Determination of Moisture in Welding Flux

Determination of Moisture in Welding Flux — Practical Application Note Summary

Significance of the Topic

Monitoring moisture in welding flux is critical for weld quality and consumable performance. Moisture in flux decomposes during welding to release diffusible hydrogen, which can lead to hydrogen embrittlement, increased cracking susceptibility, and degraded mechanical properties of welds. Quantitative moisture analysis is therefore an essential quality control measure for flux manufacturers, welding supply operations, and inspection laboratories, and it supports compliance with standard procedures such as AWS A4.4M.

Objectives and Study Overview

This application note documents a validated procedure for determining moisture in welding flux using the LECO 812 Series moisture determinators (M812/CM812). Objectives include demonstrating instrument configuration, sample preparation, furnace sequencing to separate free and crystalline moisture, calibration and verification practices meeting AWS A4.4M acceptance criteria, and presenting representative analytical results from typical flux samples.

Methodology and Procedure Summary

Analytical principle:

• Moisture released from heated flux samples is carried by oxygen through the furnace and quantified by infrared measurement of H2O in the evolved gases.
• The instrument can operate as moisture-only (M812) or simultaneous moisture and carbon (CM812).

Sample preparation and handling:

• Use representative, homogeneous samples prepared per AWS A4.4M. Follow SDS for safety.
• Pre-bake collection boats (quartz or nickel) at 1100 °C for 5 min to remove residual carbon; cool and store in a desiccator. Handle boats with clean tongs only.
• Condition Fluorhib (502-156) by baking at 1000 °C for 10 min and store over anhydrous desiccant; if not used within 24 h, re-bake.
• Layer ~1.5 g baked Fluorhib, deposit the sample, then cover with ~1.5 g Fluorhib in the combustion boat to trap evolved moisture and ensure reproducible desorption.

Furnace sequencing (key steps):

• Calibration step: constant 200 °C, oxygen carrier gas, peak monitor on, minimum 120 s, maximum 600 s.
• Weld flux analysis sequence composed of three furnace steps:
  - Free moisture: constant 105 °C, hold 360 s (peak monitoring off).
  - Crystalline moisture (ramp): ramp from 105 °C to 1000 °C at 120 °C/min in oxygen (peak monitoring off during ramp).
  - Crystalline moisture (hold): constant 1000 °C hold 300 s (peak monitoring off; peak finding enabled to separate free vs crystalline contributions).

Instrument and operational settings (representative):

• Purge flow: 4.00 L/min; analytical flow: 0.75 L/min; check delay: 20 s.
• Afterburner temperature: 850 °C; reagent heater: high.
• Nominal sample mass entry typically 1.0000 g but sample masses are adjusted per expected moisture (see recommended masses below).
• Baseline and integration parameters set to wait for baseline stability, with short integration delay and use of endline integration per method.

Calibration and verification:

• Calibration performed as a linear, force-through-origin calibration using LECO calcium oxalate LCRM reference (example: ~0.075 g of LCRM with known moisture content).
• Verify calibration by analyzing a capillary tube containing ~5.0 mg distilled water; AWS A4.4M requires moisture recovery of 95–105% for acceptance prior to sample analysis.

Recommended sample masses (practical guidance):

• Use larger sample masses for very low expected moisture to improve detection (example: ~5.0 g for <0.20% expected water).
• For moderate moisture levels, use smaller masses (typical guidance ranges shown in the application note span from 5.0 g down to 0.5 g depending on expected water percentage).

Used Instrumentation

Equipment and accessories detailed in the note:

• LECO 812 Series moisture determinators: M812 (moisture-only) and CM812 (moisture + carbon).
• Accessories: quartz combustion boats (781-335) with nickel liners (782-059) or nickel boats (625-505-430); Fluorhib 502-156; anhydrone desiccant 501-171-HAZ; capillary/melting point tubes for calibration verification.
• Oxygen as carrier/combustion gas, IR detector for H2O quantitation, and afterburner to ensure complete oxidation of evolved species.

Main Results and Discussion

Representative analytical data illustrate method precision and the ability to resolve free and crystalline moisture fractions:

• Welding Flux 880 (Beige): mean total moisture ~0.178% (free ~0.051%, crystalline ~0.127%), standard deviations on the order of 0.002–0.005% indicating good repeatability.
• Welding Flux 780 (Brown): mean total moisture ~0.064% (free ~0.016%, crystalline ~0.048%), low variability (s ≈ 0.001–0.002%).
• E7018 Weld Flux: mean total moisture ~0.536% (free ~0.139%, crystalline ~0.397%), s ≈ 0.007 total moisture.

Discussion points:

• The two-step furnace program effectively partitions easily liberated (free) moisture from bound or crystalline moisture evolved at higher temperatures, which is important for understanding how flux will behave during welding.
• Calibration and verification criteria required by AWS A4.4M (95–105% recovery on water spike) were met using LECO reference materials and the capillary water check, demonstrating method accuracy.
• Pre-baking and desiccator storage of boats and trapping media are critical to minimize blank contributions and maintain low detection limits.

Benefits and Practical Applications

• Provides a standardized, reproducible procedure for moisture quantitation in welding flux aligned with AWS A4.4M requirements.
• Enables routine QA/QC of flux batches to prevent hydrogen-related weld defects and to control consumable shelf-life and storage practice.
• Dual-function systems (CM812) allow simultaneous carbon measurement when needed, increasing laboratory throughput and data richness.
• Robust to heterogeneous, inorganic flux matrices that may be challenging for some titration-based methods.

Future Trends and Potential Uses

Anticipated developments and extensions of the method include:

• Greater automation and autoloading to improve throughput and traceability in high-volume production laboratories.
• Improved software algorithms for peak deconvolution to enhance separation between free and bound moisture peaks, especially for complex flux chemistries.
• Integration with LIMS and manufacturing QC systems for real-time process control and batch release decisions.
• Expanded use of multimode analyzers combining moisture with other elemental determinations (e.g., carbon) and potential comparison studies with coulometric Karl Fischer for specific matrix classes.

Conclusion

The LECO 812 Series method described provides a validated, AWS-compliant approach for reliable moisture determination in welding flux. Key strengths are the clear separation of free and crystalline moisture, robust calibration/verification practices (95–105% recovery requirement), and procedural controls (pre-baked boats, conditioned Fluorhib) that ensure low blanks and good repeatability. This makes the procedure suitable for routine QC, troubleshooting of weld defects related to hydrogen, and for manufacturers aiming to ensure consistent consumable performance.

Reference

1. AWS A4.4M: Standard Procedures for Determination of Moisture Content of Welding Fluxes and Welding Electrode Flux Coverings.
2. LECO Corporation application note: Determination of Moisture in Welding Flux (Instrument: 812 Series), LECO Corporation, St. Joseph, MI, USA.
3. LECO reference materials and product documentation: LCRM, LRM and accessory specifications (Fluorhib, quartz/nickel combustion boats, anhydrone desiccant).