Potential-pH diagrams: boundaries, slopes and corrosion

For an element in water, the map of predominance domains (dissolved species) or existence domains (solids) in the plane: pH on the horizontal axis, $E$ on the vertical one. It is drawn at $25\ ^\circ\text{C}$ for a fixed plotting concentration $c_{\text{tra}}$. Upwards, the high oxidation states; to the right, the basic species (hydroxides, oxides).