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What Does Acid Pickling in Galvanized Square Tubing Refer To?
Square and rectangular tubes, collectively referred to as "rectangular tubes," include both square tubes (with equal side lengths) and rectangular tubes (with unequal side lengths). These tubes are produced through a series of processing steps starting with strip steel. The process involves unpacking the steel strip, flattening it, curling it into a circular shape, welding it to form a round tube, then rolling it into a square or rectangular shape, and finally cutting it to the required length.
Acid Pickling Process:
Acid pickling is a method used to remove oxide films and rust from metal surfaces by immersing the workpiece in a strong acid solution. The fundamental principle involves a chemical reaction between the acid solution and the iron oxides (such as Fe₃O₄, Fe₂O₃, FeO) on the metal surface, resulting in soluble salts.
The acids used for pickling include sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, chromic acid, hydrofluoric acid, and mixed acids, with sulfuric acid and hydrochloric acid being the most common. Industrial-grade sulfuric acid typically contains 75% to 97% H₂SO₄ by mass. Steel components are generally pickled in a 10% to 20% (by volume) sulfuric acid solution at 40°C. When the iron concentration in the solution exceeds 80 g/L or the ferrous sulfate concentration exceeds 215 g/L, the pickling solution should be replaced. Room temperature pickling with 20% to 80% (by volume) hydrochloric acid solution is also common, as it minimizes over-corrosion and hydrogen embrittlement.
Due to the significant corrosive effects of acids on metals, corrosion inhibitors are added to the pickling solution. These inhibitors reduce the amount of acid used, enhance pickling efficiency, and prolong the service life of thermal equipment by mitigating metal corrosion and preventing hydrogen diffusion into the metal, which causes hydrogen embrittlement.
After acid pickling, the metal surface becomes silver-white and passivated, which enhances its corrosion resistance. The process typically involves managing acid discharge, requiring enterprises to conduct environmental impact assessments and engage qualified companies for waste acid recycling. Waste acid treatment methods in the steel industry include neutralization, iron sulfate precipitation, organic solvent extraction, dialysis, and ion exchange.
Applications:
Galvanized square tubes are widely used in various fields, including:
These tubes' versatility and corrosion resistance make them essential in many industrial and commercial applications.