Rapid analysis of residual elements in carbon steel piping from hydrofluoric acid alkylation units

Importance of the topic

Residual element (RE) concentrations in carbon steel—notably chromium (Cr), copper (Cu), and nickel (Ni), and secondarily Nb, Mo, Sn, V, Ti, Sb, As, and Pb—are critical determinants of material performance in petrochemical environments. Increasing use of recycled scrap in steel production raises RE levels, which can reduce corrosion resistance and other mechanical properties. In hydrofluoric acid (HF) alkylation units, elevated RE (particularly the sum Cr+Cu+Ni) has been implicated in preferential weld corrosion and failures; industry guidance (API RP 571 and RP 751) and case studies recommend strict limits (typically 0.15–0.20% sum RE) to mitigate risk.

Objectives and overview of the study

The application note evaluates the capability of the Thermo Scientific Niton XL5 Plus handheld X-ray fluorescence (XRF) analyzer to rapidly detect and quantify residual elements in carbon steel piping used in HF alkylation units. Specific goals were to determine limits of detection (LODs) and limits of quantification (LOQs) for Cr, Cu and Ni at short measurement times, demonstrate an appropriate minimum measurement duration for reliable pass/fail screening against a 0.15% RE threshold, and compare handheld XRF readouts to certified laboratory material test report (MTR) values.

Methodology

The procedure prioritized realistic field conditions and rapid throughput while maintaining accuracy:

• Sample preparation: Surface contaminants (oxide, paint, oil, grease) were removed because surface oxidation and coatings can bias XRF measurements.
• Instrument setup: The Niton XL5 Plus was configured with a pseudo-element feature to compute the sum Cr+Cu+Ni automatically and with a pass/fail threshold at 0.15%.
• Measurement times evaluated: LODs and LOQs were calculated for 5, 10, 15 and 20 second measurement times using statistical analysis of background counts (LOD = 3σ; LOQ = 10σ) to assess the trade-off between speed and analytical performance.
• Verification: Ten repeated 10-second measurements were collected on a carbon steel sample; results were averaged and compared to the certified MTR values.

Used instrumentation

The key analytical tool was the Thermo Scientific Niton XL5 Plus handheld XRF analyzer. Relevant features influencing performance included:

• Compact geometry, 5 W miniaturized X-ray tube and large-area silicon drift detector with a graphene window for enhanced sensitivity to trace elements.
• Integrated micro and macro cameras and small-spot analysis for accurate sample positioning and documentation.
• Rugged, splashproof and dustproof housing suitable for field use.
• Software features such as the pseudo-element calculation and customizable workflows for pass/fail screening.

Key results and discussion

LODs and LOQs (per element and for the summed RE) improved with longer measurement time. Representative outcomes summarized verbally:

• At 5 s: individual-element LODs ~0.022–0.028% and summed RE LOD ~0.074%; LOQ for the sum was ~0.25%.
• At 10 s: individual-element LODs ~0.013–0.019% and summed RE LOD ~0.045%; LOQ for the sum reached ~0.15% (aligning with the 0.15% screening threshold).
• At 15–20 s: summed LOQs improved further to ~0.12% (15 s) and ~0.10% (20 s), allowing more precise quantification below the threshold when needed.

Practical verification with ten independent 10-second readings produced:

• Average Cr = 0.063% (std. dev. 0.003%), Cu = 0.047% (std. dev. 0.006%), Ni = 0.072% (std. dev. 0.004%).
• Average sum RE (Cr+Cu+Ni) = 0.182% (std. dev. 0.005%), matching the MTR sum of 0.182% within experimental uncertainty.

These data indicate that a 10-second measurement provides sufficient sensitivity and precision to discriminate material above or below the critical 0.15% RE screening value for this sample set, provided surfaces are properly cleaned. The pseudo-element and pass/fail functionality facilitate rapid field triage while the analyzer can also perform full chemical analysis and positive material identification (PMI) when needed.

Benefits and practical applications of the method

Handheld XRF with the Niton XL5 Plus offers several operational advantages in refinery and inspection contexts:

• Rapid onsite screening for RE content to identify parts or welds at elevated corrosion risk without destructive sampling or lengthy lab turnaround.
• Low operator fatigue and access to restricted geometries due to compact, lightweight design.
• Flexible workflow and automated sum calculation enable consistent pass/fail decisions against regulatory or company thresholds (e.g., 0.15% RE).
• Capability to perform PMI and broader alloy chemistry checks in addition to RE screening, aiding acceptance of new materials and assessment of in-service equipment.

Future trends and potential uses

Key directions and opportunities for this approach include:

• Further improvements in detector sensitivity, tube power and software algorithms to push LOQs lower at shorter measurement times, enabling faster screening across more elements.
• Integration of geotagged imaging, cloud reporting and digital workflows to support asset management, traceability and automated compliance reporting.
• Expanded reference libraries and calibration models for matrix effects in a broader set of steels and coatings to reduce sample-prep burden.
• Combined strategies using handheld XRF for initial screening and targeted laboratory analysis for borderline or critical cases to balance speed and absolute accuracy.

Conclusion

The Niton XL5 Plus handheld XRF analyzer demonstrated capability for fast, reliable residual element screening in carbon steel relevant to HF alkylation service. With appropriate surface preparation and a minimum practical measurement time of 10 seconds, the instrument achieved summed RE LOQ at the 0.15% threshold and produced results consistent with certified laboratory values. This enables effective field triage to identify materials at elevated corrosion risk and supports PMI and QA/QC workflows in refining and petrochemical operations.

References

1. Tim Munsterman and Anelsy G. Mayorga, Preferential Corrosion of Welds in HR Service, Hydrocarbon Processing, September 2004, pp. 113-119.
2. American Petroleum Institute, API Recommended Practice 751-2021, Safe Operation of Hydrofluoric Acid Alkylation Units, Washington, DC.
3. American Petroleum Institute, API Recommended Practice 571-2020, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, Washington, DC.