Development of New Austenitic Stainless Steel Cathode Grade for More Efficient Copper Electroplating

Austenitic stainless steel Supra 316L/1.4404 has been utilized for decades as permanent cathode plates in the last stages of pure copper production. Outokumpu Europe has a long experience in supplying cathode specific steel for this use. The development of more efficient copper electroplating material was initiated in order to increase the lifetime and reduce the cost fluctuation of permanent cathode plate steels. Permanent cathode plates are immersed in a corrosive environment, i.e. in sulphuric acid solution where it is expected that the steel does not corrode or dissolve during the operation. The solution may consist of corrosive elements like chlorides, which together with elevated temperatures increase the material requirements. The corrosion resistance of industry standard Supra 316L/1.404 is achieved by a combination of ~17% chromium and 2.0% molybdenum alloying. Additionally, nickel content must be minimum of 10% in order to achieve austenitic non-magnetic microstructure at the room temperature. Recently developed austenitic stainless steel grade Supra 316plus/1.4420 is designed in such manner that the corrosion resistance is achieved by increased chromium (20.3%) and nitrogen (0.20%) levels. Additionally, the nickel content was reduced down to 8.6% and molybdenum down to 0.7% while retaining fully austenitic microstructure. This paper shows the development of cathode specific Supra 316plus/1.4420 product. Copper electroplating and adhesion performance have been compared to the typical Supra 316L stainless steel in a laboratory environment by test plating and by measuring the separation force. Surface micrographs showcase the importance of surface microtopography on the copper nucleation, growth and adhesion on top of stainless steel. Mechanical properties are also compared and advantages in the copper stripping operation are reviewed.