Formation and Stabilization of Passivation Film

 

Initial Formation of Passivation Film: In stainless steel, chromium is the key element for the formation of the passivation film. When the stainless steel is exposed to an oxygen - containing environment, the chromium atoms on the surface will react with oxygen to form a dense chromium oxide (Cr₂O₃) thin film. This film is very thin, usually only a few nanometers thick, but it can effectively isolate the stainless - steel substrate from external corrosive media such as oxygen, water, and various chemicals.

 

Stabilizing Role of Nickel: Nickel plays a role in stabilizing and strengthening the passivation film in this process. Nickel atoms can be incorporated into the lattice structure of the chromium oxide film, changing the electronic structure and chemical properties of the film. This incorporation makes the passivation film more compact and reduces the defects and pores in the film. For example, in nickel - containing 304 stainless steel, the presence of nickel makes the passivation film more stable than that of nickel - free stainless steel and better able to resist the penetration of corrosive ions such as chloride ions.

 

Repair of Damaged Passivation Film: In the actual service environment, the passivation film may be locally damaged due to mechanical damage, chemical erosion, or other reasons. At this time, nickel can promote the diffusion of chromium elements to the damaged area. Due to the presence of nickel in the stainless - steel matrix, it can act as a "medium" to guide chromium atoms to move quickly to the damaged area, so that new chromium oxide can be re - formed at the damaged area, timely repair the passivation film, and prevent the further development of corrosion.

 

 

 

 

 

 

 

GL F500 extra High Strength Chemical Composition Tempering and Quenching
Grade	The Element Max (%)
	C	Si	Mn	P	S	Al(min)	N
GL F500	0.18	0.55	1.60	0.025	0.020	0.015	0.020

 

 

 

 

 

Change of Corrosion Potential

 

Fundamentals of Electrochemical Principles: From an electrochemical perspective, metals will form a corrosion cell in an electrolyte solution. In this cell, the electrode potentials of different metals determine their roles in the corrosion process. Metals with lower electrode potentials are more likely to lose electrons and be corroded as anodes; while metals with higher electrode potentials are relatively more difficult to be corroded.

 

Effect of Nickel on Potential Increase: The addition of nickel can increase the electrode potential of stainless steel, making it more tend to have the potential characteristics of noble metals (such as gold and platinum). In the stainless - steel - electrolyte solution system, the anodic reaction activity of nickel - containing stainless steel is reduced. For example, in a structure with multiple metal connections, such as a stainless - steel pipe connected to a carbon - steel support, the carbon steel has a lower electrode potential and is easily corroded as an anode. However, due to the increased potential of nickel - containing stainless steel, the potential difference between it and carbon steel is reduced, thereby reducing the risk of galvanic corrosion between them. This change in potential enables nickel - containing stainless steel to better maintain its integrity in a complex electrochemical corrosion environment.

 

 

 

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