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Suspended magnetite particles in boiler waters were found to deposit in orifice headers, impeding flow and causing inefficiencies and boiler tube failures from overheating. Particle interactions due to the zeta potential developed on the colloidal magnetite particles could also be a factor in deposition. To study the mechanism behind the deposition of suspended magnetite on stainless steel, a high temperature high pressure electrophoretic deposition cell with provisions to test 304 stainless steel substrates has been developed. The tests, conducted at pH25 °C of 9.3 in simulated boiler conditions in terms of temperature, pressure and water chemistry. The deposition was monitored using in-situ Electrochemical Impedance Spectroscopy (EIS) and post-test surface analyses were carried out on the substrate. The results of the tests concurred with the theory on oxide particle behavior in aqueous systems and colloidal stability. The specific conditions in the boiler that favor deposition have been successfully replicated.

Under the high pressure high temperature (HPHT) conditions in boilers, iron oxides, like magnetite and hematite, are the most common corrosion products. Apart from just degrading the base metal in the feedwater system, corrosion products can also be transported downstream, where they can potentially deposit on surfaces in the boiler. Deposition of magnetite  on orifice plates in lower headers of forced circulation boilers leads to improper flow patterns, poor heat transfer and in some cases lead to tube failures. Field data suggests that the boiler operating conditions, like temperature, pressure and boiler water chemistry could influence the deposition of magnetite from the aqueous stream on to the metal surface. It has been experimentally shown previously that boiler water conditioned to pH25 ⁰C 9.3 using NH4OH, favors the deposition of suspended magnetite nanoparticles on 304 stainless steel surfaces at 300 ⁰C and 100 bar. As an extension, the work presented here, explores the effects of additional chemistries on the deposition process and behavior of suspended magnetite particles. This study looks at the influence of all-volatile treatment (AVT) regime with the addition of solid alkali to the boiler water. The aim is to understand the influence of the phosphate treatment-addition of sodium triphosphate-and caustic treatment-addition of sodium hydroxide- on the deposition process with respect to the conventional AVT regime. In addition, the study also investigates the influence of solid alkali addition on the suspended magnetite and hence the nature of the deposits formed on the stainless steel substrate. The boiler environment was simulated in an autoclave pressure vessel. The test solutions were prepared in de-aerated conditions to closely match the boiler feedwater in terms of chemistry. The concentrations of sodium triphosphate and sodium hydroxide used were decided based on the standard operating guidelines followed in the industry. A standard three electrode assembly was used to monitor the deposition process using Electrochemical Impedance Spectroscopy (EIS). The nature of the deposits were investigated using surface characterization techniques. The results showed a clear influence of the zeta potential and double layer compression on the behavior of the suspended particles and their affinity to deposit at 300 ⁰C and 100 bar. The results will add to the existing work on understanding magnetite deposition and suspended particle behavior in high pressure high temperature environments.

Suspended magnetite [Iron (II,III) Oxide] particles in boiler water deposit on the walls and orifice plates in boilers, impeding flow and causing inefficiencies. Operating data suggests that reducing conditions and boiler water chemistries in the boiler enhances development of magnetite deposits. The inter particle interaction due to the zeta potential developed on the colloidal magnetite particles could also be a factor in deposition. In order to study the mechanism behind the deposition of suspended magnetite on stainless steel a high temperature high pressure electrophoretic deposition cell has been developed. An autoclave pressure vessel with graphite O-ring gaskets is serve as the electrophoretic deposition cell simulating the boiler conditions (300 °C and 9 MPa). Carefully prepared test solution imitating boiler water chemistries containing suspended magnetite particles are pumped in to the autoclave. The test solution is prepared in an anaerobic environment to prevent oxygen contamination. The tests  simulate the boiler conditions in terms of temperature, pressure and boiler water chemistry. The experimental setup has been designed to allow the sampling of test solution to monitor the pH, dissolved oxygen level and solution potential. 304 Stainless Steel samples were placed in the autoclave to serve as the substrate for deposition. A three-electrode assembly, consisting of 304 SS substrate (working electrode), a counter electrode and a quasi-reference electrode, has been used to characterize the deposition process using in-situ Electrochemical Impedance Spectroscopy measurements. The goal is to understand the influence of boiler water chemistries on the surface charge properties of the suspension. The surface properties can be related to the stability of suspended matter, which in turn relates to the deposition of particles on the substrate. The growth of the deposits is characterized using in-situ Electrochemical Impedance Spectroscopy measurements. Post-test analysis of the steel sample was carried out using Scanning Electron Microscopy (SEM) techniques. Preliminary tests have been carried out with a test solution pH of about 9.4, controlled by adding NH4OH (All Volatile Treatment). Deposition was observed on all samples from all the tests. In-situ EIS measurements have been recorded at test conditions of 300 °C and 9 MPa over the duration of the test. The results will be significant in understanding the inter-particle interactions and electrophoretic deposition at high temperature-high pressure conditions.