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D5723-95 – Standard Test Method Technical Guide
🔬 Excitation and Detection Principles
The X-ray fluorescence (XRF) method described in this practice relies on the interaction of primary radiation with the chromium coating and its substrate. The primary radiation, generated by an X-ray tube or a radioisotope, excites the sample, resulting in the emission of fluorescent X-rays at energies characteristic of the elements present. For chromium, these X-rays are detected by a system that includes a radiation detector with pulse amplification and counting electronics, which discriminates between different energy levels in the secondary beam. This allows for the quantitative measurement of the chromium treatment weight.
⚠️ Safety: Per the standard’s scope, users must establish appropriate safety, health, and environmental practices and determine applicability of regulatory limitations prior to use.
⚙️ Calibration and Standards
Accurate determination of coating weight requires calibration using primary standards with known coating weights. A linear relationship typically exists between the secondary radiation intensity and the coating weight within the desired range. This relationship is expressed through a conversion factor, which represents the number of X-ray counts per unit coating weight (e.g., mg/ft² or mg/m²). The calibration must be established for each individual combination of substrate and chromium-containing treatment, often provided by the treatment supplier, using standards with the same composition as the test specimens.
| 🟦 Parameter | 📏 Description |
|---|---|
| Primary Radiation Source | X-ray tube or radioisotope |
| Detected Radiation | Fluorescent X-rays characteristic of chromium |
| Energy Discrimination | Detector set to energy region of chromium X-rays |
| Common Coating Weight Units | mg/ft² or mg/m² |
💡 Tip: Since the relationship is substrate-dependent, always calibrate using standards that match the test specimen’s substrate and treatment composition.
📊 Coating Weight Determination
The coating weight is determined by measuring the number of fluorescent X-rays in the energy region characteristic of chromium. The detection system is configured to count these X-rays, and the count rate is correlated to the coating weight via the calibration curve. If a linear relationship exists, the coating weight is calculated using the conversion factor. Advanced XRF apparatus may include data handling systems for direct readout, but the calibration curve must be pre-established. The exact relationship must be determined for each coating-sub