Electrochemical Behavior of AISI 304SS with Particulate Silica Coating in 0.1 M NaCl

Abstract

This paper presents electrochemical behavior of AISI 304 stainless steel with a silica layer in a stagnant bulk solution of 0.1 M NaCl. Layers composed of densely packed 350 nm diam silica particles were deposited cathodically on stainless steel at a constant voltage by electrophoretic deposition (EPD). Quite smooth and crackfree silica layers less than about 80 μm in thickness were obtained and the thickness of the layer depended linearly on the deposition time. It is proposed that silica layers deposited by EPD can be used as simulated particulate layers to investigate localized corrosion of corrosion-resistant alloys under atmospheric environments. Electrochemical properties of silica-coated stainless steel samples in 0.1 M NaCl were investigated. The cathodic polarization behavior depended on the thickness of the silica layer; the limiting current density for oxygen reduction reaction decreased with increasing silica layer thickness. The effect of the silica layer on anodic polarization behavior was not remarkable.One type of atmospheric corrosion involves the interaction of a humid environment with a contaminated surface. Water from the humid environment can be absorbed onto surfaces covered with fine dust particles containing soluble salts, creating a saturated deliquescence brine layer.1-4 Such a layer could be extremely corrosive, even to corrosion-resistant alloys (CRAs) if the temperature were high enough, due to the salt concentration and high availability of oxygen. Furthermore, it is possible that precipitates or rock particulates could act as a crevice former and cause localized corrosion of the substrate metal. The objective of this work is to generate, in a reproducible and controllable fashion, simulated artificial deliquescence brine layers and to investigate localized corrosion behavior of CRAs under such an environment.In this study, electrophoretic deposition (EPD) of silica particles suspended in solution was utilized to create simulated particulate layers on the surface of CRAs. EPD is a very useful technique to deposit ceramic particles on metal surfaces; one can get ceramic layers with thicknesses on the order of μm to mm by EPD. [5][6][7] Furthermore, the thickness of the ceramic layers can be controlled easily by changing the applied voltage or deposition time. Therefore, EPD can be utilized to create simulated particulate layers on CRAs in a reproducible and controllable fashion.In general, when EPD of ceramic particles is conducted on a metallic substrate, such as stainless steel (SS), the substrate should be polarized either at a positive voltage or a negative voltage with respect to the counter electrode, depending on the zeta potential or isoelectric point of the particles to be deposited and the pH of the suspension. Because the isoelectric point for silica is quite acidic, 8, silica particles in a neutral suspension carry negative charges on their

Pitting Corrosion of Silica-Coated Type 304 Stainless Steel Under Thin Electrolyte Layers

“…The coating was visually homogeneous with no obvious cracks. The relationship between coating thickness and electrophoretic deposition time determined previously [7][8] was observed in this work, and the 30-s deposition time corresponds to a coating thickness of approximately 17 μm.…”

“…22 However, when the pit is covered not only with the undermined passive layer but also with a silica coating, the pit covering might be more robust so that metastable pits can grow for a longer time before rupture occurs and repassivation ensues. In prior work with similar silicacovered Type 304 SS samples, metastable pits were observed during potentiodynamic polarization in bulk chloride solution, 8 as well as during dryout experiments under thin layers of electrolyte at OCP. 7 The medians of the distributions for times to initiation of stable pits are very similar for bare and silica-coated samples.…”

“…Polarization experiments in bulk electrolytes indicated that the limiting current density for oxygen reduction decreased because of the silica coating, which acted as a slight diffusion barrier. 8 However, the silica layer had no effect on the pitting potential and the passive current density. Under thin electrolyte layers, the particles did not influence pit initiation but slowed the pit propagation process.…”

“…3 Tada and Frankel investigated the influence of silica particles on the localized corrosion behavior of Type 304 (UNS S30400) (1) SS. [7][8] A thin layer of silica particles was electrophoretically deposited on the sample and corrosion measurements were conducted in bulk solution or under a thin electrolyte layer. Polarization experiments in bulk electrolytes indicated that the limiting current density for oxygen reduction decreased because of the silica coating, which acted as a slight diffusion barrier.…”

Effects of Particulate Silica Coatings on Localized Corrosion Behavior of AISI 304SS under Atmospheric Corrosion Conditions

“…The procedure to create the silica layers on 304SS was described in detail in a previous paper. 2 After depositing the silica coating, the samples were stored in a desiccator until just before the atmospheric corrosion experiments. The test electrolyte solutions applied on the samples were either 0.5 M NaCl or x M MgCl 2 (x = 0.05-3).…”