Dipped the pretreatment articles into zinc bath , after an immersion period the protective zinc coating will be formed on the surfaces of articles. This technology is called hot-dip galvanizing. In fact, hot dip galvanizing is a reaction process between iron and liquid zinc. When articles are dipped into zinc bath at a temperature of 450 Celsius, the liquid zinc reacts with the steel surfaces by inter-diffusion to form Fe-Zn alloy layers. Since galvanizing forms a metallurgical bond between the zinc coating and the underlying steel, the galvanized coating is adherent to the base steel very effectively and compactly, which provides the coating with excellent protection against corrosion and damage. Therefore, hot-dip galvanizing is universally applied for protecting steel product.

Unique advantages

Formation of Fe-Zn alloy layers: in liquid zinc, both of Fe-Zn alloy layers and pure zinc layer are formed on the surface of the base steel. The coating can isolate the steel from the corrosion and oxidization environment.

100% reliability: after chemical pretreatment the steel articles can be contacted thoroughly and react with liquid zinc, thus the even and reliable alloy layers without any defect are formed.

Good adherence: The metallurgic bond between zinc coating and steel article makes the coating compactly adhere to the underlying steel. In addition, the protective coating is ten times thicker than normal electroplated zinc coating. Hence, it has some impact resistance and also has good wear-resistance and corrosion resistance.

Low cost and excellent quality: The zinc-coated articles have smooth surfaces, pleasing appearances, having service life of 15 to 50 years or more. No maintenance and repair will be needed during the period. Safety and reliability are guaranteed. The hot dip galvanizing process is the best way to provide protection for steel articles from corrosion in atmosphere.

Different rates of corrosion under different atmospheres environment

Environment Corrosion rate (range) (g/m²/y) Average corrosion rate (g/m²/y) Life of protection (years) (Average) Heavy industry area 28 - 40 34 16 Urban area 12 - 18 15 36 Coastal area 11 - 14 13 42 Rural area 5 - 12 9 60 Mountain area 3 - 8 6 90 Dry air area 2 - 5 4 135 Note: The life of protection is concluded by the coating weight of 600g/m².

Reason of the deformation

The residual stress would be caused during the process of casting, forging, welding and machining of steel articles, and the residual stress will be released and make the articles deformed when the articles are dipped in hot zinc bath. On the other hand, if the different regions of galvanized article are cooled unevenly during the quenching in water after galvanizing, the deformation will also occur. But it's very difficult for galvanizer to entirely predict and avoid the deformation of the articles in these cases.

Tendency of the deformation

If articles have the same shape, the thinner and longer they are, and the more serious the deformation will be.

If articles have same thickness or length, the deformation will be more serious in the order: I-steel, channel steel, angle steel, and steel plate.

For a tube, deformation in the length will be more serious than that in the diameter; and only a little deformation is caused on the tubes with square and rectangle section.

Prevention methods

According to the reason and tendency of the deformation, the slim and long structure should be avoided as possible, and assembled components should be used as possible at designing process. If necessary, the following methods can be used for reducing deformation: strengthening the structure by supplementary temporary brace; adopting symmetry welding method and tempering for reducing the stress after welding; disconnecting components of the steel structure before HOT-DIP galvanizing; choosing the right hanging position and suitable cooling rate.

The hermetic hollow articles cannot be galvanizing

The first reason is the density of liquid zinc is 6.6g/cm³. If there is air inside articles, the articles will be floating on the surface and cannot be immersed into the liquid zinc; Secondly, when articles are forced to dip into liquid zinc, there will be great danger of explosion during galvanizing if some residual water is introduced in the pretreatment process. The remedy is to make inlets and outlets for air and liquid zinc in some suitable positions. Please refer to figure 1 about construction of inlets and outlets.

Organic compounds wipe by component surface

Some organic compounds, plastics and oils, such as paint, epoxies, mineral oil, animal oil and vegetable oil on the article surfaces, cannot be removed by normal treatment, the special treatments, such as high-temperature alkali solution washing, flame burning, grit blasting, should be used before galvanizing.

Welding flux residues and casting sands should be removed completely

Welding flux residues and casting sands should be removed completely because they have bad effect on the quality of zinc coating, or even prevent the process of galvanizing. So it is necessary to remove these foreign inclusions by polishing, grit blasting, or by using tools like pneumatic chisel.

Allowing enough clearance for moving parts

Allowing enough clearance for moving parts: When a galvanized assembly incorporates moving parts (such as drop handles, shackles, and shafts) a radial clearance of not less 1.6mm must be allowed to ensure full freedom of movement following galvanizing. The size of clearance varies according to different requirements (see the underside images)

The structure of article in which where are large overlapped areas

A clearance will be created when shapes or plates overlap each other. All edges should be seal welded when overlapped areas exist. Otherwise acid will fill in the clearance and then ooze out affter galvanizing, resulting in discolored article and damaged coating. (please see the underside image)

When an overlapped area is unavoidable, make a gas outlet on the surface of one of the overlapped parts (an outlet of 6mm in diameter in each 100cm²-area.), at the same time, seal weld the edges (please see the underside image)

For a long and hollow article

Hermetic structure should not be applied to hollow articles welded by various types of pipes and tubes. Make air vents, inlets and outlets to allow all solutions and melted zinc to drain freely. The desirable position for them is on opposite sides or diagonal sides of the articles. The diameters of air vent should be 25% of the internal diameters or the diagonal lines of the articles. Details of position and size should be set on the agreement of purchaser and galvanizer. (please see the underside image)

Hanging holes and ears for hoisting

If articles cannot be hoisted by tools like chain, hook or basket, hanging holes or ears for hoisting should be prepared in advance. Details should be set on the agreement of buyer and supplier.

There are three major quality indexes for inspection on zinc coating

Weight

Every country has its own standard specifications for the coating weight according to different thickness of articles. For some special articles, such as particularly heavy or light articles, the coating weight requirements may be consulted between buyer and supplier. The coating weight is the weight in each square meter area of the coating (g/m²). Normally the coating thickness is measured by magnetic method (EN ISO 2178). (μm). The conversion formula between coating weight and the thickness is 1g/m² = 0.14μm(micron).

Uniformity

The coating should be uniform and continuous. The uniformity of coating can be tested by dipping the sample in the standard copper sulphate solution. The coating shall withstand several one-minute dips in the copper sulphate solution without the formation of an adherent red spot of metallic copper upon the basis metal. BS729 provides the samples shall be subjected to four successive dips in the solution each lasting 1 minute. Each country may prescribe dip times according to situation.

Adhesion

The coating should be adherent enough in order to prevent the coating from peeling or flaking in regular operation condition. Normally, bending test and hammering test are provided for testing the adherence of coating. Any test for adhesion shall be agreed between purchaser and galvanizer.

The evaluation on the appearance of coating is very important. The coating required should be complete, firm, smooth and has a standard thickness. However, the appearance of coating of fabricated article differs from mechanically wiped products such as galvanized sheet, wire and pipe. The corrosion resistance of coating will not be affected by the gloss, color, and the presence or absence of spangle. The following section is evaluation about the appearance:

Dark gray coating with gray areas or networks

Steel contains certain chemical elements like carbon (C), silicon (Si), phosphorus (P), manganese (Mn) and so on. With increasing Si, P content in steel the Fe-Zn reaction rate increases and the Fe-Zn alloy layers can grow up to the surface of the coating. Galvanizing the steel containing high Si, the accelerated Fe-Zn reaction results in the formation of the coating with matte gray patches or networks. The dark-gray coating has less pleasing appearance, but usually it is thicker than bright coating and is corrosion resistance is never reduced.

Rough coating

Normally rough coating is formed due to the uneven growth rate of Fe-Zn alloy layers. The distribution of Si on the surface of steel is not uniform, causing various thicknesses of Fe-Zn alloy layers. Furthermore, if the surfaces of steel are not smooth, a rough coating is created after galvanizing. However, the rough coating will not affect the usage of products. If buyers have special requirement on the roughness of coating, they should consult with galvanizing experts and provide materials that have uniform chemical compositions and smooth surface.

Lumpiness and runs: When liquid zinc draining back into zinc bath is delayed due to the shape and size of articles, redundant liquid zinc accumulates on the article, lumpiness and runs will be formed. The additional zinc, though wasteful, is clearly not detrimental except in those instances where a smooth finish is essential. If buyers have special requirements, they should discuss with suppliers to accept improved design and treatment method for galvanizing.

Dross protrusions: Dross protrusions are formed due due to zinc dross particles (Fe-Zn alloy) which floats in the zinc bath and adheres to the surface of article during galvanizing. Because dross has a corrosion rate similar to zinc's, it has little effect on the normal life of the coating. A few and dispersed dross protrusions do not affect the usability of zinc coating. Yet excessive and over large dross protrusions will seriously destroy the completeness and appearances of the coatings.

Wet storage strain (white rust)

White rust is mainly the complex compound of zinc oxide and zinc hydroxides. It occurs when the galvanized products are exposed to a humid or moist environment without access to freely circulating air. Galvanizers are not responsible for this problem unless a large amount of white rust had already occurs before the products leave the factory. If the products are exposed in rain and unventilated environment during the transport and storage, white rust is most likely to form. If the thickness of the coated layer still meets the required standard after removing the white rust, the coating function will be not affected, the products will be still qualified. One way to avoid white rust is to stack the products in a dry and well-ventilated environment during the storage and transport, and avoid the products to overlap each other. Covers should be placed on the top of the articles; the other way is to passivate the articles after galvanizing.

Black spots (uncoated areas)

Inadequate pretreatment of the steel surface is the major cause of uncoated areas. However some common causes of black spots, such as welding flux residues do not removed clearly; rolling defects in steel (laminations, laps, folds, and noon-metallic impurities rolled in to the steel surface); cannot be controlled by galvanizer. Therefore each country has a relevant standard in which the size of the uncoated area permitted to repair and the materials permitted for repairing the uncoated area are provided. General speaking, if black spot is less than 3mm in width and 40mm² in area, it may be renovated by the use of good quality zinc-rich paint or by zinc thermal spraying according to the relevant regulation.