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Office of Naval Research - Stress Corrosion Cracking Control Plans - III - Copper Alloys

ALLOY NOMENCLATURE 
mhe atomic arrangement in crystals of pare copper is 
face—centered cubic, and as long as this arrangement is 
retained in copper alloys, they are designated alpha alloys , 
such as alpha brass. If a brass contains more than about 
3" zinc, grains of a second phase termed beta appear 
among the alpha grains. The beta phase has the atoms 
arranged in a body—centered cubic lattice. Brasses con— 
taining both phases are termed alpha—beta brasses. Above 
about 408 zinc the alloys consist of all beta grains and 
are termed beta brasses. Beta brass alloys are rare in 
commerce, but a few are produced in Europe. 
Copper alloys are not strengthened by heat treatment, 
with few exceptions, notably beryllium copper. But they 
are commonly strengthened by cold working. 
The term bronze was once reserved for copper alloys 
in which the principal alloy addition was tin, but the 
term has been debased to the point that its use serves 
little more than to exclude the near ly ail—copper alloys 
and the nearly pure binary copper—zinc alloys (hr-asses} .

A principal environment causing SCC in copper alloys , 
although not the only one, IS generally believed to involve 
arnrnoniacal compounds. SCC in the copper/ammoniac-l system 
has often been termed "season cracking" by leading authorities 
but there is a lack of unanimity in defining this term by 
various authors and nomenclature committees. Therefore 
to avoid any possible confusion the term "amnoniacal SCC" 
will be used here instead, though even so it should be 
understood that the word "ammoniacaln is a convenience 
intended to include amines and all other species which can 
react with copper to produce the cupric—arnrnonium complex 
ion or perhapa btructuraily similar complexes. 
If -one learns how to avoid ammoniacal SCC, a large 
proportion of the practical SCC threat to copper alloys 
will have been prevented. Not only is it considered a 
principal SCC hazard to copper alloys, but it is also the 
SCC system for which we have the most intercomparable 
laboratory data and field experience by far. We Will 
there Eore treat ammoniacal SCC as the principal topic of 
this chapter, thereafter treating the Other alloy/environment 
systems which do not fit into the major group. These other 
systems, though they have been responsible 
for fewer SCC failures than ammoniacal SCC, 
be extremely troublesome, as will be seen. 
historically 
can nevertheless 
It should be 
especially noted that recent studies with copper Sulfate