Stainless steel passivation is the process of making a material ‘passive’, by the deposition of an oxide layer that sticks onto the metal surface. Generally, the passivation process for stainless steel is done after fabrication, to enhance the natural corrosion resistance.
It is a known fact now that the industrial carbon steel tools, along with their work efficiency, also introduce an unacceptable risk of contamination from rust (chemically called iron oxide) as well as chrome particles. Thus, it becomes necessary to employ appropriate hand tools for installation and maintain sterile processing line machinery for a good manufacturing practice. A protective process for the same is called passivation.
In simple terms, it is the process whereby the material in question is made ‘passive’, as a result of the deposition of an oxide layer that sticks onto the metal surface. In terms of corrosion, stainless steel passivation is the impulsive formation of a hard non-reactive surface film that ceases further corrosion. Now this layer is usually a few nanometers thick and can be either oxide or nitride. Generally, the passivation process for stainless steel
is done after fabrication, to enhance the natural corrosion resistance.
Under idyllic conditions, the original oxide film will totally cover the entire surface and in actual practice, the microscopic iron particles from the cutting tools are actually transmitted during the machining. In order to maximize safeguarding against corrosion, the freshly fabricated stainless steel is immersed in a passivating bath composed of nitric or citric acid. This procedure results in a superior endurance of the stainless steel, in all environments, even in salt spray exposures in the marine industry applications!
The Process Of Passivating Stainless Steel
Generally, the conditions required for stainless steel passivating process are reported in what they call ‘pourbaix diagrams’. Certain corrosion inhibitors induce the formation of passivating layer on the metal surface on which they are applied. On the other hand, some compounds such as the dissolving molybdates and chromates form a non-reacting, low solubility films on the surface of the metals.
The specific materials which are important for stainless steel passivation essentially include aluminum, silicon, nickel and ferrous materials. It is known that pure aluminum forms a thin film on the metal surface which is nothing but aluminum oxide, when it comes in contact with air oxygen (it’s a process called oxidation). This layer acts as a physical barrier against corrosion and prevents further oxidation in any environment. Hence, chromate conversion coatings are used as a means for passivating aluminum as well as cadmium, silver, copper, zinc, magnesium etc.
Similarly, a strong passivating oxide formation on the surface is directly beneficial to the performance of silicon, and this is help important especially in the field of microelectronics. The passivating surface layers like silicon dioxide, silicon nitride and titanium oxide significantly help reduce surface recombination. The similar applications for ferrous materials and nickel also apply.
A typical stainless steel passivating process (which is nothing but cleansing of stainless steel) would involve cleaning with the help of sodium hydroxide and then citric acid which is followed by nitric acid. Then there is a complete water rinse. With the help of this process, the film on the surface of the stainless steel is restored, the metal particles and dirt are removed, and the welding generated particles are eliminated.
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