These include leaded brasses, such as UNS C3800, manganese bronze and the leaded nickel silvers.  The lead is usually present at 1 to 2 wt.% to improve machineability.  The lead particles are fine and are distributed more or less uniformly throughout the matrix. 

Sometimes these alloys are used for components that operate above the melting point of lead (~328°C).  The author has seen these alloys used for items such as gas jets in a baker's oven and steam glands in a power station.  At temperatures greater than 328°C the lead is very mobile and over a period of weeks to months it diffuses through the matrix and forms a small number of large lead particles.  Where these are at the exposed surface, the liquid lead encourages fast diffusion and rapid corrosion can occur if air is present.

The solution is to use lead-free copper alloys or a chromium steel, such as the 7% or 9%Cr grades.