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What means are required to perform acid pickling of SS vessels? Which standards need to be followed? When is it necessary?

Pickling process is mostly done on Stainless Steel material after welding when oxidation may have occurred resulting in depletion of chromium (& formation of chromium oxide). Those heat affected areas are also called heat tints, formed after welding process. Hot rolling and other fabrication operations can also result in heat tinted layers which can easily be attacked by corrosive fluids when in service.
Regarding pickling of SS vessels, this process may be undertaken at manufacturer's workshop if any vessel undergo high welding operations and heat tints are formed. In service SS vessels if repaired by welding are not pickled but if repair job includes extensive welding which may form heat tints, may be spray pickling is one option.
Normally, pickling is done on SS piping (intended to carry highly corrosive solutions like carbamate, urea etc.) after complete fabrication and erection of lines. Circulation pickling is adopted, this method involves circulating the pickling mixture through the system.
ASTM A380 states some standard practices for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems.

Hope this answers your question.

Ashfaq,
Which chemical / reagent is used for pickling? what is the basic mechanism involved in this process? any particular temperature requirements for pickling? I suppose that the main intention is to create a localized chromium oxide layer for protection against corrosion. Any specific practical example please?
Regards - Qaisar

Mixtures of nitric and hydrofluoric acids are used for pickling of stainless steels. For in-situ pickling of smaller areas, electromechanical cleaning is one method and the other is direct brushing on pastes or gel.
The basic mechanism is to remove a thin layer of metal (heat tinted layers) from the surface of stainless steels.
Temperature of the mixture may range from 30 to 70 deg C.
Practical examples include immersion pickling or spray pickling of larger SS parts at manufacturer's place and brush-on gel pickling of smaller parts by specialized people. Very common one is circulation pickling after erection of heavy SS piping.

Hope this answers your question.

Thank you buddy.
Now the things are more clear. Last but not the least, are the acids used for pickling, dangerous to chromium oxide layer of stainless steel? what are the parameters / units used to represent the degree / quality of pickling?
- Qaisar

Very professional.

Ashfaq,
How we can identify the requirement of pickling. Is it necesary to have heat tints during welding or it is a general process done after welding every SS vessel / piping?

Also it would be a great learning if some one can share how to gauge the effectiveness of Pickling process being done on a particular weld joint? Can it be checked by performing Ferrite Testing?

Dear Nabeel,
Heat tints are always visible if formed on SS after welding with high heat input and poor shielding. The extent of affected area would determine whether pickling is required or not. Not every pipe/vessel is required to be pickled after welding operation, usually good welding conditions, effective shielding / purging of stainless steel and good welding technique & welders do not allow heat tints to be formed.
Ferrite testing would not give you any idea about how effective was your pickling process. Its the originality of stainless steel material which would be the true representative of effective pickling. Steel shall appear stainless from its appearance and there shall not be any sign of heat tints like in the figure attached.

Hope this will help.

Dear Qaisar,
The acids used for pickling process do not impair the chemical composition of stainless steels, they remove the metal formed in the form of heat tints which can be a potential localized area for corrosion attack.
Degree of effective pickling depends on some factors mainly the cleanliness of area to be pickled. Circulation pickling cannot be quantitaitvely measured, only input parameters are maintained like composition of mixture (acids to be used), flow and time of circulation. Brush on gel type pickling plus immersion pickling may have certain units defined by industrial specialists.