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Up Topic Welding Industry / Technical Discussions / pickling bath
- - By sims78 Date 07-30-2007 16:48 Edited 07-30-2007 17:12
Hi there,

I'm new in this forum and i have a question for a work about a pickling bath for a stainless steel duplex (1.4362)! we know that the solutions of nitric (HNO3) and hydrofluoric (HF) acids are used but we have a doubt the % of each that must be used! i searched the web and i found some information but isn't enough explicit to begin with the process! Also because of the type of steel and the conditions of the bath (namely temperature) we want to try it before!

Thanks!
Parent - - By G.S.Crisi (****) Date 07-30-2007 19:50
Take a look at the answer I've posted in 10/18/2002 to the question "Etching stainless steels" that "kam" posted in 10/9/2002 on the Technical section of this Forum.
It says exactly how to prepare the etching solution.
Giovanni S.Crisi
Sao Paulo - Brazil
Parent - - By Mwccwi (***) Date 07-30-2007 22:30
ok, now I'm confused he said pickleing / you said etching- aren't the y different Pickeling = cleaning  etching = corroding?
Parent - - By chuck meadows (***) Date 07-30-2007 22:46
Pickling is a nitric/hydrofluoric/sulphuric acid solution used to remove the chromium depleted oxide layer generated by the heat from the welding process. Pickling paste will also remove and restore the steel when pitting corrosion is on the surface of the steel. It is a surface solution, not an intergranular solution. There are pickling baths, pickling pastes, and pickling spray gels. Pickling is not a cleaning process, as it will not work through grease, dirt, etc. The surface must be clean for pickling to work properly.
Parent - - By DaveBoyer (*****) Date 07-31-2007 04:00
Chuck, does the use of these pickling solutions leave the material in a passive state, or is there another step that must be done for that?
Parent - By chuck meadows (***) Date 07-31-2007 15:53
Dave,
  Once the S/S has been pickled it is now considered to be in the passive state. Before pickling is done, the surface should be clean and free of any contaminants for the pickling solution to work properly.
Parent - - By Mwccwi (***) Date 07-31-2007 11:12
Mr. chuck meadows,
Here's what lead me to associate pickling to cleaning - read the stentance begining with carbon steel

http://en.wikipedia.org/wiki/Pickling_(metal)
Parent - - By chuck meadows (***) Date 07-31-2007 15:55
The pickling, (nitric, hydrofluoric, and sulfuric acid mixture), is not appropriate for carbon steels, only stainless steels.
Parent - - By G.S.Crisi (****) Date 07-31-2007 19:35
Chuck,
first of all, let me tell you that I'm most happy to see you back in the Forum. Believe me, I've remembered you in my prayers all of these days.
Second, let me desagree with you. Pickling is used also for carbon steel. Example: before putting a turbine generator or a turbo compressor in operation, the lube oil piping system must be pickled to remove the oxyde, rust and debris in the pipes inner surface. This is true also for oil refineries and petrochemical plants, where some piping systems must be pickled before putting them in sevice. 
Back in my days of erector engineer I took part in the erection of a few turbine generators, turbo compressors and one oil refinery and remember very well that the operation was carried out. What I don't remember, though, is the chemical composition of the pickling solution. It consisted in acids to remove rust, solvents to remove oil and grease and inhibitors whose function was to let the acids "eat" the rust and oxyde but prevented them from attacking the carbon steel. But I don't remember the percentages.
Giovanni S. Crisi
   
Parent - By chuck meadows (***) Date 07-31-2007 20:02
Giovanni,
  Thank you, my friend, for your prayers. I should have specifically said that the pickling solution formulated for stainless steel is not recommended for carbon steel. Sorry for any confusion. Thanks again..

Chuck
Parent - - By G.S.Crisi (****) Date 07-31-2007 19:16
You're right, Mw, I don't know what I was thinking of when I answered your posting. As we say in Brazil: "confundi as bolas" (I was confused about the balls, an expression originated from snooker games). Sims78, do you use that expression also in Portugal?
Giovanni S. Crisi
 
Parent - By sims78 Date 08-01-2007 07:44
Hi Crisi,

Sure we have that expression in here! :)
Parent - - By sims78 Date 07-31-2007 07:57
Crisi,
I'm from Portugal! :)
I read the post your refer but the SS that i'm using is the duplex SAF2304 (ASTM A240) that have 23% Cr and 4% of Ni! The % of the acids are different depending of the SS used, is true?
I read some articles about the composition of the acids and the most of them doesn't refer to this type SS and if it does, only gives some indications, not the %! Because we want to pickle big pieces the best way to do is with the bath! The acids must be pure, right? For others SS with the Cr < than 16% they gave a value of % of 15-25% of nitric and 1-8% of hydrofluoric!

Chuck,
About your post: pickling will turn the SS into anti-corrosion state (passivated), but i didn't knew that you had to clean it before the process! We used a TIG machine with an electrode to weld the pieces. In this case what is the type of cleaning product used? And if we don't clean it before the pickling process is not efficient? I read other post that you answer and i see that you're a major expert in the pickling process! Hope you can help me with this! :)
Parent - - By chuck meadows (***) Date 07-31-2007 16:06
When I say that the piece should be clean, I mean free from any contamination. Pickling is  normally used to remove the "heat tint" which is actually a chromium-depleted-oxide surface layer that is a result of the heat from the welding process.

The Avesta pickling solution is consistent whether you are pickling 2304 Duplex or 316L stainless steel. It is designed to remove that oxide layer regardless of the grade of stainless steel. Pickling will remove the oxide layer, and working with the oxidation in the atmosphere, will reform or "self-heal" the surface and reform the chromium layer to the surface that is necessary for maximum corrosion resistance. You are right, it is a combination of nitric and hydrofluoric acid with a small amount of sulphuric acid.

Pickling bath is normally used for small pieces that are dipped in a vat (or tub) and then dipped in a vat of water to remove the acid. Larger pieces are normally done by a spray gel process. But, whatever works for your particular scenario.   
Parent - - By sims78 Date 07-31-2007 17:07
I understand now what you are saying! About the pickling solution, we want to try to make the bath and see how it works in the pieces! Do you know approximately the % of these components?
In a 1st stage we will make our own bath, but in the future, if the production of the pieces goes well, sure we will use solutions already prepared!

Sure the pieces can be dipped in the bath! :) we want a solution that could pickle more than one piece at the time!
Parent - - By chuck meadows (***) Date 07-31-2007 19:23
The pickling solution is very carefully formulated, so be extremely careful if you plan to mix your own. You can easily cause more damage than good. The exact formulation is proprietary, so be careful.
Parent - - By Stephan (***) Date 07-31-2007 20:41
Chuck,

sims78 may allow me to make a short sidestep "off" his interesting topic here, but it is a heartfelt pleasure to read your great posts again!

Best regards to you,
Stephan
Parent - By chuck meadows (***) Date 07-31-2007 21:39
Stephan,
  Thank you very much, my friend. It is certainly good to be feeling like joining back in on the Forum. Thank you..

Chuck
Parent - - By sims78 Date 08-01-2007 07:57
Yes, i know Chuck! At the time in not income-producing to buy the bath, because, as i said, we are just starting with this new product in stainless steel and we don't know the market will react (strong competition). My research is to set an approximate value! I understand that the formulation is proprietary...
I have a pickling handbook from and there they have a lot of information about all the pickling methods, even the % for general SS! I see is very risky to try it ourselfs! If someone did a pickling bath and could share the results that will be a great help!
Parent - By chuck meadows (***) Date 08-01-2007 13:46
Good luck.
Up Topic Welding Industry / Technical Discussions / pickling bath

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