Alkaline base descaler to replace HCL 2

[limct]

Hi all,
In order to enaure zero hydrogen evolution during the descaling process of screws (material is Chrome Molydenium Alloyed steel, SCM435, through hardened and tempered to property class 8.8), I'm looking for an alkaline base descaler which is suitable to replace HCL. Can anyone recommend?

Thank you,
ct

 

[unclesyd]

Come back with what caused the scale or how bad is it.
Also give the quanity you need to descale.
Where are you located.

 

[limct]

The screws will undergo black oxide process after the descaling.

I'm in Malaysia.

 

[kenvlach]

There is no alkaline descaler that will allow you to completely replace an acid dip. Why do you think that people have kept using acid despite knowing of the hydrogen embrittlement problem for about 80 years? Every single hot black oxide plant I have ever seen has an HCl dip (& rinse) prior to the black oxide. It is also necessary for stripping the oxide when rework is necessary.

An oxide scale has negligible solubility in alkaline solutions, the only way they can be very effective is as electrocleaners (e.g., Oakite 90), which forms gas bubbles at the solution-metal interface and can partially get underneath and mechanically lift of the scale. Not effective if you have a smooth hard heat treat scale.* Even then, it is more effective to alternate between anodic and cathodic (only cathodic causes H at the metal).
*Better to avoid scale formation in the first place!!

You can add an inhibitor to HCl or sulfuric acid, such as one of the 'Rodine' products from Henkel, which will greatly minimize the hydrogen embrittlement risk. But, some inhibitors interfere with some plating processes, so best to check with the supplier first (I think no problem for black oxide, though).

http://www.henkelsurftechna.com/

Another brand is 'Pickle Pal' from Columbia Chemical:

http://www.columbiachemical.com/

 

[unclesyd]

Totally agree with "Kenvlach's" post especially with the statement of not getting the scale in first place. This is the easiest way around your scale problem. To do this may not be the cheap up front but will pay big dividend in the long run. If the scale is from heat treating (mill scale) it is going to take an inhibited acid as "Kenvlach" states. A general statement on removing heat treating scale on a part is detrimental to the part in some way. This is especially true on alloy fasteners.

There are several options available to you.

Fix you heat treating facilities to prevent/minimize the scale. This is better option.

Same acid + inhibitor

Change acid + inhibitor, this might be problem depending on the type scale.

Acid Mix + inhibitor, again depending on your scale.

(Inhibitor meaning a complete package of inhibitor, wetting agent, rinse agent, etc.}

Electrolytic, very high initial cost. Maybe a problem depending on your scale.

The rust removing ability of alkaline products is very limited and is primarily used for removal of "flash rust". Alkaline cleaners though much better for the metal require much more attention as far as the maintenance of the bath and rinse system.

No matter what system you use, The proper maintenance of the baths is essential.

As stated get with a supplier of pickling materials and discuss your problem with him. You will need a technical representative not a salesman. When we mixed our acids we used the Rodine products with great success. If i remember it was Rodine 92 for HCl.