Low-melting-point bismuth- (Bi-) based alloys have recently been proposed for plug-and-abandonment (P&A). Previous experiments have shown the feasibility of BiSn [58-wt% Bi and 42-wt% tin (Sn)] and BiAg [97.5-wt% Bi and 2.5-wt% silver (Ag)] alloy plugs in moderate temperature wells, both across shales and across the shale/sandstone sequence. These were validated in linear and cylindrical wellbore cavity geometries for various differential setting pressures for alloy over air. Until now, all of the experiments for setting alloy plugs have been conducted with air as the wetting fluid. Given the lack of adhesion between minerals and alloy, our concept for providing bond strength and integrity has hinged on providing a bicontinuous structure through alloy penetration into the pore network. For shales, with a positive setting pressure, anchors on the surface, in lieu of pores, have proven to be adequate. With results obtained under excess alloy pressure, we have quantified the effect of setting pressure on the alloy/shale bond quality. With brine as the wetting fluid, imposing an excess pressure on the alloy has not been demonstrated previously. This paper is the continuation of our previously published papers (Zhang et al. 2020a, 2020b), and our objective here is not only to show the possibility of forming a plug under brine but also to quantify the plug’s quality with and without an excess alloy pressure. We first describe an apparatus that controls alloy and brine pressures independently through a semipermeable piston assembly and demonstrate forming alloy plugs in a brine-filled borehole cavity. Based on pressure decay tests across the plug, we demonstrate that wellbore integrity is possible only with a positive alloy pressure over that of brine.