  We aim to analyze the chemical evolution of the Small Magellanic Cloud adding
12 additional clusters to our existing sample having accurate and homogeneously
derived metallicities. We are particularly interested in seeing if there is any
correlation between age and metallicity for the different structural components
to which the clusters belong. Spectroscopic metallicities of red giant stars
are derived from the measurement of the equivalent width of the near-IR calcium
triplet lines. Cluster membership analysis was carried out using criteria that
include radial velocities, metallicities, proper motions and distance from the
cluster center. The mean cluster radial velocity and metallicity were
determined with a typical error of 2.1 km/s and 0.03 dex, respectively. We
added this information to that available in the literature for other clusters
studied with the same method, compiling a final sample of 48 clusters with
metallicities homogeneously determined. Clusters of the final sample are
distributed in an area of ~ 70 deg^2 and cover an age range from 0.4 Gyr to
10.5 Gyr. The metallicity distribution of our new cluster sample shows a lower
probability of being bimodal than suggested in previous studies. The separate
chemical analysis of clusters in the six components (Main Body, Counter-Bridge,
West Halo, Wing/Bridge, Northern Bridge and Southern Bridge) shows that only
clusters belonging to the Northern Bridge appear to trace a V-Shape, showing a
clear inversion of the metallicity gradient in the outer regions. There is a
suggestion of a metallicity gradient in the West Halo, similar to that
previously found for field stars. It presents, however, a very large
uncertainty. Also, clusters belonging to the West Halo, Wing/Bridge and
Southern Bridge exhibit a well-defined age-metallicity relation with relatively
little scatter in abundance at fixed age compared to other regions.
