Stainless steel is an alloy of iron that contains at least 10.5% chromium and less than 1.2% carbon. It is resistant to staining, rusting, and corrosion due to the formation of a passive layer of chromium oxide on its surface. However, this passive layer can be damaged by various factors, leading to tarnishing.
Formation of the Passive Layer
When stainless steel is exposed to oxygen, a thin layer of chromium oxide forms on its surface. This layer is protective and prevents further oxidation of the iron in the alloy. The thickness and stability of the passive layer depend on several factors, including the chromium content of the steel, the temperature, and the presence of certain chemicals.
Factors that Damage the Passive Layer
Several factors can damage the passive layer on stainless steel, including:
– Chlorides: Chloride ions are particularly damaging to stainless steel as they can break down the passive layer. This is why stainless steel is not suitable for use in marine environments or where it will be exposed to salt.
– Acids: Acids can also dissolve the passive layer, leading to tarnishing. Stainless steel is not recommended for use in acidic environments or with acidic foods.
– Heat: Excessive heat can damage the passive layer and make stainless steel more susceptible to tarnishing.
– Mechanical damage: Scratches or other mechanical damage can break the passive layer, exposing the underlying metal to oxygen and moisture.
Mechanisms of Tarnishing
When the passive layer is damaged, oxygen and moisture can reach the underlying metal, leading to tarnishing. The following mechanisms are involved:
– Oxidation: Oxygen reacts with the iron in stainless steel to form iron oxide, which is a reddish-brown color.
– Corrosion: In the presence of moisture, iron oxide can combine with water to form rust, which is a flaky, reddish-brown substance.
– Pitting: Pitting is a localized form of corrosion that can create small holes in the surface of stainless steel. This can occur when chloride ions are present and the passive layer is damaged.
Prevention of Tarnishing
To prevent tarnishing, it is important to protect the passive layer on stainless steel. This can be achieved by:
– Keeping stainless steel away from chlorides and acids.
– Avoiding excessive heat.
– Cleaning stainless steel regularly with a mild detergent and water.
– Applying a protective coating to stainless steel if it will be exposed to harsh conditions.
