1. What is the most authoritative and reliable method for detecting the zinc content on galvanized coils?
The most authoritative and reliable method for trade settlement, and the sole arbitration standard for resolving quality disputes, is the gravimetric method (or dissolution weighing method).
Its core principle is as follows: A sample of galvanized sheet with a known surface area is cut and weighed precisely. Then, it is immersed in a hydrochloric acid solution with added corrosion inhibitors. After the zinc coating is completely dissolved, it is removed and weighed again. The difference between the two weighings is the mass of the zinc coating. Dividing this difference by the precise surface area of the sample yields the zinc content per unit area (grams per square meter).
This method is a destructive testing procedure, relatively complex, but its results are directly determined by mass, eliminating instrument calibration bias. Therefore, it has the highest accuracy and serves as the benchmark for all online and field testing methods.
Second: Since the gravimetric method is the most accurate, why are other non-destructive testing methods used more often?
Because while the gravimetric method is accurate, it has two major drawbacks: first, it requires cutting and sampling, which damages the product and prevents 100% inspection; second, the entire process is time-consuming, from sampling and dissolving to drying and weighing, which cannot meet the needs of real-time feedback and rapid process adjustments on production lines.
Therefore, in actual production, to pursue efficiency and universality, non-destructive testing methods are more commonly used. These methods do not damage the product and can complete a measurement in a few seconds to tens of seconds, making them very suitable for production...
3. How do X-ray fluorescence spectrometry (XRF) and magnetic thickness measurement work? What are their respective advantages and disadvantages?
XRF works by irradiating a galvanized sheet with X-rays, exciting zinc atoms to produce characteristic fluorescence. By analyzing the intensity of this fluorescence, the amount of zinc on the sheet can be accurately calculated. Its advantages include extremely fast detection speed (results in only 2-3 seconds), high accuracy (laboratory-grade relative standard deviation as low as 0.092%), and the ability to analyze multiple elements simultaneously. However, its disadvantages include expensive equipment, requirements for sample shape, and the need for periodic calibration with standard plates.
Magnetic thickness measurement works by utilizing the change in magnetic flux between the probe and the steel substrate to directly measure the physical thickness of the non-magnetic zinc layer. Once the thickness value (unit: micrometers, μm) is obtained, multiplying it by the density of zinc (7.14) easily yields the zinc content (g/m²). Its advantages include portable instrument, simple operation, and low cost, making it the most commonly used tool for production line inspections and on-site acceptance. Its disadvantages are also obvious: the measurement accuracy is slightly lower, with an error of about ±3% to 5%, and it is easily affected by the surface roughness, curvature and impurities of the substrate.
4. How to monitor and control the zinc coating amount in real time during production?
This relies on an online coating weighing system installed at the end of the galvanizing production line. These systems typically integrate beta-ray or X-ray technology to perform non-contact, continuous scanning measurements on the high-speed running strip.
These systems measure the zinc layer distribution across the entire width of the strip (including the upper and lower surfaces) and feed the data back to the central control system in real time. After calculating the current average zinc coating amount and uniformity, the system automatically compares it with the target value and adjusts the air knife parameters accordingly, achieving closed-loop control of the zinc layer quality and ensuring stable production.
In addition, handheld XRF or magnetic thickness gauges are often used for production line sampling inspections, cross-validating data with the online system to ensure process stability.
5. How to ensure the accuracy of test results and avoid disputes caused by different methods?
To ensure the accuracy of test results, the core strategy in production is "tiered adoption and cross-validation," mainly focusing on the following three points:
Standardization First for Traceability: All tests, whether destructive or non-destructive, must be strictly performed in accordance with national standards (such as GB/T 1839-2008) or international standards. This is the cornerstone for ensuring the unified traceability of the entire testing system and data.
Comparative Standardization for Improved Accuracy: The factory uses the gravimetric method to calibrate the zinc coating amount on standard plates of different specifications. These standard plates are then used to calibrate online and handheld testing equipment in the production area. During production, the gravimetric method is also used regularly to verify the test data of online and handheld equipment. Based on the deviation adjustment algorithm, it is ensured that the benchmarks of non-destructive and destructive methods remain highly consistent.
Multi-Point Measurement for Authenticity: Because the zinc coating may be uneven in the width direction of the plate, single-point measurement results are unreliable. Therefore, regardless of the testing method used, multiple measurements should be taken at several typical locations on the steel plate (such as the edge, center, etc.) and the average value should be taken to truly and objectively reflect the zinc coating level of the entire galvanized coil.