Proton exchange membrane (PEM) fuel cells are being developed for future NASA missions. These fuel cells use a bipolar construction, with internal manifolds to conduct product water and coolant water. The wetted surfaces of the manifolds present large voltage gradients across the product water and coolant water passages, which can induce water electrolysis in the manifolds of full-scale stacks. If not controlled, shunt currents lead to parasitic power losses and corrosion of the fuel cell metal surfaces; therefore, it is important to understand and characterize the effects of shunt currents. This analysis also applies to electrolyzers, flow-batteries and other devices where an electrolyte manifold experiences voltage gradients. In this work, electrochemical characterizations were performed to estimate these characteristics under relevant operating conditions. A numerical solution of the shunt currents is presented, and the distribution of water electrolysis and corrosion is described.