Deposition of Suspended Magnetite in High Temperature High Pressure Boiler 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.