Corrosion protection of mild carbon steel media in phosphate grinding mill using impressed current technology

Corrosion plays a significant role in wear of mills used to grind acidic phosphate slurry at pH 2–4 in Florida fertilizer plants. Approximately half of the grinding media wear results from corrosion or oxidation dissolution of metal surfaces in grinding. Cathodic protection using impressed current was investigated to reduce the wear rate. The effect of polarization potential on corrosion rate and required current density were studied using a specially designed ball mill whose electrochemical potential can be controlled. Experimental results indicate that the total wear rate was reduced by 42–46% for 1018 carbon steel when a potential of −1.0 V was applied. The required current density to effectively reduce the wear rate was 210 mA/m2 in pH 3.1 solution, 180 mA/m2 in pH 6.8 solution, and 160 mA/m2 in pH 9.2 solution. Scanning electron microscopy (SEM), and X-ray diffraction (XRD) methods were used to investigate the surface morphology, corrosion products, and composition of metals during phosphate grinding with and without cathodic protection. SEM results indicated that pitting corrosion was the main corrosion type. XRD images suggested that Fe2O3 was the main corrosion product in pH 3.1 solution without cathodic protection, and the corrosion was significantly reduced when potential of −1.0 V was applied.