2011 IR&D Annual Report

Corrosion Measurements in Fuel Systems, 18-R8203

Principal Investigators
James F. Dante
Gary Bessee

Inclusive Dates:  01/01/11 – Current

Background — Corrosion in fuel systems has become a widespread problem in recent years for underground storage tank manufacturers and operators at fuel dispensing stations nationwide as well as engine/engine part manufacturers. Some of these issues appear to be related to the introduction of ultralow sulfur diesel fuel (ULSD). One of the major difficulties in studying corrosion of metals in fuel systems is the inherent low conductivity of the fuel environment, which makes conventional electrochemical techniques to measure in situ, real-time corrosion rates virtually impossible. Another significant 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. While several hypotheses have been developed to explain the corrosion failures observed in the field, no confirmed root cause has been established to date.

The objectives of the project are to 1) validate a method for measuring corrosion rates in fuel systems, 2) investigate the effect of dew points, water content and fuel chemistry on the corrosivity of ULSD, and 3) determine some of the parameters that lead to phase separation in ethanol/gasoline blends.

Approach — This project addresses the measurement issues by employing a recently developed corrosion sensor technology to measure corrosion rates in fuels. The sensor system (SwRI® multielectrode array sensor, MAS) is able to measure corrosion rates in thin electrolyte layers, such as ones forming in a fuel containing very small amounts of water. This sensor technology has never been used in non-aqueous environments. 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 MAS technology.

Accomplishments — A corrosion test cell and multi-electrode probe were fabricated. Corrosion currents in a surrogate fuel with varying amounts of "aggressive" ethanol were measured. Data indicated a significant increase in corrosion current with increasing ethanol concentration. Initial testing to measure corrosion currents in diesel fuel has begun. Data indicates that corrosion currents are very small in systems with small amounts of contamination in the water phase. Investigations continue in an attempt to understand the levels of ionic contaminants and the effect of diesel fuel composition on corrosion currents.

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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 10 technical divisions.