Corrosion Measurements in Fuel Systems, 18-R8203
James F. Dante
Gary B. Bessee
Inclusive Dates: 01/01/12 – Current
Background — Corrosion in fuel systems has become a widespread problem in recent years for underground storage tank manufacturers and operators as well as engine and engine part manufacturers. Some of these issues appear to be related to the introduction of ultralow sulfur diesel fuel (ULSD). One of the difficulties in studying metals corrosion in fuel systems is the low conductivity of the fuel, which makes conventional electrochemical techniques to measure in situ, real-time corrosion rates virtually impossible. Another technical issue is the phase instability of ethanol/gasoline blends, such as E10, in the presence of water.
As low as 0.5 vol percent water in the dispensing line can cause phase separation, leading to severe corrosion and off-spec blend. This phenomenon has been found to be very sensitive to the composition of the blend, water content and temperature, but no operating boundaries for phase stability have been established. Moreover, no confirmed root cause has been established to explain the corrosion failures observed in the field. The objectives of the proposed project are to 1) validate a method for measuring corrosion rates in fuel systems, 2) investigate the effect of dew point, water content and fuel chemistry on the corrosivity of ULSD, and 3) determine the conditions and parameters that lead to phase separation in ethanol/gasoline blends.
Approach — The proposed research project addresses the measurement issues by employing a multi-electrode array sensor (MAS™) to measure corrosion rates in fuels. The MAS are able to measure corrosion rates in thin electrolyte layers, such as the ones forming in fuel. Carefully selected model fuels of known composition will be used to investigate the environmental effects that increase the corrosivity of ULSD compared to other diesel fuels. Two approaches will be employed to study the phase separation of ethanol/gasoline blends. Thermodynamic calculations will first be carried out using a mixed solvent electrolyte model to define parameter boundaries of phase instability. Then, the corrosion properties of different blends will be measured using the MAS technology.
Accomplishments — Corrosion currents in a surrogate fuel with varying amounts of "aggressive" ethanol were measured using the MAS. Data indicated a significant increase in corrosion current with increasing ethanol concentration. Low-sulfur diesel (LSD) and ultra-low sulfur diesel (ULSD), both filtered and unfiltered, were also investigated. A small amount of tap water, DI water, or artificial contamination water (ACW) was mixed with the fuel during corrosion testing. Although corrosion is a function of the ionic contamination, probe data in diesel fuel indicate that corrosion currents are very small in systems with small amounts of contamination in the water phase. For example, no corrosion was observed in the presence of DI water while higher currents were observed in ACW. Further, no difference in corrosion behavior could be observed between LSD and ULSD fuels regardless of the composition of the added water phase. Comparatively higher current and anodic charge transfer were measured in filtered fuel in the presence of tap water. When using ACW, the filtering process did not affect the results due to the dominant effect of the low pH of the ACW. The average water content of both LSD and ULSD has been measured and was found to be very similar up to 50°C. Above 50°C, ULSD contained 50 to 100 ppm (20 to 25 percent) extra water. Thermodynamic modeling was performed using OLI focusing on ethanol and tert-amyl methyl ether (TAME) as oxygenate, and toluene, hexane and pentane as hydrocarbon. The results were integrated into the OLI Analyzer Studio software and will be used to predict ethanol drop out from the blend.