1. the dissolution of zinc: Zn - Zn 2+ + 2 e-; in the case of chemical dissolution in an acid medium, we would also have: 2 H + + 2 e- ⁇ H 2 , that is the global reaction: Zn + 2 H + ⁇ Zn 2+ + H 2 .
• the oxalation of metal surfaces is likely to be implemented by one of the following techniques: by dipping, by coating or by spraying.
• the dipping technique consists of scrolling at high speed (80 to 100 m / min) a strip of galvanized steel in a tank containing a solution comprising only oxalic acid and possibly a wetting agent.
• the thickness of the zinc oxalate layer must be greater than approximately 0.7 ⁇ m, which corresponds to a grammage of the order of 2 g / m 2 of zinc oxalate.
• the strip moving at high speed (80 m / min) the treatment time making it possible to obtain a layer of zinc oxalate capable of improving the cold forming ability of the surface thus treated, is very short. , on the order of 1 to 5 s.
• the sheet After application of this solution, the sheet can be rinsed and dried in a conventional manner. It is then coated with a thin layer of QUAKER6130 type oil to provide temporary protection against corrosion.
• the duration of the treatment is not long enough to reach the expected gain in stamping, whether the product is rinsed or not;
• the oxalate deposit comprises oxalic acid which has not reacted with zinc but which will react with the layer of oil with which the product is subsequently coated, if it is not removed by rinsing; in this case the performance of the oil is greatly deteriorated. This being the case, whether the product has been rinsed or not, the layer deposited is not thick enough to lead to an improvement in the drawability of the product.
• the coating technique consists of scrolling at high speeds (80 to 100 m / min) a strip of galvanized steel between two rotating coating rollers, which soak in two tanks containing a solution comprising only oxalic acid optionally added with a wetting agent.
• the thickness of the zinc oxalate layer is governed by the amount of material deposited by the rollers, and therefore by the roller-sheet distance and the application time of the oxalic acid solution is also very short, on the order of a second.
• the application of the treatment solution by coating without rinsing before drying makes it possible to access a more homogeneous distribution of the conversion layer than the application of the solution by dipping, and grammages lower than 0.5 g / m 2 , or even less than or equal to 0.1 g / m 2 , may then be sufficient to obtain the optimal pre-lubricating properties.
• the concentration of the oxalic acid solution is between 0.3 and 0.8 mol / l, so as to obtain layers of zinc oxalate on the substrate which are sufficiently thick.
• the use of highly concentrated oxalic acid solutions has drawbacks:
• the concentrated acid solutions are aggressive towards the treatment installation;
• the treatment tanks are generally made of stainless steel, and the rollers for coating the solution are made of rubber or polyurethane.
• the zinc oxalate layer formed is dried by dryers brought to 180 ° C, and placed just below the treatment tanks.
• the heat released by the dryers first causes the aqueous solutions of oxalic acid contained in the tanks to evaporate, then secondly the precipitation of oxalic acid. Solutions of a milky appearance which are unsuitable for the desired oxalation reaction are then obtained fairly quickly. We must therefore stop the production line, clean the tanks and reload them with a clean solution of oxalic acid.
• step 2 of oxalation can only occur if step 1 of dissolution has been previously initiated, which is a classic and general scheme of treatments. conversion; to increase the speed of oxalation to a level compatible with the speed of movement of steel sheets in industrial installations, it is therefore necessary to increase the speed of dissolution of the zinc (step 1) while maintaining under conditions of oxalate precipitation (step 2).
• a layer is thus obtained of a mixture of zinc oxalate, of complex of Zn type (HC 2 O 4 ) 2 and of residual oxalic acid.
• this surface is subsequently protected against temporary corrosion by an oil layer, the oil reacts with the residual acid functions; a poor resistance to temporary corrosion of the surfaces thus treated is then observed.
• the object of the present invention is therefore to provide a method for treating zinc-coated steel strips using ecological oxalation solutions, so as to obtain deposits of zinc oxalates having good properties of pre- lubrication (therefore of sufficient thickness), while significantly increasing the speed of oxalation, and avoiding or limiting the aforementioned drawbacks.
• the subject of the invention is a method for forming a layer of zinc oxalate on the surface of a strip or of a metal sheet coated with a layer of zinc or of zinc alloy, with the exception of zinc-iron alloys, by means of an aqueous oxalation solution containing oxalic acid, characterized in that said solution is an aqueous solution of oxalic acid at a concentration of between 5.10 ⁇ 3 and 0, 1 mole / l containing at least one compound and / or ion of a metal oxidizing zinc at a concentration between 10 " 6 and 10" 2 mole / l, and optionally a wetting agent.
• the concentration of oxidizing ions is lower than the concentration threshold from which precipitation of the corresponding metal is observed.
• the oxalic acid concentration is preferably between 5.10- 3 and 5.10- 2 mol / l.
• the concentration of zinc oxidizing compounds and / or ions in said solution is preferably between 10 -6 and 10 -3 mole / l.
• the at least one ion is chosen from the group comprising Ni 2+ , Co 2+ , Cu 2+ ,
• the grammage of said layer of zinc oxalate is between 0.05 and 3 g / m 2 .
• the subject of the invention is also a method of lubrication and temporary protection of a galvanized sheet, characterized in that it comprises a step of surface oxalation treatment according to the invention, followed by a step of application. a layer of oil.
• a step of surface oxalation treatment according to the invention, followed by a step of application. a layer of oil.
• the invention also relates to a method of stamping a galvanized sheet characterized in that it comprises, prior to stamping, a lubrication step according to the invention.
• the subject of the invention is finally a strip or a metal sheet coated with a layer of zinc, then coated with a layer based on zinc oxalate obtained by the oxalation process according to the invention, characterized in that said oxalate layer comprises at least 99% zinc oxalate.
• zinc-plated surface of a steel sheet or strip is intended to mean a surface essentially coated with zinc, or with a zinc-based alloy, with the exception for this invention of zinc-iron alloys.
• the conversion layer obtained comprises at least 99% of zinc oxalate.
• the concentration of compounds and / or metal ions oxidizing zinc is between 10 " 6 and 10" 2 mole / l, preferably between 10 " 6 and 10 ⁇ 3 mole / l.
• concentration of metal ions less than 10 -6 mole / l the effect of these ions on the speed of oxalation is not significant.
• the metal ion is chosen from the group of ions listed in Table II; this table also indicates the value of the normal potential of the redox couple (ion / corresponding metallic element or other ion) in volts (V) compared to the Normal Hydrogen Electrode (“ENH”).
• Table II Ions usable in the oxalation solutions according to the invention.
• the oxalation treatment bath can include wetting agents and inevitable impurities.
• the procedure is carried out in a conventional manner, for example by dipping, by spraying, or by coating; the application step is followed by a drying step; between the application step and the drying step, the treated sheet can be rinsed.
• the optimal composition of the bath (concentrations of oxalic acid and metal ions) and the morphology of the deposit obtained based on oxalate depend on the application conditions; these conditions are adapted in a manner known per se to obtain the grammage of deposition based on oxalate necessary for obtaining the desired properties, for example pre-lubricating properties.
• the minimum thickness required is of the order of about 0.7 ⁇ m, which corresponds to a grammage of the order 2 g / m 2 of zinc oxalate; the application of the treatment solution by coating without rinsing before drying makes it possible to access a more homogeneous distribution of the conversion layer and grammages below 0.5 g / m 2 , or even less than or equal to 0.1 g / m 2 , may then be sufficient to obtain the optimal pre-lubricating properties.
• the oxalate-based deposit obtained on the zinc-plated surface of the sheet provides properties comparable to those of conventional oxalate-based deposits of the prior art, at least in the following respects: - comparable prelubricant effects: absence of grazing on friction, significant reduction in the coefficient of friction (> 50%) compared to the same oiled sheet without prior oxalation.
• the method according to the invention makes it possible to widen the “operating range” of the treatment, that is to say the range of concentrations of oxalic acid which make it possible to obtain a sufficiently pre-lubricating deposit; for example :
• the range obtained with the addition of zinc oxidizing ions according to the invention is between 5.10 "3 to 0.8 mole / liter. This effect facilitates the management of oxalation baths in industrial application.
• the deposit obtained is in the form of cubic crystals, or, in the case of thicknesses less than 0.1 ⁇ m, in the form of flakes; the average size of these crystals can be quite different, in particular depending on the conditions of application of the treatment solution:
• the deposit according to the invention has a carbon content of approximately two times greater than that of a deposit carried out under the same conditions but without the addition of metal ion oxidizing zinc to the oxalation solution (analyzes based on deposits made using an oxalation solution containing 0 , 1 mole / l of oxalic acid).
• Galvanized steel sheet used USICAR TM galvanized steel sheet, coated by electrodeposition of a zinc layer with a thickness of about 7.5 ⁇ m, degreased in an alkaline medium.
• Bath temperature unless otherwise indicated, room temperature (around 25 ° C).
• Areal density of dry deposition obtained unless otherwise indicated, 2 g / m 2 by dipping (or about 0.7 ⁇ m), 0.1 to 0.3 g / m 2 by coating.
• the measurement used generally corresponds to the average of the coefficients of friction measured during the test.
• Step 1 Using a three-electrode assembly (the treated galvanized steel test piece against a stainless steel electrode, a reference electrode saturated calomel "ECS"), is carried P beach- dissolution of the zinc deposit and coating by applying a potential of - 800 mV / DHW to the treated galvanized steel test piece; when the current is canceled, it is considered that all of the zinc has passed into solution in the electrolyte; in the solution obtained which has a very acid pH, it is considered that the zinc oxalate is entirely uninhibited according to the reaction:
• Step 2 To the resulting solution was added a few drops of manganese sulfate to catalyze the redox reaction; the HC 2 O 4 oxalic acid is then assayed using a potassium permanganate solution of known normality, according to the reactions:
• the volume of the electro-dissolution solution, the treated surface of the test piece, the surface density of the oxalate-based deposition of the test piece is calculated in a manner known per se. initial and / or the average thickness of this deposit.
• test specimens are placed in a climatic chamber corresponding to DIN 51160, which simulates the corrosion conditions of an outer coil of sheet metal coil or of sheet metal cut into sheets during storage.
• climatic chamber corresponding to DIN 51160, which simulates the corrosion conditions of an outer coil of sheet metal coil or of sheet metal cut into sheets during storage.
• test pieces are individually suspended vertically. Visual observation of the samples makes it possible to quantify the degradation of the coating by the appearance of white rust as a function of the number of successive cycles of exposure. Scores are stopped when at least 10% of the total surface area of the sample is affected by white rust.
• FIG. 1 represents the variation in the thickness of the zinc oxalate dip coating ( ⁇ m) as a function of the duration of the oxalation treatment, ie here the duration of soaking (s), for different concentrations of oxalic acid -
• FIGS. 2A, 2B and 3 illustrate Examples 1 and 2: on the ordinate, potential for abandonment (measured in mV relative to a Saturated Calomel Electrode: “DHW”) of a sheet of galvanized steel as a function of the time (s) on the abscissa, measured from the instant of immersion of the sheet (time zero) by chronopotentiometry at near-zero current.
• DHW Saturated Calomel Electrode
• the purpose of this example is to illustrate, according to the invention, the effect of the addition, at very low concentration, of Ni 2+ ions in the treatment solution on the oxalation rate of the zinc-coated sheet, in using here, still while soaking, different treatment solutions at 25 ° C. containing the same proportion of 0.5 mol / l of oxalic acid; Ni 2+ ions are oxidizing zinc.
• the sheet steel electrode is in the form of a circular disk with a surface area of 0.2 cm 2 ; during the measurement, the electrode is rotated at 1250 rpm.
• Curve C (comparative) relates to a solution of the prior art, without the addition of zinc oxidizing ions; it shows a first phase of regular increase in potential up to approximately 100 seconds followed by a second phase of slight slow and regular decrease; in the first phase, we note that the oxalation speed is very low in the first moments and then increases regularly (increase in the slope of the curve); this very low oxalation speed reflects a phenomenon of temporary inhibition of the galvanized surface which the invention precisely makes it possible to limit.
• Curves A and B relate to solutions according to the invention, containing oxidizing ions of zinc; they show that the oxalation is almost instantaneous, which indicates that very small quantities of Ni 2+ ions added to the solution make it possible to limit or even eliminate this inhibition phenomenon, to considerably increase the reactivity of the zinc-plated surface , and greatly increase the speed of oxalation.
• FIG. 2B shows that this effect results from a synergy between the C 2 O 4 2_ ions and the Ni 2+ ions; the results reported relate to the following treatment solutions:
• the purpose of this example is to illustrate that only the ions which are oxidizing zinc bring, even at low concentration, this synergistic effect and make it possible to increase the speed of oxalation.
• Example 1 the abandonment potential measurement of the same galvanized steel sheet dipped in the treatment solution to be evaluated is used.
• solutions containing only 0.05 mole / l of oxalic acid are used here, always at 25 ° C; for all solutions (except the reference, B), the concentration of ions added is 10- 3 mole / l.
• concentration of ions added is 10- 3 mole / l.
• the purpose of this example is to find out in which concentration range the oxidizing ion of the zinc added to the treatment solution is effective in catalyzing and accelerating the oxalation of the galvanized surface.
• Example 2 the potential curves are plotted when a galvanized steel electrode is abandoned in solutions comprising 0.05 mole / liter of oxalic acid and different concentrations of NiCI 2 ranging between 10 -7 and 10 " 1 mole / liter; we note that the catalytic effect of Ni 2+ ions occurs as soon as the NiCI 2 concentration reaches 10 -6 mole / liter; this effect is always observed for higher concentrations, up to at 10 -2 mole / liter; beyond this concentration, a chemical nickel deposit is observed with the eye.
• FIG. 4 illustrates, from top to bottom, the profiles of Ni + 58 , O " 16 and ZnO + 80 obtained by ion mass spectroscopy (" SIMS ”) on an oxalate-based deposit produced according to the invention ( curves A) and on a deposit made under the same conditions but without the addition of oxidizing metal ions (curves
• the curves indicate the intensity of the signal as a function of the erosion time ionic (0 to 25 min.), ie depending on the depth from the extreme surface.
• the erosion time (sputter time) in English) extends to 25 minutes and corresponds to a depth of the order of 1 to 1.5 ⁇ m approximately.
• the nickel added to the oxalation bath is present in the thickness of the deposit made in the presence of Ni 2+ at a conception at least 5 times higher than in the thickness of the reference deposit; the nickel detected in the reference deposit corresponds to the nickel of the inevitable impurities present in the bath.
• the addition of Ni 2+ in the oxalation bath increases the proportion of zinc in the oxidized state Zn 2+ in the deposit, which confirms that this addition promotes the dissolution and oxidation of zinc (to Zn 2 + ) of the surface to be treated and makes it possible to increase the thickness of the deposited layer.
• the purpose of this example is to illustrate the possible synergies between the deposition based on oxalate and a lubricating oil, in particular in the case where this oil contains fatty esters and / or calcium carbonate.
• Fatty esters are classic components of lubricating oils; calcium carbonates are conventional additives to these oils, dispersed and emulsified in the oily phase in general using alkyl sulfonates or alkyl aryl sulfonates; the usual term for this mixture is "overbased calcium sulfonate".
• QUAKER 6130 oil used in the pre-lubricating effect evaluation procedure contains, in addition to oil olefinic or paraffinic mineral, the two components at the same time: approximately 16% of fatty esters and approximately 5% of calcium carbonate.
• Friction tests are carried out (point 2, ⁇ METHODS above, here, with a constant clamping pressure of 400 10 +5 Pa) on zinc-plated test pieces not treated by oxalation and on test pieces treated by coating according to the invention so as to obtain an oxalate-based deposit of grammage of the order of 0.3 g / m 2 .
• the purpose of this example is to illustrate that the galvanized sheets treated according to the invention (application of the solution according to the invention by the coating technique) and then coated with a thin film of QUAKER 6130 oil have good behavior both in stamping and in temporary corrosion.
• Table IV results of behavior in humid heat and stamping.
• the free acid functions of the layer would react with the sulfonate functions of the oil (corrosion inhibiting compounds) by an acid-base reaction.
• the oil would be depleted in corrosion-inhibiting species and would no longer be able to ensure its protective function against corrosion.
• the grammages of the zinc oxalate layers deposited on the treated zinc-coated surface are close to the targeted grammage (0.2 g / m 2 ), and lead to good behavior in humid heat, as well as to excellent stamping behavior.

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