Hot dip galvanizing is to make molten metal react with iron matrix to produce alloy layer, so that the matrix andPlatingCombine the two.Hot galvanizing is the first step to carry outpickling, in order to removeferric oxide, after pickling, passammonium chlorideorZinc chlorideWater solution or mixed water solution tank of ammonium chloride and zinc chloride for cleaning, and then sent toHot dip platingSlot.Hot dip galvanizing has the advantages of even coating, strong adhesion and long service life.
Steel materials, which are the most widely used in industry, will be corroded to varying degrees when used in atmosphere, seawater, soil, building materials and other environments.According to statistics, the annual loss of steel materials due to corrosion in the world accounts for about 1/3 of its total output.In order to ensure the normal use of steel products and extend their service life, the corrosion protection technology of steel has been widely valued.
Hot dip galvanizing is one of the most effective means to delay the environmental corrosion of steel materials. It is to dip the cleaned and activated steel products into the molten zinc solution, and through the reaction and diffusion between iron and zinc, coat the surface of steel products with zinc alloy coating with good adhesion.Compared with other metal protection methods, the hot-dip galvanizing process has incomparable advantages in the protection characteristics of the combination of physical barrier and electrochemical protection of the coating, the bonding strength of the coating and the substrate, the compactness, durability, maintenance free and economy of the coating, and its adaptability to the shape and size of products.At present, hot-dip galvanized products mainly include steel plate, steel strip, steel wire, steel pipe, etc., of which hot-dip galvanized steel plate accounts for the largest proportion.For a long time, the hot-dip galvanizing process has been favored by people because of its low plating cost, excellent protection characteristics and beautiful appearance. It is widely used in automobile, construction, household appliances, chemical industry, machinery, petroleum, metallurgy, light industry, transportation, power, aviation, marine engineering and other fields.[1]
Process Introduction
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The principle of hot dip galvanizing is simply to clean the cleanedironPiece, via plating aidWetting actionImmersed in a zinc bath, the steel reacts with molten zinc to form an alloyed coating.A good hot-dip galvanizing operation should be that each process should be under strict control and give full play to the function of the process.Moreover, if the operation of the previous process is not good, it will cause chain adverse reactions of the subsequent process, which will greatly increase the operating cost or cause undesirable hot-dip galvanized products.If the pretreatment is poor, the molten zinc cannot react with the steel properly and completely, forming the most perfect zinc coating structure.If the post-treatment is poor, the appearance of the galvanized film will be damaged and thecommodity valueEtc.[2]
advantage
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1. The whole steel surface is protected, and the melted zinc is easily and evenly covered on the inside of the pipe fittings in the depression or any corner where the coating is difficult to enter.
Hot dip galvanizing
2. The hardness of zinc coating is greater than that of steel.The Eta layer on the top layer has only 70 DPN hardness, so it is easy to be impacted and indented. However, the Zeta layer and delta layer on the bottom layer have 179 and 211 DPN hardness values respectively, which are higher than the 159 DPN hardness value of iron material, so their impact resistance and abrasion resistance are 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, the hardest to construct, and the most vulnerable to injury, so they often need to be maintained again.
4. Even if it is caused by great mechanical damage or other reasons.A small part of the zinc layer will fall off and the iron base will be exposed. At this time, the surrounding zinc layer will play a rolesacrificial anodeTo protect the steel here from erosion.On the contrary, for other coatings, rust will be generated immediately and spread to the underside of the coating rapidly, causing the coating to peel off.
5. The consumption of zinc layer in the atmosphere is very slow, about 1/17 to 1/18 of the corrosion rate of steel, and can be estimated.Its service life is much longer than any other coating.
6. The coating life is mainly determined by the coating thickness under a specific environment.The thickness of the coating is determined by the thickness of the steel, that is, the thicker the steel is, the thicker the coating is. Therefore, the steel parts with medium thickness in the same steel structure must also have thicker coatings to ensure a longer life.
7. Due to beauty, art, or use in a specific severely corrosive environment,Zinc coatingThe above paint treatment duplex system can be applied again. As long as the paint system is correctly selected and easy to construct, its anti-corrosion effect is better than that of single painting andHot-dip galvanizingThe service life is 1.5~2.5 times better.
8. In addition to hot-dip galvanizing, there are several other methods to protect steel with zinc coating. Generally, hot-dip galvanizing is the most widely used method with the best corrosion prevention effect and the best economic benefit.[1]
Judgment criteria and main causes
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Adhesion amount
Hot dip galvanizing
Corrosion resistance is mainly determined byZinc coatingTherefore, the measured thickness is often the main basis for judging the quality of galvanizing. The galvanized layer has different reactions due to the different composition, structure and structure of the steel surface, and the angle and speed of entering and exiting the zinc solution also have a great impact.Therefore, it is practically impossible to predict a completely uniform coating thickness.Therefore, measuring the amount of adhesion can never be judged by a single point (position). It is meaningful to measure the average weight of zinc adhesion (g) per unit area (m2).There are many methods to measure the adhesion, such as destructive sectioning metallographic observationpicklingNon destructive film thickness gauge methodElectrochemical methodEstimation of weight difference between incoming and outgoing goods.Film thickness gauge method and acid washing method are commonly used.
Film thickness gauge is the most common and convenient method to measure the thickness of zinc layer by magnetic induction. Its basic condition is that the steel surface must be smooth and complete to get more accurate figures.Therefore, it is unlikely to have an accurate figure at the corners of steel or rough, angled steel parts or castings.The raw iron material used for ordinary iron castings is Danggui zero base material. If the figure is fairly accurate, the casting is absolutely inaccurate.
Pickling method is the most accurate method used in the formal inspection report. Only when slicing, we must pay attention to the fair choice of upper and lower parts to get accurate figures.However, it also has disadvantages, such as time-consuming, difficult to obtain the area of complex steel, and too large parts cannot be pickled completely.Therefore, it is enough to make full use of the film thickness gauge to control the on-site manufacturing process, and use the pickling method to do the final detection.[1]
Homogeneity
The most rusty 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 uniformity test method is generally usedcopper sulphateHowever, this method is very problematic for testing the zinc coating film composed of zinc layer and alloy layer.This is because the dissolution rate of zinc layer and alloy layer in copper sulfate test solution is different, and the alloy layer is also different due to the difference of zinc/iron ratio.Therefore, it is not very reasonable to judge the uniformity by the repeated times of a certain immersion time.
Hot dip galvanizing
Therefore, in recent European and American specifications and JIS, there is a tendency to abolish this test method, replacing uniformity with distribution, giving priority to visual or tactile inspection, and checking the distribution status with a film thickness gauge when necessary.Because it is difficult to measure the area of small components with complex shapes and to obtain the average film thickness, sometimes the copper sulfate test method must be used as a reference, but the copper sulfate test must not be used to replace the purpose of adhesion measurement.[1]
Solidity
The so-called firmness refers to the tightness between the galvanized layer and the steel. It mainly requires that the galvanized components should not be peeled off in the process of sorting, transportation, storage and use. The general inspection methods include hammering, bending and rolling.
The hammering method is to strike the test piece with a hammer to check the coatingmembraneThe state of the surface.Fix the test piece so as not to make the hammer stand at the same height and level. The hammer is centered on the stand so that the vertical position of the handle weight falls naturally. Hit 5 points in parallel at an interval of 4mm, and observe whether the skin film peels off for judgment.However, the test shall not be conducted within 10mm from the angle or end, and the same place shall not be hit more than twice.This method is the most common and applicable to the solid test of zinc, aluminum and other coatings.Other methods, such as extruding and wrapping, are seldom used, so they are not mentioned for the time being.
The common people often have a misconception that in order to measure solidity, they often take twoGalvanized steelKnock each other with corners and observe the peeling of corners to judge.If there are just a few thick zinc particles at the corners.If it is not handled well in the operation, the thick zinc particles will certainly peel off if it is knocked hard.Therefore, this method cannot be used to determine the tightness of normal galvanized coating film and iron base.
AdhesionHomogeneityAnd firmness, which are the quality inspection items of hot-dip galvanizing defined in the general specification.It is also the standard of general formal inspection report.Relevant specifications.[1]
