Pitting Corrosion of SS304L in Droplets of MgCl2 Solution

A droplet of a salt solution on the surface of a corrosion resistant alloy will lose water due to a temperature increase or a decrease in the relative humidity.  The chloride concentration thereby increases until reaching a critical concentration, at which point pitting corrosion can initiate, and is accompanied by a sharp drop in the open circuit potential (OCP). In this study the pitting corrosion behavior of stainless steel 304 under an electrolyte droplet was investigated using a height-controlled Kelvin Probe (KP). It was thereby possible to determine the pit initiation time from the corrosion potential and the chloride concentration at the point of initiation from the progression of the drop dimensions during drying.

A drop of dilute MgCl₂ solution was placed on the surface of a SS304L sample in the KP chamber. The RH in the chamber was then controlled to be the value of saturated MgCl₂, 34%, causing the drop to lose water and increase in chloride concentration.  The corrosion potential of the steel under the drop was followed with time as was the height of the drop using the height-controlled KP.  The initial size and concentration of the drops were varied in different experiments

A typical OCP transient exhibited a rather constant value and then a precipitous decrease, which is associated with the initiation of pitting. A single pit was found in every experiment, and the location of the pit in the drop volume was random. The height change at the center of the drop decreased linearly with time at the beginning of exposure due to water evaporation. The drop was found to have the shape of a spherical cap.  Using an equation for the volume of a spherical cap, and knowing the initial drop volume and concentration, it was possible to determine the drop concentration at the time of pit initiation. The median initiation concentration decreased with increasing initial droplet volume probably because a larger droplet has a higher possibility of covering a more-susceptible defect site for pit initiation.

The available cathodic current was calculated using a series of assumptions and compared to the anodic current determined by analysis of the pit volume as a function of time determined from different pits grown for different times.  The anodic current was initially less than the available cathodic current, but both decreased with time, and they converged after a period.  The pit morphology was observed to change at approximately this time from a shallow round disk shape found initially, to deeper attack at one part of the disk in the shape of an ear at later times.  The mechanism of pit initiation and growth are discussed.

Support by the Science & Technology Program of the Office of Civilian Radioactive Waste Management (OCRWM), U.S. Department of Energy (DOE), is gratefully acknowledged. The work was performed under the Corrosion and Materials Performance Cooperative, DOE Cooperative Agreement Number: DE-FC28-04RW12252. The views, opinions, findings, and conclusions or recommendations of authors expressed herein do not necessarily state or reflect those of the DOE/OCRWM.