Hot dipping process

Another familiar commercial method is the immersion or hot-dipping process. The article to be coated is immersed in a molten metal bath. Usually httie else is done to change the properties of the coating, which adheres to the surface upon removal of the article from the bath. For a successful coating, an alloying action must take place between the components to some extent. Zinc and tin coatings are appHed to sheet steel by hot-dipping. 

At first sight, the answer would seem to be to increase the thickness of the zinc layer. This is not easily done, however, because the hot dipping process used for galvanising is not sufficiently adjustable and electroplating the zinc onto the steel sheet increases the production cost considerably. Painting the sheet (for example, with a bituminous paint) helps to reduce the loss of zinc considerably, but at the same time should vastly decrease the area available for the cathodic protection of the steel and if a scratch penetrates both the paint and the zinc, the exposed steel may corrode through much more quickly than before. 

Mechanical assemblies may be made rigid by hot-dip processing since all crevices should be filled with the coating metal, and this will also provide a seal where required. 

It is essential that the effect of heating of the basis metal during the hot-dipping process be borne in mind at the design stage, especially with aluminising when temperatures of about 700°C are reached and also with... 

Hot-dipped tin coatings are difficult to apply outside the thickness range of 8-38 jtm and hot-dipped zinc coatings do not normally greatly exceed 50 m in thickness. Hollow sections and excessively large articles may be impractical to handle by the hot-dipping process and very thin sections may be subject to much distortion. 

Steel sheets are aluminized by a hot-dip process similar to galvanizing. The principal applications for such a product are furnaces and ovens, automobile mufflers, and other equipment requiring heat and corrosion resistance. When a sheet which has been coated with aluminum by a hot-dip process is exposed to a temperature over l,000°F (538aC). the aluminum forms an iron-aluminum alloy which is heat- and corrosion-resistant. 

In the hot dip process, the sheets or other articles to be coated must be free from scale, dirt, grease, etc., and arc usually prepared by pickling and washing before immersion in molten zinc commercially known as speller. Articles fabricated from iron and steel sheets and wire are hand-dipped. Sheets and wire are handled mechanically. 

Tin—lead coatings (10—60 wt % tin) can be applied by hot-dipping or electrode position to steel and copper fabricated articles and sheet. A special product is teme plate used for roofing and flashings, automobile fuel tanks and fittings, air filters, mufflers, and general uses such as covers, lids, drawers, cabinets, consoles for instruments, and for radio and television equipment. Teme plate is low carbon steel, coated by a hot-dip process with an alloy of tin and lead, commonly about 7—25 wt % tin, remainder lead. Electroplating is another possibility. 

The hot dip process is used to coat wire baskets, brackets etc., and depends on preheating the object to be coated (100-120 °C) and dipping it into the plastisol where the PVC gels around the object. Cold dip and spray coating is used on fabrics, such as lined gloves, or metal, such as underbody sealant. These are subsequently gelled in an oven. The use of an infrared system to cure PVC sealant compound has also been introduced (343). 

Galvanized Steel. This steei is coated with zinc to protect against corrosion. The coating is usuaiiy done by a hot dip process. 

All diffusion coatings, irrespective of the coating technique, are characterized by the existence of an alloy layer in many cases, this comprises several discrete layers [42]. The extent of alloying is dependent on the coating process, and the precise nature of this layer is discussed below. In addition, part of the coating may be of the unalloyed coating metal (e.g., as obtained by conventional hot-dip processes). The properties of the coating will therefore be a complex function of the individual properties of the discrete layers and their interfaces. 

Zinc is a white metal, fairly hard and brittle, becoming very brittle when heated above 200 °C. When exposed to moist air it forms a thin, white, protective layer of basic salts for this reason it is applied to iron by a hot dip process, called galvanizing. Zinc is attacked by dilute acids (more slowly when the metal is pure) and alkalis. It is rarely used alone as a material of construction, but is used for galvanizing, as a constituent of alloys such as brass and bronze, and for building materials. 

Izeki, T., et al. (1992). Development of a new hot dip process for Zn-5% A1 alloy coatings on steel structures and evaluation of its corrosion resistivity. Proc. 1st Asian Pacific Gen. Galv. Cong., pp. 158-166. 

Terneplate is a tin-lead alloy coated sheet steel, and is produced either by hot dipping or electrodeposition. The hot dipping process with a chloride flux is used to produce most temeplates. The coating layer, whose electrode potential is more noble than that of the steel substrate, contains 8-16% Sn. Since the electrode potential of the coating layer is more noble than the steel substrate, it is necessary to build a uniform and dense alloy layer (FeSn2) in order to form a pinhole free deposit. 

The base metal for tin-plating is low carbon steel. The term tin-plate is reserved for a low carbon steel strip coated on both sides with a thin layer of tin. In recent years, the hot dipping process for tin plating has been replaced by electroplating because of the improved film properties obtained by the latter process. 

Cold-rolled sheet steel coated on both sides with a lead-tin alloy by a continuous hot-dip process. 

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