Passivation; What? Why? How?

What is Passivation?

Passivation is a post-fabrication process used to build up a passive, protective layer on a component’s surface to help increase it’s chemical or corrosion resistance. Although this process can be done on many ferrous materials, it is most commonly used with stainless steel.

Passivation plays a critical role when it come to stainless steel components. Although stainless steel has self-passivating properties which allows it to form a corrosion resistant layer on it’s surface, when stainless steel undergoes any form of fabrication process the material loses this protective layer. Passivation helps to restore this!

Passivation is a fairly quick process, 20-30 minutes, and as a result can be used as an effective process in many use cases, to restore the passive layer on metallic components after processes such as grinding, polishing, machining and welding. However, due to its quick process time, passivation can also be used as a preventive measure between manufacturing stages. This regular passivation throughout stages of production can help prevent failure of the component later in service.

Advantages and Disadvantages of Passivation

Advantages

• Improves corrosion resistance
• Improves durability
• Removes contamination from a components surface
• Fast process time
• No change in surface appearance

Disadvantages

• Complex and difficult process to masterGoldilocks process; too little and the process is uncompleted, too much a phenomenon know as ‘flash attack’ occurs where corrosion can occur and runaway, just right… your winning!
• Thorough cleaning of components before passivation process is critical.
• Cannot mix grades of stainless steel in common baths
• Toxic compounds produced in nitric acid process

Passivation, Pickling or both?

Both passivation and pickling are processes used for cleaning metallic components, the significant difference between them is, pickling is a pre-treatment process whereas passivation is a post-treatment process.

Pickling is used to clean and remove oxides from a materials surface before fabrication processes such as welding to reduce heat-affected zones.

Passivation not only cleans a material’s surface is primarily used to restore the passive oxide layer after fabrication as a means to increase corrosion resistance. In order for the passivation process to have maximum effectiveness, the component or sub-assembly must be as clean as possible. Any form of surface contamination can lead to uneven application. Pickling can be used to achieved this!

Passivation Process

1. Cleaning
   All surface contamination (dirt, rust, grease or oil) is removed. As discussed, sometime this can be pickling!
2. Passivation
   Traditionally, components are immersed in nitric acid baths and sodium dichromate is added to the solution to encourage the formation of oxygen to build the passive layer.A modern alternative to nitric acid is critic acid. Components are immersed in critic acid and the oxygen formation is promoted via ultrasonic machines. This process, although slower, is much safer than the nitric acid process with fewer toxic hazards.
3. Rinsing
   Residual acid solution is removed by rinsing in a suitable solvent
4. Oxidation
   Components are exposed to chemicals such as potassium ferricyanide, copper sulphate and salt spray to encourage the formation of the metal oxide layer.