The corrosion behavior of mild steel in soft alkaline cooling water in relation to phosphorus-free corrosion inhibitors

Abstract

Water as a coolant for industrial products or processes is widely used. Efficient operation of Open Recirculating Cooling Water Systems (ORCWS) relies on controlled cooling water conditioning programs that prevent corrosion, fouling, scaling and microbial growth. Water intensive industries feel greater pressure to decrease their water demand as water resources become increasingly stressed and discharge restrictions become more stringent. Soft alkaline cooling water (SACW) might be a good alternative for the conventional water and chemicals intensive ORCWS operation. However, this type of water requires complete reevaluation of the conditioning programs. Therefore, this research focused on the corrosion processes in SACW and a beginning was made in evaluating non-toxic and phosphorus-free corrosion inhibitors.

The research comprised the production of synthetic cooling water (SCW) and the evaluation of corrosion of mild steel in this type of water, with and without inhibitors. The selected corrosion inhibitors were NovoTraqua® without phosphorus (NT0) and sodium silicate (Si200). PHREEQC simulation were used to validate the SCW composition. A desk study was carried out to evaluate the applicability of corrosion indices. The average corrosion rate and inhibition efficiency were determined by mass loss tests of mild steel specimens with an immersion time of up to 4 weeks. The development of passivity was followed by open circuit potential (OCP) measurements. The pitting susceptibility was tested using cyclic potentiodynamic anodic polarization (CPP) measurements. 3D digital microscopy, SEM/EDS and XRD were used to evaluate the surface morphology and the composition of the top layer.

The addition of NT0 or Si200 to the SCW resulted in the inhibition of uniform corrosion but no complete protection against localized corrosion was realized. Si200 was the best performing inhibitor, also for inhibiting crevice corrosion. The immersion of mild steel in SCW resulted in natural passivation by the formation of a Fe2O3 oxide layer. The SCW did not have the ability to immediately repassivate the specimens after localized breakdown of the protecting passivating layer. The addition of NT0 or Si200 reduced the aggressiveness of the environments at the pits but did not accomplish complete repassivation.

Lab experiments with SCW can provide initial insights in the behavior of mild steel and can contribute to the decision making in more cost intensive research on alternative operation of ORCWS. Better understanding of the protection mechanisms would be obtained when more advanced techniques like Fourier Transform Infrared Spectroscopy (FTIR) and Electrochemical Impedance Spectroscopy (EIS) are used. Although cooling water corrosion inhibition by means of silica is a mature research topic, it is recommended to proceed with research on silica as a corrosion inhibitor in the specific conditions of SACW.