Comparison of Aqueous and Gelled 3.5% NaCl Electrolytes for Assessing the Corrosion Resistance of Thermal Spray Stainless-Steel Coatings in Electrochemical Corrosion Tests

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.

Quasi Non-Destructive Quality Assessment of Thermally Sprayed AISI 316L Coatings Using Polarization Measurements in 3.5% NaCl Gel Electrolyte

“…For the measurements, both a 3.5% NaCl gel electrolyte and an adapted measuring cell based on the three-electrode arrangement were developed. An earlier study [8] already proves that the measurement setup is suitable to avoid an influence of the substrate on the polarization curve in contrast to the use of aqueous electrolytes and provides a comparison of the test method described here and the conventional one (aqueous electrolyte, Ag/AgCl sat. KCl as reference electrode).…”

“…This leads to an out-ofcontrol increase of the real measurement area, which results in a systematic error when determining important characteristic values such as the corrosion current density. Therefore, the measurements are often influenced by the substrate material used to such an extent that the corrosion behavior of the coating can no longer be accurately assessed [8][9][10]. However, even if the infiltration of the coating does not extend to the coating-substrate interface, slight effects such as those caused by minor microstructural changes can hardly be detected because of the undefined measurement area.…”

Parameter Optimization and Mechanical Properties of Laser Cladding of 316L Stainless Steel Powder on G20Mn5QT Steel

“…Previously, many surface modification techniques [5][6][7][8] have also been used to repair other materials and thus improve their service life. Examples include surface remanufacturing [9], thermal spraying [10], electroplating [11], and vapor deposition [12]. These techniques enhance the surface properties of the substrate without changing the overall properties of the substrate material.…”