For hardenable and therefore low-alloy stainless steels such as 1.4021 or 1.4034 or for stainless steels with special chemical stability requirements, an artificial passivation layer has become indispensable to protect against possible corrosion. In this project, the possibilities of innovative passivation processes were analysed. The main focus was on the influence of different production processes on the formation of passivation layers.
In this joint project the influence of the manufacturing processes milling, blasting, scotching, electropolishing and laser welding on the formation of passivation layers of stainless steels was investigated. The stainless steels 1.4301 were used as a known representative of the higher alloyed stainless steels and 1.4021 as a common representative of the lower alloyed stainless steels.
The corrosion resistance of stainless steels can be improved by suitable passivation. The thickness of the passivation layers is only a few nanometers. The success of a passivation process usually depends on a large number of parameters. These include, for example, the cleanliness of the material surface, the chemical composition of the material on the surface depending on the pre-treatment and, last but not least, the passivating agent itself including the associated process parameters.
The citric acid process (CitriSurf 2250) from MKK, a process from Borer Chemie, the nitric acid process and the SolNit® process from Härterei Gerster were used as the passivation process.
The quality of passivation layers was evaluated by measuring the chemical composition of the material on the surface and examining the topography using photoelectron spectroscopy (XPS or ESCA), scanning electron microscopy (SEM) and profilometry. The investigations on passivated stainless steel surfaces were performed according to the standards 90120403B-STD (XPS) and 90120401B-STD (SEM).
In addition, the element concentrations of selected samples were determined using depth profiles using SNMS. This was particularly necessary because an evaluation of the corrosion resistance exclusively via the iron-chromium system appears insufficient in the case of nitrogen implantation. Especially with the SolNit® process used in this project, however, the nitrogen dissolved in the material structure could be a significant parameter for corrosion resistance.
In order to connect to conventional analytical methods for the evaluation of passivation layers, electrochemical tests such as rest potential measurements and cyclic voltammetry were carried out on selected samples.
The characterisation of the influence of the manufacturing processes on the chemical composition and structure of the material surfaces was supplemented by investigations of the surface morphology on the basis of microsections using FIB technology (Focussed Ion Beam).
Tests of medical devices in the field of passivation layers are offered as a service for industry using our accredited test methods in the field of material, surface and interface analysis.
- 19 Firmen, vorwiegend aus der Medizintechnik