At the very beginning of the processing process, in order to get rid of white plaque, the method of drying the galvanized surface is applied. In the presence of white plaque on the surface, the corrosion process will continue to operate and will subsequently damage the base steel and the galvanized surface itself.
After drying the galvanized surface, further processing will depend on the area of white plaque. After a little drying, the product itself must be left in the open. Since white plaque turns into zinc, this is due to the reaction to carbon dioxide, which is contained in the air. With a limited flow of air, plaque is removed using a stiff wire or nylon brush.
How to prevent the formation of zinc plaque
A heavy and medium degree white coating may be present on the galvanized coating, which must be removed to avoid the formation of zinc plaque, since this significantly affects the life of the product itself, the galvanized coating and steel. The plaque is removed with a wire or nylon stiff brush, after which it is necessary to treat with a special washing solution. After that, the zinc thickness of the coating is measured, which allows you to accurately establish the strength of the coating on a steel product. In some cases, heavy and moderate plaque can turn black, which means that galvanizing of steel should be repeated.
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Features of galvanized metal welding
The need to remove zinc coatings from metals arises, as a rule, during welding. Every welder knows that it is problematic to weld galvanized surfaces and even the thinnest layer of zinc can greatly degrade the strength properties of the weld. An admixture of zinc entering the weld pool leads to pore formation and the appearance of crystallization cracks in the weld, that is, it does not allow a dense and uniform weld to be obtained. In addition, during welding, zinc begins to evaporate under the influence of a high-temperature arc, and its toxic fumes can cause severe poisoning and even suffocation of the welder, especially when working in a poorly ventilated area.
Methods for removing zinc from welded surfaces
Mechanical. Unwanted zinc coatings on metal surfaces can be removed with an abrasive scrubbing wheel. Sometimes it’s enough to strip the part to a metallic sheen with a brush with a hard wire bristle (cord brush).
Thermal. An effective way to remove a metal coating is to burn zinc with a high-temperature flame, for example, using a gas torch. In this case, zinc toxicity should be remembered and all safety measures taken.
Chemical. When welding critical structures, preliminary cleaning of surfaces from zinc is best done in a weak solution of hydrochloric acid (5%). Zinc is more active than steel, so it effectively reacts with acid and is easily removed from hardware. To prevent acid corrosion of steel, 1% urotropine can be added to the solution. The part to be welded is immersed in a room temperature diluted acid bath. The process of purification (etching) lasts several minutes. When it is completed, and this will be seen by the attenuation of the hiss, the metal is treated with alkali to neutralize the acid, for example, with a solution of soda (1g / 50ml), then washed with water and dried until the moisture evaporates completely.
Many welders for chemical etching of welded galvanized parts use a proven and effective tool - spent acid battery electrolyte, which is nothing but an aqueous solution of sulfuric acid. Just a few minutes in such a bath and from the zinc coating on the metal there will be no trace.