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Pickling & Passivation, start-up of a pickling bath

{ 12 februari, 2020 }

This procedure describes the start-up of a pickling bath for stainless steel parts.  For this set-up an initial order was placed of 2000 Ltrs. Pickling Batch Concentrate.

Start-up of the bath

 

a bath was used with a content of 4000 Ltrs.  The bath was filled as follows :

  • 2000 Ltrs tap water
  • 2000 Ltrs Pickling Bath Concentrate

Temperature of the bath :

  • After filling the bath : Temp of 23°C
  • After 1hour : Temp of 20°C
  • After 12hrs : Temp of 17°C
  • After 36hrs : Temp of 13°C

Pickling Time of Stainless Steel 304 & 316 is between 60 and 90 minutes at room temperature of around 20°C.

Per 10°C less the pickling time will double up
Per 10°C more the pickling time will be half

For this reason it is important to monitor the temperature of the bath on a regular base.

Next to the improvised Pickling bath also a small rinsing bath was used unt lthe installation of the special ordered rinsing bath.  This rinsing bath is to have a first simple rinsing once the parts come out of the pickling bath.

At start the rinsing bath, filled with normal tap water, has a neutral pH (7) as normal tap water is used.

The parts contain residue of the Pickling product and the pH will decrease.

Once a pH of 3,5 to 3 is reached, it is recommended to renew the rinsing bath with fresh tap water

Submerging the parts

 

The parts need to be completely clean before they can go in the bath.  The parts are immersed in the bath and it is important to make sure that the pieces don’t touch the bottom of the bath or each other.

The parts have to be in the bath min. 60 to 90 minutes at a temperature of around 20°C.

The parts already have a very nice surface after rinsing in the bath.
However on the weld we can clearly see the black remaining.

This is iron which came to the surface after pickling, which adheres aggressively to the weld surface.

Rinsing with high pressure

 

After the rinsing in the bath it is very important to rinse thoroughly with high pressure (min. 150 bar).

There are 2 reasons:

  • To rinse/neutralize the acidity completely of the pickling bath
  • To remove all remaining iron from the surface

FINAL RESULT WHEN PERFORMING THE PICKLING PROCESS CORRECT

BEFORE

AFTER

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