All grinding wheel and wire brushes need to be made for stainless steel. They should be kept separate and not used on carbon steel. Grinding wheel and wire brushes that have been used on carbon steel should never be used on stainless steel as this will contaminate the surface.
You should blast the surfacd with a nonmetallic abrasive and then passiviate the surface with a nitric acid solution to clean the carbon steel from the surface. Then you could perform a feroxyl test the surface for free iron.
The contamination is usually due to poor shop practices but it is easy to contaminate the plate surface. Plate clamps, chain hooks and even grinding dust from nearby operations can cause problems. That why we found it is usually easier to blast and passivate the surface after completion to insure no free iron. Good luck.
Another point is that machinery such as plate rolls, bending brakes, shears, and ironworkers are made with carbon steel components. So with that, and grabs and chains, and with grinding dust from nearby, it's nearly impossible to totally prevent carbon steel contamination under most circumstances. Even your used wooden dunnage and concrete floors get imbedded with carbon steel.
I'm inclined to agree with R Johnson. I think it's easier to assume you will have contamination and then take care of it after fabrication and welding is complete.
CHGuilford
is stainless spray paint an option? it's quite expensive, I have seen some food grade equipment with painted areas where there has been something welded on & the stainless has turned blue, it covers nicely.
I agree with the previously posted comments. There are a lot of potential sources for iron contamination.
The Nickel Development Institute (NiDI) has at least two very good free publications regarding iron contamination. #10004 "Fabrication and Post Fabrication Cleanup of Stainless Steels" and #11007, "Guidelines for the welded fabrication of nickel-containing stainless steels for corrosion resistant services". Go to www.nidi.org. These papers deal with detection and removal of the contamination as well.
Regards.
By -
Date 08-30-2001 18:09
You already have a number of good replies regarding the contamination, so I will not again talk about this. There are however other possibilities.
We must keep in mind that under conditions where there is no Oxygen present, your stainless steel can corrode. This can typically happen under coatings, in the presence of reducing environments and even where differential aeration cells can form. (e.g. Where water is allowed to "stand" on the surface of the S/Steel.)
Where is you component rusting? Is it generally all over the surface, in small isolated spots, under operating conditions, next to welds, under standing water or in crevices?
Regards
Niekie Jooste
Sasolburg South Africa
The free iron that is left on the material could probably be removed with abrasives and followed up with a pickling and passivating operation that would remove the iron (Most of it) and return the chromium oxide layer to the SS.
The pickling and passivating is done with acid and instructions for this operation can probably be found on the internet or from some industry organization such as ASM.
The stainless steel prefix implies that the 304 stainless is more resistant to rusting not rust proof.
Certain household cleaners will cause stainless steel to rust, especially in the event of low oxygen content atmospheres (you need the oxygen to build up a new protective Chromium oxide layer)
Have you used any solvent degreasers for cleaning the product surface, if so it may have reacted with the material, and procedures should be undertaken to remove residue of these hash chenicals.
The rust may also be carbon steel particles staining the stainless' surface after oxidizing thus giving the impression of rusting.
