  (ABRIDGED) Globular clusters trace the formation and evolution of the Milky
Way and surrounding galaxies, and outline their chemical enrichment history. To
accomplish these tasks it is important to have large samples of clusters with
homogeneous data and analysis to derive kinematics, chemical abundances, ages
and locations. We obtain homogeneous metallicities and alpha-element
enhancement for over 800 red giant stars in 51 Galactic bulge, disc, and halo
globular clusters that are among the most distant and/or highly reddened in the
Galaxy's globular cluster system. We observed R ~ 2000 spectra in the
wavelength interval 456-586 nm and applied full spectrum fitting technique. We
compared the mean abundances of all clusters with previous work and with field
stars. We used the relation between mean metallicity and horizontal branch
morphology defined by all clusters to select outliers for discussion. We find
our metallicities are comparable to those derived from high-resolution data to
within sigma = 0.08 dex over the interval -2.5 < [Fe/H] < 0.0. We also find
that the distribution of [Mg/Fe] and [alpha/Fe] with [Fe/H] for the 51 clusters
follows the general trend exhibited by field stars. It is the first time that
the following clusters have been included in a large sample of homogeneous
stellar spectroscopic observations and metallicity derivation: BH 176, Djorg 2,
Pal 10, NGC 6426, Lynga 7, and Terzan 8. In particular, only photometric
metallicities were available previously for the first three clusters, and the
available metallicity for NGC 6426 was based on integrated spectroscopy and
photometry. Two other clusters, HP 1 and NGC 6558, are confirmed as candidates
for the oldest globular clusters in the Milky Way. The technique used here can
also be applied to globular cluster systems in nearby galaxies with current
instruments and to distant galaxies with the advent of ELTs.
