Rusted stainless steel castings 3

[joeswoes]

I have 304 stainless steel castings that are developing rust. They have already been pickled (how and in what, I don't know). Can anyone guide me to the corrective action ? I've heard of passivating, sandblasting, electropolishing, and at this point, I don't have a clue as to the most cost-effective solution. Also, directing me to anyone that does this corrective operation is also appreciated.

 

[mcguire]

The observation of red rust on a stainless steel like 304 should be interpreted as an indication of the presence of iron contamination on the surface, unless some more complicated and unlikely hypothesis can be demonstrated.
The resolution is to remove the iron by pickling it off; some call it passivation. Use nitric acid, phosphoric acid, citric acid, etc. Specific instructions can be found at websites like stainlesscenter.com.

 

[arunmrao]

The red stains can be due to grinding dust embedded on the surface or due to the from shot blasting. Please clean the surface by sand blasting and possibly pickle it to remove the outer skin layer. This will prevent any further rust formation.

 

[mcguire]

I agree with arunmrao. There are a lot of places to pick up iron contamination in a foundry. Shot blast, grinding dust, iron oxide. It's very difficult to keep stainless iron-free if non-stainless ferrous castings are also produced.

 

[Astroboy]

Has it been welded? You might be seeing a heat effected zone near a weld.

 

[grayseal]

joeswoes,

This may be too late but there may be another reason the castings are rusting. If the chemistry is not controlled or final heat treatment is not correct, an excessive amount of ferrite may be present in the casting.

grayseal

 

[mcguire]

The ferrite phase is also stainless and will not rust on its own. For red rust to be present there MUST be iron contamination.

 

[majordud]

All of the above are correct. Free Fe is necessary for rust on a 304 casting. Blasting, machining fluids, poor pickling, etc can cause this. A quick test to determine whether or not free Fe is present is the Copper Sulfate test. If free Fe is present, the Copper quickly plates onto the surface, replacing the Fe. Quick & visual. Most 304 castings will have about 12-15% ferrite, so they will be slightly magnetic. Can you measure or determine degree of magnetism, or chemistry? In other words, are you sure its 304?

 

[badashp]

Joeswoes,
Have you tried shotblasting with Stainless steel shots or Grit after heattreatment and before passivation?
I think that might help.

 

[majordud]

Caution with this last one. If in fact your castings are contaminated with free Iron, then the only thing blasting with stainless shot will do, is contaminate the stainless shot with the free Iron. Then, not only will the casting rust, the shot will rust, and anything subsequently blasted with the same shot will be similarly contaminated. You must prevent, or remove the free Iron. Use the Copper Sulfate test to be sure. Also, contact
Dan Englebert
Technical Manager
Imagineering Enterprises, Inc.
South Bend, Indiana
or their web site for more information about pickling/passivation.

 

[badashp]

Interesting observation Majordud but as i said last time will not Stainless Grit mixed with shot or better still grit only make any difference? Also are your observations true for all grades of Stainless steel or for 304 specifically ??

 

[majordud]

It does not matter what the base metal is. Could be Nickel, Cobalt, 316 SS or Aluminum, or any other non-rusting material. If you put free, metallic Fe (which can come from many sources, such as machining fluids, blasting shot or grit that have previously been in contact with carbon steel, low alloy steel, or similar)on it, the Fe will rust. Period. You must chemically remove the Fe, or be smart with your procedures & prevent it from happening. That is the nature, & purpose of Passivation (which includes pickling).
In short, control your processes, or add cost.

 

[kenvlach]

joeswoes,
Are you still there? You have been given lots of good advice (although even SS grit blasting is dubious).

My questions:
1) What is the environment & application?
2) How long did it take for the rust to appear?

In some environments, regular washing is necessary to remove dirt, salt and air pollution fallout in general.
The Coastal Corrosion Technical Bulletin is helpful in both the initial treatment and the maintenance of SS to avoid rusting.

http://www.assda.asn.au/Teastain.pdf

 

[bruv]

An alternative to the copper sulphate test can be found in ASTM A380, Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems.

Section 7 (Inspection after Cleaning) gives a couple of tests for detection of iron contamination. We have used the Ferroxyl Test very succesfully on titanium alloys.

Bruv