Introduction
Acid pickling is a process used extensively in iron and steel industry for surface treatment and cleaning of metals like iron and steel. In this process, the iron or steel surface is treated with chemical solutions known as pickling solutions or acid pickling solutions to remove rust, scale, oxides and other impurities from the metal surface. This process plays a very important role in preparing the metal surface for further processing like galvanization, enameling, painting etc. Let us explore more about the acid pickling process used in steel rolling mills.
Acid Pickling Process
The acid pickling process consists of following main steps:
Cleaning and degreasing: In the first step, the metal surface is cleaned and degreased by using alkaline solutions or organic solvents to remove oil, grease and other contaminants from the surface. This prepares a clean surface for acid pickling.
Immersion in pickling acid: In this step, the cleaned metal is immersed in tanks containing pickling acids like hydrochloric acid, sulfuric acid etc. concentrated at 10-25% depending on the type of impurities. The pickling acid dissolves and chemically removes rust, scale and other impurities from the metal surface through chemical reactions.
Rinsing: After acid pickling, the metal is thoroughly rinsed with clean water to remove any remaining acid from the surface. This prevents further corrosion of the metal.
Passivation: Some metals like stainless steel and aluminum undergo passivation process after acid pickling roll to form a protective oxide layer on the surface that prevents further corrosion.
Drying: Finally, the pickled metal is dried using hot air blowers to remove moisture from the surface before subjecting it to the next process.
Acid Pickling Solutions
Different types of acids are used for pickling of metals based on the type and composition of impurities on the metal surface. Some commonly used pickling acids are:
Hydrochloric acid: HCl acid is mainly used for pickling of carbon steel and low alloy steel to remove iron oxide scale. It has a high pickling rate.
Sulphuric acid: H2SO4 acid is another general purpose pickling acid which can remove various oxides from steel surface. It is mainly used on stainless steel.
Nitric acid: HNO3 acid is used for pickling of stainless steel to remove chromium oxide and passivate the surface. It provides shine and corrosion resistance to stainless steel.
Acid mixtures: Mixtures of acids are also used for more effective pickling in some cases. For example, pickling of galvanized steel uses a mixture of HCl and H2SO4 acids.
Process control in acid pickling
There are several factors which need strict control in acid pickling process to get best results:
Acid concentration- The concentration of pickling acid is maintained between 10-25% as per the requirement. Higher acid is used for treating heavily scaled metal surfaces.
Temperature- The pickling is generally carried out at room temperature but heating the acid solutions speeds up the pickling process. Excessive temperature should be avoided to prevent hydrogen embrittlement of some metals.
Pickling time- It depends on factors like metal composition, scale thickness, acid concentration etc. Longer exposure to acid removes scales completely but may also attack the base metal.
Agitation- Stirring or jetting air into the pickling tanks ensures even exposure of entire metal surface to acid for uniform attack.
Rinsing- Proper rinsing of pickled metal surface with clean water is important to remove all acid traces completely from the surface.
Drying- Drying the metal after pickling and rinsing prevents further oxidation and corrosion of the pickled clean surface.
Strict Process and effluent control must be practiced to avoid hazardous conditions in pickling.
Uses and Advantages of Acid Pickling
Some key applications and advantages of acid pickling process are:
– Removes rust, scales and impurities from ferrous and non-ferrous metal surfaces before coating, plating, enameling or painting.
– Cleans the metal surface for further fabrication processes like bending, stamping, welding etc. by removing oxides and contaminants.
– Passivates stainless steel and provides shine and corrosion resistance.
– Promotes better adhesion of paints, coatings and plating on metal surfaces due to increased surface roughness.
– Restores original surface of rusty metal and extends its service life by removing rust completely.
– Uniformly treats complex metal profiles due to effective agitation and acid circulation methods.
– Eco-friendly alternate to mechanical scale/rust removal methods such as sand blasting.
– Provides consistently clean surfaces at high production rates in industries.
– Extends equipment life and improves appearance of metal components used in various applications.
In modern steel mills, automated and computer controlled acid pickling lines capable of handling continuous and varying product streams have revolutionized surface cleaning operations through precise process control. Acid pickling ensures highest functional and aesthetic standards are met for steel products before further processing. It represents a key technology enabling consistent production of high quality steel.
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