No abstract

In power plant cooling systems, water droplets and condensate films form due to heat transfer through cooling tube walls. Condensate films are known to cause flow accelerated corrosion on carbon steels used in air-cooled condensers. Corrosion is further accelerated by droplets suspended in the accelerating steam that impinge on walls, T-joints, or valves, further damaging protective oxide layers on pipe walls. Droplet impingement and flow accelerated corrosion were studied using a modified rotating cylinder electrode system coupled with electrochemical impedance spectroscopy. Surface liquid films caused by droplet impingement were found to correlate directly with flow accelerated corrosion caused by condensate films. In the absence of a stable liquid film, droplet impingement increased corrosion rates and resulted in pit formation. Select corrosion inhibitors were found to be ineffective under flow accelerated corrosion or droplet impingement.

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.

One aspect of effective steam/water cycle chemical treatment programs is their ability minimize corrosion and corrosion related failures. In this study, we examine how corrosion product layers on the surface of tubing samples impact corrosion rate for a series of test specimens prepared from boiler/steam piping provided from operating power plants. Corrosion rates were estimated using the following electrochemical techniques: linear sweep voltammetry (LSV), linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS). Two sources of Tafel slopes were the standard assumptions used if the corrosion mechanism is not known and those obtained from LSV curves. In total, five variations of corrosion rate measurements were compared. For these corrosion tests, water chemistries representative of conditions that induced online hydrogen damage and offline pitting were used. Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDS/EDX) were the electron microscopy techniques applied to examine the corrosion product layers. Using these techniques, the microstructure and elemental composition of the tubing sample corrosion product layers were examined. These techniques could be utilized to evaluate the effectiveness or reduction in corrosion rates with the application of film forming products.

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.