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Why Does Stainless Steel Rust and How to Avoid it?

There is a common misconception that stainless steel does not rust. Stainless steel has higher resistance to corrosion compared to other metals however it is not completely free from this problem. Stainless does not mean it never stains. It just implies that it stains less. Read on to know as to how and why stainless steel rusts.

What Causes Stainless Steel to Rust?

Stainless steel can remain intact for years under normal environmental conditions. It only corrodes when kept or used constantly under harsh atmospheric conditions. Stainless steel comes in different qualities including Austenitic, Ferritic and Martensitic among others. Different elements are used in different measures to create these. Each of these types of stainless steel has different degrees of corrosion resistance. Highly alloyed stainless steel is less likely to corrode even during stern conditions. However, in any case when the limit of resistance is crossed, the rust formation will occur. The degree of rust resistance is dependent on the elements the stainless steel constitutes of.

Corrosion Mechanisms of Stainless Steel

Knowledge about the corrosion mechanism of stainless steel helps in understanding why it rusts. Corrosion mechanism in stainless steel has been divided into six broad categories. Here is a look at these:

General Corrosion

General corrosion occurs when the pH value of stainless steel is less than

Crevice Corrosion

Crevice corrosion takes place when the level of oxygen is very low in a crevice. If the stainless steel structure is constantly exposed to a solution where chlorides are likely to accumulate then this kind of corrosion can be a problem.

Pitting Corrosion

Pitting corrosion occurs when stainless steel is exposed to environment containing chlorides.

Stress Corrosion Cracking

Stress corrosion cracking is caused when tensile stresses are coupled with certain climatic conditions.

Bimetallic Corrosion

Stainless steel stands at risk of bimetallic corrosion when dissimilar metals in a common electrolyte come in contact with the other. Exposure to heavy rain can lead to bimetallic corrosion.

Intergranular Attack

Heating stainless steel at 450 to 850 degree Celsius can lead to corrosion. Exposure to such high intensity of heat causes the carbon in steel to transform into grain boundaries. This is referred to as intergranular attack.

How to Avoid Stainless Steel from Corroding?

As mentioned above, stainless steel is highly resistant to rust. It corrodes only when exposed to harsh conditions. Different grades of stainless steel corrode in different conditions. It is thus imperative to choose the quality of stainless steel based on the environmental conditions they are likely to be exposed to in order to avoid corrosion as much as possible. Here are few things to bear in mind while choosing the stainless steel grade:

  • The corrosive environment under which the stainless steel is likely to be used
  • The kind of welding that needs to be carried out on it
  • The temperature of operation it would be exposed to. High temperature leads to high corrosion rate and thus higher grade of stainless steel must be employed in such condition.

So, now you know stainless steel can also rust though the chances are not as high as in other metals. Besides, you can avoid stainless steel corrosion by maintaining some caution as discussed above.

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