Passivation processes are generally controlled by industry standards, the most prevalent among them today being ASTM A 967 and AMS 2700. These industry standards will generally list several typical “types” of passivation processes that can be used, with the specific method to be decided between the customer and vendor. The “Method” refers to either the use of a nitric acid-based passivating bath (Method 1), or a citric acid based bath (Method 2.) The various ‘Types’ found listed under each method refer to differences in acid bath temperature and concentration. Sodium dichromate is often required as an additive to promote oxidation in certain ‘types’ of nitric-based acid baths.
Common among all of the different specifications and types are the following steps: Prior to passivation, the parts must be cleaned of any contaminants and generally must undergo a validating test to prove that the surface is ‘clean.’ The part is then placed in an acidic passivating bath that meets the temperature and chemistry requirements of the Method and Type specified between customer and vendor. (Temperatures can range from ambient to 140 degrees Fahrenheit, while minimum passivation times are generally around 20 to 30 minutes). The parts are then neutralized using a bath of aqueous sodium hydroxide and then rinsed with clean water, dried, and the passive surface is validated using exposure to humidity, elevated temperature, a rusting agent (salt spray), or some combination of the three. However, proprietary passivation processes exist for martensitic stainless steel, which is difficult to passivate, as microscopic discontinuities can form in the surface of a machined part during passivation in a typical nitric acid bath. The passivation process removes exogenous iron, creates/restores a passive oxide layer that prevents further oxidation (rust), and cleans the parts of dirt, scale, or other welding-generated compounds (e.g. oxides).
It is not uncommon for some aerospace manufacturers to have additional guidelines and regulations when passivating their product that exceed the requirements in a national standard. Often, these requirements will be flowed down using NADCAP or some other accreditation system. Various testing methods are available to determine the passivation (or passive state) of stainless steel. The most common methods for validating the passivity of a part is some combination of high humidity and heat for a period of time, intended to induce rusting. Electro-chemical testers can also be utilized to commercially verify passivation.
