Corrosion in Liquid Cooling Systems with Water-Based Coolant - Part 2: Corrosion Reliability Testing and Failure Model

This paper reports the corrosion mechanism active in microchannel cold plates used in a liquid cooling system and proposes a kinetic model describing the rate of corrosion induced failure as a function of testing conditions. The corrosion failure mechanism investigated in this paper is galvanic corrosion because the cold plate is typically made of Cu and is assembled using brazing alloys and there exists a galvanic potential between the Cu and the brazed area. A series of experimental characterizations indicates that the brazed joint is subjected to galvanic attack when exposed to a coolant, a mixture of water and propylene glycol (PG), with a galvanic potential sufficient to dissolve the braze component with an accelerated rate. Various testing on the galvanic corrosion finds that the braze (a ternary alloy of Cu, Ag, and P) becomes an anode in the galvanic pair and loses the component element by the process of dissolution. This type of galvanic corrosion is found to exist even with corrosion inhibitors present in the coolant, necessitating the corrosion assessment methodology that can predict the rate of cold plate failure with the use of the "accelerated testing" and the prediction model. Our research leads to the development of the micro-galvanic cell testing as well as the zero resistance ammeter (ZRA) methodologies. Our investigation with these testing methodologies presents clear evidence showing that the galvanic corrosion is the most active and serious form of corrosion in the cold-plate with the galvanic pair exerting as high as similar to 0.3V anodic potential on the brazed joint. It is also found that the rate of corrosion can be further accelerated with temperature and the external potential purposely applied across the Cu and the braze. The resulting galvanic corrosion kinetics collected in the form of current may be used to predict the corrosion rate at use conditions as they are found to follow the form of an Arrhenius-type kinetics model with a consideration of the corrosion acceleration factor.