Galvanised steel sheets are coated with a layer of metallic zinc to prevent corrosion on the surface of the steel and to extend its service life.
① Hot-dip galvanised steel. The thin steel plate is dipped into a molten zinc bath, so that a thin steel plate with a layer of zinc adheres to the surface. At present, the main use of continuous galvanizing process production, that is, after the rolling of the steel plate continuously immersed in the zinc-containing liquid galvanizing tank, made of galvanized steel.
② Alloy galvanised steel sheet. This steel sheet is also manufactured using the hot dipping method, but is heated to approximately 500°C immediately after discharge from the tank to form an alloyed film of zinc and iron. This type of galvanised steel has good coating adhesion and weldability.
(iii) Electro-galvanised steel sheet. Galvanised steel sheets produced by the electroplating method have good processing properties. However, the coating is thinner and less resistant to corrosion than hot-dipped galvanised sheets.
④ Single-sided and double-sided differential galvanised steel sheets. Single-sided galvanised steel sheet is a product that is galvanised on one side only. It has better adaptability than double-sided galvanised sheet in terms of welding, painting, rust-proof treatment and processing. To overcome the disadvantage of one side not being galvanised, there is another layer of galvanised sheet on the other side, i.e. double-sided differential galvanised sheet.
⑤ Alloy and composite galvanised sheet steel. It is made from zinc and other metals such as aluminium, lead and zinc and is used to manufacture alloy or even composite coated steel sheets. This type of steel sheet has not only excellent rust protection properties but also good coating properties.
In addition to the five types mentioned above, there are also coloured galvanised steel sheets, printed and coated galvanised steel sheets, PVC laminated galvanised steel sheets, etc., but the most commonly used at present is still hot-dipped galvanised steel sheets.
(1) Throughout the service life of galvanised sheeting, the first corrosion to occur is the oxidation of the surface galvanised layer, resulting in white rust. After a longer period of time, the white rust on the surface will further react with carbon dioxide and other impurity gases in the moist air to form black spots. When the galvanised sheet is used for a long period of time and the corrosion of the galvanised layer becomes more severe, the steel matrix loses the sacrificial corrosion protection of zinc and begins to oxidise to form red rust. Once the steel matrix begins to oxidise, the corrosion rate becomes very rapid and the galvanised sheet ends its service life.
(2) In addition, there are two special cases of corrosion in galvanised sheet metal, known as blackening. One is that after a period of use, the matt part of the galvanised sheet will blacken due to the influence of elements such as lead in the galvanised layer. The blackened galvanised layer still has a protective effect on the steel substrate. It only has a certain effect on the life span. This situation has improved considerably after the current stopped using lead and switched to antimony to promote the formation of flash. Another situation is the transport of galvanised sheet. Due to the low coiling tension and the looseness between the layers of steel strip, the parts of the coil in contact with the skid rub against each other and cause oxidation. After wiping off the white oxide, galvanised black spots are produced on the surface of the galvanised sheet. This condition can damage the passivation film and make the galvanised layer thinner, significantly reducing the service life of the galvanised sheet and also affecting its appearance.
(3) Galvanised sheet has excellent corrosion resistance in dry or non-polluted environments. However, in contaminated environments the service life will be greatly reduced and it must be pre-coated into coloured sheets prior to use.
(4) In general, corrosion of galvanised sheet is not caused by uniform corrosion of all the galvanised layers on the surface, but rather the coating corrodes first where the coating is poorly bonded to the substrate, resulting in severe localised corrosion and loss of serviceability. From this point of view, the adhesion of the galvanised layer, especially the overall adhesion, is more important than the thickness of the galvanised layer. If the local adhesion of the galvanised layer is poor, even if the galvanised layer is thicker, it will start to rust due to poor adhesion. This is similar to the principle of a bucket, which is something that the galvanising production technician must understand.
The biggest difference between galvanised sheet and galvanised aluminium sheet is the difference in the coating. Galvanised sheet has a uniformly distributed layer of zinc material on its surface, which acts as an anodic protection for the base material, i.e. an alternative corrosion protection for the zinc material. In addition to the base material used, the internal base material can only be damaged if the zinc corrodes completely.
The surface coating of aluminised zinc sheets consists of 55% aluminium, 43.5% zinc and a small amount of other elements. Under the microscope, the surface of the aluminised zinc coating has a honeycomb structure. The “honeycomb” made of aluminium contains zinc. In this case, although the aluminium-zinc coating also acts as an anodic protection, on the one hand the zinc content is reduced and on the other hand the zinc material is encapsulated in the aluminium and is not easily electrolysed, so the anodic protection is greatly reduced. As a result, the cut edges will rust very quickly once the Alu-Zinc sheet has been electroplated, at which point the cut edges are essentially lost and protected. For this reason, aluminium and zinc plated sheets should be cut as little as possible. After cutting, the edges should be protected with rust-proof paint or zinc-rich paint to ensure the longevity of the sheet. Extension
Galvanised sheets have a more attractive surface than galvanised sheets and are more expensive than galvanised sheets. Small lace is more expensive than large lace.
Galvanised sheet steel has a uniquely smooth, flat and ornate surface. The base colour is silvery white and has excellent corrosion resistance. The normal service life of Aluzinc sheeting is up to 25a. It has good heat resistance and can be used at temperatures up to 315°C. Good adhesion between the coating and the paint film. It has good processing properties. It can be punched, cut, welded, etc. The surface conductivity is very good. Good
The composition of the coating consists of 55% aluminium, 43.4% zinc and 1.6% silicon by weight respectively. The production process of galvanised steel is very similar to that of galvanised steel, which is a continuous melt coating process. The colour coated product has excellent adhesion and flexibility.
Galvanised aluminium sheet has twice the heat reflectivity of galvanised steel. We can use it as an insulating material. Alu-Zn sheet also has good heat resistance, withstanding temperatures above 300 degrees Celsius, very similar to the high temperature oxidation resistance of aluminised steel, and is commonly used in chimneys, ovens, lamps and fluorescent light covers. As 55% AL-Zn is less dense than Zn, it has the same weight and thickness as gold plating.
(1) There are two types of packaging: galvanised steel sheets cut to length and galvanised steel sheets in rolls. They are generally packed in tinplate, lined with moisture-proof paper, and tied with an iron waist on the outside. Tie firmly to prevent the inner window galvanised steel sheets from rubbing against each other
(2) The specifications, types, codes and relevant product implementation standards (described below) indicate the thickness, length, overall width and permissible tolerances of the galvanised steel sheets strongly recommended for use. In addition, the total width and length of the sheet and the total width of the roll can be specified on request.
(3) Appearance of the surface layer: Galvanised steel sheets have different surface conditions during the coating process due to different treatment processes, such as plain flower, fine flower, flat flower, no zinc flower, pickled phosphate on the surface layer, etc. Galvanised steel sheets cut to length and galvanised coils should not have the disadvantages of dangerous applications (detailed below), but steel coils are allowed to have multiple abnormal parts, such as welding and welding positions.
(4) Hot dip galvanising quantity Hot dip galvanising quantity indicator value: Hot dip galvanising quantity is a reasonable way of indicating the thickness of the zinc layer on galvanised steel sheets. There are two types of double-sided hot-dip galvanising of the same quantity (i.e. equal thickness hot-dip galvanising) and different double-sided hot-dip galvanising (i.e. different thickness hot-dip galvanising). The company’s hot dip galvanising capacity is g/m2.
(5) Physical properties ① Compressive strength test: In general, only galvanised steel sheets used for construction, stretching and deep-drawing can have compressive strength properties. (2) Tensile test: An important new item for considering the properties of sheet steel. However, there is no consistency in the regulations for various galvanised steel sheets around the world. It is generally stipulated that after bending the galvanised steel sheet 180 degrees, the zinc layer on both sides of the surface shall not be removed and the bottom of the sheet shall not be cracked or fractured.
(6) The organic chemical composition of hot-dipped galvanised steel sheets is specified by national standards worldwide. If not required in Japan, it is also required in the UK. Finished product inspection is not normally carried out.
(7) Plate shape considers the merits of plate shape with two index values, namely perpendicularity and long knife curve. There are certain requirements for the larger control values for the perpendicularity and long knife curve of the sheet.
(1) There are many kinds of coils on the market. When selecting, you must first observe the surface of the sheet and choose one with a smooth, even surface and no obvious scratches or particles to avoid problematic sheets.
(2) The panels have a uniform surface with a clear and regular grain. Due to these characteristics, the raw materials used in the boards are of high quality. A confusing and irregular pattern on the surface of a circuit board indicates uneven heating during processing, so do not purchase this product.
When buying, avoid problems such as scratches, particles and uneven distribution of patterns on the surface. Choose a large, regular company for your selection so that you can avoid some quality problems.
(1) Improves the surface flatness of the galvanised sheet. After smoothing, the flatness of the galvanised sheet is improved to a certain extent and the waveform of the galvanised sheet is partially eliminated. At the same time, slag or other uneven spots on the surface can be flattened out, making the surface smooth.
(2) Adjusting the surface roughness of the galvanised sheet. By controlling the surface roughness of the finishing machine’s work rollers, the original uneven roughness of the galvanised sheet surface can be unified to within a certain range. This improves the paintability of the product and the lubricity of the process.
(3) Improving the surface colour of galvanised sheet. For both shiny and non-shiny products, smoothing allows the galvanised sheet to achieve a more uniform glossy surface. For some private companies using large flake products as colour coated sheets, the flake bumps and different gloss levels formed by the large flake crystallisation can be smoothed and unified and the flake will become blurred, facilitating the covering of the flake in the painting process.
(4) Improve the processing performance of galvanised sheet. Through the smoothing process, the yield plateau of the galvanised sheet can disappear or become inconspicuous, thus preventing metal slip lines from appearing in subsequent processing.