A synergistic effect exists between sodium potassium tartrate (SPT) and Zn2+ in controlling corrosion of carbon steel immersed in rain water collected from roof top and stored in concrete tank. The formulation consisting of 50 ppm of SPT and 25 ppm of Zn2+ has 91% corrosion inhibition efficiency (IE). As the immersion period increases IE decreases. Polarization study reveals that SPT-Zn2+ System functions as a mixed inhibitor. AC impedance spectra reveal that a protective film is formed on the metal surface. FTIR spectra indicate that the protective film consists of Fe2+-SPT complex and Zn(OH)(2). A formulation consisting of 50 ppm of SPT, 25 ppm of Zn2+ and 50 ppm of a biocide, N-cetyl-N,N,N-trimethyl ammonium bromide, has 98% corrosion inhibition efficiency and 100% biocidal efficiency.