PROCESS FOR THE PASSIVATION OF TINNED STAINLESS STEEL STRIP WITH A CHROME-FREE PASSIVATION FILM

Abstract: Process for the passivation of tinned stainless steel strip with a Chrome-free passivation film, characterised in that it comprises the step of dipping, optionally applying a cathode current, the tinned strip in a passivation bath comprising at least one salt of a transition metal selected from Cobalt, Titanium, Zirconium and Molybdenum. The present invention further relates to the passivated tinned strip thus obtainable, to the tinned strip passivated and painted with Chrome-free paint, and to the use thereof for the production of tinned goods.

Description: "PROCESS FOR THE PASSIVATION OF TINNED STAINLESS STEEL STRIP WITH A CHROME-FREE PASSIVATION FILM" DESCRIPTION The present invention relates to the field of the treatments for the passivation of tinned stainless steel strip with a Chrome-free passivation film, in order to improve the environmental impact of the related technologies . As it is known, coated steels, prior to the end use thereof, should be passivated when employed tel quel, or be subjected to a surface treatment in order to improve the adhesion of optional coatings or some performances, like, e.g., the corrosion resistance. In the state of the art, the most widely adopted passivation or surface conversion treatments utilize solutions containing Chrome in the form of sodium dichromate . These treatments are aimed both to the conversion of the metallic substrate having a protective function in mildly aggressive environments, and to the preparation of the surface in order to improve the adhesion of subsequent protective layers. The passivation coating may be construed as a mixture of corrosion products of the metallic substrate and of various reduced species deriving from the conversion bath. Typical examples of passivation treatment are chromate/phosphate-based systems, accounting for a considerable share of the market of surface conversion processes and products. The main passivation techniques provide the mere dipping of the product to be passivated, or the applying of a cathode current, often allowing more rapid forming times, thereto. The need to improve the environmental impact and the severity of the Acts regulating the treatment of toxic substances, like, e.g., the heavy metals, and hence the need to treat the scouring waters and the exhausts, has raised the demand of a passivation treatment of the metal surfaces which be totally void of toxic substances. The tinned stainless steel strip is, e.g., conventionally passivated with Chrome salt (Sodium dichromate, Na 2 Cr 2 0 7 ) -based solutions. As Chrome is a highly toxic substance entailing dire environmental impact problems, there ensues the problem of replacing the Chrome-based passivation with other likewise effective passivations based on Chrome-free solutions. The present invention meets this demand. In fact, an object of the present invention is a process for the passivation of tinned stainless steel strip with a Chrome-free passivation film, characterised in that it comprises the step of dipping, optionally applying a cathode current, the tinned strip in a passivation bath comprising at least one salt of a transition metal selected from Cobalt, Titanium, Zirconium and Molybdenum. In an embodiment of the invention, the passivation bath is an aqueous solution of Cobalt sulfate heptahydrate (CoS0 • 7H 2 0) , Potassium Titanium fluoride (K 2 TiF 6 ) and Sodium nitrate (NaN0 3 ) . In a variant of this process, the process features are as follows: passivation bath composition expressed in g/1 - CoS0 4 .7H 2 0 10-15 - K 2 TiF 6 10-15 - NaN0 3 4-8 and water q.s. to 100; bath temperature 50-65°C; cathode current density 0.10-0.15 A/dm 2 ; and exposure time 1-5 s. In a second embodiment of the process according to the invention, the passivation bath is an aqueous solution of Potassium Titanium fluoride (K 2 TiF 6 ) and Sodium nitrate (NaN0 3 ) . In a variant of this process, the process features are as follows: passivation bath composition expressed in g/1 - K 2 TiF 6 10 - 15 - NaN0 3 4 - 8 and water q.s. to 100; bath temperature 50-60°C; cathode current density 0.10-0.15 A/dm 2 ; and exposure time 10-30 sec. In a third embodiment of the present invention, the passivation bath is an aqueous solution containing Zirconium sulfate. In a variant of this process, the process features are as follows: passivation bath composition - Zr(S0 4 ) 2 20 g/1 - H 2 S0 4 0,5 M and water q.s. to 100; bath temperature 50-60°C; cathode current density 0.1-0.2 A/dm 2 and exposure time 5-10 s. In a fourth embodiment according to the present invention, the passivation bath is an aqueous solution containing Titanium oxalate. In a variant of this process, the process features are as follows: passivation bath composition expressed in g/1 - Ti 2 (C 2 O 4 ) 3 .10 H 2 0 15-20 - K 2 C 2 0 4 .H 2 0 3-8 and water q.s. to 100; bath temperature 50-65°C; cathode current density 0.1-0.2 A/dm 2 and exposure time 5-10 s. In a fifth embodiment according to the present invention, the passivation bath is an aqueous solution containing Sodium molybdate. In a variant of this process, the process features are as follows: passivation bath composition expressed in g/1 - Na 2 Mo0 4 15-20 - Na 2 S 2 0 3 2-4 and water q.s. to 100; bath temperature 40-45°C; cathode current density 0.1-0.15 A/dm 2 and exposure time 5-10 s. A further object of the present invention is a tinned stainless steel strip, passivated with the abovedisclosed process. Another further object of the present invention is a tinned stainless steel strip, passivated with the abovedisclosed process, painted with Chrome- free paints. Finally, another object of the present invention is the use of the tinned stainless steel strip, passivated and painted without use of Chrome, for the production of tinned goods. Hereto, the present invention has merely been outlined. Hereinafter, with the aid of the following Figs, and of the example, a more detailed description of specific embodiments thereof will be given, aimed at making apparent the purposes, the features, the advantages and the operation modes thereof. Fig. 1 shows the potentiodynamic polarization curves in citrate buffer, at pH 6, scanning speed 0.2 mV/s, 50°C, for a tinned strip, not passivated (dotted curve), passivated with a Chrome-based solution (broken-line curve) , and passivated according to the invention (solid curve) with the following process features: passivation bath containing 15 g/1 CoS0 4 -7H 2 0, 15 g/1 K 2 TiF 6 and 5 g/1 NaN0 3 (electrolyte 1) ; bath temperature 50°C; current density applied 0.15 A/dm 2 ; and exposure time 1 s. Fig. 2 shows the potentiodynamic polarization curves in citrate buffer, at pH 6,0, scanning speed 0,2 mV/s, 50°C, for a tinned strip, not passivated (dotted curve) , passivated with Chrome-based solutions (broken-line curve) , passivated according to the invention (solid curve) with the following process features: passivation bath containing 15 g/1 K 2 TiF 6 and 5 g/1 NaN0 3 (electrolyte 2); bath temperature 50°C; current density applied 0.14 A/dm 2 ; exposure time 30 s. Fig. 3 shows an EDS (X-ray Dispersive Energy Spectrometry Measurements) spectrum of an untreated tinned strip, to be subjected to the process for the passivation according to the invention. Figs. 4 and 5 show the GDOES (Glow Discharge for Optical Emission Spectrochemical Analysis) profiles for samples passivated with the electrolyte 1 and with the electrolyte 2 according to the invention, respectively. EXAMPLE A tinned strip to be subjected to the process for the passivation according to the invention is characterised with the EDX (X-ray Dispersive Energy Spectrometry Measurements) technique. The related spectrum, shown in Fig. 3, reveals the presence of Tin as a massive element and of Iron. The Chrome content, highlighted by atomic absorption spectroscopy, ranges from 0.58 to 0.67 mg/m 2 . This tinned strip is subjected to passivation according to the invention, in aqueous solution at 50 °C containing 15 g/1 K 2 TiF 6 and 5 g/1 NaN0 3 , with a cathode current density equal to 0.14 A/dm 2 and an exposure time of 30 s. The corrosion behaviour of samples of tinned strip thus passivated is studied by potentiodynamic polarization curves and exposition tests in a 30 g/1 sodium chloride solution for 24 hours at 25°C. The potentiodynamic polarization curves are constructed by measuring taken with a conventional glass cell at 25°C in a solution saturated with pure Nitrogen. The counterelectrode is a Platinum sheet and, as reference electrode, a saturated calomel electrode (SCE) is used. The electrolyte is a citrate buffer, at pH 6,0. The scanning speed is 0.2 mV/s. The polarization curves are plotted after the stabilization of the corrosion potential (about 90 min) . Fig. 2 shows the potentiodynamic polarization curve of the tinned strip passivated as abovedisclosed (solid curve) , in comparison to the potentiodynamic polarization curves of tinned strip not passivated (dotted curve) and of tinned strip passivated with a Chrome-based solution (broken-line curve) . The result of the exposure tests confirms the results yielded by the polarization curves. The samples passivated according to the abovedescribed embodiment of the invention exhibit optimum behaviour. The surface of samples of tinned strip passivated as aboveindicated was analysed by SEM (Scanning Electron Microscopy) , EDS (X-ray Dispersive Energy Spectrometry Measurements) , GDOS (Glow Discharge for Optical Emission Spectrochemical Analysis) (profile shown in Fig. 4), and XPS (X-ray Photoelectron Spectroscopy) techniques. The results of these surface analyses highlight features comparable to those of the surfaces of tinned strip passivated with Chrome-based solutions.

Claims: CLAIMS 1. A process for the passivation of tinned stainless steel strip with a Chrome-free passivation film, characterised in that it comprises the step of dipping, optionally applying a cathode current, the tinned strip in a passivation bath comprising at least one salt of a transition metal selected from Cobalt, Titanium Zirconium and Molybdenum. 2. The process for the passivation of tinned stainless steel strip with a Chrome-free passivation film according to claim 1, wherein the passivation bath is an aqueous solution of Cobalt sulfate heptahydrate (CoS0 4 -7H 2 0) , Potassium Titanium fluoride (K 2 TiF 6 ) and Sodium nitrate (NaN0 3 ) . 3. The process for the passivation of tinned stainless steel strip with a Chrome-free passivation film according to claim 2 , wherein the process features are as follows: passivation bath composition expressed in g/1 - CoS0 4 - 7H 2 0 10 - 15 - K 2 TiF 6 10 - 15 - NaN0 3 4 - 8 and water q.s. to 100; bath temperature 50-65°C; cathode current density 0.10-0.15 A/dm 2 ; and exposure time 1-5 s. 4. The process for the passivation of tinned stainless steel strip with a Chrome-free passivation film according to claim 1, wherein the passivation bath is an aqueous solution of Potassium Titanium fluoride (K 2 TiF 6 ) and Sodium nitrate (NaN0 3 ) . 5. The process for the passivation of tinned stainless steel strip with a Chrome-free passivation film according to claim 4, wherein the process features are as follows: passivation bath composition expressed in g/1 - K 2 TiF 6 10-15 - NaN0 3 4-8, and water q.s. to 100; bath temperature 50-60°C; cathode current density 0.10-0.15 A/dm 2 ; and exposure time 10-30 s. 6. The process for the passivation of tinned stainless steel strip with a Chrome-free passivation film according to claim 1, wherein the passivation bath is an aqueous solution containing Zirconium sulfate. 7. The process for the passivation of tinned stainless steel strip with a Chrome-free passivation film according to claim 6, wherein the process features are as follows: passivation bath composition - Zr(S0 4 ) 2 15-20 g/1 - H 2 S0 4 0,5 M and water q.s. to 100; bath temperature 50-60°C; cathode current density 0.1-0.2 A/dm 2 and exposure time 5-10 sec. 8. The process for the passivation of tinned stainless steel strip with a Chrome-free passivation film according to claim 1, wherein the passivation bath is an aqueous solution containing Titanium oxalate. 9. The process for the passivation of tinned stainless steel strip with a Chrome-free passivation film according to claim 8, wherein the process features are as follows: passivation bath composition expressed in g/1 - Ti 2 ( C 2 O 4 ) 3 . 10 H 2 0 15 - 20 - K 2 C 2 0 4 . H 2 0 3 - 8 and water q.s. to 100; bath temperature 50-65°C; cathode current density about 0.10-0.20 A/dm 2 and exposure time 5-10 s. 10. The process for the passivation of tinned stainless steel strip with a Chrome-free passivation film according to claim 1, wherein the passivation bath is an aqueous solution containing Sodium molybdate . 11. The process for the passivation of tinned stainless steel strip with a Chrome-free passivation film according to claim 10, wherein the process features are as follows: passivation bath composition expressed in g/1 - Na 2 Mo0 4 15 - 20 - Na 2 S 2 0 3 2 - 4 and water q.s. to 100; bath temperature 40-45°C; cathode current density 0.1-0.15 A/dm 2 and exposure time 5-10 s. 12. A tinned stainless steel strip, characterised in that it is obtainable with the process recited in claims 1 to 11. 13. The tinned stainless steel strip, passivable according to claim 12, characterised in that it is painted with Chrome-free paints. 14. A use of the tinned stainless steel strip, passivated and painted with Chrome- free paints according to claim 13, for the production of tinned goods.

Inventors: DE CRISTOFARO NORMA, DE CRISTOFARO, NORMA
Applicants: CT SVILUPPO MATERIALI SPA, DE CRISTOFARO NORMA, CENTRO SVILUPPO MATERIALI S.P.A., DE CRISTOFARO, NORMA
Application year: 2001
Main classification: C25D
IPC codes: C23C22, C25D9