At its essence, the science behind corrosion is quite fascinating and it boils down to basic chemistry. When a refined metal, like stainless steel, is exposed to the atmosphere, it’s inherently unstable. It attempts to return to its more stable state or ‘ore’ state. This transformation is brought about through chemical reactions which are influenced by specific environmental factors. Let’s delve deeper into this.
Metals like stainless steel are composed of countless microscopic grains which meet at grain boundaries. When exposed to a certain corrosive environment, these boundaries can get attacked leading to intergranular corrosion. Essentially, the stainless steel’s passive film gets disrupted and the underlying metal begins to corrode.
Another phenomenon involves the formation of concentration cells on the surface, usually a pit or crevice, where there is a difference in oxygen concentration. This kind of localized corrosion, called crevice corrosion, is especially damaging as it can often go unnoticed.
In other cases, certain elements in an alloy may turn out to be more reactive and get preferentially corroded leaving behind a porous, weakened structure. This is called selective corrosion or selective leaching. Galvanic or bimetallic corrosion, on the other hand, occurs when dissimilar metals are in contact with each other in a corrosive environment. One metal (the anode) corrodes faster than the other (the cathode) causing the former to deteriorate at a much higher rate.
The final method we’ll discuss is stress corrosion cracking, where the combined effect of tensile stress and a corrosive environment leads to cracks and ultimate failure in equipment and other fixtures.
Now you might wonder, if corrosion is an inevitable natural process, can it be prevented or at least significantly delayed? The good news is, yes—through various methods such as cathodic protection, usage of inhibitors, controlling the environment with maintenance solutions etc. But before we dive into these methods, let’s first understand each type of corrosion in detail in the upcoming sections.
