Determination of Hydrogen in Aluminum

Significance of the topic

The hydrogen content of aluminum and its alloys critically affects casting integrity and the mechanical performance of finished products. Hydrogen dissolves readily in molten aluminum but its solubility falls sharply on solidification, causing porosity, blisters and other defects when gas becomes trapped. Reliable determination of total, surface and bulk hydrogen is therefore essential for quality control in foundries, rolling mills, and other aluminum-processing operations to prevent failures and meet specification limits.

Objectives and overview of the application note

This application note describes a validated workflow for measuring hydrogen in aluminum using the LECO H836EN inert-gas fusion analyzer consistent with ASTM E2792. The method separates surface and bulk hydrogen by a stepped furnace program, supports calibrations (including steel reference materials when aluminum CRMs are scarce), describes sample preparation and operational parameters, and reports typical results demonstrating repeatability at low-ppm levels.

Methodology — principles and workflow

Principle:
The method uses inert-gas fusion (argon carrier) with an electrode/impulse furnace to thermally extract gases from a weighed aluminum sample placed in a graphite crucible. Evolved water and hydrogen are managed by chemical traps; carbonaceous reaction products (CO/CO2) are converted and scrubbed. A molecular sieve separates H2 from N2 and other light gases, and a thermal conductivity detector measures H2.

Key analytical concepts:

• Stepped furnace heating separates surface-associated hydrogen (released early) from bulk hydrogen (released at higher energy), enabling integration of two distinct peaks.
• Chemical traps remove moisture (H2O) and convert CO to CO2; remaining CO2 is scrubbed before detection of H2.
• Calibration can be established using available aluminum reference materials or, when aluminum CRMs are limited, steel CRMs with adjusted furnace parameters.

Sample preparation

Proper sample prep is emphasized to avoid surface contamination and hydrogen pickup. Main recommendations:

• Section and machine samples to uniform dimensions (maximum/ideal 10.5 x 24 mm) using a water-cooled cutoff saw and lathe.
• Remove the original surface completely (lathe chuck and tool must be free of lubricants).
• Recommended lathe speed: 600–700 rpm.
• Rinse in acetone and dry with warm air if needed; handle prepared samples only with clean tweezers and analyze promptly.

Used instrumentation

The instrument and ancillary hardware used are described as follows:

• Primary analyzer: LECO H836EN hydrogen determinator (electrode/impulse furnace).
• Carrier gas: Argon.
• Detection: Thermal conductivity (TC) cell following molecular sieve separation.
• Gas cleanup: Reagents to remove moisture (H2O), convert CO to CO2 and a CO2 scrubber.
• Sample containment: 764-330 graphite crucibles; electrode tip 611-351-183; tin flux pellets (761-739) used for steel calibration only.
• Accessories: crucible tweezers (766-053), sample tweezers (760-138), lathe for sample finishing.

Method parameters and furnace programs

Standards and references: ASTM E2792 is cited as the standard test method. Two furnace program families are used depending on calibration material:

Steel calibration (typical settings):

• Outgas: constant current ~900 A (30 s cycles with brief cool times).
• Analysis: 850 A constant for reference-material analysis in a two-step furnace method.

Aluminum sample analysis (five-step power-controlled method):

• Outgas power: ~4500 W (30 s cycles).
• Step 1 — surface H liberation: 950 W for 20 s.
• Step 2 — pause to separate surface/bulk: 0 W for 60 s.
• Step 3 — complete surface removal: 450 W for 70 s.
• Step 4 — bulk H liberation: 1400 W for 35 s.
• Step 5 — reduced power to avoid Al boiling into electrode: 0 W for 150 s (cool/hold).

Peak integration strategy separates two peaks: Peak 1 (surface hydrogen) is integrated from a fixed start to end time, and Peak 2 (bulk hydrogen) is linked to the end of the previous peak and extends to the end of sample data.

Instrument-specific settings (timing, integration delays, and comparator thresholds) are defined in the H836EN operator manual and must be adjusted for line voltage and sample behavior to avoid crucible burn-through and to improve recovery.

Procedure summary (blanks, calibration, sample analysis)

Sequence overview:

1. Prepare the instrument and consumables per operator manual.
2. Establish steel calibration blanks (if calibrating on steel): run three or more blank replicates using tin flux pellets in crucible.
3. Establish aluminum sample blanks: run three or more blank replicates with an empty graphite crucible.
4. Perform calibration/drift correction: run three or more calibration replicates (typically steel CRMs when aluminum CRMs are limited), entering sample masses and IDs; use tin flux for steel calibrations.
5. Verify calibration by analyzing a different CRM to confirm results fall within tolerance.
6. Analyze prepared aluminum samples: weigh, enter mass/ID, and run each sample following the five-step furnace method. Clean electrodes as required between runs.

Operational notes:

• Tin flux pellets are used only for steel calibrations, not for aluminum analyses.
• Blanks for each material type are used to set baseline corrections.
• Perform analyses promptly after sample preparation to limit surface hydrogen uptake.

Main results and discussion

Typical data reported in the note were obtained with a linear calibration forced through the origin using LECO steel LCRM pins (6.9 ppm and 1.3 ppm verification). Key observations from representative sample sets:

• Measured total hydrogen values for tested aluminum rods clustered near 0.15 ppm (mg H/kg Al).
• Surface hydrogen contributions ranged approximately 0.05–0.08 ppm, while bulk hydrogen contributions were roughly 0.07–0.10 ppm in the presented datasets.
• Reported sample means (two groups) were about 0.154 ppm (total) with standard deviation 0.049 ppm and 0.147 ppm (total) with standard deviation 0.031 ppm. Surface and bulk components showed respective standard deviations indicating reasonable repeatability for routine QC at low-ppm levels.
• A consolidated reported value example: 0.068 ± 0.018 mg H/kg Al (this appears to be a specific averaged or target value cited in the note).

Discussion points:

• Separation of surface and bulk peaks by the stepped furnace allows identification of hydrogen introduced by surface contamination or handling versus hydrogen trapped within the metal matrix.
• Using steel reference materials for calibration is practical when aluminum CRMs are limited, but furnace parameters and outgas conditions must be adjusted accordingly and verified with secondary checks.
• Careful control of crucible condition, electrode cleanliness, and sample handling are essential to obtain low-ppm reproducible results.

Benefits and practical applications

Practical advantages of the presented approach include:

• Ability to quantify both surface and bulk hydrogen, aiding root-cause analysis for casting defects and sheet blisters.
• Low detection capability at sub-ppm (mg/kg) levels suitable for industrial QC and acceptance testing.
• Compatibility with standard workflows and ASTM E2792, enabling reliable comparison between labs.
• Operational flexibility to use alternative reference materials with appropriate calibration adjustments.

Applications:

• Foundry process control (degassing effectiveness, melt treatment validation).
• Incoming material inspection for rolled or cast aluminum products.
• Failure analysis where hydrogen-induced porosity or blistering is suspected.

Future trends and potential applications

Areas for development and extension include:

• Improved availability of aluminum-certified reference materials to simplify calibration and reduce reliance on steel CRMs.
• Integration with automation and sample-handling robotics to reduce operator-induced surface contamination and increase throughput.
• Coupling inert-gas fusion with complementary detectors (e.g., mass spectrometry) for better speciation and sensitivity at ultra-low hydrogen concentrations.
• In-line or near-line monitoring of hydrogen in process melts using rapid sampling and data analytics for real-time process control.
• Advanced furnace control algorithms and sensor feedback to further minimize crucible burn-through while maximizing hydrogen recovery.

Conclusion

The LECO H836EN inert-gas fusion method, implemented with a stepped furnace program and proper sample preparation, provides a robust means to measure total, surface and bulk hydrogen in aluminum at low-ppm levels. Adherence to recommended handling, blanking and calibration procedures—including verification when using non-aluminum CRMs—ensures repeatable results appropriate for industrial quality control and failure analysis. Ongoing enhancements in reference materials, automation, and detector technologies will further strengthen hydrogen analysis capabilities for the aluminum industry.

Reference

1. Talbot DEJ. The Effects of Hydrogen in Aluminum and Its Alloys. Maney; London: 2004.
2. Hess PD, Turnbull GK. Proceedings of the International Conference on the Effects of Hydrogen on Materials Properties and Selection and Structural Design. Champion; 1973. p. 277–287.
3. Ransley CE, Talbot DEJ. The routine determination of the hydrogen content of aluminum and aluminum alloys by the hot-extraction method. Journal of the Institute of Metals. 1955–1956;84:445.