Nitric acid passivation is most useful in enhancing the corrosion resistance of freshly machined surfaces.
On the surface of stainless steels, there is an extremely thin transparent film. Nevertheless, it is tenacious, uniform, stable and passive. It imparts to the surface the property of passivity, normally associated with noble or inert metals and it is to this passive film that stainless steels owe their superior corrosion resistance.
The film will form spontaneously, or repair itself if damaged, both in the air due to the presence of oxygen, or when immersed in solutions, provided there is sufficient oxygen or oxidizing elements present. The basic passivation treatment for stainless steel is exposure of a clean surface to air. However, there is much practical evidence which shows that passivity, and therefore corrosion resistance, is enhanced if the passive film is formed by the action of oxidizing acid solutions. Nitric acid is such an oxidizing acid and is always used for passivation treatments. Nitric acid does not corrode stainless steel, does not alter critically dimensioned parts and will not remove heat tint, embedded iron or other embedded surface contamination. Nitric acid passivation is most useful in enhancing the corrosion resistance of freshly machined surfaces.
The standard nitric acid passivating solution is made up and used as follows: 10 to 15 percent by volume of nitric acid (HNO3) in water. Quickest and best passivation results if used at 150°F (65°C) for the austenitic (300 series) stainless steels, and 120°F(50°C) for the ferritic and martensitic(400 series) plain chromium stainless steels. The immersion time is approximately 30 minutes, followed by thorough water washing.
It is, however, appreciated that it is not always possible to fully immerse fabrications in a hot passivating solution, and therefore, swabbing with cold acid solution is normally used where lower concentrations and temperatures of acid ease handling and application. It must be appreciated that longer contact times will be required.
For austenitic (300 series)
15% nitric acid at 65/80°F (20/25°C)
………………………..30 to 90 minutes
For ferritics (400 series)
12% nitric acid at 65/80°F (20/25°C)
………………………..30 to 45 minutes
The acid solution is swabbed on with sponges, soft paintbrushes or fine nylon pads. Continual swabbing is necessary to ensure contact over the time period. For the treatment of small localized areas, proprietary passivating pastes are obtainable and can be used. Contact time should be in accordance with suppliers’ recommendations, as the concentrations may differ from paste to paste, and the lower alloyed grades will require shorter times. Thorough water rinsing MUST follow all passivating treatments.