May 2015 Archives

#208 Hot Dip Galvanizing -1

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(1) Plating pretreatment process

The figure shown below is the basic work process of hot dip galvanizing.
To be brief, this process can be classified into the following three stages: 1) Pretreatment process to remove rust, scale, and oil content from the plating substrate surface; 2) Plating process to form a zinc film on the surface by immersing the substrate into molten zinc; 3) Finishing process to make the plated products conform to the required quality standard.
Figure

Wet processes such as alkaline degreasing are the common method in the batch treatment. On the other hand, continuous treatment such as hoop plating uses a method where the substrates pass through a furnace at 700 to 900℃ to perform annealing on steel materials as well.
Acid pickling is an important process to enhance the adhesiveness by forming an alloy layer between the zinc and the steel substrate after removing oxidized materials that have formed on the iron/steel products. Most commonly, sulfuric acid or hydrochloric acid will be used in this process.
Blasting is a method of removing rust or scale by spraying abrasive grains or glass beads with high-pressure air. This method is applied to carbon steel and high-tensile steel that are likely to suffer from hydrogen embrittlement caused by acid pickling.The adhesiveness with the zinc coating is improved by shot blasting, grit blasting, and sand blasting as they roughen the surface.Therefore, blasting may be performed for the purpose of improving adhesiveness and increasing the adhesion amount.
The purpose of performing the flux treatment after acid pickling is to facilitate a reaction of steel and zinc on the product surface. This treatment also eliminates zinc oxide on the molten zinc surface immersed in a plating bath by removing adhered substances on the steel surface or small rust generated after acid pickling.
The flux treatment has the following two methods: 1) Dry method - the metal object is preheated and dried after being soaked into the heated 10% zinc chloride ammonium solution (or ammonium chloride); 2) Wet method - molten salt of zinc chloride ammonium is formed on the molten zinc of a plating bath, and the metal object is treated with the molten salt.

Figure

#207 Hot Dipping

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Hot dipping is a surface treatment method used to form a film of molten metal on a product surface by immersing the product into a bath of molten metal at a high temperature and lifting it out after dipping.In Japan, it has long been a familiar method known as tempura plating from how the film is formed.Although the principle is simple, this method requires highly technical skills in order to make products that meet a certain quality standard.

Various metals are used for hot dipping, such as zinc, aluminum, lead, tin plating, and composites of these metals. The most frequently used method is hot dip galvanizing, which is applied for a wide range of steel products, including various tubes, plates, wire ropes, metal meshes and more.

In recent years, the industry has shifted its emphasis from a heavy, thick, long, and large structure to a light, slim, short, and compact structure. Owing to this industry trend, hot dipping rarely receives the attention it deserves, but it supports the industrial infrastructure by being used as a heavy-duty anticorrosion measure for transmission towers, large-scale bridges, and marine hardware.
Figure

Only the metals with a lower melting point than the substrate can be used for hot dipping. However, hot dipping is frequently used in corrosive environments that require a high degree of anti-corrosion protection (zinc plating) or high-temperature oxidation resistance (molten aluminum plating) because hot dipping can form a relatively thick metal film.

Hot dip galvanizing provides highly reliable corrosion resistance because of the property of zinc being electrochemically less noble than steel. Even when the substrate is scratched, a local battery is formed between the substrate metal and the plating metal where zinc serves as the sacrificial anode and protects the steel substrate from being corroded.Thus, the thicker the plating, the longer the product life.Because metals, such as tin/lead plating, being more electrochemically noble than steel will not serve as the sacrificial anode, the performance will be based on the anti-corrosive properties of the plating metals themselves.

Figure

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