The connection between some features of the metallicity gradient in the Galactic disc, best revealed by Open Clusters and Cepheids, and the spiral structure, has been explored. The step-like abrupt decrease in metallicity at 8.5 kpc (with R0= 7.5 kpc, or at 9.5 kpc if R0= 8.5 kpc is adopted) is well explained by the corotation ring-shaped gap in the density of gas, which isolates the internal and external regions of the disc one from the other. This solves the long-standing problem of a lack of understanding of the different chemical characteristics of the inner and outer parts of the disc. The time required to build up the metallicity difference between the two sides of the step is a measure of the minimal lifetime of the present grand-design spiral pattern structure, of the order of 3 Gyr. The plateaux observed on both sides of the step are interpreted in terms of the large-scale radial motion of the stars and of the gas flow induced by the spiral structure. The star formation rate revealed by the density of open clusters is maximum in the Galactic radial range from 6 to 12 kpc (with an exception of a narrow gap at corotation), coinciding with the region where the four-arms mode is allowed to exist. We argue that most of the old open clusters situated at large Galactocentric radii were born in this inner region where conditions more favourable for star formation are found. The ratio of α-elements to Fe of the sample of Cepheids does not vary appreciably with the Galactic radius, which reveals a homogeneous history of star formation. Different arguments are forwarded to show that the usual approximations of chemical evolution models, which assume fast mixing of metallicity in the azimuthal direction and ignore the existence of the spiral arms, are poor ones

We analyzed the relation between the corotation radii and the galactic radii at which breaks or changes of slope of the metallicity gradients occur in spiral galaxies. With this purpose we compiled the results from the literature on rotation curves, corotation radii and radial metallicity distributions of 27 galaxies, of which 16 were considered qualified to be studied in the context of this work. We re-scaled all references of each galaxy to a same framework in order to compare the results and to identify the radii where breaks and changes of slopes are found, when non-linear models fit the radial metallicities better than a linear model. In most galaxies we have found minima and breaks in radial metallicity near the corotation radius, revealing a significant correlation between these two radii, as it occurs in our Galaxy. The results are interpreted as a consequence of long-lived spiral structures, in which the starformation rate depends on the distance to the corotation radius, producing secular effects in the observed radial metallicity distributions.