Pickling & Passivation Procedure

[ashfaqanwer]

Can anybody please share a typical pickling and passivation procedure for stainless steels.

[ben]

Ashfaq,
Where do you want to undertake this procedure? On-site or in yard?
The complete cycle of this whole procedure is as follows:
1. Cleaning
2. Degreasing
3. Rinsing with water
4. Pickling
5. Rinsing again with water
6. Passivation
7. Rinsing with water again
8. Drying

[mechcolor]

Pickling
(a) Pickling of austenitic stainless steels especially having carbon content lower than 0.03% or stabilized (316L, 304L, 316Ti, 321) may either be done chemically in a bath or with local pickling pastes. Paste pickling is carried out by using a paint brush and the staying time shall be 30 minutes at least and removal shall be done with clean steel brush.

(b) Pickling of austenitic stainless steels with carbon content higher than 0.03% or not stabilized (304, 316) and other ferritic, martensitic, duplex stainless steels shall be done with mechanical deoxidization process.This is done with stainless steel wire brushes with resistance to corrosion at least equal to the steel being processed.

Passivation
(a) For 400 series Stainless Steel with Cr less than 16%
For matt surfaces: Use bath of nitric acid 20-50% by volume and at 40-50 deg C temperature for 20-30 minutes
For bright surfaces: Bath first with nitric acid as stated above and then with 2-6 gr by weight sodium bichromate at 20-35 deg C temperature for one hour

(b) For For 200, 300 and 400 series Stainless Steel with Cr equal or above 16%
For matt surfaces: Use bath of nitric acid 20-50% by volume and at 50-70 deg C temperature for 20-30 minutes
For bright surfaces: Same as in (a) above

[ashfaqanwer]

Thanks Ben & Mechcolor for your prompt reply.
Just want to know about three more things:
1. Cleaning practices
2. Degreasing agents
3. Picking agents, temperatures and time for bathing process

[mechcolor]

Cleaning can be done with any of three methods:
1. High pressure water jet
2. Compressed air blowing
3. Mechanical or manual brushing
Selection is based upon the extent of cleaning required

Degreasing agents vary from normal liquid thinner to emulsions or alkaline. Emulsions are good for removal of liquid penetrant testing residuals and fluid marks from cutting operation. Alkaline agents are good for removing solid contaminants and thinner is used where contamination is not of a big scale.

Pickling is done in two phases for bath process. Nitric acid and hydroflouric acids are used in 10-15% and 0.5-1.5% by volume respectively at 20-40 deg C temperature for 5-30 minutes. These percentages are kept high for SS containing Cr equal or greater than 16%.

Hope this answers your queries.

[ashfaqanwer]

That really helped.
Thanks.

[octane]

When is it necessary to pickle and passivate?

[mechcolor]

It's the decision to be made in between the manufacturer / contractor and the Client.
But following some conditions ask for the pickling and passivation operation:
1. Formation of heat tints / residual weld oxides
2. Uneven finishing of the surface due to organic contaminants
3. Contaminated water stains, marks of cutting fluids, residuals of different operations like dye penetrant testing etc.

[octane]

Thanks...