Brown Staining of Stainless Steel 2

[klop]

We recently compounded a shampoo formula in a brand new stainless steel vacuum tank. All product contact surfaces are 316L. We introduced the high active surfactant into the batch with 80°C water as the disperser (through a shear mill). After the batch was completed, dropped and cleaned, we noticed multiple brown stains all over the bottom of the tank and periodically on some of the sides of the tank. The stains are smooth to the touch and do not rub off. We have passivated three times with a 20% citric acid solution, to no avail.

Does anyone have any insight into what these stains are and are they harmful to the stainless steel?

Thanks

 

[unclesyd]

What was the surfactant?
What is the water quality used?
How was the tank heated?

As you have found out Citric acid alone probably isn't going to clean the tank.
Two things to try. If you have a surfactant or wetting agent compatible with citric acid try adding a little to the solution. You might want to try citric acid again and if so get a proprietary compounded cleaner and use a directed.
You might also want to try H3PO4 based concentrated cleaner and dilute 3::1 acid::water. Assistance from a Scotch-Brite pad may help.

I would also test an alkaline base cleaner.

This is the long approach as you tank is new and pretty.

 

[klop]

The surfactant was Texapon N70; water is ~18 meg-ohm and the tank is heated by steam in a dimpled jacket.

 

[unclesyd]

Based on my limited experience with SLES (Texapon N70) your mixing temperature seems high.

Could any of surfactant contact the hot surface sans water?
I think 60C is max at 70% and essentially the same before hydrolysis starts in solution.

As state new system was all part thoroughly cleaned prior to start up?

 

[klop]

Surfactant is educted into batch with the water through a mill........and yes, tank was passivated once installed and cleaned. It has manufactured 6 batches since its installation, but this batch is the first with the high active surfactant....

 

[EdStainless]

I doubt that you are causing any real damage. Take pictures and keep an eye on it. You will see if things are going bad.

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Plymouth Tube

 

[HEC]

Brown stains may indicate points of either mild steel contamination or surface variations. If tank is not EP polished you may consider pickling the surface stains. Pasivation will provide a stronger chrome oxide surface without essentially changing the surface chemistry. Pickling will remove any free iron from the surface that may lead to the staining. As EdStainless states The staining is likely to be cosmetic only, so monitor.

Mark Hutton