Rusty streaks on 304 ss

[corrosionman]

Hi. We constructed a large item from perforated 304 stainless sheet. The work was done with close checks on all materials used but in a workshop where there was also lots of mild steel fabrication going on at same time and with lots of grinding and disc cutting. Two days after the 304 item was finished and outside in the rain we see lots of rusty streaks. Someone suggests they are from the mild steel grinding dust. We now doing checks to see if it is possible but, more important, is there anything with which we can wash the material to clean it up - - not a polished surface - - just to get rid of unsightly marks. Thanks for listening. C M

 

[unclesyd]

I will say that with 95% certainty that the CS grinding dust is the major contributor to the rust stain problem on the SS. You need to separate the two operations.
Another possibility is that as you worked the SS you left an active surface or very fine SS particles that can quickly rust.

As for cleaning there are several methods available.

Can you come back with quantities of SS needing cleaning.

 

[corrosionman]

unclsyd, I always value your speedy replies.The "item" is a 26 ft dia sphere built up from 300 five sided plates, each flanged at the edge and bolted together. Each plate is approx 24 ins x 30 ins. fron perforated thin 304 ss
It was dismantled and palleted at the docks ready for shipment when staining was noticed. Imagine sprinkling a surface with fine pale brown paint then passing a brush over the surface to create lines of stain.
Approx 50 plates are stained. they have got a stainless steel "honeycomb" 2 inches thick affixed to inside surface by brazing.
We thought of shotblast cleaning using talc powder or even wallnut shells ( a comon remedie for soft metal polishing. Tomorrow we experiment to re-create the problem but how did it happen. You say maybe we left very fine SS particles. But can ss go rusty? Possible area to be cleaned is approx 250 squ ft. Hope this makes sense to somebody > C M

 

[EdStainless]

There are a few methods used.
The most common is to passivate with an acid. This may not remove existing stains and you might need something more aggressive. Before I suggest acid pickling there are some other options.
I have seen sucess with blasting. You must use virgin media. Both dry ice and plastic blast media work well.
You should follow blasting with a citric acid passivation to really clean the surface.

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Rust never sleeps
Neither should your protection

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

 

[unclesyd]

The honeycomb and brazing present a few problems using the easier methods. Blasting can quickly damage honeycomb, personal experience.

More soft blasting media, corn cobs.

If you have a very experienced blaster you can get him to try dust blasting, making sure to instructing him in exactly what you are trying to accomplish. Tell him to remove the rust not improve the finish. No matter the media the blaster needs to use a brush technique.
No matter what, the component being blasted needs to be isolated from the clean ones.

My usual approach on cleaning rust stains off SS is to use some propreitery H3PO4 cleaner full strength applied with a fine 3M pad. Let sit a minute or two and rinse or wipe with a sponge. On large surfaces I spray the cleaner on full strength and let sit 30 minutes and rinse.

There are also strong pickling and passivating pastes
for problems such as yours. These paste are very aggressive.

I've never had much success using Citric Acid cold but it can't hurt.

You need to isolate your CS shop from the SS shop. Along with this no common tools like grinders and brushes. Every thing use on SS should be SS if possible.

Very fine SS particles, especially from grinding operations or wear will very easily rust.

What type preparation are you using for protecting the components during shipping?

http://www.greenproducts.com/specifications.htm

http://www.black-walnuts.com/page.asp?category=nutshellproducts&subcategory=productlisting

 

[JR97]

corrosionman,

we had a similar condition on stainless steel shells a few years ago. What worked for us was to scrub the plates with deck wash solution and remove by power washing. We did free iron checks after the cleaning (wet cloth sealed against surface overnight) to verify removal.

 

[unclesyd]

Be careful with deck wash solutions as some contain Oxalic Acid and the Environmental and Health People get terribly upset when they see the label.

Oxalic Acid was banned from our site due to health concerns after 25 years of use cleaning SS heat exchangers.

 

[corrosionman]

I appreciate all your replies. Our construction is 300 "plates" ( panels) from thin perforated Stainless Steel (type 304 ) brazed to a 40 mm thick S/S honeycomb The brazing is done by a nickel powder spread on the joint line, they are pressed together and heated. Somewhere along the way a contaminated cutting disc has been used to trim to size hence the Carbon Steel " dust" and rusty streaks as it gets wet.
Tests show that concentrated Phos Acid cleans it up within minutes and we will then hot pressure wash to remove residual acid. The Client is worried about acid damage to the brazed joints and will do "peel tests" after we demonstrate our rust removal .
Your comments will be much appreciated before this job goes half way round the world to end user. CM

 

[unclesyd]

Try to get one of the proprietary industrial cleaners as it will have inhibitors that will help prevent any base metal attacks. The standard for years was Oakite 33, but any local supplier should have an equivalent.
With your exposure time there should be no problem with the brazing metal.

 

[arto]

Look at ASTM A967 "Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts"

http://www.derustit.com/

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

Check NiDI too:

http://www.nickelinstitute.org/index.cfm?la_id=1&search=true&keywords=passivation&ci_id=6579

 

[corrosionman]

We have just about solved the problem and would like to tell latest update. We obtained 10 gallons of concentrated Phos. acid ( 95% ) and immersed one test panel for 30 seconds - -all rust still there, and even after 30 minutes, and even after brushing the immersed surface with stiff brush. When we lifted it out the honeycomb was full of acid 95% has viscosity of thick engine oil. It is also very heavy, SG = 1.7 Acting on a hunch we diluted the acid right down to a ratio of acid: water 1:7 When immersed again the rusty streaks vanished in 30 seconds and the honeycomb almost self drained when lifted out. We hot water pressure jet cleaned to remove traces and all looks good now. Client was happy and all now remaining to do is the peel tests to ensure the brazed bond is still OK. It was amazing that the "dirt " from the panel just "fell out" leaving a perfect image of its shape on the tank floor.
Gentlemen, Many thanks for your interest in our problem. You helped me solve it. CM

 

[unclesyd]

You should chastise me for not reminding you that when using concentrated Phosphoric Acid from the jug you need to make the dilution usually, 3::1 acid::water. Most of the proprietary cleaners are at this concentration, though some seem to work better with a little dilution.


Appreciate the feedback on a job well done.