Hot-dip galvanizing is the reaction of a molten metal with an iron matrix to produce an alloy layer, thereby combining the substrate and the plating. Hot-dip galvanizing is to pickle the steel pipe first, in order to remove the iron oxide on the surface of the steel pipe, after pickling,
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Definition Hot-dip galvanizing is the reaction of a molten metal with an iron matrix to produce an alloy layer that combines both the substrate and the coating. Hot-dip galvanizing is to pickle the steel pipe first, in order to remove the iron oxide on the surface of the steel pipe, after pickling, it is washed by ammonium chloride or zinc chloride aqueous solution or a mixed aqueous solution of ammonium chloride and zinc chloride, and then fed. Hot dip plating tank. Hot-dip galvanizing has the advantages of uniform plating, strong adhesion and long service life.
Process Introduction The principle of hot dip galvanizing is simply to immerse the cleaned iron parts in a zinc bath by the wetting action of Flux to react steel with molten zinc to form an alloyed film.
Good hot-dip galvanizing operations should be carried out under the strict control of each process. If the operation of the previous process is not good, it will cause a chain reaction of the subsequent process, which will increase the operating cost or cause poor hot dip galvanized products. If the pretreatment is poor, the molten zinc cannot completely react with the steel normally, forming the most perfect galvanized coating structure. If the post-treatment is poor, the appearance of the galvanized coating is destroyed, and the value of the product is lowered.
advantage
1. The entire surface of the steel is protected, and the molten zinc is easily covered evenly in the inside of the tube at the recess or in any corner where it is difficult to enter. Hot dip galvanizing
2. The hardness value of the galvanized layer is larger than that of the steel. The uppermost Eta layer has a hardness of only 70 DPN, so it is easily bumped and recessed. However, the lower layer Zeta layer and the delta layer have 179 and 211 DPN hardness values ​​respectively higher than the iron material's 159 DPN hardness value, so the impact resistance and The wear resistance is quite good.
3. In the corner area, the zinc layer is often thicker than other places, and has good toughness and wear resistance. Other coatings at this corner are often the thinnest and most difficult to construct, and the most vulnerable, so they often need to be maintained.
4. Even if it is subject to great mechanical damage or other reasons. A small part of the zinc layer is peeled off and the iron base is exposed. At this time, the surrounding zinc layer functions as a sacrificial anode to protect the steel from corrosion. On the other hand, the other coatings are just the opposite, and the rust is formed immediately and quickly spreads under the coating, causing the coating to peel off.
5. The consumption of zinc in the atmosphere is very slow, about 1/17 to 1/18 of the corrosion rate of steel, and is predictable. It has a much longer life than any other coating.
6. Coating life in a specific environment, depending on the thickness of the coating. The thickness of the coating is determined by the thickness of the steel. The thicker the steel, the thicker the coating, the thicker the steel in the same steel structure must be thicker to ensure a longer life.
7. Due to aesthetics, art, or when used in a particularly corrosive environment, the galvanized layer can be applied to the duplex system. As long as the paint system is selected correctly and the construction is easy, the corrosion resistance is better than that of the separate lacquer and hot dip galvanizing. The lifespan is even better than 1.5 to 2.5 times.
8. The use of zinc layer to protect steel, in addition to hot dip galvanizing method, there are several other methods, the advantages and disadvantages of which are compared as Table 3, generally the most widely used, the best anti-corrosion effect and the best economic benefit, Hot dip galvanizing method.
Judging criteria and the amount of attachment
Corrosion resistance is mainly determined by the thickness of the galvanized layer. Therefore, the measurement thickness is often the basis for determining the quality of galvanizing. The galvanized layer reacts differently depending on the composition, structure and structure of the steel surface. The angle and speed of the solution also have a large effect. Therefore, it is practically impossible to obtain a completely uniform coating thickness. Therefore, it is absolutely impossible to measure the amount of adhesion by a single point (part). It is necessary to measure the average attached zinc weight (g) per unit area (m2).
There are many methods for measuring the amount of adhesion, such as destructive slice metallographic observation, acid washing, non-destructive film thickness meter, electrochemical method, and weight difference estimation method. Commonly used are the film thickness method and the pickling method.
The film thickness gauge is the most common method for measuring the thickness of the zinc layer by magnetic field induction. The basic condition is that the steel surface must be smooth and complete before accurate numbers can be obtained. Therefore, at the corners of the steel or rough, angled steel or castings, etc., an accurate number is unlikely. Ordinary iron parts with the original iron material Angelica zero substrate, can still get quite accurate figures, castings are absolutely inaccurate.
The pickling method is the most accurate method for formal inspection reports. However, it is necessary to pay attention to the fair choice of the upper and lower parts when slicing, in order to obtain accurate figures. However, it also has shortcomings, such as a lot of time, complex steel area is not easy to find, too large pieces can not be soaked. Therefore, it is sufficient to make full use of the film thickness gauge to control the on-site process, and to use the pickling method for the final test.
Uniformity
The most rustable part of hot dip galvanized steel is still the thinnest part of the zinc layer, so it is necessary to measure whether the thinnest part meets the standard.
The test method for uniformity is generally tested with copper sulfate, but this method is problematic for the test of the galvanized layer film composed of the zinc layer and the alloy layer. This is because the dissolution rate of the zinc layer and the alloy layer in the copper sulfate test solution is different, and the alloy layer also differs due to the difference in the ratio of zinc/iron. Therefore, it is not very reasonable to determine the uniformity by the number of repetitions of a certain immersion time.
Therefore, in recent European and American specifications and JIS, there is a tendency to abolish this test method, and the uniformity is replaced by distribution, and the visual or tactile sensation is dominant. When necessary, the film thickness gauge is used to check the distribution state.
The small-sized components are difficult to measure due to the area measurement, and it is difficult to obtain an average film thickness. Sometimes, the copper sulfate test method has to be used as a reference, but the purpose of measuring the adhesion amount cannot be replaced by the copper sulfate test.
Solidity
The so-called solidity is the adhesion between galvanized layer and steel. It is mainly required that the galvanized component has the property of not being peeled off during finishing, transportation, storage and use. The general inspection method includes hammering method, squeezing method, winding method, etc. .
The hammering method is to hammer the test piece and check the state of the surface of the plating film. Fix the test piece so that the height of the hammer support table is high and the level is the same. The hammer is centered on the support table, and the vertical position of the handle weight is naturally dropped. The hammer is struck at 5 points in parallel at 4 mm intervals to observe whether the film is peeled off or not. However, within 10mm of the distance or end, this test shall not be carried out, and the same place shall not be struck more than 2 times. This method is the most common and is suitable for solid testing of zinc, aluminum and other films. Others such as the squeezing method and the winding method are rarely used, so they are not mentioned for the time being.
Ordinary people often have a misconception, often in order to facilitate the measurement of solidity, take two galvanized steel, knocking each other at the corners, observing the corner peeling situation to judge. If there are just a few thicker zinc particles at the corners. If it is not handled well in the operation, it will be peeled off with a strong tap. Therefore, this method cannot be used to determine the adhesion of a normal galvanized coating to an iron base.
Adhesion, uniformity and firmness are the items that define the hot dip galvanizing quality inspection for general specifications. It is also the standard for general formal inspection reports. Relevant specifications.
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