Equilibrium Phae Diagram of Cu-Zn
Due to metallurgical, mechanical and thermal reasons, alloys with higher zinc content than 35% are less suitable for tube production. It is however interesting to explore what happens as copper content decreases (or alternatively the zinc content increases). This can be achieved using the Phase Equilibrium Diagram as the road map to the use of two (or more) metals. These diagrams give details on conditions of thermodynamically distinct phases that coexist in equilibrium with each other.
Alpha (α) is the solid solution phase which is pure copper with a zinc element inside a single phase. The properties of α brass can give us the right amount of strength and elongation we prefer for tubes and headers and many other products. Pure copper is too soft. Because it is existing as a stable single-phase metal these properties are predictable and it has good corrosion properties. We use 30-70 brass for tubes and headers to stay inside a single α phase. Moving from left to right, as we increase the zinc content the α structure becomes saturated and a second phase drops out. Between approximately 36% and 48%, we are in an α-β region where there are 2 phases. The heterogenous lattice is highly susceptible to dezincification and corrosion since this is a bi-metal solution with high energy crystal boundaries from zinc and impurities moving to the borders in transition. Also, as with any two metals in contact, the galvanic potential for corrosion is high, particularly with anodic zinc. It should be noted that when β is small and dispersed effectively around the α phase, it may become an efficient strengthening phase and give this brass some desirable physical properties. Moving from left to right, as we increase the zinc content, the α single phase solid solution disappears and the next single phase metal forms with a new structural compound making up the lattice. This is β which is a single-phase metal that has the stability and improved corrosion resistant properties of any single-phase metal.