Passivation and Corrosion

October 21, 2016

Passivation is a term that refers to a material becoming less affected by the operating environment. Generally passivation is used to strengthen and preserve the appearance of easily corroded metals.

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A classic example of passivation layers in action is pure aluminum metal, seen above. That is because pure aluminum quickly forms a thin layer of aluminum oxide on its surface upon contact with oxygen in the atmosphere.

Passivation is a term that refers to a material becoming less affected by the operating environment. During passivation, a thin film outer layer of a shielding material is applied, either through a chemical reaction with the base material, or built through spontaneous oxidation in the air. Generally passivation is used to strengthen and preserve the appearance of easily corroded metals. However, passivation can pose a challenge in applications such as electrochemical water treatment, where passivation can reduce effectiveness by increasing circuit resistance. The inert surface layer formed by passivation is sometimes called the native oxide layer and can vary with both material and the length of time that the material has been exposed to its operating atmosphere.

The mechanism of passivation (including the growth of the oxide layer over time) can depend on multiple factors such as the ratio of the volume of oxide to the volume of the parent metal, the mechanism of oxygen diffusion through the oxide, and the chemical potential of the oxide relative to the parent metal. However, if the oxide is crystalline, grain boundaries can allow oxygen to easily diffuse through the passive layer and reduce the effectiveness of the oxide layer in preventing corrosion. Therefore, amorphous oxide coatings can be very effective passive layers because they do not have grain boundaries.

A classic example of passivation layers in action is pure aluminum metal. That is because pure aluminum quickly forms a thin layer of aluminum oxide on its surface upon contact with oxygen in the atmosphere. Unfortunately, aluminum alloys do not have the same protection against corrosion. If an aluminum alloy must be passivated, alclading, chromate conversion coating, or anodizing can be used. During alclading, a thin layer of pure aluminum is bonded to the surface of the aluminum alloy. The pure aluminum layer quickly oxidizes to protect the bulk alloy from corrosion. Alternatively, during chromium conversion coating, a chromate layer is applied to the alloy surface, which then oxidizes to for a passivation layer. Finally, anodization is an electrolytic process that produces a thick, robust oxide layer to protect the bulk material underneath from corrosion.

Natural passivation layers can be extremely useful in protecting a material from dullness and, more seriously, corrosion. However, not all materials are able to form a natural oxide layer for corrosion prevention. When that is the case, it is important to treat the material in a way that allows it to form a protective, corrosion resistant layer.

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