Corrosion in renewable fuels has become an emerging challenge for refineries that are adapting their facilities to produce and process biofuels, renewable diesel (HVO), hydrotreated vegetable oils, and other bio-based feedstocks. Although these alternatives contribute to the decarbonization of the energy sector, they also introduce new operating conditions and degradation mechanisms that were not always taken into account during the plants’ original design.
The transition to lower-carbon fuels requires modifying existing processes, repurposing equipment, and processing streams with chemical compositions that differ significantly from conventional petroleum. In this context, understanding how these new fuels interact with metallic materials is essential to preserve mechanical integrity, optimize inspection programs, and ensure safe and reliable operations.
Corrosion from renewable fuels
Traditional refineries were designed to process fossil crude oils whose chemical properties are well understood. However, incorporating renewable feedstocks such as vegetable oils, animal fats, used cooking oil (UCO), pyrolysis oils, and other bio-oils introduces oxygenated compounds, organic acids, water, and contaminants that significantly modify the corrosive environment inside process units.
Unlike conventional hydrocarbons, most renewable streams contain high oxygen levels and exhibit a less stable chemical composition, promoting oxidation processes, the formation of acidic compounds, and thermal degradation. These conditions can increase the aggressiveness of the environment toward carbon steels and other materials used in process equipment.
Why does corrosion increase during the transition to renewable fuels?
Co-processing or fully converting refinery units to renewable fuels (such as renewable diesel (HVO), sustainable aviation fuel (SAF), or biodiesel) introduces corrosion mechanisms that differ from those encountered with conventional crude oil.
| Factor | Degradation mechanism | Most affected equipment |
|---|---|---|
| Free Fatty Acids (FFA) | High-temperature acid attack similar to naphthenic acid corrosion. | Distillation towers, preheat furnaces. |
| Higher water content | Corrosion under insulation (CUI) and water dew point pitting corrosion. | Heat exchangers, storage tanks. |
| Contaminants (Chlorides and Phosphorus) | Chloride stress corrosion cracking (SCC). | Stainless steel piping and hydrotreating reactors. |
The severity of corrosion depends on factors such as feedstock quality, pretreatment efficiency, operating temperature, pressure, water content, and the presence of metallic impurities or chlorinated compounds.
How do organic acids affect bio-oils?
One of the main challenges associated with processing bio-oils is the presence of organic acids, primarily carboxylic acids, which are naturally formed during biomass decomposition or generated during thermochemical processes such as pyrolysis.
These compounds reduce the pH of the medium and promote uniform corrosion of carbon steel, particularly when free water or condensation is present. In addition, the combination of elevated temperatures and acidity can accelerate the degradation of heat exchangers, process lines, storage systems, and auxiliary equipment.
The corrosive potential is commonly assessed using the Total Acid Number (TAN), a parameter widely used to estimate the acidity of oils and biofuels. Although a high TAN value does not necessarily indicate a higher corrosion rate, it is an important indicator for evaluating the potential aggressiveness of a process stream.
In bio-oils produced through fast pyrolysis, the oxygen content may exceed 30%, promoting the formation of chemically unstable mixtures that require corrosion-resistant materials or stabilization processes before industrial processing.
Challenges of renewable diesel in converted units
Renewable diesel, commonly known as Hydrotreated Vegetable Oil (HVO), is produced through hydrotreating processes similar to those used in conventional hydrodesulfurization units. For this reason, many refineries have chosen to retrofit existing facilities rather than build new processing plants.
However, this conversion does not completely eliminate corrosion risks. Renewable feedstocks contain phosphorus, sodium, potassium, calcium, chlorides, and other contaminants that may poison catalysts, promote deposit formation, and increase localized corrosion if they are not effectively removed during pretreatment.
Furthermore, the temporary coexistence of fossil and renewable feedstocks within the same facility can create operating conditions that differ from those originally considered during the design stage. Changes in chemical composition, operating temperatures, and intermediate products alter the degradation mechanisms affecting piping, reactors, and heat exchangers.
As a result, converted units require dedicated inspection programs capable of evaluating material performance under these new service conditions.
Material selection to control corrosion during the transition
Material selection is one of the most effective strategies for controlling corrosion during the transition to renewable fuels. Although carbon steels continue to be widely used because of their economic advantages, their performance may be insufficient in environments characterized by high acidity, elevated water content, or oxygenated compounds.
In critical areas, austenitic stainless steels, corrosion-resistant alloys (CRAs), internal coatings, and other specialty materials may be required to withstand more aggressive operating conditions. Material selection should consider not only chemical resistance but also weldability, mechanical properties, life-cycle cost, and maintenance requirements.
The decision should not rely solely on fuel composition but also on parameters such as temperature, pressure, flow velocity, solids content, chloride concentration, and the corrosion mechanisms expected throughout the asset’s service life.
How should the corrosion management plan be updated for renewable fuels?
The incorporation of new feedstocks requires reviewing traditional corrosion management programs. Damage mechanisms that predominated during conventional petroleum processing may change significantly when bio-oils, HVO, or other renewable fuels are introduced.
The first step is to identify emerging threats by analyzing the damage mechanisms expected in each process unit. Subsequently, risk matrices, corrosion circuits, inspection programs, and monitoring strategies should be updated accordingly.
Leading refineries are increasingly implementing online sensors, real-time corrosion monitoring, artificial intelligence-based predictive models, and Risk-Based Inspection (RBI) methodologies to optimize inspection intervals and prioritize the most critical assets.
It is also advisable to periodically review operating procedures, monitor feedstock quality, track parameters such as TAN, water content, chlorides, and metallic contaminants, and verify that the selected materials remain suitable for the new operating conditions.
The energy transition is not only about producing more sustainable fuels but also about adapting asset integrity strategies to ensure that refinery facilities can operate safely and reliably for decades to come.
Conclusions
The transition to renewable fuels represents an important opportunity to reduce carbon emissions across the energy sector, but it also introduces new challenges related to industrial asset integrity. Corrosion in renewable fuels does not necessarily follow the same mechanisms observed during conventional petroleum processing, making a specific assessment of the risks associated with each feedstock and process essential.
The presence of organic acids, water, oxygen, and contaminants in bio-oils and renewable feedstocks can increase the aggressiveness of the operating environment and accelerate the degradation of critical equipment if appropriate mitigation measures are not implemented. Proper material selection, continuous monitoring, risk-based inspection, and the continuous updating of corrosion management programs are essential to maintain refinery reliability throughout this transformation process.
As the refining industry adopts new technologies to produce renewable diesel, sustainable aviation fuel (SAF), and other advanced biofuels, corrosion management must evolve accordingly by integrating digital technologies, intelligent monitoring systems, and mechanical integrity principles that ensure safe, efficient, and sustainable operations.
References
- American Petroleum Institute. (2023). API Recommended Practice 571: Damage Mechanisms Affecting Fixed Equipment in the Refining Industry. API Publishing Services.
- Association for Materials Protection and Performance (AMPP). (2023). Corrosion Control in Refining and Petrochemical Facilities. AMPP.
- International Energy Agency. (2024). Renewables 2024: Analysis and Forecast to 2030. IEA.
- International Organization for Standardization. (2023). ISO 8217: Petroleum products — Fuels (class F) — Specifications of marine fuels. ISO.
- Knothe, G., Krahl, J., & Van Gerpen, J. (Eds.). (2021). The Biodiesel Handbook (3rd ed.). AOCS Press.
- Speight, J. G. (2023). The Chemistry and Technology of Petroleum (6th ed.). CRC Press.
- ASTM International. (2023). ASTM D664: Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration. ASTM International.
