Corrosion Measurements in Fuel Systems, 18-R8203
James F. Dante
Inclusive Dates: 01/01/11 – 01/01/13
Background — Corrosion in engines and underground fuel storage tanks has become a widespread problem in recent years. This appears to be related to the introduction of ultralow sulfur diesel fuel (ULSD). However, corrosion in fuel is difficult to study due to its low conductivity, making in situ, real-time conventional electrochemical techniques impractical. 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% 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. No confirmed root cause has been established to explain the corrosion failures observed in the field.
Approach — The objectives of the project are to validate a method for measuring corrosion rates in fuel systems, investigate the effect of dew points, water content and fuel chemistry on the corrosivity of ULSD, and determine the parameters leading to phase separation in ethanol/gasoline blends. Multielectrode array sensor (MAS™) technology will be used to measure corrosion rate in fuels. MAS allows measurement of corrosion rates in thin electrolyte layers, such as ones forming in fuel. Carefully selected model fuels 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 — The final report was submitted in December 2012. The summary of findings is as follow:
- An increase in the aggressiveness of surrogate gasoline was linked to ethanol additions using MAS.
- In diesel fuel systems, corrosivity is governed by the aggressiveness of water contaminants. A decrease in pH to a value of 5 has the largest effect on corrosivity.
- No difference was found in the corrosivity of LSD and ULSD fuels under the conditions tested. The corrosivity of a diesel/water mixture is increased when the diesel fuel is first filtered. This may be the result of the removal of corrosion inhibitors (among other constituents) for the filtered fuel. Note that after filtering, the difference between the corrosivity of the two types of fuels is still indistinguishable. Also note that we have not studied the ability of these fuels to support microbiological growth.
- For temperatures below 40°C, there is no difference in the ability of LSD and ULSD to hold water for diesel fuels tested. Filtering the fuel removes polar molecules resulting in a decrease in the ability of the fuel to hold moisture and thus promoting phase separation. Filtering has the same effect on both the LSD and ULSD fuels tested here.
- While the corrosivity of the diesel fuels tested here are primarily a function of contaminant water chemistry, aggressiveness of gasoline containing ethanol is at least in part a function of the corrosivity of ethanol.
- The MAS probe in its current configuration was unable to detect the onset of phase separation from gasoline blends or contaminated diesel fuels although it was very successful in measuring corrosion processes.