The non-rusting properties of stainless steels are attributable to a very thin, invisible oxide film that inherently covers the surfaces of the part and prevents corrosion from occurring. Theoretically, a just-machined, polished or pickled part will acquire this film quickly from atmosphere. In practice, however, such fabricated parts may be contaminated with small particles of foreign material that must be removed to impart the full properties of stainless steel. As an example, residual cutting tool fragments may be impinged into the machined part during manufacture. The primary purpose of passivation is to remove surface contamination and restore optimum corrosion resistance of stainless steel alloys. Passivation IS NOT a scale removal treatment and will not affect part tolerances.
Stainless parts that have been carburized or nitrided shall not be passivated. Additionally, some stainless steel types such as 303, 416, 420, 430, and 440 are problematic and may flash attack, during passivation. A commom practice of Alkaline – Acid – Alkaline processing is used as a means of resolving such concerns.
There are many Types of passivation solution make-up formulated specifically for the type of material being passivated. Most specifications have a table or chart defining recommending the best method.
Many specifications require post process validation testing to ensure process effectiveness. Testing methods may require one of the following:
- High Humidity Test (24 hours)
- Salt Fog Test (2 hours)
- Copper Sulfate Test
- Potassium Ferricyanide Test (Feroxyl)