Galvanizing refers to a surface treatment technology that coats a layer of zinc on the surface of metals, alloys or other materials for aesthetics and anti-rust effects. The main method used is hot-dip galvanizing.

Zinc is easily soluble in acid and alkali, so it is called amphoteric metal. Zinc hardly changes in dry air. In humid air, a dense basic zinc carbonate film will be formed on the surface of zinc. In the atmosphere containing sulfur dioxide, hydrogen sulfide and marine atmosphere, the corrosion resistance of zinc is poor, especially in the atmosphere of high temperature, high humidity and organic acid, the zinc coating is easily corroded. The standard electrode potential of zinc is -0.76V. For the steel substrate, the zinc coating is an anodic coating. It is mainly used to prevent the corrosion of steel. The quality of its protective performance is closely related to the thickness of the coating. After the zinc coating is passivated, dyed or coated with light protection agent, its protection and decoration can be significantly improved.

Main Feature

1. The appearance is smooth, without zinc tumors and burrs, and is silvery white;

2. The thickness is controllable and can be selected arbitrarily within 5-107μm;

3. No hydrogen embrittlement and no temperature hazard, which can ensure the mechanical properties of the material remain unchanged;

4. It can replace some processes that require hot-dip galvanizing;

5. Good corrosion resistance, neutral salt spray test up to 240 hours.

Application range Steel nails, iron nails, fasteners, water pipe joints, scaffolding fasteners, steel wire rope horse-steel chucks, etc.

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In the plating tank filled with galvanizing solution, the cleaned and specially pretreated parts to be plated are used as the cathode, and the plated metal is used to make the anode, and the two poles are respectively connected to the positive and negative poles of the DC power supply. The galvanizing solution is composed of an aqueous solution containing metal plating compounds, conductive salts, buffers, pH regulators and additives. After electrification, the metal ions in the galvanizing solution move to the cathode under the action of potential difference to form a coating. The metal of the anode forms metal ions into the galvanizing bath to maintain the concentration of the metal ions being plated [1]. In some cases, such as chrome plating, it is an insoluble anode made of lead or lead-antimony alloy, which only plays the role of transferring electrons and conducting current. The concentration of chromium ions in the electrolyte needs to be maintained by regularly adding chromium compounds to the plating solution. During galvanizing, the quality of the anode material, the composition of the galvanizing solution, temperature, current density, energization time, stirring intensity, precipitated impurities, power waveform, etc. will all affect the quality of the coating, and timely control is required.

Folded hot-dip galvanized

At present, the main method of galvanizing the surface of steel plate is hot-dip galvanizing.

Hot-dip galvanizing is developed from the older hot-dip galvanizing method. Since 1836, when hot-dip galvanizing was applied to industry in France, it has a history of 170 years. However, the hot-dip galvanizing industry has developed on a large scale with the rapid development of cold-rolled strip steel in the past three decades.

The production process of hot-dip galvanized sheet mainly includes: raw sheet preparation → pre-plating treatment → hot-dip galvanizing → post-plating treatment → finished product inspection, etc. According to the habit, it is often based on the pre-plating treatment method.

The hot-dip galvanizing process is divided into two categories: off-line annealing and in-line annealing, namely wet method (single-sheet hot-dip galvanizing method), off-line annealing (single-sheet hot-dip galvanizing method), hot-dip galvanizing Huilin (Wheeling) method (strip continuous hot-dip galvanizing method), in-line annealing Sendzimir (Sendzimir) method (protective gas method), improved Sendzimir method, United States steel method (same as Japan Kawasaki method), Selas (Selas ) method and Sharon method.

Cold galvanized

Cold galvanizing is also called electrogalvanizing. It uses electrolytic equipment to degrease and pickle the pipe fittings, then put them into a solution composed of zinc salt, and connect the negative electrode of the electrolytic equipment. Place a zinc plate on the opposite side of the pipe fittings and connect them to the electrolytic The positive pole of the equipment is powered on, and a layer of zinc is deposited on the pipe fittings by using the directional movement of the current from the positive pole to the negative pole. The cold-galvanized pipe fittings are processed first and then galvanized.

Mechanical galvanizing process principle

In the rotating drum filled with plated parts, glass balls, zinc powder, water and accelerators, the glass balls as the impact medium rotate with the drum, and friction and hammering with the surface of the plated parts generate mechanical and physical energy. Under the action of the accelerator, the coated zinc powder is "cold welded" to the surface of the plated part to form a smooth, uniform and meticulous coating with a certain thickness.

The technical standard adopts American ASTM B695-2000 and military C-81562 mechanical galvanizing technical standard.

Comparison between electro-galvanized and hot-dip galvanized

Galvanized

1. Principle

Because zinc is not easy to change in dry air, but in humid air, a very dense zinc carbonate film can be formed on the surface, which can effectively protect the interior from corrosion. And when some reason causes the coating to be damaged and a small substrate is exposed, the zinc and the steel substrate form a micro-battery, making the fastener substrate a cathode and protected. It is widely used in industries such as automobile transportation, but what is needed is a trivalent chromium passivation layer and a zinc-nickel alloy plated sealing coating to reduce the harmful and toxic layer of hexavalent chromium passivation.

2. Performance characteristics

The zinc coating is thicker, the crystal is fine, uniform and non-porous, and the corrosion resistance is good; the zinc coating obtained by electroplating is relatively pure, and it corrodes slowly in the mist such as acid and alkali, which can effectively protect the fastening sand piece substrate. After acid passivation, it will form white, colored, army green, etc., which are beautiful and elegant, and have a certain degree of decoration. Due to the good ductility of the galvanized layer, it can be cold punched, rolled, bent, etc. without damage. plating.

3. Application scope

The fields involved in electro-galvanizing are becoming more and more extensive. The application of fastener products has been widely used in machinery manufacturing, making galvanized chain link fences, electronics, precision instruments, chemicals, transportation, aerospace, etc., which are of great significance in the national economy.

Hot dip galvanized

1. Principle

The hot-dip zinc layer is formed by zinc in liquid state in three steps. The surface of the iron base is dissolved by the zinc liquid to form a zinc-iron alloy phase layer; the zinc ions in the alloy layer further diffuse to the substrate to form a zinc-iron mutual-soluble layer; the alloy layer The surface is covered with zinc layer.

2. Performance characteristics

A thick and dense layer of pure zinc covers the surface of the steel fastener, which can prevent the steel substrate from contacting any corrosive solution and protect the steel fastener substrate from corrosion. In the general atmosphere, a thin and dense layer of zinc oxide is formed on the surface of the zinc layer. It is difficult to dissolve in water, so it has a certain protective effect on the steel fastener matrix. If zinc oxide and other components in the atmosphere form insoluble zinc salts, the anti-corrosion effect is more ideal; it has a zinc-iron alloy layer, which is densely combined, and exhibits unique corrosion resistance in marine salt spray atmospheres and industrial atmospheres; due to the combination Strong, zinc-iron mutual solubility, strong wear resistance; due to the good ductility of zinc, its alloy layer is firmly attached to the steel substrate, so hot-dip galvanizing can be used for cold punching, rolling, drawing, bending, etc. The forming process does not damage the coating; after the steel structure is hot-dip galvanized, it is equivalent to an annealing treatment, which can effectively improve the mechanical properties of the steel substrate, eliminate the stress of the steel forming and welding, and facilitate the turning of the steel structure; The surface of the fastener after galvanizing is bright and beautiful; the pure zinc layer is the most plastic galvanized layer in hot-dip galvanizing, its properties are basically close to pure zinc, and it has good ductility.

3. Application scope

This plating method is especially suitable for strong corrosive environments such as various strong acids and alkali mist.

Influencing factors of folding galvanizing

In the actual production process, the common factors affecting the speed and quality of galvanizing are:

(1) The pre-treatment is not thorough. There is an oxide film on the surface of the workpiece, which affects the normal deposition of zinc.

(2) Poor electrical conductivity. The current is consumed in the wire and the current distributed to the surface of the workpiece is too small.

(3) The workpiece has high carbon content. High-carbon steel, iron castings, etc. will reduce the hydrogen precipitation potential, accelerate the hydrogen evolution on the surface of the workpiece, and reduce the current efficiency.

(4) The workpiece is bound too densely. During galvanizing, the workpiece is partially shielded and the coating is too thin.

(5) The bath temperature is low. When the temperature of the plating solution is low, the distribution current density is correspondingly reduced, and the deposition rate of the coating is bound to be reduced.

(6) The content of sodium hydroxide in the bath is too high. When the sodium hydroxide content is high, the current efficiency decreases accordingly.

(7) The content of additives in the plating solution is low. Low additive content will affect the dispersion ability, and the coating will appear too thin locally.

(8) The area to be plated is underestimated, and the current density delivered during plating is too small.

(9) The workpiece suspension method is improper, and the distance between the workpiece and the zinc anode is too large, and the position should be adjusted.

(10) The workpiece is too corroded. Reducing the hydrogen precipitation potential will reduce the hydrogen evolution acceleration current efficiency on the workpiece surface, thereby affecting the zinc deposition rate. A proper amount of corrosion inhibitor should be added to the pickling solution. If the scale is too thick in the local area, it should be removed by mechanical method first, and more inspections should be made during the pickling process.

(11) Anode passivation. The effective area is reduced, affecting the normal distribution of current.

(12) Sodium hydroxide content is low. If the sodium hydroxide content is low, the current density cannot be increased, and the anode is passivated.