Analysis of Silicon in Steel to Prevent Sulfidic Corrosion Failures

Significance of the topic

Carbon steels with low silicon content (<0.10 wt% Si) are known to be particularly susceptible to hydrogen-free sulfidation corrosion in many refining process units. Such failures are a frequent cause of leaks, unplanned outages and safety incidents. Rapid, reliable in-field screening of silicon concentration in piping and equipment is therefore critical for prevention, maintenance prioritization and verification of correct alloy installation as recommended by industry guidance such as API RP 939-C and PMI programs.

Objectives and overview of the study

This application note evaluates the performance of the Thermo Scientific Niton XL5 Plus portable X-ray fluorescence (XRF) analyzer for fast, accurate determination of silicon in steels to identify materials at elevated risk of sulfidic corrosion. The goals were to demonstrate agreement with laboratory reference data, characterize repeatability at low Si levels, and define practical measurement parameters and surface-preparation practices for reliable field use.

Methodology

Certified reference standards and prepared steel samples were measured after removing surface contaminants and oxides. The study emphasized proper surface preparation because oxidation, paint, grease or oil can bias XRF readings. Typical cleaning used a right-angle die grinder with zirconia/alumina abrasive discs to remove oxide layers and coatings in the measurement area. Measurements were performed using the Niton XL5 Plus with combinations of the main and light filters and measurement durations of 9, 13 and 33 seconds (reported as 3 s main filter plus 6/10/30 s light filter portions). Data quality objectives determined the minimum required analysis time.

Used instrumentation

The central instrument under evaluation was the Thermo Scientific Niton XL5 Plus portable XRF metal analyzer. Key hardware and functional features used in the study include:

• Compact, lightweight design for easier access to constrained test points.
• 5 W X-ray tube and optimized compact measurement geometry.
• Silicon drift detector (SDD) with a graphene window to enhance sensitivity to light elements (Mg, Al, Si, P, S).
• Integrated camera and small-spot capability for accurate sample positioning and image capture.
• Rugged, splashproof/dustproof housing suitable for refinery environments.

Main results and discussion

The analyzer demonstrated excellent agreement with laboratory reference values and resolved Si concentrations around the critical 0.1 wt% threshold used to assess sulfidation susceptibility. Key findings:

• Accuracy: Correlation between Niton XL5 Plus results and certified laboratory values showed a high coefficient of determination (R2), indicating strong agreement (very good correlation to lab results reported in the note).
• Repeatability and precision: Measurements on two low-Si standards (reference ~0.058 wt% and ~0.147 wt% Si) at three different total measurement durations showed the following aggregated behavior: average measured Si ≈ 0.060–0.061 wt% across durations, with the standard deviation decreasing as measurement time increased (SD: 0.016 at 9 s, 0.009 at 13 s, 0.004 at 33 s). This demonstrates improved precision with longer counting times, particularly important for values near the 0.1 wt% screening threshold.
• Speed: With appropriate surface preparation, the analyzer was able to distinguish pass/fail relative to the 0.1 wt% Si threshold in under 9 seconds for clear-cut cases. For borderline results close to the threshold, extended measurement time (e.g., 33 s) improves confidence through lower variability.

Practical considerations: surface oxidation and coatings materially affect Si readings—oxide layers grow faster as Cr content decreases—therefore mechanical removal of oxide/paint/grease prior to measurement is essential. Contaminants common in refinery environments (paints containing Ti, Zn, Ca; greases containing Mo additives) can introduce positive biases and must be cleared from the analysis spot.

Benefits and practical applications of the method

The Niton XL5 Plus portable XRF delivers several practical advantages for refinery inspection and asset integrity work:

• Rapid field screening for low silicon content to identify parts at higher risk of sulfidation corrosion, supporting preventive maintenance and replacement decisions.
• High sensitivity to light elements enables reliable Si detection at concentrations relevant to industry thresholds (0.1 wt% Si).
• Small-spot analysis and integrated imaging allow accurate targeting of prepared metal surfaces even in confined locations.
• Short measurement times permit high-throughput surveys while longer counts can be used for borderline cases to improve precision.
• Rugged design suits refinery environments, reducing the need to remove components for lab analysis.

Future trends and potential applications

Portable XRF analyzers with improved light-element sensitivity will continue to expand in refinery and industrial asset integrity roles. Anticipated developments and applications include:

• Tighter integration with digital workflows for automated logging, geotagging and laboratory cross-checks to support asset management systems.
• Improved detector and filter designs to further lower detection limits and reduce measurement times for light elements.
• Advanced calibration routines and machine-learning approaches to correct for matrix effects, surface condition variability and common contaminants.
• Broader adoption in pre-commissioning, turnarounds and continuous monitoring programs where rapid alloy verification reduces risk and downtime.

Conclusion

The Thermo Scientific Niton XL5 Plus portable XRF analyzer provides accurate, repeatable and rapid silicon measurements in steel that align closely with laboratory reference data. With proper surface preparation, the analyzer can reliably distinguish steel above or below the 0.1 wt% Si threshold used to assess sulfidation corrosion risk in less than 9 seconds for many samples; longer measurement durations reduce variability for borderline cases. These capabilities support practical, on-site alloy verification and risk mitigation efforts in refining and related industries.

Reference

Thermo Fisher Scientific. 2021. Analysis of Silicon in Steel to Prevent Sulfidic Corrosion Failures Using the Niton XL5 Plus XRF Analyzer. Application Note. Boston, MA: Thermo Fisher Scientific.