We report a detailed optical investigation of the wetting phenomena in fluid metalrich KxKCl1-x solutions for temperatures up to 820 degrees C and x greater than or equal to 0.86. To this end the fluid sample-sapphire interface has been probed by second harmonic generation (SHG) with continuous variation of temperature T at various x. At coexistence a wetting transition near T-w approximate to 500 degrees C is signaled by a clear change in the temperature dependence of the SH intensity. This observation is in agreement with previous ellipsometric measurements. The precise location of the transition in these systems is strongly influenced by the presence of oxygen impurities segregating at the interface. This is manifested in a strong enhancement of the SH signal in the vicinity of 450 degrees C. In the homogeneous metal-rich phase the SH intensity shows a pronounced maximum as a function of temperature for T> T-w. The occurrence of these maxima is explained consistently by assuming a salt-rich microscopic wetting film in between the metallic bulk fluid and the sapphire window whose thickness changes from thick to thin with increasing temperature. The line connecting these maxima positions in the x-T plane exhibits all the characteristics of a prewetting line. In particular, it is curved away from the coexistence curve as has been predicted theoretically for systems with screened Coulombic interactions, but is found here for the first time experimentally.