Electrochemical studies under thin electrolyte layers using a Kelvin probe

In this study, electrochemical experiments were performed under thin electrolyte layers using a Kelvin Probe (KP). In the first part of this dissertation, cathodic and anodic polarization experiments of stainless steel 304L were conducted under a thin layer of chloride containing solution. Cathodic polarization curves exhibited a limiting current density associated with oxygen reduction. The limiting current density varied with solution layer thickness over a finite range of thickness. Anodic polarization curves on 304L in a thin layer of chloride solution resulted in pitting corrosion. The breakdown potential did not vary with solution layer thickness. However, the thin layer was observed to increase in volume remarkably during pit growth owing to the absorption of water from the high humidity environment into the layer with ionic strength increased by the pit dissolution. Furthermore, pitting of stainless steel 304 under droplets of MgCl2 solution was monitored. Droplets of different volumes of MgCl2 solution were placed on the steel surface and exposed to a constant low relative humidity (RH). As the concentration increased during exposure of the drop to low RH, the open-circuit potential (OCP) and the shape change of the drop were monitored by the KP. Pit initiation was detected by a sudden decrease in the OCP. Pits initiated earlier under small droplets than under large drops. The chloride concentration at initiation was found to be between 3.0 and 8.4 M for droplets with a starting concentration of 0.88 M Cl-. The initiation concentration increased when the initial concentration of the droplet was higher. The anodic current demand of pits growing at OCP decreased with time as did the available cathodic current. When the current demand exceeded the available cathodic current, the active pit area decreased. The pit stability criterion for OCP pits as expressed by the product of the current density and the pit radius, i&dota, was found to be much lower than for pits in dilute solutions. A mechanism for pit formation and growth under droplets of MgCl2 solution was proposed. Additionally, pitting corrosion behavior of stainless steel 304 under an electrolyte droplet with a layer of silica particles on the surface was investigated by using a KP. Droplets of 2.5 M MgCl2 solution were placed on an electrophoretically silica-coated steel surface and exposed to a constant low relative humidity. Due to evaporation of water, the chloride concentration increased. At a certain value pitting initiated, which was detected by a sudden drop in open circuit potential. Metastable pits repassivated slower under the silica particle layer than on bare stainless steel. Pits on silica-coated SS304 initiated within a narrow chloride concentration and time range unlike pits on bare SS304. Pit growth rate was not influenced by the silica layer.In the second part of this dissertation, the electrochemical behavior of magnesium-rich primer (MgRP) on AA2024-T3 was investigated with a KP. MgRP was developed based on the mechanism of Zn-rich primer for steel. It was suggested that the Mg pigment in the organic matrix acts as sacrificial anode and the aluminum substrate is cathodically protected. To get a deeper understanding of this mechanism different Mg-rich coating systems were compared with each other and galvanic corrosion experiments were performed between bare AA2024-T3 and AA2024-T3 coated with MgRP. The electrochemical properties of Mg-rich coatings primarily depend on the polymer matrix whereas the source of Mg pigment plays a secondary role. Mg-rich primer acts like an insulator under dry conditions. Water has to penetrate the polymer-pigment network for the coating to act as a sacrificial anode. This results in an activation time for cathodic protection. The ability of Mg-rich primer to protect the aluminum substrate depends on the coated/bare aluminum ratio. Basic or cathodic corrosion of AA2024-T3 is possible for samples coated with Mg-rich primer. Thin electrolyte layer experiments and cathodic polarization curves in different gases showed that CO2 in high concentration was able to buffer pH on AA2024-T3 surface and no basic corrosion occurred. However, amount of CO2 available in the air was not enough to buffer OH- from Mg corrosion products and oxygen reduction. Cathodic or basic corrosion took place.