Parts from stainless steels owe their chemical resistance to the functional properties of their surfaces. They are responsible for the corrosion resistance and also for other properties like adhesion of substrates or ease to clean. The passive layer forms the phase boundary between metal and environment. It is formed either spontaneously during the fabrication or can be produced in an accelerated way by specific chemical treatments. The quality of the passive film is dependant of the alloying content of the metal, but also largely influenced by other factors like fabrication processes, surface topography and type of mechanical and chemical surface treatment. The passive layer is also a kind of living system whose properties and stability is largely influenced by the environment where it is used in. So far, there are no methods to characterize these functional surface properties in a generally applicable way. Combination of special electrochemical techniques in combination with measurements of the surface energy have shown to be useful tools. This permits to characterize and to classify the functional properties of surfaces which have been produced by mechanical and special chemical and electrochemical treatments.
The functional properties of materials are determined by their behaviour in contact with various substances i.e. the corrosion and tribological behaviour as well as the adhesion of the substances to the metal. The latter behaviour is responsible for the cleaning properties of the equipment. The various grades of stainless steels are widely used in food, CPI and pharmaceutical industries because of their corrosion resistance and their ease in fabrication1. Additionally, as they can be rolled with shiny and even surfaces with low roughness they are supposed to be easy to clean. Additionally, they can be electropolished to produce non-sticky surfaces. This procedure which is effective but also time consuming and costly is often applied in paper machines and pharmaceutical industries2. Though a low roughness is often considered to be essential for the adhesion properties is has been shown that the processes how the surface was prepared is as important as the roughness. This also applies for corrosion properties as is reported by Volmer3 and Ladwein and Guempel4,5. Due to these complex interrelationships it is not trivial to characterize and to quantify the functional surface properties. Measuring the roughness by mechanical or optical procedures is not good enough. Measuring of surface energies by means of wettability tests can be another alternative. Goellner et al6. proposed to use electrochemical noise as this approach is very sensitive for even slight variations of the surfaces and the passive layers and therefore can help to differentiate between various types of surface structures7,8.
Test Materials Test specimen were cut in sizes 20x20 mm from a cold rolled sheet (3 mm thick) of stainless steel 1.4404 (AISI 316L) in the as delivered condition. A copper wire was spotwelded from the backside. Surface preparations were applied by wet-grinding the specimen on a metallografic disc using emery paper of various grits.
