What you are referring to is commonly called passivation. Nitric acid is commonly used for this, but I do not recall what concentration. There are other acids in use also.

Following the welding of stainless steel, it is generally required to pickle and passivate the affected surface adjacent to the weld. The pickling operation removes the "burned" oxide layer while the passivation helps to re-develop a new oxide layer.

The pickling is usually performed by a reducing acid while the passivation is done by an oxidizing acid. These days however, it is usual to use one product that performs both the pickling and passivation in a single operation. Any of the major welding suppliers will be able to supply this. (Just make sure that the solution/paste is Cl free.)

Regards
Niekie Jooste