Producing Platinum Group Metal-Doped Stainless Steel for Improved Corrosion Resistance

In general, stainless steels have good corrosion resistance. However, under certain aggressive acidic conditions, the chromium oxide film which imparts its characteristic corrosion resistance can dissolve. Alloying with platinum group metals (PGMs) (such as palladium) can enhance the stability of the passive film by the mechanism known as cathodic modification - meaning that the material can be used in more aggressive environments. Studies have shown that cathodic modification can be achieved by two different approaches: bulk alloying and surface alloying steels with PGMs. Despite demonstrations of effective cathodic modification of stainless steels, this technique has not achieved commercial success. A major reason why bulk alloying with PGMs is not a widely implemented solution to corrosion is that it is a very costly technique because the volume of PGMs required.

Alloying the stainless steel surface with PGMs is a less costly alternative; however, research has shown that this technique also possesses limitations. Materials created in this way can degrade over time leading unpredictable corrosion performance, meaning it is unlikely to be able to provide effective long-term corrosion protection. This PhD project aims to instead use a powder metallurgical approach to producing cathodically modified stainless steel for enhanced corrosion resistance.

Field assisted sintering, a powder metallurgical technique, has been successfully used to produce near-fully dense volume alloyed samples of ruthenium and palladium doped 316L and 17-4PH stainless steel. Currently, these specimens are being corrosion tested in varying test solutions to assess the effectiveness of these samples. This work forms the first step towards minimising the total volume of PGMs required to attain cathodic modification.

In general, stainless steels have good corrosion resistance. However, under certain aggressive conditions, the chromium oxide film which gives stainless steel its characteristic corrosion resistance can dissolve. Alloying with platinum group metals (PGMs) can improve the stability of the oxide film by the mechanism known as cathodic modification. This means that the material can be used in more aggressive environments. Studies have shown that cathodic modification can be achieved by two different approaches: bulk alloying and surface alloying steels with PGMs. However, the high costs and unpredictable corrosion performance associated with these two approaches mean that cathodically modified stainless steels have not achieved commercial success. This PhD project aims to instead use a powder metallurgical approach to producing cathodically modified stainless steel for enhanced corrosion resistance. Field assisted sintering, a powder metallurgical technique, has been successfully used to produce near-fully dense volume alloyed samples of ruthenium and palladium doped 316L and 17-4PH stainless steel.

The novel aspects of this project show real promise for overcoming the cost and reliability issues associated with PGM-doped stainless steels

Natasha Sweeney Fort

As with all PhD projects, there is a great deal of uncertainty and risk associated with the outcome of this study. This study has, however, demonstrated the versatility of field assisted sintering, demonstrating its ability to sinter materials not typically produced in such a way.

Equipment used:

• FCT D50 FAST/SPS Machine – Royce Discovery Centre, University of Sheffield
• Mastersizer 3000 - Royce Discovery Centre, University of Sheffield
• Electrochemistry experiments carried out in Engineering A building, University of Manchester
• Zeiss Ultra Scanning Electron Microscope, Johnson Matthey Research Centre, Sonning Common
• LECO Elemental Analyzer, Johnson Matthey Research Centre, Sonning Common
• Zeiss Gemini 2 Focused Ion Beam, Johnson Matthey Research Centre, Sonning Common
• Tescan Vega3 LMU Scanning Electron Microscope, Sorby Centre, University of Sheffield

Biography:

Natasha Sweeney Fort is an EngD researcher in the Department of Materials Science and Engineering at the University of Sheffield, Sheffield, UK. Natasha’s PhD project on PGM-doped stainless steels is sponsored by Johnson Matthey and Globus Metal Powders. Her electrochemical work has led to the development of close ties with the corrosion department at the University of Manchester, Manchester, UK. Prior to her postgraduate studies, Natasha gained an MEng in Mechanical Engineering at the University of Surrey, Guildford, UK. During her degree, she founded the Engineers without Borders Surrey chapter and was nominated for the Ricardo Most Promising Female Engineer award. Her interest in materials was piqued by her Master’s project on the ‘Optimisation of Indium Tin Oxide Thin Films Produced by Magnetron Sputtering’.