Low chromate corrosion inhibitor with borate and hexametaphosphate

Chromate, despite being an efficient and versatile inhibitor for aqueous systems, is being phased out primarily because of its toxicity and carcinogenic nature. An effort is made in this work to substitute chromate by borate using Cr-51 and radioactive tracer technique. Mild steel is found to develop a thicker Cr containing protective film as chromate is sys tematically reduced from 100 to 60%, and substituted by borate. The film thickness decreased as chromate is reduced further, but the inhibitor still protects mild steel up to 5% chromate, at 303 K, at least up to 6 h. The open circuit potential shows predominance of anodic plarization. On attempting to reduce chromate further by substituting with hexametaphosphate, the Cr-uptake on the sample surface increases further showing the synergistic effect. The combination of inhibitors protects mild steel best at the ratio of chromate, borate and hexametaphosphate of 5:40:55 even in the presence of 500 ppm Cl- and 200 ppm SO42- ions, with predominance of cathodic polarization. The inhibitor formulation is more effective, less costly and contains chromate within the permissible limit.