Zinc is soluble in both acid and alkali, so it is called amphoteric metal. Zinc is hardly changed in dry air. In humid air, the zinc surface will form a dense basic zinc carbonate film. In the atmosphere containing sulfur dioxide, hydrogen sulfide and marine atmosphere, zinc corrosion resistance is poor, especially in high temperature and high humidity containing organic acids, zinc coating is easy to be corroded. The standard electrode potential of zinc is -0.76V. For the steel substrate, zinc coating is an anodic coating, which is mainly used to prevent the corrosion of steel. After passivation, dyeing or coating with light protection agent, the protective and decorative properties of zinc coating can be improved significantly.
Zinc plating refers to the surface treatment technology of plating zinc on the surface of metal, alloy or other materials to have a beautiful appearance and antirust effect. The methods used are hot-dip galvanizing and cold galvanizing.
- Hot-dip galvanizing
In the bath containing the galvanizing bath, the parts to be plated after cleaning and special pretreatment are used as the cathode, and the anode is made of coated metal. The poles are connected with the positive and negative poles of the DC power supply respectively. The zinc plating bath is composed of an aqueous solution containing a metal plating compound, a conductive salt, a buffer, a pH regulator, an additive, etc. After electrification, the metal ions in the galvanizing solution move to the cathode to form a coating under the action of potential difference. The metal of the anode forms metal ions into the zinc plating bath to maintain the concentration of the plated metal ions. In some cases, such as chromium plating, is the use of lead, lead-antimony alloy made of insoluble anode, it only plays the role of electron transfer, conduction current. Chromium ion concentration in the electrolyte is maintained by periodic addition of chromium compounds to the bath. When galvanizing, the quality of anode material, the composition of galvanizing solution, temperature, current density, electrification time, stirring intensity, precipitated impurities, power waveform and so on will affect the coating quality, and need timely control.
- Cold galvanizing
Cold zinc plating is also known as electrolytic zinc plating, is the use of electrolytic equipment will pipe after oil removal, pickling, into the zinc salt solution, and connected to the negative electrode of the electrolytic equipment, A zinc plate is placed on the opposite side of the pipe and connected to the positive electrode of the electrolytic equipment to turn on the power supply. A layer of zinc is deposited on the pipe by the directional movement of the current from the positive to the negative electrode. The cold-plated gi pipe fittings is processed first and then galvanized. Mechanical galvanizing In a rotating drum containing plating parts, glass balls, zinc powder, water and an accelerator, the glass balls as the impact medium are rotated with the drum, Friction and hammering with the surface of the plating part produces mechanical physical energy, in the role of chemical accelerators, the zinc powder coated on the plating surface “cold welding” to form a smooth, uniform and detailed coating with a certain thickness.
Hot dip galvanizing is developed from the older hot dip method. It has been 170 years since France applied it to industry in 1836. However, the hot dip galvanizing industry has been developed in a large scale in recent thirty years with the rapid development of cold rolled steel strip.