Pickling and Passivating of Welded Carbon Steel Pipes

A. I've personally never heard of passivating carbon steel, Winky. It seems like there would be a lot less call for zinc plating, galvanizing, phosphatizing, painting and powder coating if such a thing could be done; I don't think it can. You might look up Corten® steel though, or see if ASTM A569 gives any specifics on the pickling.

A. I have some experience for carbon steel passivation using Sodium Nitrate [affil links] solution. This will create a thin film of oxide which can prevent the carbon steel from corrosion. However I am not sure how long/effective this technique is.

A. Pickling and passivation of carbon steel is relatively uncommon when compared to stainless steel, however I have been involved with large scale pickling and passivation of carbon steel piping for installation at a plant in Mozambique. The pickling and passivation was carried out in Durban (South Africa) and I was in charge of the entire project. The finished product was then shipped up to Maputo by road.

Regards,

Q. Hi Hisham; hi Mike. We understand what pickling is (the removal of scale), why it is done, and how it is done. But I am not understanding what you mean by "passivation" of mild steel, i.e., what it is, why it is done, or how it is done. What property is claimed for passivated mild steel whereby you could say "this item is passivated, and that one is not"? Thanks.

Regards,

A. Hi, B. A problem here is that "passivation" doesn't have an exact meaning. The most common meaning applies to stainless steel, where nitric acid or citric acid alternatives serve to chromium enrich the surface and form a chrome oxide skin. So, when people talk about passivation of carbon steel, what exactly do they mean -- they certainly don't mean chromium enrichment.

Some people call chromate conversion coating of zinc plating "passivation", and others call phosphatizing of steel "passivation". Phosphatizing or a similar treatment does offers some degree of rust prevention.

Can you provide context? Texts such as the Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon] provide fairly detailed information about pickling and passivation. Good luck.

Regards,

A. Pickling & passivation is a term specified for the stainless steel and this means the removal of the corrupted protective layer of chromium oxide and regeneration of the new protective layer of the chromium oxide and pickling being done by treating the surface by a mixture of HNO3 and HF with different % depending on the stainless steel grade (304, 316, 316L, 316H, 316Ti, duplex, etc.) and passivation is done spontaneously by oxygen in air and can be accelerated up by using an oxidizing agent like HNO3 (10%).

But carbon steel is different and is called chemical cleaning, and this is the removal of the scales and foulants, and passivation is to stabilize the free iron remaining after pickling phase to convert it to magnetite -- and this is done by oxidizing like sodium nitrite at pH 9.5 (use ammonia [on eBay or Amazon] to raise the pH) or hydrogen peroxide.

A. I recently 'passivated' carbon steel pipe, its intended use is for a thermal fluid. Fabrication/welding done before chemical cleaning. But firstly had to remove light surface rust by using 10% hydrochloric acid. HCl did a great job and removed other surface contaminants. Use of HCl is not for the faint hearted! I then applied a 1% sodium nitrite solution with a high pressure spray. Finish appearance was a dull grey color. It's important to note that rust will reappear quickly if left unprotected.

A. Hello Bhie.
Paul's posting was from 8 years ago, so he may not be reachable or responsive. But the way I read it the sodium nitrite was a separate spray.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Mohammed,
Carbon steel cannot be passivated in the usual sense of the word that is used for stainless steel or a chromate conversion coating. For the closest thing I've ever seen to a claim of passivating stainless steel, see the responses from Trevor Tipton and Dan Brewer at
www.finishing.com/118/22.shtml

adv.

Hi Muhammed,
Please contact me directly. We have a mild (acid free) method of passivating carbon steel. The solution should be heated slightly. The process takes about 20 minutes. The process works fine at our customers facilities for cleaning, passivating large inner surfaces of the carbon steel tanks.

A. Mohammed,
Yes, in general, any treatment process where you are talking about maintaining contact between the metal surface and a chemical solution for a period of time can be performed by immersion, circulating through a pipe system, continuous spraying in a tank (aka CIP), or using a thickened/gel version of the solution.

There may be exceptions but I expect very few.

A. Hi Sara. I may be misunderstanding you, but you cannot passivate assemblies consisting of stainless steel and carbon steel. The purpose of the passivation of stainless steel is to dissolve any carbon steel on them, and the nitric acid used for passivation of stainless steel will react violently upon steel parts.

Regards,

A. Sara,
The only way you could possibly do it is with a passivation paste type product, applying it as much as possible only to the stainless and keeping it off the carbon steel.

A. Hi Arfat. Although you're certainly welcome to ask for personalized help, "pickling paste" is a commodity item offered by numerous vendors. If you Google that term you will immediately see everything from brief 6-step instructions, to youtube video demonstrations, to multi-page technical articles. Good luck!

Regards,

A. Arfat,
It's worth it occasionally to review our terms here.

Pickling is another term for acid descaling, as opposed to mechanical descaling like sand blasting or wire wheel grinding. The main objective is to remove surface scale or discolorations that look bad, or to give the surface some kind of uniform finish or etched look.

Passivation (for stainless) refers to stripping away as much surface iron as possible to increase the corrosion resistance.

Note that a traditional nitric-HF pickling formula imparts a fair bit of passivation, and a nitric acid passivation formula tends to do a decent amount of pickling. On the other hand, citric acid is much safer to handle but only passivates with no pickling. If the surface appearance is already what you want it to be, then that is all you need.

Relevant standards here are ASTM A380 and ASTM A967.

A. Hi Arfat,

adv.
Contrary to some other vendors who sell pickling paste, we deliver know-how of the pickling and passivation paste composition. You will be able to produce "summer" paste variation, "winter" paste variation, passivation only paste, hard oxide after welding paste, "green" paste (environmentally friendly), paste for 316 ss, paste for 400-series, for carbon steel and even paste for titanium. We will also teach you on how to use pickling pastes for cleaning large tanks or vessels.

You will also receive various solutions know-how for pickling and passivation by submerging, solutions that "fool-proof" that do not impact actual metal strength and surface image.

It is justifiable to have your own mixing shop if you deal with large volume pickling during metal parts fabrication, or your shop bids for regular cleaning of large tanks for bio, pharmaceutical or food or beverage industry.

Bama Technologies from Turbigo, Italy, acquired our technologies more than 10 years ago. Cheap home-made pickling pastes and solutions helped them to beat their competitors and to establish treatment plants in Morocco, and other locations in Europe.

Contact us for more information.

A. We did passivation of carbon steel pipe for the transport of liquid oxygen through the line.

  Thanks Dan. Personally, my problem remains the same for almost 20 years now: although people talk in general terms about nitrates and boilers and so on, what exactly is the surface condition that defines when carbon steel has become 'passivated'. What can we analyze or measure? How do we test that it has been done 'properly'?

When we talk about passivation of stainless steel, we refer to specifications ASTM F86, AMS2700,ASTM A967 and others, and it is clear what steps are involved, exactly what the process does, and how to measure whether it has been done successfully or not. Even the galvanic series shows a clear & measurable distinction between passivated and non-passivated stainless steel surfaces.

Similar specifications & observations might exist for 'passivation of carbon steel' but I don't personally know anything of them and would appreciate if somebody could help us out with specification numbers, tests, measurements, and things like that :-)

Regards,

A. Refer to AS 1627.5 (2003)
for the Australian Standard on
METAL FINISHING
Preparation & Pretreatment of Surfaces
PART 5 - Pickling