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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.
Hard and wear resistant coatings deposited by PVD techniques have been characterized for decades for their capabilities to protect steel substrates from corrosion. In the present work the effect of Mg incorporated into TiN coatings is described in terms of the corrosion behavior as well as the mechanical and structural properties.
TiN and TiMgN films with Mg contents between 10 and 35 at.% were deposited onto mirror polished 100Cr6 (1.3505) steel samples with 2.5 and 5 μm thickness by using DC magnetron sputtering. The corrosion protection capabilities of the coatings were characterized by neutral salt spray (NSS) test, considering the amounts and sizes of growth defects inherent in each coated sample as determined by a recently developed optical scan method (Large Area High Resolution mapping). The defect data were statistically analyzed for improved interpretation of NSS test results. Chosen growth defects were additionally analyzed by focused ion beam technique. Furthermore the coating composition and morphology, the hardness and the tribological behavior were characterized.
Polished steel samples coated with 2.5 μm TiMgN containing about 35 at.% Mg were in the plane free of corrosion after 24 h in a NSS test. TiMgN with 10 or 20 at.% Mg only provided a slightly improved corrosion protection in relation to pure TiN coatings, which was limited to certain types of growth defects. The highest Mg containing coatings exhibited a decreased hardness down to 1200 or 1800 HV depending on type of deposition (HV 1200: Ti- and Mg-target with rotating substrate holder, 1800: Mg-plugged Ti-target with static substrate holder), but also showed a strongly improved wear resistance against Al2O3 related to pure TiN. By analyzing the NSS test results it was found that the corrosion behavior of the coated samples did not only depend strongly on the Mg content, but also on the sample individual defect concentrations. Therefore this subject is extensively discussed.
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.