AISI 304 - Glass Blasting - Colour change 2

[jesper1]

I use glass blasted stainless steel (AISI 304) in the production.
Unfortunately it sometimes turns dark (instead of stay grey/silver).

After glass blasting the surface is cleaned (alkaline cleaning), the supplier claims it is passivated.

Does anybody know the mechanisme of this colour change – and how to avoid colour change ?

 

[unclesyd]

A lot of questions.

Is the surface dark gray before the alkaline cleaning?
What is the composition of the alkaline bath, time,temperature and rinsing?
Is there any enhanced drying of the part?
Is there a passivation procedure after cleaning?
Have you checked the profile of the of the affected parts?
Do you recycle the glass beads?
So you blast Al in the same cabinet?
Is this confined to one set of parts or is it random?

I don't think that SS is passivated by the alkaline bath. Passivation requires a specific acid bath or long exposure to air.
There is one possibility that crops up quite often and that is the air for the blasting process is contaminated and the contamination isn't removed by the cleaning process. You should see this after the blasting.

 

[EdStainless]

What are they using for passivation?
take some samples and treat them in warm 20% Nitric acid and see what the surface looks like.
My hunch is that they have imbeded some material.

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Plymouth Tube

 

[jesper1]

I appreciate your answers

Our products are made by subsuppliers - I do not know everything about the processes.

The surface is light grey after glassblasting

The cleaning is performed at 55 °C, 5 minutes, pH about 9,0, the cleaning bath is also supposed to give some corrosion protection (carbon-steel – not relevant in this production)

After cleaning the parts are dried in an oven

No passivation after cleaning (passivation = dip into acid bath) – I will do some after-passivation on parts and test them (test: climate chamber, cyclic with condensed water part of the time or ??)

Checked the profile of the affected parts: no geometrical change, other tests ??

Glass beads: recycled - as the other machines in the production line: only used for our stainless steel parts

The colour change is periodical in periods more batches are affected

 

[unclesyd]

I would first see if you have any drag out form your cleaning bath and overwhelming your rinse tank/s. I would pull a few parts and air dry after the rinse tank to look for any residue. This is kinda iffy but it may show something.
Can you get a MTR of the materials use in the fabrication. Though it may be limits sulphur in at the higher limits has been known to cause cleaning problems.
I would also check the bead blasters to see if someone might be running a unauthorized plant test.

Could you tell what the end use of the part is?