As we know, the price of different types of sheets, including acid washing sheets, depends on various factors, including the country and brand of the manufacturer, the method of production and its dimensions. Acid washing sheet is one of the most widely used alloy sheets that is produced from hot rolled sheet or black sheet. It is common to use this product in cases where the appearance and quality of the surface are important. The surface of the black sheet, which has reached the desired thickness by hot rolling, contains grease and contaminants. These fats and contaminants are removed from the sheet with the help of an acid, which is often hydrochloric acid. This process is called acid washing or flaking and the sheet obtained from this process is called acid washing sheet. With this process, surface coals and some of its surface impurities are removed, resulting in a glossy surface. This product is produced in different ways and in different dimensions and thicknesses, which we will discuss in this article.
What is the purpose of hot sheet acid washing?
The hot or black sheet undergoes high temperatures during the hot rolling process, which makes these products have a surface with oxide shells. The presence of these shells on the surface of the product makes the background, color and thickness of the sheet heterogeneous. In such cases, acid washing on a hot plate can greatly help to descale it and bring the surface to a good quality. Acid washing is done for this purpose for those acid washing sheets that enter the consumer market directly after production. But the acid washing process is done for other purposes as well. The black sheet on which the cold rolling operation is to be performed is also acid washed. The oxide shells formed on the surface of the black sheet are often hard and anti-wear. Therefore, performing cold rolling operations on such a surface will damage the polished surface of cold rolled rollers and will take them out of tolerance and will cause problems in the overall cold rolling process. Even if the oxides on the surface of the black sheet have little adhesion and are loose, it will lead to double the difficulty in the cold rolling process. For this reason, acid washing is required for all sheets before cold rolling.
The type of acid used in the acid washing process
The acid leaching process requires a dilute mineral acid to separate the oxides and contaminants from the metal surface from the base metal and dissolve it. In this process, acids such as phosphoric, hydrofluoric, nitric, hydrochloric and sulfuric acids are used alone or in various combinations. Nitric acid washing is performed at room temperature and therefore heating costs will be eliminated in acid washing with it. During the acid washing process, hydrogen penetration can lead to defects in the part, but using nitric acid will not be a problem. Iron salts can also form during the process, and using this type of acid for acid washing can prevent it. When using this acid, foam forms if its temperature exceeds the ambient temperature. Another disadvantage of this acid is its high corrosive power, which can cause damage to equipment and also has a very high disposal and recovery cost. In the past, sulfuric acid was the main material used in the acid washing process and has been used in acid washing of carbon alloys. The effective concentration of this acid varies with temperature changes. That is, with increasing temperature, low-concentration acid has an effect equal to high-concentration acid at low temperature. This acid is recyclable and reusable, and its use can reduce costs; However, the possibility of hydrogen penetrating the part and causing defects in it is high when working with this acid. Today, hydrochloric acid is mostly used to produce this product. As far as the most common use of this type of acid in the world belongs to acid washing units.
Advantages of using hydrochloric acid over sulfuric acid
Using hydrochloric acid is more economical and much cheaper than sulfuric acid.
This acid has a so-called higher acid leaching rate and dissolves more oxide, so a cleaner surface will be obtained by using it. Another advantage of this feature of hydrochloric acid is the saving of acid consumption in acid wash tanks.
Hydrochloric acid attacks less on the surface of the steel as the base metal. This advantage will result in less metal wasting.
This acid, in reaction with iron oxide, produces iron salts, but the amount of precipitated salt is small and does not cause harm.
This acid dissolves less hydrogen and reduces the likelihood of defects in the part.
The operating temperature for hydrochloric acid is lower than for sulfuric acid. The optimum operating temperature for hydrochloric acid is 70 to 80 ° C. While this temperature range for sulfuric acid is 100 to 150 degrees Celsius.
The only disadvantage of hydrochloric acid over sulfuric acid is that it is volatile. For this reason, hydrochloric acid containers are isolated to prevent it from escaping. Acid-washed sheets produce smoke at high temperatures, polluting the surrounding air and damaging the respiratory system. It is also very strong and can damage equipment.