Galvanizing quality of hot-dip galvanized steel pipes
Hot dip galvanizing, also known as hot-dip galvanizing and Hot-dip galvanization, is an effective way of metal corrosion prevention, mainly used on metal structures and facilities in various industries. It is to immerse the derusted steel parts in molten zinc at around 500 ℃, so that a zinc layer is attached to the surface of the steel components, thereby achieving the purpose of corrosion prevention.
Hot dip galvanizing was invented in the mid-19th century and has a history of over 300 years, developed from the hot tin plating process. Until today, hot dip galvanizing is still the most common and effective process measure in steel corrosion prevention methods. The galvanized steel pipes born under this technology have played an important role in the fields of fire protection, environmental protection, water supply, electrical, and steel structures.
The quality of galvanizing has a significant impact on the functionality and cost of hot-dip galvanized steel pipes. If the hot-dip galvanized steel pipe has problems such as insufficient galvanized thickness, uneven galvanized layer, insufficient zinc adhesion, and missed plating, it can cause corrosion of the steel pipe, reduce its physical and chemical properties, shorten its lifespan, and even cause safety hazards.
The quality of galvanizing is closely related to the hot-dip galvanizing process: the purity of the zinc ingot, the surface treatment of the steel pipe before plating, the temperature of the zinc bath, the hot dip time, and the post-plating treatment all affect the quality of galvanizing. The indicators for judging the quality of galvanizing mainly include:
1、 Appearance of galvanized pipe fittings
The surface of high-quality galvanized pipe fittings should have a complete coating, consistent color, uniform transition, and no sags, drips, or excess lumps on the surface. The surface of the pipe fittings shall not have defects such as missed plating or exposed iron.
2、 Galvanized layer thickness
The thickness of the zinc layer on galvanized pipes is an important indicator for judging the quality of galvanizing, usually measured in units μ M represents the weight of the galvanized layer per square meter (g/m2), and the conversion formula is: 1g/m2=0.14 μ M. Due to the different magnetic and polarizability between zinc and steel, the determination of zinc layer thickness can be non-destructive measured using a zinc layer thickness gauge. To determine the weight of the zinc layer, it is necessary to scrape and sample the zinc layer, and measure it using chemical methods such as antimony chloride method.
3、 Uniformity of zinc coating
High quality galvanized coating has uniform texture and is free of missing plating or post plating damage. Galvanized pipe fittings can be immersed in a certain concentration of Copper(II) sulfate solution for a certain number of times and time. If there is no red leaching substance (metal copper) on the surface, it can be considered that galvanized pipe fittings are free of missing plating and post plating damage defects.
4、 Adhesion of zinc coating
The adhesion of the galvanized layer is a standard for measuring the firmness of the bonding between the galvanized layer and the steel pipe. After the steel pipe is immersed in the zinc bath for a certain time, the surface of the steel pipe will react with the zinc plating liquid to form a zinc iron Mixed layer. Scientific and sophisticated galvanizing process will form a zinc iron Mixed layer with sufficient thickness to improve the adhesion of zinc layer. The adhesion of the galvanized layer can be monitored by lightly tapping with a rubber hammer, preferably to ensure that the coating does not fall off after a certain number of taps.
At present, there is a certain misconception about the quality of galvanized steel pipes in China, with a one-sided emphasis on the bright surface color of the galvanized layer. As mentioned earlier, the quality of galvanizing is related to many factors. In order to achieve a bright surface color, it is necessary to lower the temperature of the zinc bath and prolong the soaking time, which will reduce the adhesion of the zinc coating. Although aesthetically pleasing, from a practical perspective of anti-corrosion, a decrease in adhesion can lead to damage to the galvanized layer, which is not worth the loss. The hot dip galvanizing standard in our country only stipulates that the surface color of the galvanized layer is consistent, and for the sake of aesthetics, the galvanized steel pipe color is required to be bright, without any theoretical basis, which is neither scientific nor economical.
