Corrosion testing : a comparison between active screen and D.C. treatments

Abstract

It has been proved, by numerous studies, that low temperature plasma surface treatments give advantageous results when applied to stainless steels. These treatments are accredited with imparting better hardness and wear resistance to the material treated without adversely affecting its inherently good corrosion resistance properties. Most study groups focus on the investigation of a nitrogen S-phase, thus leaving the carbon S-phase scarcely explored. Although the nitrogen S-phase and carbon S-phase share numerous similarities each alloyed layer has its own distinct features. This work investigates the localised corrosion resistance properties of carbon S-phase produced on medical grade stainless steel when using two different set-ups. The two set-ups which were compared make use of direct current plasma technology and active screen plasma technology respectively. Using established optimised treatment temperatures and parameters, precipitate-free carbon S-phase was created with both technologies. Crevice corrosion, together with other forms of localised corrosion, may at times cause the premature failure of an object during its operation. Medical devices cannot afford the luxury of failing prematurely within the human body. The results oblained from corrosion resistance tests conducted on treated samples gave a positive response, thereby encouraging further possible use of such treatments in biomedical environments. The aim of the crevice corrosion resistance tests was to be able to fully characterise and rank the possible resultant corrosion effects on the S-phase layers produced. Purposely-built corrosion cells were designed, constructed and used for the testing. Long term chloride solution tests resulted in heavy crevice attack for untreated material, whilst vast improvements were seen on the treated surfaces. A distinction in results was visible between the direct current and the active screen with the latter suffering from negligible surface damage. Apart from crevice corrosion, intergranular corrosion is a major concern for carburised samples. Low temperature plasma treatments, conducted at much lower temperatures than other treatments, should mitigate susceptibility. Boiling acid tests conducted had an adverse effect on the treated surfaces. Treated samples suffered severe, uniform corrosion attack in boiling sulphuric acid. This therefore limits its use in high temperature acidic environments. This limited the possibility of drawing any conclusions with respect to the intergranular susceptibility of carbon S-phase. The results obtained for both within an intrahumanlike environment and in aggressive halide solutions showed no surface damage with excellent corrosion resistance behaviour. Carburising proved to be beneficial using both treatment techniques. Improved results for crevice corrosion resistance of the medical grade, austenitic stainless steel untreated material were obtained. It was confirmed that using an active screen set-up results in a better surface composition, more resistant to selected localised corrosion attack than that produced using only a direct current furnace hence encouraging their use in commercial medical application.