Passivation/Pickling vs. Mechanical Cleaning of 2205 s.s.

By bhoheisel Date 02-20-2007 15:38

We have a project with a 2205 stainless steel tank and the question of pickling and passivating to remove heat tint and iron contamination arose. The tank is fabricated and in place and the tank is used in a limestone slurry environment. I was wondering if any one has data or recommendations/concerns about mechanical cleaning vs. pickling/passivation or combination of both to improve corrosion resistance and remove scale and iron contamination pits.

Thanks,

By chuck meadows (***)

Date 02-21-2007 00:27

Pickling will do the most thorough job. Mechanical means will not necessarily remove all the oxides. Sometimes, depending on the mechanical means employed, it will actually either smear the oxides or can possibly embed them even further in the grain structure. Mechanical means would be OK if it is followed by a final pickling process.

Chuck

By gyadon

Date 02-21-2007 13:09

Been there, done that. As QA/QC CWI on this project I did everything I could to get the craft and supervision to clean as they go. Keep the iron contamination, spatter and heat tint to a minium. Well as everyone knows the CWI does not know anything. Well the end of the project came and guess who was right. This project was mechanically cleaned by alot of tiger paws and and blasting. Before everyone starts in on me I know and recommended Passivation but I can only go so far. The problem I see with passivation is the size anyone have any input on that? If I ever do another one of these I will carry a big stick.
Thanks Chuck for I have learn much from reading your post as well as other.
Gary

By G.S.Crisi (****)

Date 02-21-2007 22:22

bho,
pickling and passivating are different things: by pickling you clean the metal surface to "as new" (or almost "as new") condition. By passivating you stabilize the metal surface chemically, so it won't oxidize nor rust in contact with atmosphere.
Chuck Meadows please correct me if I'm wrong.
Giovanni S. Crisi
Sao Paulo - Brazil

By chuck meadows (***)

Date 02-22-2007 04:32

Giovanni,
As you said, pickling is the act of removing the oxides that restrict the oxidation in the atmosphere from working with the stainless steel chemistry to form the chromium protective surface that is necessary to prevent rusting. After pickling, the surface is now considered as passive. Actually, "passivation" is known as decontamination. The passivation liquid is applied to the steel as the last step before shipping. It "reinforces" the passive surface by removing any smudges, contaminants, free iron, etc. from the surface. This is explained in the Avesta Weldiing Manual. I agree with what you said, but possibly I just said it in a different way.

Chuck

By bhoheisel Date 03-12-2007 15:29

thanks for the update