1.What are the principles and procedures of the file test?
Principle: Pure zinc is relatively soft; when filed, the chips are bright and flaky. Zinc-iron alloy layers (such as galvanic coating) are very hard and brittle, producing grayish-black powdery chips.
Procedure: On the edge or inconspicuous area of the galvanized coil, file the surface at a certain angle with a new file and observe the shape and color of the chips.
Judgment:
Bright, flaky chips: Indicates a pure zinc layer with high purity and a high content of free zinc.
Grayish-black powdery chips: Indicates a zinc-iron alloy layer where zinc has diffused with the steel substrate, forming brittle intermetallic compounds (such as FeZn7, FeZn13), with almost no free zinc on the surface.
Characteristics: Simple, fast, and low-cost, but it is a destructive test, and the results are somewhat subjective.
2.What are the principles and operations of the copper displacement test?
Principle: Zinc displaces copper ions from a copper salt solution by utilizing the metal activity order. A distinct replacement reaction occurs on the surface of a pure zinc layer, forming a reddish-brown copper film. On the other hand, since zinc-iron alloy layers have little or no free zinc on their surface, the replacement reaction is weak or almost nonexistent.
Procedure: Place a drop of copper sulfate solution on a clean galvanized sheet. Allow it to sit for a period of time (e.g., 30 seconds to 1 minute) and observe the surface color change.
Evaluation:
Rapid development of a uniform copper-red color indicates a high free zinc content and high zinc purity.
No change or only faint, sparse red spots indicate a zinc-iron alloy layer with extremely low free zinc content.
Features: Rapid, intuitive, and destructive testing (the test point may be contaminated).
3.What is the principle and detection content of glow discharge spectroscopy?
Principle: This is one of the most commonly used and accurate methods for analyzing coating composition and thickness. It uses ions generated by a glow discharge to bombard the sample surface, causing material to sputter and stimulate luminescence. By analyzing the wavelength and intensity of the spectrum, the elemental composition and distribution of each layer from the surface to the substrate can be precisely determined.
What it can detect:
Zinc layer depth profile analysis: This method clearly shows the concentration changes of elements such as zinc, iron, and aluminum from the outermost layer to the iron matrix.
Confirming purity: If the outermost layer is pure zinc, the GDOES spectrum will show that this area is almost 100% zinc. Only in the transition layer close to the iron matrix does the iron content increase sharply.
Detecting impurities: It can accurately detect the content of trace elements such as aluminum, lead, and antimony in the zinc layer.
Features: It is highly accurate and provides depth information, making it an authoritative detection method. However, the equipment is expensive and must be performed in a laboratory.
4.What are the principles and detection capabilities of a scanning electron microscope combined with an energy dispersive spectroscopy (EDS) instrument?
Principle: Using an electron beam to scan a sample cross-section, SEM provides high-resolution microscopic morphological images, while EDS allows for qualitative and quantitative elemental analysis of microregions of interest.
What Can Be Tested:
Observing Coating Structure: Clearly distinguishing between pure zinc layers, alloy layers, and the steel substrate.
Micro-Area Composition Analysis: Point analysis or surface scanning can be performed on pure zinc and alloy layers, yielding precise elemental composition.
Features: Highly intuitive, enabling both morphological observation and compositional analysis. However, sample preparation is complex (requiring cross-sectional metallographic specimens), and this also falls under the category of laboratory testing.
5.What are the principles and detection contents of X-ray fluorescence spectrometry?
Principle: X-rays are used to illuminate a sample and the wavelength and intensity of the resulting characteristic X-rays are measured for qualitative and quantitative elemental analysis.
What it can detect:
It is primarily used to measure coating thickness, but with calibration, it can also quantitatively analyze the main elements (Zn, Fe, Al, etc.) in the zinc layer.
It is very effective for analyzing the composition of the topmost layer.
Features: It is fast and non-destructive, suitable for online or on-site testing, but is insensitive to changes in the composition of the underlying layers.