Passivation treatment of tinplate

Abstract: A chromium free passivation film for tinplate. The film is produced electrolytically by immersing tinplate in an aqueous solution of a soluble compound selected from potassium titanium oxalate, titanium sulphate and zirconium sulphate. The film so produced has sulphide stain resistance comparable to that produced by conventional chromium bearing passivating solutions.

Description: This invention relates to passivation treatments for applying chromium-free passivation films to tinplate. Passivation is a treatment in which chemically active surfaces of tinplate are coated with impervious films of a passive material by anodic or cathodic polarisation significantly to reduce the active state of such surfaces. It is of course extremely important that tinplate used in the canning industry is chemically inactive. In commercial production, strips of blackplate are passed continuously through a series of treatment tanks to clean, plate and chemically treat the plated material prior to use. Conventionally, passivation fluids are chromium-based aqueous solutions, typically sodium dichromate. Like many chromium-based solutions, sodium dichromate is a hazardous fluid which represents a potential health risk to line operatives and is environmentally unfriendly. There is therefore on both health and environmental grounds an increasing need to develop chromium-free passivation treatments. Commercially any such treatment must meet a range of performance criteria equivalent to or better than those of chromium based passivation treatments. Also, any such treatment should be capable of application under tinning line conditions similar to those used for chromate-based treatments, these including a typical treatment time of approximately one second. The present invention sets out to provide a chromium-free passivation treatment which meets these and other criteria. According to the present invention in one aspect there is provided a passivation treatment for tinplate in which the passivating solution is an aqueous solution of a soluble compound selected from potassium titanium oxalate, titanium sulphate and zirconium sulphate, the passivation treatment being characterised in that the concentration of titanium sulphate in solution is in the range from 15 to 30 gms per litre. Preferred concentrations of potassium titanium oxalate in solution are from 5 to 20 gms per litre. Preferred concentrations of zirconium sulphate In solution are from 10 to 30 gms per litre. In a further aspect the invention provides a passivation treatment for tinplate in which the passivating element is either titanium or zirconium. The invention will now be described by way of example only with reference to the accompanying diagrammatic drawings in which:- Figure 1 is a schematic diagram of a plating line for conducting passivation treatments in accordance with the invention; and Figure 2 is a graph comparing depth profile analyses of passivating elements in accordance with the invention with the conventional passivating element sodium dichromate. The plating line illustrated in Figure 1 comprises a plurality of guide rollers 1 for transporting a strip of blackplate 2 continuously from a coiled roll 3 to an exit coil 4 via a multiplicity of tanks. These tanks include a cleaning tank 5, rinsing tanks 6, a pickling tank 7, a plating tank 8 and an electrolytic chemical treating tank 9. Towards the centre of the plating line is a reflow 11 to each side of which are positioned a drag-out bath 12 and quenching bath 13. A welding station 15 is positioned between the roll 3 and the entrance to the plating line. The plating line illustrated is generally conventional. In use, strip is passed continuously through the plating line with a thin passivating film being applied to both sides of the tinplate as it passes through the tank 9. The line speed is typically 300-350 metres per minute with a treatment dwell time in tank 9 of one second. Static electrodes are provided in the tank 9, deposition of the passivating film being achieved by cathodic reaction. The width of the strip 2 is typically 1 metre and above and the treatment temperature is typically 65°C. Hitherto the passivating solution present in the treatment tank 9 has been chromium-based and in particular sodium dichromate. In the present invention a chromium-free passivating solution is employed, this being selected from aqueous solutions of zirconium sulphate, potassium titanium oxalate and titanium sulphate. For zirconium sulphate a solution concentration in the range 10 to 30 gms per litre is preferred, a typical concentration being 21 gms per litre. For potassium titanium oxalate a solution concentration in the range 5 to 20 gms per litre is preferred, a typical concentration being 14 gms per litre. For titanium sulphate a solution concentration in the range 15 to 30 gms per litre is preferred, a typical concentration being 21 gms per litre. Thus, the passivating element is either titanium or zirconium. Depth profile analyses of each of the passivating elements are shown in the graph of Figure 2. It was found that all three chromium-free treatments produced thicker oxide levels than a conventional chromium-based passivation treatment. By way of example, aqueous solutions of titanium sulphate in a concentration of 21 gm/litre, potassium titanium oxalate in a concentration of 14 gm/litre and zirconium sulphate in a concentration of 21 gm/litre were prepared. These solutions were in turn used in trial passivation treatments of tinplate, the bath temperature, cathodic application and pass time being respectively 65°C, coulomb density 6.4 c/dm 2 and one second. Strip speed for each trial was assumed as 180 metres/minute and the pass time through the passivation bath was one second. Sulphide stain resistance of all three treatments was found marginally to be comparable to that of a treatment using sodium dichromate. No discolouration of the tinplate occurred. The weldability range of tinplate treated in accordance with the invention was generally higher than for that treated using sodium dichromate. No abnormal weld features were observed. After two months storage, tinplate treated in accordance with the invention exhibited less passive film damage than tinplate treated using sodium dichromate. Tinplate produced by all these chromium-free treatments exhibited improved adhesion when compared with tinplate treated with sodium dichromate. It was found that surface concentrations of titanium and zirconium were lower than for chromium. All three treatments produced thicker oxide levels than that produced with sodium dichromate. In summary, the trials show that treatments in accordance with the invention provide effective alternatives to sodium dichromate passivation and can be applied using current tinplate technology; that potassium titanium oxalate, titanium sulphate and zirconium sulphate based treatments meet requirements for alternative chromium-free passivation for tinplate; that it is possible to weld tinplate passivated in accordance with the invention at commercial settings; that treatments in accordance with the invention give improved lacquer adhesion compared to chromium-based treatments; that passive films produced in accordance with the invention provide more abrasion resistance than passive films produced using sodium dichromate; that treatments in accordance with the invention provide treatments of comparable performance to chromium-based treatments; that surface chemistry for treatments in accordance with the invention exhibited reduced amounts of passive film elements compared to chromium-based treatments; and that total oxide film thickness of tinplate treated in accordance with the invention were greater than that produced by chromium-based treatments. It will be appreciated that the foregoing is merely exemplary of treatments in accordance with the invention and that modifications can readily be made thereto without departing from the true scope of the invention as set out in the appended claims.

Claims: A passivation treatment for tinplate in which the passivating solution is an aqueous solution of a soluble compound selected from potassium titanium oxalate, titanium sulphate and zirconium sulphate, the passivation treatment being characterised in that the concentration of titanium sulphate in solution is in the range from 15 to 30 gms per litre. A passivation treatment as claimed in Claim 1 wherein the concentration of potassium titanium oxalate in solution is in the range from 5 to 20 gms per litre. A passivation treatment as claimed in Claim 1 of Claim 2 wherein the concentration of zirconium sulphate in solution is in the range from 10 to 30 gms per litre. A passivation treatment for tinplate characterised in that the passivating element is either titanium or zirconium.

Inventors: MCALISTER JOHN ANTHONY, REES IEUAN STEPHEN, MCALISTER, JOHN ANTHONY, REES, IEUAN STEPHEN
Applicants: CORUS UK LTD, CORUS UK LIMITED
Application year: 1996
Main classification: C25D5
IPC codes: C23C22, C25D5, C25D9