Wissenschaftliche Artikel der BAM
The corrosion behavior of galvanized steels and zinc components under atmospheric exposure depends mostly on the corrosion product‐based cover layer formation under the prevailing conditions. The use of agar‐based gel electrolytes makes it possible to use electrochemical methods to obtain a characteristic value from these cover layers that describe their current and future protective capacity. It is shown here that different states of galvanized steel can be distinguished very well under laboratory conditions and that this method is also suitable for use under practical conditions. Based on the characteristic values and assuming future time of wetness, it is very easy to draw up a forecast for the future corrosion rate, which provides plausible values.
Nickel‐free high‐nitrogen‐alloyed stainless steels like the P2000 (X13CrMnMoN18‐14‐3) were developed to enhance the strength and corrosion resistance of austenitic stainless steels like 304 and 316 while keeping the typical high ductility. The mechanical and corrosive properties of P2000 were investigated and compared with 304 and 316 to highlight the application opportunities of this new alloy. The microstructure of the solution‐annealed condition was characterised by electron backscatter diffraction and the mechanical properties were studied by uniaxial tensile tests, Charpy impact tests and hardness measurements. The passivation behaviour was analysed using the electrochemical potentiodynamic reactivation, whereas the pitting corrosion resistance was compared by pitting potentials and pitting temperatures. However, secondary thermal influences or suboptimal heat treatment can impair the corrosion resistance due to the precipitation of secondary phases and the resulting sensitisation. Thermodynamic calculations and artificial ageing treatment in the range of 500–900°C for up to 100 h were used to determine critical time–temperature parameters for sensitisation. The microstructure of the various aged states was evaluated by scanning electron microscopy and compared with the degrading corrosion resistance characterised by the KorroPad method.
Hard coated steel components are used in a wide application range, mostly for protective, wear resistant and decorative purposes. Despite of these coatings being regarded as relatively dense, there is always a high risk of localized corrosion when a coated low alloyed steel component encounters a surrounding high humidity atmosphere or even an aqueous medium. An approach to enhance the corrosion properties is the addition of magnesium to physical vapor deposited hard coatings, like TiN. It has been found that there is a remarkable increase in corrosion resistance in dependence of magnesium content of the TiMgN coating and its surface properties. In this work the authors will explain the underlying corrosion protection mechanisms by means of electrochemical and analytical studies. The positive impact of magnesium in the coating relates on its preferred dissolution vs. steel. This causes the potential to shift to more negative direction with respect to the steel substrate and additionally leads to a temporarily passivation of the steel due to alkalization of the surrounding electrolyte by formation of magnesium hydroxide.
The paper presents a new method for the determination and evaluation of tafel factors using gel-type electrolytes and electrochemical frequency modulation technique (EFM). This relatively new electrochemical method offers the possibility to determine both polarization resistances and tafel factors within one measurement and in short measuring intervals. Starting from a comprehensive parameter study it is shown that a direct relationship between the two values exists that can be described mathematically. This contribution presents the determined tafel factors for the system gel-type electrolyte/zinc and discusses their applicability and their limits.
Limitations are encountered regarding the electrolyte when studying atmospheric corrosion reactions on materials that form protective layers on their surface or tend to passivate. For example, the reconstruction of a thin wet film, as well as the interpretation of electrochemical measurements in so-called bulk solutions (a "mass" of electrolyte), often proves to be difficult in view of corrosion behavior under atmospheric conditions. A new approach to this problem in corrosion research includes the use of gel-type electrolytes as an alternative to bulk electrolytes. Gel-type electrolytes form a thin wet film on the surface of the material, whereby the naturally formed protective layer is affected similar to atmospheric conditions and can be examined in an almost non-destructive way. Through electrochemical instrumentation specific values such as polarization resistances and corrosion currents can be determined providing information on kinetics of surface layer formation and stability of surface layers formed under the influence of a thin wet film. Thus, new test methods can be developed that supply a better understanding of corrosion processes in specific atmospheres.
In this article different zinc coatings and aluminium alloys were investigated, their naturally formed protective layers were electrochemically characterized and corrosion relevant values were determined by using a gel pad based on polysaccharide. Corrosion relevant values allowed the differentiation of various coating systems and could describe the current protective effect provided by the coating. It is shown that gel-type electrolytes influence protective layers and coatings considerably less than corresponding bulk electrolytes and that an atmospheric wet film is approached by these test conditions. From the results it is evident that gel-type electrolytes represent a viable and promising field in corrosion research.