Abstract
Corrosion testing with gel electrolytes gained attention in the past decade due to the advantage of almost non-destructive and in situ electrochemical measurements of bulk materials. Regarding thermal spray coatings, gel electrolytes offered the opportunity to prevent the infiltration of the typical microstructural features such as pores and microcracks. Using the example of stainless-steel AISI 316L coatings deposited by high velocity air fuel (HVAF) spraying on mild and stainless-steel substrates, the electrochemical corrosion behavior was analyzed in 3.5% NaCl electrolytes in an aqueous and gelled state. In this context, potentiodynamic polarization tests were carried out in a three-electrode corrosion cell, which was adapted for gel electrolyte testing. Gelling was realized with a technical gelatin. The characteristic corrosion values, such as open circuit potential, corrosion potential, and corrosion current density, revealed for the gelled state that the influence of the substrate material used could be eliminated and thus, the coatings itself could be characterized. In contrast, the coating specific microstructure and substrate material significantly affected the potentiodynamic polarization curve in the 3.5% NaCl aqueous electrolyte. Optical microscopy of the coating surfaces and cross-sections proved that the corrosion attack caused by aqueous electrolytes could be mimicked with the gel electrolyte.
Highlights
Gel electrolytes are increasingly considered in corrosion science with the focus on visual corrosion inspection for specific applications and electrochemical-instrumented measurements [1–12]
This study focuses on the influence of the substrate material on the measured corrosion values using potentiodynamic polarization tests in aqueous and gelled 3.5% NaCl electrolytes
Both coatings reveal a dense microstructure with distinguishable particle boundaries, i.e., the spraying parameters enabled a good compaction with few pores
Summary
Gel electrolytes are increasingly considered in corrosion science with the focus on visual corrosion inspection for specific applications and electrochemical-instrumented measurements [1–12]. The advantages addressed are an easier handling than using liquid electrolytes, e.g., for geometrical complex structures, almost non-destructive measurements, and the imitation of specific atmospheres. Further applications are corrosion assessment at sculptures and monuments [4,8] and reconstruction of specific atmospheric conditions like moisturizing films on zinc coatings [6,12] or artificial mud [11]. With regard to thermal spray coatings, Kutschmann et al showed in a previous study that an infiltration of the characteristic microstructures, such as pores and cracks, of arc and plasma-sprayed AISI 316L coatings can be prevented when using gel electrolytes in electrochemical-instrumented corrosion tests [13]. The corrosion attack can be transferred to the surface and the corrosion properties of the coating itself and the applied spraying conditions are revealed
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