  We analyze the production of the element Cr in galactic chemical evolution
(GCE) models using the NuGrid nucleosynthesis yields set. We show that the
unusually large [Cr/Fe] abundance at [Fe/H] $\approx 0$ reported by previous
studies using those yields and predicted by our Milky Way model originates from
the merging of convective Si-burning and C-burning shells in a 20 $M_\odot$
model at metallicity $Z=0.01$, about an hour before the star explodes. This
merger mixes the incomplete burning material in the Si shell, including
$^{51}$V and $^{52}$Cr, out to the edge of the carbon/oxygen (CO) core. The
adopted supernova model ejects the outer 2 $M_\odot$ of the CO core, which
includes a significant fraction of the Cr-rich material. When including this 20
$M_\odot$ model at $Z=0.01$ in the yields interpolation scheme of our GCE model
for stars in between 15 and 25 $M_\odot$, we overestimate [Cr/Fe] by an order
of magnitude at [Fe/H] $\approx$ 0 relative to observations in the Galactic
disk. This raises a number of questions regarding the occurrence of Si-C shell
mergers in nature, the accuracy of different simulation approaches, and the
impact of such mergers on the pre-supernova structure and explosion dynamics.
According to the conditions in this 1D stellar model, the substantial
penetration of C-shell material into the Si-shell could launch a
convective-reactive global oscillation, if a merger does take place. In any
case, GCE provides stringent constraints on the outcome of this stellar
evolution phase.
