Judgment Standard For Galvanized Channel Steel

1. Attachment amount
The corrosion resistance mainly depends on the thickness of the zinc coating, so measuring the thickness is often the main basis for determining the quality of zinc coating. The zinc coating has different reactions due to the composition, organization, and structure of the steel surface, and the angle and speed of entering and exiting the zinc solution also have a significant impact. Therefore, it is unlikely to obtain a completely uniform coating thickness in advance. Therefore, measuring the adhesion amount cannot be judged by a single point (location), and it is necessary to measure the average adhesion zinc weight (g) per unit area (m2) in order to be meaningful.
There are many methods for measuring adhesion, such as destructive slice metallographic observation, acid washing, non-destructive film thickness measurement, electrochemical method, and estimation of weight difference between import and export. The commonly used methods are film thickness measurement and acid washing.
Film thickness gauge (coating thickness gauge) is the most common and convenient method for measuring zinc layer thickness using magnetic field induction. Its basic condition is that the steel surface must be smooth and intact in order to obtain more accurate numbers. Therefore, it is unlikely to obtain an accurate number at the edges and corners of steel or rough, angled steel parts or castings. When using raw iron as the base material for ordinary iron parts, if a fairly accurate number can be obtained, the casting will definitely not be accurate.
The acid washing method is the most accurate method used for formal inspection reports, and only when slicing, attention must be paid to the fair selection of the upper and lower parts in order to obtain accurate numbers. But it also has its drawbacks, such as time-consuming, difficult to obtain complex steel areas, and inability to fully pickle large parts. Therefore, it is sufficient to make full use of the film thickness gauge to control the on-site process, and to use the acid washing method for final testing.
2. Uniformity
The area where hot-dip galvanized steel is most prone to rust is still the thinnest part of the zinc layer, so it is necessary to test whether the thinnest part meets the standard.
The uniformity test method usually uses copper sulfate test, but this method is very problematic for testing the zinc coating film composed of zinc and alloy layers. This is because the dissolution rate of the zinc layer and the alloy layer in the copper sulfate test solution is different, and the ratio of zinc to iron in the alloy layer also varies. Therefore, it is not very reasonable to determine uniformity based on the number of iterations for a certain soaking time.
Therefore, there has been a recent trend towards abolishing this test method in European and American specifications and JIS, replacing uniformity with distribution, focusing mainly on visual or tactile inspection, and only checking the distribution status with a film thickness gauge when necessary.
Due to the difficulty in measuring the area and obtaining the average film thickness of small components with complex shapes, it is sometimes necessary to use the copper sulfate test method as a reference, but the copper sulfate test cannot replace the purpose of adhesion measurement.
3. Solidity
The so-called solidity refers to the tightness between the galvanized layer and steel. The main requirement is that the galvanized components must have a non peeling property during sorting, transportation, storage, and use. The general inspection methods include hammering, bending, and rolling.
The hammering method is to use a hammer to strike the test piece and check the surface condition of the coating film. Fix the test piece to prevent it from falling due to the height and level of the hammer support platform. The hammer is centered on the support platform, causing the handle weight to fall naturally in the vertical position. Strike 5 parallel points at 4mm intervals to observe whether the skin film is peeling. However, this test shall not be conducted within 10mm of the angle or end, and no more than 2 blows shall be made at the same location. This method is the most common and suitable for testing the firmness of zinc, aluminum, and other coatings.