| Popis: |
Leaded (Pb) brass components are widely used in water pipeworks for their good machinability and low costs. However, even the so-called “lead-free”, potable-water grade of brass is known to cause excessive Pb leaching into water after common soldering and brazing during pipe installation works. Despite the serious public concerns on drinking-water safety worldwide, the metallurgical pathways of lead leaching from such brass have remained unknown. In this work, synchrotron radiation X-ray imaging shows that in the initial state of potable-water grade brass samples, Pb exists mainly in the form of isolated or linked together particles (some at triple points of brass grain boundaries), with sizes ranging from sub-micron to several microns. Then, on heating to ~ 200°C to simulate soldering, surfacing of Pb contents happens rapidly in minutes, with diffusion pathways including a novel interpenetrating Pb-brass structure with orientation relationship (11\(\stackrel{-}{1}\))α−brass//(220)Pb; [011]α−brass//[\(\stackrel{-}{1}\)13]Pb, and voids left behind in the brass matrix after Pb surfacing. On heating to 700°C to simulate brazing, the original isolated or connected Pb particles melt and expand in volume, with the Pb content forced into the brass lattice preferentially along {101}α−brass planes, forming Pb phase of low sphericity or even large sheets. Finally, on immersion in water to simulate leaching, the surfaced Pb particles are oxidized to form PbO needles along the normal direction of {\(\stackrel{\text{-}}{\text{2}}\stackrel{\text{-}}{\text{2}}\text{2}\)}PbO planes, which are then easily washed away to increase the Pb content in the water. Some of these experimental findings are rationalized by molecular dynamics using a Cu-Pb potential system. The results here unveil, for the first time, how Pb leaches into water from the so-called “lead-free” grade brass. |
