Passivation is an engineering and physical process which refers to the metals becoming passive. It means to be less affected by the surrounding such as air and water. The process to passivate, includes shielding outer layer of the metal. This layer can be applied by any process like micro coating or oxidation. To Passivate is to use a light coating of a protective metal which is against corrosion. This can be done is specific conditions only and microelectronics is used. To Passivate is generally for strengthening and preserving the look of metals.
In general when the metals are exposed to air; they are for a hard and inert surface, like in silver, iron. The corrosion which occurs gives a rough surface by removing some amount of metal. This is by dissolving the environment or porous coating of the material. In some cases the corrosion coating will reduce the corrosion. This depends on the material and surroundings. Like for metals zinc, aluminum it forms deep corrosion and the main reason is passivate. The thickness of oxide layer increases with time and this is mainly because of the volume of the oxide to parent metal. The main mechanism is the diffusion of oxygen in to the metal and oxide interface and its potential to make oxide for. If the boundary of the oxide layer is crystalline then it gives a path for oxygen to reach the unoxidized metal. So, vitreous oxide coatings are done so they don’t have proper grain boundaries. This can retard the oxidation. There are few conditions which are essential for the passivity of the metal. There are corrosion inhibitors which help in formation of passivity layer.
Steel and Stainless Steel Passivity
For ferrous materials like steel, which are corrosive resistant in nature, passivity is not really required. But the stainless steel is not completely resistant to rust. The main thing is that the corrosion resistant steels have small spots on the surface which get rusted due to grain boundaries which gets embedded by foreign matter and allow the water to oxidize few iron in the spot with chromium. This process is called rouging. Few grades of stainless steels are resistant while many parts go for passivity. Passivation is a process which has to be done only under controlled industrial standards. The most common one is ASSTM A967 and AMS 2700. These industrial standards have many types of passivity and can be used in a method which has to be decided by customer and supplier. The methods are either by the use of nitric acid base or citric acid based bath. In these two also there are different methods which vary the acid bath with temperature and the concentration. The most common one used is Sodium dichromate to promote oxidation. For aerospace manufacturers, there will be additional guidelines and rules for passivity. These products have different standards and they should not exceed the national standard. They are generally followed using NADCAP and other accreditation systems. There are many testing methods which can determine passivity for stainless steel. The common methods differ from validating the passivity of part with combination with other part which has high humidity and heat. Sometimes electrochemical testing can also be done.