chemcials for cleaning or descaling of stainless steel

[patholahan]

I have a few basic questions on stainless steel cleaning and descaling -
1. Should trichloroethylene be used to clean stainless steel products?
2. What effect do chlorides have on stainless steel? Can trichloroethylene contribute to chloride stress corrosion cracking?
3. How prevalent is the use of trichloroethylene as a degreasing agent?
4. What is the most common cleaning or descaling method or chemical used to prep stainless steel for passivation?

If anyone has additional resources or references they can point me to, that would be immensely appreciated as well. Thanks in advance.

 

[ash9144]

Trichloroethylene would expose the metal to chlorides and could potentially lead to cracks. Any solvent would work for degreasing. If you are already using some hydrocarbon in your process just use that so you can recover it.

If you are looking to descale and passivate nitric acid works well in stainless service.

 

[mxmaciek]

Well base for your case will be EPRI guidelines for bolier's chemical cleaning.
Similar source of information from EU will be GVB standard (available in english and german).
Both are describing the cleaning and pasivation procedures used in power industry for various materials for HP, IP and LP drums and pipelines.
Hope this will help.

 

[EdStainless]

I issue with Trichlor is usually related to residual gunk on the surface or fumes. I would expect you to see discoloration or pitting before you see CSCC, but I have seen all of them.
In many plants Trichlor is used to degrease and then parts are moved fairly quickly to Nitric Acid for passivation. Warm 20% nitric works very well. Follow that with a DI water rinse.

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Plymouth Tube

 

[unclesyd]

If you have to descale your SS you can start with a pickling bath of HNO3 + HF in various ratios. I would start with 15% HNO3 + 2 HF and go either up or down on the concentrations. You want to stay as low as possible with the HF and do not go above 3.5% HF. The higher the HNO3 the brighter the stainless will be after rinsing. The time in the bath is also dependent on the scale, a little longer for heavy scale and shorter for delicate parts with a thin scale. Good bath control and a dedicated operator is essential.
I would use DI for bath makeup and all rinsing.

A note on pickling SS with HNO3 + HF is that after immersion in the bath sometimes the scale will appear to still be there. Sometimes it takes an aggressive rinse to remove loosened scale. If this is the case don't allow the scale to dry down as this will normally require another trip through the pickling bath. We use a combination of steam and/or hot water.

We used triclean for years to degrease very precision SS parts with no problems. We used it in the solvent spray mode and the vapour degreaser mode. We switched to Freon form several years until it was banned and at present are using an alkaline based cleaner that requires an extra step.

The 20% HNO3 as a passivating bath mentioned above is right on.

As your appear to be new to this process I would definitely get someone well versed in the handling of these acids to be involved in the setting up and operation of any cleaning, descaling, and passivating of your SS. Do be afraid to ask about any part of the process.