Stainless steels are the group of steel alloys which exhibit significant resistance to corrosion.Stainless steels are defined as steels containing 10.5% or more of chromium. This is so since it has been established by experiments that a minimum of 10.5% chromium is required to make stainless steel sufficiently resistant to corrosion.
Contrary to the popular belief that stainless steel is produced by the addition of Ni, it is the addition of chromium which makes steel “stainless”.
Contrary to the popular belief that stainless steel is produced by the addition of Ni, it is the addition of chromium which makes steel “stainless”.
How does Stainless Steel Work ?
As explained above, stainless steel is mild steel with 10.5% or more chromium. Mild steel, when exposed to atmosphere, starts rusting. Rusting of steel is the formation of iron oxide and the process of oxidation of any metal is called corrosion. The corrosion product of mild steel is not protective and if mild steel is left unprotected (by painting or other means), the entire component will eventually turn to rust!
The corrosion product of stainless steel is chromium oxide, which forms automatically on the surface of stainless steel due to the high affinity of chromium to combine with oxygen. This layer of chromium oxide is passive (chemically inert), tenacious (strongly attached to the surface of SS) and self renewing.
This layer is only about 130 angstroms (1A = 10-10m) thick and protects the base stainless steel from corrosion. The self renewing property of the film means that if the film gets removed or damaged (as happens when the surface is machined or scratched), the film forms again by itself! All that is required is oxygen which, as you know, is present in air ! This is what makes stainless steel special.
As explained above, stainless steel is mild steel with 10.5% or more chromium. Mild steel, when exposed to atmosphere, starts rusting. Rusting of steel is the formation of iron oxide and the process of oxidation of any metal is called corrosion. The corrosion product of mild steel is not protective and if mild steel is left unprotected (by painting or other means), the entire component will eventually turn to rust!
The corrosion product of stainless steel is chromium oxide, which forms automatically on the surface of stainless steel due to the high affinity of chromium to combine with oxygen. This layer of chromium oxide is passive (chemically inert), tenacious (strongly attached to the surface of SS) and self renewing.
This layer is only about 130 angstroms (1A = 10-10m) thick and protects the base stainless steel from corrosion. The self renewing property of the film means that if the film gets removed or damaged (as happens when the surface is machined or scratched), the film forms again by itself! All that is required is oxygen which, as you know, is present in air ! This is what makes stainless steel special.
Type of Corrosion ?
1. Pitting corrosion
Mild steel corrodes with a uniform brown stain all across the surface. Stainless steel on the other hand generally corrodes at only spots resulting in small pits on the surface. This is termed as pitting corrosion and is one of the most commonly encountered types of corrosion in stainless steels.
2. Crevice corrosion
Crevice is a narrow gap. In a stainless structure/assembly crevices can be present due different reasons like space under a washer or bolt, gap between sheets bolted together, gap between components welded intermittently etc.
Crevice corrosion takes place even in environments which are safe from pitting corrosion point of view. Since the amount of oxygen available in the crevice is not sufficient for the formation of chromium oxide layer, there is no protection for the stainless steel surface in the crevice. Once corrosion starts in a crevice, the corrosion products accelerate further corrosion.
3. Inter granular corrosion
When stainless steels are subjected to the temperature range of 450 to 900oC, chromium combines with the carbon content of stainless steel. The speed at which this happens depends on the percentage of carbon present in the stainless steel and is shown in the chart below.
This happens in the grain boundaries and the process is called sensitization. This makes free chromium unavailable for the formation of chromium oxide and makes the grain boundaries vulnerable to corrosion. Since corrosion takes place along the grain boundaries, this type fo corrosion is called inter granular corrosion. Weld areas are generally susceptible to this form of corrosion due to the exposure to sensitizing temperature range.
4. Corrosion from Mild Steel / Iron Contamination
If mild steel or iron particles are embedded in stainless steel, stainless steel will corrode.
The iron particle rusts initially and chlorides from the atmosphere will then combine with the corrosion products forming ferric chloride which in turn makes stainless steel corrode.
1. Pitting corrosion
Mild steel corrodes with a uniform brown stain all across the surface. Stainless steel on the other hand generally corrodes at only spots resulting in small pits on the surface. This is termed as pitting corrosion and is one of the most commonly encountered types of corrosion in stainless steels.
2. Crevice corrosion
Crevice is a narrow gap. In a stainless structure/assembly crevices can be present due different reasons like space under a washer or bolt, gap between sheets bolted together, gap between components welded intermittently etc.
Crevice corrosion takes place even in environments which are safe from pitting corrosion point of view. Since the amount of oxygen available in the crevice is not sufficient for the formation of chromium oxide layer, there is no protection for the stainless steel surface in the crevice. Once corrosion starts in a crevice, the corrosion products accelerate further corrosion.
3. Inter granular corrosion
When stainless steels are subjected to the temperature range of 450 to 900oC, chromium combines with the carbon content of stainless steel. The speed at which this happens depends on the percentage of carbon present in the stainless steel and is shown in the chart below.
This happens in the grain boundaries and the process is called sensitization. This makes free chromium unavailable for the formation of chromium oxide and makes the grain boundaries vulnerable to corrosion. Since corrosion takes place along the grain boundaries, this type fo corrosion is called inter granular corrosion. Weld areas are generally susceptible to this form of corrosion due to the exposure to sensitizing temperature range.
4. Corrosion from Mild Steel / Iron Contamination
If mild steel or iron particles are embedded in stainless steel, stainless steel will corrode.
The iron particle rusts initially and chlorides from the atmosphere will then combine with the corrosion products forming ferric chloride which in turn makes stainless steel corrode.
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