Hydrofluoric Acid Corrosion

General Information

Hydrofluoric acid (HF) finds application in diverse industrial processes, serving as a crucial component in the production of fluorinated compounds. Moreover, it plays a key role in the electronics industry, where it is employed for the precision etching of glass and silicon. Additionally, HF is instrumental as a catalyst in specific chemical reactions, particularly in the alkylation process. The following chapter is mostly oriented to refining applications (alkylation), however some information is common for other process industries.

Hydrofluoric acid (HF), belonging to the family of halide acids alongside HCl or HBr, generally exhibits lower aggressiveness towards metallic materials, particularly carbon steel, when in its dry form. In comparison to dry HCl, dry HF is typically less corrosive. Nevertheless, it is crucial to acknowledge that even dry HF can induce corrosion in carbon steel. Therefore, caution should be exercised, and the potential for dry HF corrosion should not be underestimated.

It is also important to highlight that HF environment may also generate potential for cracking, especially on welds between dissimilar metals exposed to HF.¹ Atomic hydrogen, generated as a byproduct of the reaction between hydrofluoric acid (HF) and steel, has the potential to diffuse through the crystal matrix of the metal. Following recombination into molecular hydrogen, the resultant hydrogen can induce typical cracking phenomena and associated damages, including stress-oriented hydrogen-induced cracking (SOHIC) and stepwise cracking (SWC).²

Both dry (anhydrous) hydrofluoric acid (HF), with water content typically less than 400 ppm, and aqueous HF, often shipped in concentrations of 49% and 70%, exhibit extremely hazardous properties. These substances have the potential to cause severe burns to skin tissue, eyes, and lungs. Therefore, specialized safety precautions and handling procedures are necessary when dealing with hydrofluoric acid due to its high toxicity and corrosive nature.^{3 4}

Due to its extreme health hazards, corrosion prevention and mitigation in HF alkylation units constitute a critical aspect of the overall process safety approach for these units. Table 1 shows the most common areas in alkylation unit that are prone to HF corrosion.

Table 1 Potential locations for HF corrosion in HF-alkylation units.^{1 2}

Hydrofluoric Acid Corrosion is governed by a combination of several factors like chemical species and concentration, temperature, materials and/or flow regimes.

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Tools

Below are our user-friendly calculators and integrity tools to estimate the HF corrosion of Carbon steel and Alloy 400.

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HF-Corrology®

HF-Corrology® is designed to provide rapid corrosion rate estimates for carbon steel and Alloy 400 in HF service. It allows you to evaluate the impact of key process parameters like acid concentration and temperature, on corrosion rates.

Note: Calculation results are currently hidden. Register for Free Trial to unlock Corrosion Rate and Service Life Results.

Integrity Risk Indicator (Corrology®-IRI: HF)

The Challenge: Moving Beyond Linear Estimates

In aggressive HF environments, traditional linear corrosion tracking often fails to capture the rapid transition to accelerated damage. As wall thickness decreases, risk does not grow steadily - it accelerates exponentially. Relying on basic calculations can lead to either premature equipment replacement or, worse, unexpected failure.

The Solution: The HF Integrity Risk Indicator (IRI-HF)

The IRI-HF quantifies asset risk by calculating corrosion rates, service-life context, and empirical remaining-life indicators through a margin-aware, API 581-inspired engine. This tool functions as a sensitivity analysis engine, allowing you to evaluate how shifting process variables impact Carbon Steel and Alloy 400 in Predictive Mode to determine the most suitable operating window.

The Advantage

Access a high-speed, agile modeling engine designed for rapid, defensible engineering decisions:

• Non-Linear Risk Scaling: Utilize a logarithmic-exponential transformation that reflects the true physical reality of risk escalation in late-life degradation states.

• Dynamic Inspection Credit: Input your Inspection Effectiveness (Class A-E) to see how high-quality data reduces uncertainty and optimizes maintenance intervals.

• Auto-Derived Prior Confidence: Prior confidence is automatically computed from the uninspected time interval (t_uncertainty): <3 years → High, 3–5 years → Medium, ≥5 years → Low. This applies in both Predictive and Inspection modes, ensuring API 581-consistent conservatism without manual override.

• Precision Structural Floors: Take control of your model by setting Manual T-Min structural floors to match your equipment’s unique geometry.

• Expert Calibration: Gain confidence with a model structured against a 6-point lifecycle truth set, ensuring accuracy from low-risk early life to critical thin-wall scenarios.

• API 581-Aligned Risk Bands: Status and score-pill color are determined by the log probability of failure (log P_f) rather than the 0–100 score scale alone. Thresholds follow API 581 PoF magnitude ranges: log P_f ≤ −5 → Very Low; −5 < log P_f ≤ −4 → Low Risk; −4 < log P_f ≤ −3 → Moderate; log P_f > −3 → Critical. This ensures that improvements in inspection effectiveness are always reflected in the displayed risk band, even in high-age or thin-wall scenarios where the numerical score is saturated.

Note: The Integrity Risk Indicator (IRI-HF) shown below is a static preview of our Professional+ engine. Register for Professional+ to unlock IRI-HF tool.

References

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