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HomeIndustry InformationWeld treatment and zinc layer protection for galvanized steel drums

Weld treatment and zinc layer protection for galvanized steel drums

2020-04-07

Weld treatment and zinc layer protection for galvanized steel drums

Shanghai Xugong Coating Technology Co., Ltd. Xu Guoxing

The galvanized barrels we produce are divided into hot-dip galvanized barrels (some barrel factories are called front galvanized barrels or original zinc barrels) and galvanized barrels (some barrel factories are called post-galvanized barrels).

Zinc is easily reacted with an acid and reacts with a base, so it is called an amphoteric metal. It is well known that zinc reacts on the surface of zinc layer to form ZnO, Zn(OH)2 and basic zinc carbonate under atmospheric corrosion conditions, which will slow down the continued corrosion of zinc to some extent. This layer of reactants will destroy and form new reactants. When the zinc layer is severely damaged and the iron matrix is compromised, the zinc reacts electrochemically on the substrate. The standard potential of zinc is -0.76V, and the standard potential of iron is -0.44V, so that zinc and iron form a microbattery, zinc is gradually dissolved as an anode, and iron is protected as a cathode. This is the protective effect of the galvanized layer on the steel.

Zinc hardly changes in dry air. In the environment containing sulfur dioxide, hydrogen sulfide gas and maritime climate, the corrosion resistance of zinc is poor, especially in high temperature and high humidity and organic acid containing environment, the galvanized layer is easily corroded. The most obvious example, the zinc surface of our electrogalvanized steel drums is often oxidized in a humid environment, producing basic zinc carbonate (also known as white rust) and even rust spots. Therefore, whether it is hot-dip galvanizing or electro-galvanizing, it is necessary to carry out anti-corrosion treatment on the surface of the zinc layer.

Hot-dip galvanizing, also known as hot dip galvanizing, is a method in which steel components are immersed in molten zinc to obtain a zinc metal coating. Hot-dip galvanizing is a result of the older hot-dip method. Since the application of hot-dip galvanizing in France in 1836, it has been more than 170 years old. Especially in the past three decades, with the rapid development of cold-rolled strip steel, it has been developed on a large scale.

The production process of hot-dip galvanized sheet mainly includes: preparation of original board → pre-plating treatment → hot dip plating → post-plating treatment → finished product inspection.

Hot-dip galvanizing coverage is good, the coating is dense, and there is no organic inclusions. When the iron workpiece is immersed in the molten zinc solution at high temperature, the solid solution of zinc and α-iron (body center) is first formed at the interface. When the zinc is saturated in the solid solution, the two elements of zinc and iron diffuse and spread. The zinc atoms in the (or infiltrated) iron matrix migrate in the matrix lattice and gradually form an alloy with the iron. Since the electroplated zinc layer is formed by the deposition of free zinc particles, the zinc layer is relatively loose and cannot form such an alloy, so the quality of the electroplated zinc layer is worse than that of hot-dip galvanizing.

In order to enhance the corrosion resistance of the zinc layer, the steel mill has passivated, dyed and coated the oil film on the surface of the galvanized layer when producing the hot-dip galvanized steel sheet (so-called post-plating treatment), which significantly improves its corrosion protection. Sexual and decorative.

Hot-dip galvanized sheets have different surface conditions due to different treatment methods in the coating process, such as ordinary zinc flowers, fine zinc flowers, flat zinc flowers, zinc-free flowers, and phosphated surfaces.
Whether hot dip galvanized or electrogalvanized, galvanizing is a commonly used and effective method for indicating the thickness of the galvanized sheet zinc layer. Of course, the greater the amount of galvanizing, the better its corrosion resistance. The unit of galvanizing is g/m2, and the regulations for galvanizing of galvanized steel sheets in Japan, the United States and Germany are detailed in the table. Our standards are also based on these indicators.

Galvanized steel sheet index (unit: g/m2)

JISG3302

Code

Z12

Z18

Z22

Z25

Z27

Z35

Z43

Z50

Z60

Zinc plating

120

180

220

250

270

350

430

500

600

ASTMA525

Code

A40

A60

G60

G90

G115

G140

G165

G185

G210

Zinc plating

122

183

183

275

351

427

503

564

640

DIN1716

Code

100


200


275

350

450


600

Zinc plating

100


200


275


350

450

600

The hot-dip galvanized barrels we produce are made of galvanized sheet, and the barrel is welded to be edging. This presents a zinc-free weld treatment problem.

In fact, this problem has been solved in the Shanghai Chemical Co., Ltd. as early as the advent of the hot-dip galvanized barrel, that is, the molten zinc spray after sandblasting to remove the oxide layer. There are also manufacturers specializing in the production of sandblasting and melt-spraying equipment. However, the dusty dust during the sand blasting and the pollution during the spraying of zinc have discouraged many barrel factories.

So, is there a better way? There are three methods that have been tried by manufacturers, and the results are not bad, but the process needs to be improved.

First, the polyethylene powder method.

Polyethylene (PE) is the most widely produced variety of general synthetic resins, including low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE) and some products with special properties. Polyethylene is a white waxy translucent material that is soft and tough, lighter than water, non-toxic, and has excellent dielectric properties. Not soluble in any known solvent at room temperature. Polyethylene has excellent chemical stability and is resistant to various chemicals such as hydrochloric acid, hydrofluoric acid, phosphoric acid, formic acid, amines, sodium hydroxide and potassium hydroxide at room temperature. The molecular weight of 3 million to 6 million ultra high molecular weight polyethylene (UHMWPE) can even be used as a body armor.

The melting point of polyethylene increases with increasing density, ranging from 120 to 136 °C. When the steel barrel body is down from the seam welder, it is slightly air-cooled for a while, then the polyethylene powder is sprayed on the weld, and then the excess powder outside the weld is brushed with a steel wire, and then the flame gun is used to spin back and forth. Just take a polyethylene powder belt. However, we should pay attention to two points here. The first is to spray the powder and wait for the barrel to cool down before it can enter the cuff, the ribs and corrugations and other subsequent processes, otherwise it is easy to stick the polyethylene; second, it is best to use high Density polyethylene powder, because low density polyethylene is relatively hard and brittle.

Of course, there are many powders that can be used for weld coating, such as: ethylene-propylene copolymer, EVA, ethylene-butene copolymer, copolymer of ethylene-other olefin (such as octene POE, cyclic olefin), ethylene - no Saturated ester copolymers (EAA, EMAA, EEA, EMA, EMMA, EMAH), epoxy, polyester, and the like.

Second, the plastic film method

There are many types of plastic film, which can be covered by hot melt of steel barrel welds, preferably polyethylene film, because its performance has been introduced above. Further, a polyester film, a nylon film, a polyvinyl chloride film or the like can also be used.

The method of hot-melt coating of plastic film is to cut the film into strips slightly wider than the width of the weld seam. After the steel drum body is slightly air-cooled under the welder, the film strip is covered and then fired with a flame gun. Just one click. Of course, like the powder method, it is necessary to wait until the barrel is cool before entering the next process. However, it should be noted that the plastic film should not be too thin or too narrow, because the plastic is easily deformed and contracted when heated, too thin and too narrow, which may cause the weld to cover the dew point and the exposed edge after heat shrinkage.

Third, hot melt adhesive method

It melts when it is hot, and it becomes solid when it is cold. This is the characteristic of hot melt adhesive. Different grades of hot melt adhesives have different hot melt temperatures. Hot-dip galvanized barrel welds are coated with hot melt adhesive to apply this characteristic.

The main component of hot melt adhesive is EVA resin, which is then added with paraffin, rosin and other additives. EVA resin is an ethylene-vinyl acetate copolymer. In general, the performance of an EVA resin mainly depends on the content of vinyl acetate on the molecular chain. Generally, the content of vinyl acetate (VA) in the EVA resin is 5% to 40%. Compared with polyethylene, EVA introduces vinyl acetate monomer into the molecular chain, thereby reducing high crystallinity, improving flexibility, impact resistance, filler compatibility and heat sealing performance. EVA resin is non-toxic, soft, tough, elastic and transparent, excellent in anti-aging, chemical and ozone resistance, and maintains its softness, good processability and heat sealability at low temperatures. the use of.

When we use it, we need to cool the barrel from the welder to about 200 °C. At this time, just wipe the hot melt glue stick back and forth on the weld, and a film will be formed at the weld. And the film can be trimmed with heat. Of course, as in the previous two methods, it is necessary to wait for the weld to cool before entering the next process, otherwise it will be stuck.

Another convenient and convenient method is to apply a two-component, self-drying 891 coating at the weld, which is described in more detail below.

Electrogalvanizing is carried out in a plating bath containing galvanizing solution. The pretreated component to be plated (such as our steel drum) is used as a cathode, and the anode is made of plated metal. The two poles are respectively connected with the positive and negative poles of the DC power source. . The galvanizing bath is composed of an aqueous solution containing a plating metal compound, a conductive salt, a buffer, a pH adjuster, and an additive. After energization, the metal ions in the galvanizing solution move to the cathode (such as our steel drum) under the potential difference to form a coating. The metal of the anode forms metal ions into the galvanizing bath to maintain the concentration of the plated metal ions. (In some cases, such as chrome plating, it is an insoluble anode made of lead, lead-bismuth alloy, which only acts to transfer electrons and conduct current. The concentration of chromium ions in the electrolyte depends on the plating solution periodically. The chromium compound is added to maintain.) When galvanizing, the quality of the anode material, the composition of the galvanizing solution, the temperature, the current density, the energization time, the stirring strength, the precipitated impurities, the power waveform, etc. all affect the quality of the coating, so it is necessary Control at the right time.

The galvanizing solution has two types of cyanide plating solution and cyanide-free plating solution. The cyanide plating solution is divided into micro cyanide, low cyanide, medium cyanide, and high cyanide. The cyanide-free plating solution includes an alkaline zincate plating solution, an ammonium salt plating solution, a sulfate plating solution, and an ammonia-free chloride plating solution. The cyanide galvanizing solution has good plating ability, and the obtained coating layer is smooth and meticulous, and has been used for a long time in production. However, due to the high toxicity of cyanide, the environmental pollution is serious. In recent years, low-cyanide, micro-cyanide and cyanide-free galvanizing solutions have been used, but the quality of galvanizing is not as good as that of cyanide plating.

Therefore, the galvanized layer of our electrogalvanized steel drum has no passivation, dyeing and post-treatment of the oil film as hot-dip galvanized steel. On the other hand, the zinc layer structure is looser and more complete than hot-dip galvanizing. It is galvanized with a non-cyanide bath, so it is more susceptible to oxidation and corrosion in wet or corrosive environments. So how can the galvanized layer of the electroplated zinc drum reach or be close to the aesthetic and corrosion resistance of the hot-dip galvanized steel drum? The usual method is to apply 891 electro-galvanized anti-corrosion coating on the surface of the electro-galvanized steel drum.

891 electro-galvanized anti-corrosion coating is a steel barrel developed on the basis of anti-corrosion advantages of integrated epoxy coating, PVF coating, pure phenolic coating, polyurethane coating, polyester coating and amino alkyd coating. Special coatings. The patent number is ZL 01 2 45930 . 5, which is the only patented product for domestic steel barrel special coatings.

891 electro-galvanized anti-corrosion coating is a two-component room temperature curing coating. Its film formation is achieved by the addition and crosslinking reaction of two components in the coating. The reaction can be carried out at room temperature or under low temperature baking. get on. This reaction is a direct addition reaction in which no by-products are released and the shrinkage is small. Therefore, the coating has excellent compactness, and the dense network structure coating film has excellent corrosion resistance.

After coating a thin layer of transparent 891 electro-galvanized anti-corrosion coating on the outer surface of the electro-galvanized steel drum, not only the overall anti-oxidation and anti-corrosion performance of the electro-galvanized steel drum is greatly enhanced, but also its appearance is bright and gorgeous. After doing it very quickly, it has been widely used in many barrel factories. Since only a very thin layer is applied, the unit consumption is very low. It is said that several barrel factories are manually painted with a wool stylus. One kilogram of 891 paint can be applied to twenty 200 liter galvanized steel drums. The cost of the combination is only about one dollar per steel drum.

In addition to the outer coating for electroplated zinc steel drums, there are many barrel mills that use 891 paint for the inner coating of open steel drums, making the interior of the steel drum bright and hygienic (with a white napkin and no trace of waste) It is also anti-corrosive and rust-proof. Such steel drums will not rust in the wet and corrosive environment for several years.

Some barrel mills spray a layer of 891 paint on the painted surface of the painted steel drum, which increases the brightness of the outer paint and greatly enhances the corrosion resistance of the outer paint, making the steel drum corroded. In the gas environment, and when the steel drum is filled with corrosive chemical liquid and splashed onto the surface of the steel drum paint, the paint on the surface of the steel drum is still intact.

In addition, the barrel mill incorporates aluminum silver paste into the 891 paint, and mixes it and sprays it on the weld of the hot-dip galvanized barrel. It is said that the effect is also good, and the color and vision are similar to those of the hot-dip galvanized layer. However, the aluminum paste needs to be diluted in cyclohexanone before it can be incorporated into the 891 coating. It should be noted here that it is aluminum silver paste (slurry) rather than aluminum powder.

Since the 891 electro-galvanized anti-corrosive coating can be cured without baking, it is especially suitable for some barrel-making plants that do not have baking equipment. For units that already have baking equipment, energy can also be saved and costs can be reduced.

When using 891 electro-galvanized anti-corrosion coating, mix the two components in a ratio of 1:1 and mix well. Leave it for 10 minutes before use (so that the bubbles generated during agitation run away). This kind of paint is applied to the inner surface of the steel drum and is fully exposed to oxygen. It is dried at normal temperature (25 ° C) for 10 minutes, and it is dry for half an hour. The higher the temperature, the faster the curing, but when the temperature is lower than 10 ° C The curing time is greatly extended. Therefore, many barrel factories apply 891 paint in steel drums and then cure them at 180 °C ~ 220 °C for 15 minutes, which is said to be very effective. Units without baking equipment If X-891 coating is applied to closed steel drums, it is best to seal the bottom cover after a few days depending on the temperature, so that it can be fully cured in full contact with oxygen.

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