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Q. we are making 316L stainless steel tube for vacuum environment, which requires a high surface quality. We do a mechanical mirror polishing to the CNC-processed tube followed by conventional cleaning and rinsing. For some reason, we need to plate a layer of Ni on the outside surface of the tube, before plating, the tube was pickled, but the pickling process always damages the inside mirror-finished surface of 316L tube and leads to a higher surface roughness which is no good for working in vacuum environment.

According to my knowledge, pickling is used to remove the oxide layer on stainless steel after welding or processing at high temperature. The mechanical mirror polishing could not produce thick oxide layer on the surface, I am wondering if the mirror-finished 316L tube can be electroplated without pickling process. If the pickling process is indispensable, is there any method to reduce the damage to surface during pickling?

Hao Wang
- WuXi China
October 23, 2023

A. Hello Hao,
Stainless steel does require an aggressive, highly acidic, Wood's Nickel strike for the plating to adhere.

I may be misunderstanding the situation, but why can't you simply plug the ends of the tubing before plating with vinyl or silicone plugs? It's done all the time.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

Q. Thank you for your reply, I rethink the whole finishing process and the main problem is as follows: the 316L stainless steel tubes are used in electrovacuum enviroment, it demands ultralow surface roughness and no oxide layer on the surface.

After mechanical polishing, the original oxide layer was removed and surface roughness also meets the requirements. however, the mirror-finished 316L can naturally generate a thin layer of oxide in the air, the next pickling process could remove this new oxide layer and prepare a good surface for plating, but damage the surface roughness inside of the tube, if we plug the ends of the tube during pickling and plating process, the thin oxide layer on the surface inside the tube can not be removed.

What can I do to solve this problem; can I just polish the inside surface again after plating, or is there any simple method to remove the thin oxide layer without damaging the surface quality after plating, such as using acetic acid to clean the inside surface?

Hao Wang [returning]
- WuXi China
October 25, 2023

A. Hi again, Hao. Yes, I think you can dip the plated tubes into citric acid without harming the plating.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

Environmentally-friendly citric acid based Processes for Passivation of Stainless Steel
stellar Citrisurf citric acid  passivation

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