Biomass derived oils, made from decomposition and conversion of biomass feedstocks, are proposed as alternative sustainable fuels and potential precursors to valuable chemicals. Bio-oils are produced using different approaches including hydrothermal liquefaction and fast pyrolysis of biomass feedstocks. The composition of bio-oils varies depending on the biomass source as well as on the production method. Bio-oils also contain various functional groups including carboxyl, ketone, aldehyde, phenol etc. Among the functional groups, compounds containing oxygenated species such as carboxyl, phenol and ketone groups can be particularly corrosive to metallic and polymeric materials used for transportation, production, and storage of bio-oils. Formic and acetic acids are some of the most prevalent carboxylic acids present in fast-pyrolysis bio-oils and pose a concern for corrosion of metal alloys. In the present study, we varied the amounts of carboxylic acids in a forest residue fast pyrolysis bio-oil to assess their effects on corrosion of type 410 stainless steel (410SS). To study the effect of formic and acetic acids on corrosion in bio-oils, we exposed 410SS specimens in FR3 (forest residue, fast pyrolysis bio-oil) in short term (48 hour), mild temperature (50 °C) exposures. 410SS specimens were polished, weighed and exposed to fast pyrolysis bio-oil (FR3) with baseline modified total acid number (AMTAN) value of 50.1 mg of KOH per gram of bio-oil. Using a piece of Teflon string, specimens were suspended in approximately 25 g of bio-oil at 50 °C for 48 hours. The oil samples were then spiked with either formic or acetic acids. Capillary electrophoresis was used to quantify formic and acetic acids present in the bio-oil. The area under the curves were determined and correlated to concentration of each specie using standards. Electrochemical Impedance Spectroscopy (EIS) was also performed to investigate corrosion behavior of 410SS in FR3 bio-oil at room temperature. A miniature glass cell & Luggin capillary to minimize ohmic drop within the bio-oil phase was utilized. Reference mercury sulfate electrode and platinum mesh counter electrode were used for EIS measurements.

The results of short-term (48 hour, 50°C) exposures of 410SS coupons in bio-oils with addition of small carboxylic acids are shown in Figure 1. Mass change plot vs the concentration of formic acid is shown in the left graph. We observed linear increase in mass loss of 410SS with increasing formic acid concentration in the bio-oil. This is expected given our previous results that showed corrosivity of formic acid towards low chromium alloys. Iron formate was the main corrosion product that formed on the surface. However, acetic acid shows the opposite effect on mass change data as shown in the graph on the right in Figure 1. Increasing addition of acetic acid resulted in decreased mass loss. This result is intriguing because acetic acid unexpectedly behaves as a corrosion inhibitor. We are currently performing more experiments to study this effect and to understand the reason for seemingly inhibitory effect of acetic acid on corrosion of 410 stainless steel. Mass changes and cross-sectional examination of post-exposure SS410 specimens, using scanning electron microscopy equipped with energy dispersive x-ray spectroscopy, will also be presented. Impedance data from EIS measurements were analyzed as another means to assess corrosion performance of SS410 in the bio-oil and used for comprehensive corrosion assessment along with other results.

Funding provided by DOE Bioenergy Technologies Office.