Passivation of SS Vessel

[SnTMan]

Hello All.

I am on a project where the customer is VERY concerned about both carbon contamination and general internal cleanliness of a large, 304 SS vessel.

For the first vessel (prior to my joining the company) the process was to clean and clean and clean some more mostly using mechanical means. Very time consuming.

Unfortunately, the shop itself is not one of the cleaner places I've seen, lots of large CS equipment being fabricated all over the place.

I'm wondering if it might be cost effective to not take a lot of special measures during fabrication and just passivate after fab is complete.

Anyone out there got any experience with passivating large vessels?

Any experience with citric acid passivate?

Thanks in advance,

Mike

 

[metengr]

Mike;
I can discuss one aspect of your post and that is fabrication. It would not be cost effective nor practical to simply rely on a pickle/passivation operation as a substitute in following good fabrication practices for austenitic stainless steels.

There is too much at stake during fabrication that can result in corrosion problems after the vessel is placed into service. The fabrication shop needs to segregate fabrication of stainless steel vessels to avoid surface contamination, period.

 

[kenvlach]

Agree with metengr.
Welding is probably the most contaminant-susceptible stage of fabrication. A really bad contaminant is zinc (embrittles SS), which can come from galvanized equipment, other welding, paint, etc. Of course, passivation can't clean within the metal.

Somewhere, unclesyd posted a response which detailed a regulatory need for segregating SS fabriction.

However, if you are certain that the welds are clean, I recommend the sequence: mechanical polishing, electropolishing and passivation for the vessel's interior. This is SOP for the pharmaceutical industry.

 

[unclesyd]

The thread mentioned by kenvlach is thread794-138607.

Adding a little to the referenced thread. We and the largest chemical company in the US both used the same fab shop. The big chemical company set the conditions in shop in regards to segregation and cleanliness in the fabrication and machining area. They allowed absolutely no comingling of CS and SS unless the vessel was clad and then there was a viscous set of specific rules used during fabrication. This company has the strictest requirements for segregation of metals and cleanliness during fabrication of anyone in the business.


You have to have a very vigilant QC person on the floor as mechanics will take shortcuts about the time the owner's inspector shows up.

addenda:
One of my pet peeves on SS vessels is when some idiot with a side grinder does dido’s all over the plate will prepping or planishing welds. Just recently saw a 5,000 gal reactor that had a callout for smooth welds with the minimum grinding possible. Everything was OK until the last couple of hours when the two mechanics decided to grind local high school football slogans allover the plate. Needless to say the owner was none to happy with the shop, who took a hit on the contract.

 

[MJCronin]

SnT....

I agree with what has been stated above..

You may want to consider a shop that only fabricates stainless tanks and vessels....??

-MJC

 

[SnTMan]

Good Morning All, and thanks.

Perhaps a little clarification is in order. First of all, I work for the fabricator, no sending this out.

I did not mean to imply we would not follow proper fab procedures. For example, we have lined our plate rolls with carpet. (This might make an interesting thread of its own.)

We plan to erect a "clean room" around the unit, basically a frame covered with plastic film. However, there goes the crane access. Every time we open up to bring in materials, etc. we will get dirt inside.

The thinking is to fab the outside complete, blast the unit and erect the "clean room", fab the internals, hydro, install the buundle, etc. Blasting is fine for the outside, but I am reluctant to, say, blast the inside due to the difficulty of removing all the debris afterward.

When I said "special measures" in my post I really meant extraordinary measures. I want to avoid repeatedly mechanically cleaning the unit.

So, to my original question, anybody have experience with passivating large vessels?

 

[arto]

see ASTM A-967
also:
Tuthill & Avery, “Specifying Stainless Steel Surface Treatments,” Advanced Materials Processes, Vol. 142 (6), 1992

“Cleaning & Descaling Stainless Steels” Designer Handbook #9001 American Iron & Steel Institute 1982

[available fron NiDI

http://www.nickelinstitute.org/index.cfm/ci_id/45/la_id/1.htm

]
citric:

http://www.stellarsolutions.net/products/citacid.htm

http://www.cartech.com/machining_zone/citric_pass.html

also:

http://www.derustit.com/

 

[kenvlach]

Re large vessels:

For electropolish & passivation jobs too large for our tanks (EP 6.5 feet deep, but passivation can be done horizontally), if vessel could fit through the 14 ft rollup doors, we would process by pumping solution into the tank, spray rinsing, liquids removal via suction hose if no bottom drain.

Abrasive blasting inside a vessel is cumbersome, operator needs respirator & lighting, etc. Much better to use an SS wire wheel or flapper wheel to clean welds, a small power sander to level.
You can electropolish the entire insides (if pharmaceutical), but this requires several days for precise rigging of the copper cathodes, maintaining a 1” standoff with PP insulators, extra electrical, and then removal of cathodes.

Much simpler & quicker to brush electropolish the (mechanically cleaned and polished) weld lines. This can even be done at the customer. E.g., a pharmaceutical would add 6 ft height to a cylindrical vessel (previously EPed & passivated) by welding in a 6 ft tall ring. Just necessary to do the weld lines & HAZs. (Note: the inside bottom was covered, & the worker wore booties over soft-soled shoes.) Spray rinse the EP with warm water & then DI water. A bit of passivation by wiping nitric acid passivation solution over the EPed area (mostly to ensure no phosphate or sulfate left from EP), then remove worker & anything extraneous, spray rinse, passivation spray*, rinse, DI spray rinse until conductivity meter shows << 1 ppm TDS. The customer would then clean & passivate vessel & associated piping in place.
*mostly to make sure all traces of the sticky EP solution were gone.

Since you are dealing with 304 SS, the simplest passivation is 10 vol% nitric acid at ambient T (70^oF). If you plan on doing more often and have room for a secondary containment area, get a plastic storage tank for the passivation solution. In this case, make the solution about 35 vol% nitric to meet QQ-P-35C & newer specs.

Re citric acid passivation: The difficulty is it needs heating to about 120^oF, plus you need to add a surfactant if not buying a proprietary product (e.g., CitriSurf

http://www.stellarsolutions.net/),

plus it is more expensive chemical than nitric acid. Benefit is safety.

Hope this helps,
Ken

 

[kenvlach]

There is an ASME Bioprocessing Equipment (BPE) Standard which is geared toward biotechnology and pharmaceutical processes. Maybe overdoing it bit for food service; but hey, people are just as important as bugs!
Plus, you can expand your customer base.

This CD from ASME should be useful:
"Metallic Materials of Construction for Hygienic Servic: Fabrication, Finishing & Corrosion.

Description:
This course will address both the theoretical and practical issues associated with the fabrication, finishing, and corrosion of metallic materials used in the construction of biotechnology and pharmaceutical process tubing/piping systems. Many critical details of material selection and fabrication techniques will be addressed by industry experts. The focus will be the drainability and cleanability requirements imposed by the ASME Bioprocessing Equipment (BPE) Standard. Topics such as the effect of melting practice on inclusion removal during electropolishing, a comparison of the surface oxides produced by various passivation technologies, and the response of discolored welds to standardized corrosion tests will be discussed. What you will learn: Why some materials are more difficult to electropolish than others. Exactly what a surface finish measurement of 20 Ra means. How to prevent internal heat-affected zone (HAZ) discoloration during welding of stainless steel tubing. That discoloration from welding degrades the corrosion resistance of the HAZ of 316L stainless steel. Under what conditions European materials can be used to build tubing/piping systems to the requirements of the ASME BPE Standard. How electropolishing changes the surface chemistry and topography of metallic materials. Technologies which are available to passivate the surface of stainless steels. The general content being developed for the new addition to the ASME BPE Standard, "Metallic Materials of Construction." This course is geared toward design engineers, project managers, process engineers, manufacturing engineers and technicians, and fabricators with 1 to 5 years of experience in material selection, project management, fabrication, inspection, etc.

List Price: $300.00"

 

[HEC]

Just a quick comment, all are dealing with pasivation. The issue in particular where there is a mix of mild steel and stainless steel fabrication is with mild steel contamination. Most of the methods have been mentioned above, dust, sparks from other activities, or using cutting tools previously used on mild steel where teh contamination can come from. Pasivation is the treatment used to ensure a pasive (Chrome oxide) layer is formed on the SS. This will happen without chemical but a longer time! The issue of mild steel contamination or chrome depletion (HAZ) can only be addressed by removal of the mild steel or iron rich areas. This is achieved by either mechanical means grinding, not polishing as this will tend to "wipe" the contamination and will appear later. Altermativly chemical pickling is required, I have used 1% hydrofluric acid and 5%nitric brew for this.


Mark Hutton

 

[EdStainless]

A nitric passivation will remove minor surface Fe contamination from your SS. I would prefer to see a system of rules that is managable, not pie in the sky.
Minimize contact with steel wherever possible (carpeting).
Being manic about weld clenliness. Brush EP to clean welds may be better than allowing grinding.
Reasonable dust control (plastic sheet room), esp when you are grinding on CS near by.
A final passivation with Nitric would be good practice.

What are you asked to do to demonstrate that the surface is free from free Fe? There are standard tests, but the two easiest are to cover the area of interest with diaper material (from a roll) and wet it with DI water. Let it sit over night and see if there is any rust streaking. My favorite is to use Head & Shoulders shampoo (the white version). Wipe the area with a nice layer and then mist with DI water to keep it damp. Any blue patches indicate free Fe.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm