What is galvanized screw? What are the characteristics of galvanizing?

2020-08-29 09:18 hongsheng bolt

Origin of hot dip galvanization

Word of Galvanizing

1. Galvanizing or hot-dip Galvanizing zinc on the surface of steel products to prevent corrosion is called by hot-dip Galvanizing. Galvanizing is named after the Italian scientist Luigi Galvani(1737-1798). Galvani discovered steel traps in 1791 that caused frog legs to jerk when they caught a piece of dead animal leg. Galvani mistook the frog's leg itself for storing electricity that caused it to twitch.

2. A few years later, another Italian scientist, AlessandoVolta(1745-1827), explained that frog legs were a conductor and pointed out that Galvani discovered a new way to generate electricity: junction of different metals or placed directly in acid tanks, where one metal would receive a positive voltage and the other a negative voltage (potential difference). If you connect the two metals by a wire, you can find a current flowing through the wire. Galvani and Volta are the first scientists to discover that they can produce stable currents. Since then, many scientists have been engaged in research and a new field called Electrochemistry has emerged. For example, batteries and electrolysis are made using this principle.

The Volt is called "Voltage" in honor of Volta's discovery, and its unit is called the Volt. The movement of electricity between metals by potential difference (electrochemistry) is also called GalvanicAction in memory of Luigi Galvani. Galvanizing is used to protect steel by Galvanizing zinc on the surface of steel products by using the potential difference between zinc and iron (different metals) and the electronic flow of zinc to iron.

In 1931, Tardenz Sendzimir, a Polish, discovered a process capable of changing the galvanizing industry by preheating the cold-rolled steel strips in an oxidizing atmosphere to remove the residual rolling oil from the plates and to form a thin layer of iron oxide on the plates. The steel strip is then reduced by high temperature in a hydrogen - nitrogen reducing atmosphere, so that the surface of the steel strip is suitable for galvanizing. The strip must then be cooled to a temperature suitable for galvanizing when it comes out of the oven. In 1936, the Butler Workshop of the Armco Steel Company in the United States was successfully industrialized and became known as Armco - Sendzimir Law. It was patented in the United States in 1938. What's more, this process is called Oxidizing Direct - Fired Furnace. Another advantage of the Sendzimir method is that the Flux is not needed, so aluminum (Al) can be added into the plating bath to restrain the excessive formation of hard and brittle Zn-Fe Alloy Layer, which can improve the adhesion and formability of zinc. However, due to the need to reduce the oxidation layer on the surface of the steel strip at high temperature, it is impossible to produce full hard plate. The surface of the steel strip must be clean to enable zinc to react with iron for a good coating. The purpose of plating aid is to prevent the formation of impurities and rust. The most commonly used plating aid is zinc chloride (ZnCl2) and ammonium chloride (NH4Cl) or a mixture of both. It can be divided into Wet Flux and Dry Flux.

In addition to providing the above functions, the wet plating aid can react with zinc oxide (ZnO) to form the following compounds, which float above the zinc bath to form harmless substances.

ZnO 2 + 2 (ZnCl2 NH4Cl) - > ZnCl2, ZnO + 2 ZnCl2, NH3 + H2O write

Therefore, the wet plating aid floats on the zinc bath, so aluminum (Al) cannot be added in the zinc bath to inhibit the formation of zinc-ferroalloy. Because aluminum is more active than zinc, the white smoke of aluminum chloride (AlCl3) with low melting point will disappear when it interacts with the plating aid in the zinc tank.

2 al 3 + zncl2-3 zinc + 2 alcl32a1 + 6 nh4c1-2 alcl3 nh3 + 6 + 3 h2

As a result, the content of aluminum is reduced and zinc plating aid is destroyed, so in wet mode producers, zinc-ferroalloy is thicker, the formability and adhesion is poor.

Principle of galvanizing against corrosion

Although steel has high strength, easy to process, and the advantages of economy, but easy to rust is also its biggest drawback, of course, the most effective and the most economical corrosion prevention method is galvanized steel surface. Galvanizing corrosion prevention principle has two, one is the "cover isolation" principle, the other is the "substitute corrosion" principle. Barrier Protection prevents steel from being in direct contact with air and water, thereby preventing rust. Galvanized screws are also made using this principle.