In PICKLING PROCESS : DETAIL STUDY , Pickling is term used in rolling process to remove scales and oxide layers present on the surface of hot rolled steel.
Or it is defined as during hot rolling process of steel, oxygen from the atmosphere reacts with the surface of hot rolled steel which cause formation of scaling on the surface . Made up of oxide of iron known as a scaling.
Pickling is the process to remove scaling from the surface of hot rolled steel for further processing.Scales and oxide layers are harder than steel and hence hinder the process of cold rolling of low carbon steel .
Oxide layer forms on steel during manufacturing and hot-working when surface reacts with oxygen in the air. The composition of the formed oxide scale depends on the cooling rate.
Scales on the hot rolled coil is generated at the last step of finishing mill during the cooling of coil.
It is composed of 3 well defined layer:
If the cooling rate is easily soluble then wustite (Feo) is formed on the surface.
If the cooling rate is long then there is formation of magnetite (Fe3O4).
The outer most layer is hematite. it is thin layer (Fe2O3) dissolve slowly.
It is a chemical process either done by sulphuric acid (H2SO4) or hydrochloric acid (HCL).
Pickling acid penetrates the both layer hematite and magnetite through cracks and reacts with
Wustite layer .The reaction of acid with the base metal cause formation of hydrogen bubbles .it looses
Scale from the surface. over pickling may cause mechanical failure and increase in acid consumption.
After loosing from the surface scale dissolve in the acid and it fall down the bath.
Types of pickling acid
- HCL (Hydrochloric acid)
- PHOSPHORIC ACID(H3PO4)
- SULPHURIC ACID (H2SO4)
It is the most used pickling acid. good pickling efficiency can be achieved at low temperatures. Concentration for pickling is 5-15%.inhibitor always used when pickling with HCL to protect the base Metal.
HCL should not be used with stainless steel or zinc material.
Phosphoric acid does not attack base metal aggressively. it is recommended between 15-70% concentration. it provide good surface for paint and protect metal from corrosion.
SULPHURIC ACID (H2SO4)
Recommended concentration for sulphuric acid is between 5-10%.inhibition to protect the
Base metal is recommended. extreme amount of heat is produced in the reaction due to this
Caution should be used while diluting sulphuric acid.
In this pickling process is lower as compared to HCL ACID.
Pickling cost for sulphuric acid is more.
ADDITIONAL PROCESS AND ALTERNATIVES TO PICKLING
- Wire brushing
- Abrasive blasting
- Thermal cleaning
- Mechanical de scaling
The process flow chart of pickling line given below
- Un coiler 1 and un coiler 2:
It is the starting point of pickling process. It is a continuous process due to this 2 Hot rolled coil placed in the un coiler 1 and 2.
- Flattener 1 and 2 :
After un coiler the coil will pass through flattener 1 and 2.
- Shear 1 and 2:
Passing through flattener 1 and 2 if there is any defect in the coil at particular place then shearing of that part of coil takes place. in this cutting of sheet takes place starting or end point.
- Sticher :
After this the head end of new coil is joined with the tail end of previous coil
With the help of stiching. Due to this joining the movement of continuous line remain maintained.
- Notcher :
Coils will go through notcher process.
- Ironing roll:
The other one process is ironing roll.
- Pre wash tank:
The coil will pass through pre wash tank that contains water shower for the purpose of removing dust from the surface of the coil.
- Acid tank:
Acid tank contains respective acid for the removal of scale.
- Rinse water tank:
Rinse water are provided for the removal of acid from the surface.
- Edge trimming:
For more uniform the edge trimming of coil takes place. about 30-60mm
Coil go through oiling process so that it remain preventive for certain amount of time.
At the end the pickled steel is then recoiled.
Types of pickling process
- push and pull type
- semi continuous type
- continuous type