Ethanol Stress Cracking in Carbon Steel: A Scientific Perspective on Carbon Steel Degradation

The increasing use of ethanol as a biofuel and fuel additive presents a surprising challenge to the commonly employed carbon steel in storage tanks and pipelines. This article delves into the phenomenon of ethanol stress cracking (SCC), exploring the current understanding of its damage mechanism (DM) from a purely scientific perspective.

The Stress-Ethanol Conundrum

While generally robust, carbon steel is susceptible to SCC when exposed to ethanol under specific conditions. The key factors are:

• Stress: External forces (pressure) or inherent residual stresses (welding) create a susceptibility within the steel.
• Ethanol: The chemical composition of ethanol (C?H?OH) plays a crucial role. Unlike pure ethanol, fuel-grade ethanol contains dissolved oxygen and minor impurities, which act as the primary instigators.

A Step-by-Step Breakdown of the Chemical Process

The exact DM of ethanol SCC is still under investigation, but current theories suggest a multi-step process:

1- Surface Breach: Dissolved oxygen in the ethanol reacts with the steel surface, forming a thin oxide layer. This layer, while initially protective, can become brittle and prone to micro-cracks. The oxide layer itself might have a complex composition, not simply iron oxide (FeO or Fe?O?). The presence of other elements, like impurities from the steel or contaminants in the ethanol, can alter the oxide's structure and inherent strength. These "impurity oxides" often have different crystal structures compared to pure iron oxide, making them more susceptible to cracking under stress.

2- Ethanol's Intrusion: There's a possibility that ethanol might react directly with the iron oxide at the interface between the layer and the steel. This reaction could weaken the bond between the oxide and the metal, making it easier for cracks to propagate into the steel itself. The micro-cracks allow ethanol to penetrate the steel's passive layer, reaching the underlying iron. Here's where the chemistry becomes crucial:

Potential Reactions: Ethanol molecules (C?H?OH) might interact with the iron on the atomic level, weakening the metallic bonds. The specific reactions are still being unravelled, but the weakening could involve:

• Dissociation: The ethanol molecule first gets attracted to the crack tip due to the high surface energy at that location. This adsorption process can involve various factors like hydrogen bonding between the ethanol molecule and the iron atoms at the crack face. Once adsorbed, the C-H bonds within the ethanol molecule become susceptible to further weakening due to the intense stress field at the crack tip. This stress can act to stretch and distort the electron cloud around the carbon atom, making it easier for the H atom to break away. The weakened C-H bond breaks homolytically, meaning both the carbon (C) and hydrogen (H) atoms end up with one electron each. This results in the formation of a hydrogen atom (H) and an ethyl radical (C?H??). Hydrogen atoms can also act by promoting a mechanism called Hydrogen Embrittlement. They can locate themselves at the boundaries between iron grains, effectively pushing the iron atoms apart and reducing the cohesive strength between the grains.
• Adsorption: Alternatively, ethanol molecules might adsorb onto the iron surface, altering its electronic structure and making it more susceptible to cracking. How??

If the ethanol molecule gets close enough to the iron surface, a hydrogen atom on the ethanol can form a weak bond with an oxygen atom already present on the iron surface (likely from the initial oxide layer formation). Once adsorbed, the ethanol molecule isn't just a passive observer. It can disrupt the electronic structure of the iron at the crack tip. Here's how:

The functional group (OH) in ethanol can act as an electron donor or acceptor depending on its proximity to the iron surface. This donation or withdrawal of electrons can alter the local electronic density around the iron atoms, weakening the metallic bonds between them.

The presence of the adsorbed ethanol molecule can also influence the way iron atoms bond to each other. It might distort the surrounding iron atoms' orbitals, making it more difficult for them to share electrons effectively. This weakens the overall cohesive strength of the iron lattice near the crack tip.

3- Stress Amplification: The presence of the cracks concentrates the applied stress at their tips, further accelerating their propagation. This vicious cycle leads to the growth and eventual failure of the steel component.

Oxygen's Detrimental Influence

The severity of ethanol SCC is directly linked to the amount of dissolved oxygen. Higher oxygen concentrations:

• Increase the rate of oxide layer formation: This creates more sites for crack initiation. These micro-cracks become initiation points for SCC. The higher the oxygen concentration, the faster this oxide layer forms, creating more potential weak points for cracks to nucleate.
• Enhance the reactivity at the crack tip: The presence of oxygen at the crack tip itself might participate in additional detrimental reactions. These reactions could further weaken the iron-iron bonds in the vicinity of the crack, by forming brittle iron oxide compounds that weaken the surrounding metal and make it more susceptible to breaking under stress. Unfortunately, ethanol, compared to water, has a higher capacity to dissolve oxygen. This characteristic makes it challenging to entirely eliminate oxygen from the environment, even with practices like minimising air contact during storage and transportation. The persistent presence of even small amounts of dissolved oxygen can significantly contribute to the risk of SCC.

The Water Enigma: A Double-Edged Sword

Water, often considered an enemy in corrosion, plays a complex role in ethanol SCC. While some studies suggest that very low water content (less than 1%) might exacerbate SCC, the presence of moderate amounts (up to 5%) might have an inhibiting effect. The reasons are not entirely clear, but possibilities include:

• Reduced Oxygen Solubility: Some studies suggest that extremely low water content might actually exacerbate SCC. This could be because very dry ethanol allows for higher oxygen solubility, as discussed previously. Oxygen, as we know, is a key instigator of SCC.
• Altered Crack Chemistry: Conversely, the presence of moderate amounts of water (up to 5%) might have an inhibiting effect on SCC. Water can compete with ethanol for available space, potentially reducing the amount of dissolved oxygen. This lowers the concentration of the primary SCC culprit. Water molecules might influence the way ethanol molecules adsorb onto the iron surface at the crack tip. This could potentially alter the weakening effect of ethanol on the iron-iron bonds.

There seems to be a potential parallel between the findings in methanol and the observations in ethanol SCC. Both suggest a "sweet spot" for water content that promotes SCC, existing at very low concentrations. This aligns with the notion that extremely low water content in ethanol might increase SCC risk due to higher oxygen availability. However, it's important to note that the specific water content thresholds might differ slightly between ethanol and methanol due to their unique chemical properties. More research is needed to definitively establish the critical water content range for ethanol SCC.

The Road Ahead: Mitigating the Threat

Understanding the complexities of ethanol SCC is crucial for engineers. Here are some mitigation strategies:

• Limiting Oxygen Exposure: Minimising air contact during storage and transportation can significantly reduce the risk of SCC. Practices like inert gas purging of tanks and pipelines can be highly beneficial.
• ?Material Selection: Using alternative materials less susceptible to SCC in ethanol environments might be necessary for critical components. Research into new, more resistant alloys is ongoing.
• Stress Management: Design practices that minimise residual stresses and control operating pressures are highly beneficial. Techniques like stress relief grooves or optimised weld geometries can help.
• Post-Weld Heat Treatment (PWHT): Applying a controlled heating process after welding can significantly reduce residual stresses in the steel. This reduces the susceptibility to crack initiation and propagation under stress and ethanol exposure.
• Protective Coatings: Applying internal tank linings or coatings specifically designed for ethanol service can offer a barrier against SCC. These coatings can impede the direct contact of ethanol with the steel surface and potentially alter the adsorption behaviour, reducing the risk of SCC.

Conclusion

Ethanol SCC presents a significant challenge for the safe and reliable use of carbon steel in biofuel applications. While the complete DM remains an active area of research, the current understanding highlights the critical role of stress, dissolved oxygen, and potentially water content. By implementing a combination of mitigation strategies and ongoing research, engineers can effectively address this challenge and ensure the safe and efficient utilisation of ethanol.

Take Care ??