Corrosion and fouling are considered major factors affecting the performance of water-steam cycles (WSC). Flow-accelerated corrosion (FAC), present in feed and condensate systems, is a well known corrosion mechanism, eroding and dissolving the protective magnetite layers. Fouling of the boiler, by suspended magnetite particles, is partly controlled by forces arising due to surface charging. Film forming amines (FFA) are gaining acceptance as means to control FAC. However, its performance in low pH regions is unknown. In addition, despite FFA being a surfactant, its effect on the colloidal magnetite surface charge and point of zero charge (pzc) are unknown. This research set out to determine the effect of FFAs on the formation of a protective magnetite layer and its resistance against acidic FAC, and to determine the effect of FFAs on the surface charge of colloidal magnetite. This study focused on two FFAs, Octadecylamine (ODA) and Oleyl Propylenediamine (OLDA). In 48h 230-250 ºC immersion corrosion tests magnetite layers were formed on C1010 coupons, inside a high pressure high temperature autoclave under different treatments: untreated (blank), 2ppm ODA and 2ppm Ammonia, 2ppm OLDA and 2ppm Ammonia, and only 2ppm Ammonia. 48h 150 ºC re-immersion corrosion tests were performed to test the magnetite layer performance under acidic (acetate 0.08ppm) FAC. After the corrosion tests, the layers were verified using XRD, EDS + SEM and Weigh-loss measurements. Potentiometric titrations were employed to measure the proton induced surface charge of magnetite particles (10g/L) at an ionic strength of 0.01, and 0.1 mol/kg (KNO3) in the presence or absence of ODA, or OLDA (2ppm) over a wide pH range, at 25, and 150 ºC. This gave the magnetite surface charge density curves. The pzc was determined using the inflection point of titrations (pHinfl) and common intersection point (pHcip). XRD confirmed the presence of magnetite layers on all coupons after the immersion and re-immersion tests. The SEM measured magnetite layer decrease after the re-immersion tests was: 19.1%, 14.5%, 8.6%, and 23.3% for blank, ODA, OLDA, and ammonia treatment respectively. Weight loss determined corrosion rates taken over both immersion and re-immersion tests were: 0.070, 0.057, 0.060, and 0.073 mm/y for blank, ODA, OLDA, and ammonia treatment respectively. All magnetite surface charge density curves were unaffected by the presence of ODA, and OLDA, except for ODA at 0.1 mol/kg KNO3 and 25 ºC, which resulted in a raised/neutralized surface charge density curve in the alkaline pH region. Magnetite layers formed under ODA, and OLDA additions were smoother, thinner, and more uniform compared to layers formed under an ammonia only chemistry, and blank chemistry. Layers formed under the ODA, and OLDA chemistries were better resistant against acidic FAC and offered better protection, in terms of corrosion rate. At the applied concentration ratio, and ionic strength of 0.01M, ODA, and OLDA did not affect the magnetite colloid surface charge over pH. However, both caused magnetite particles to agglomerate. At higher ionic strengths of 0.1M, ODA neutralized the magnetite surface charge in the alkaline region.

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.