Why Does Stainless Steel "Rust"?
I thought it weird. My knives rusted in the USA but they don't rust down here -- even in a salt air environment. It ain't the chili... Turns out the culprit is Chlorine. Even as little chlori...
Forum Discussion
Well, when I was in the Navy, we used to make nice looking signet rings out of Monel nuts using nothing but files and emery cloth. Of course, it took awhile, but we had lots of time on our hands.
Back on topic: How does stainless steel corrode? First lets discuss how SS avoids corrosion. To avoid corrosion, SS forms a coating of Chromium Oxide on its surface. This is done in free Oxygen very rapidly. When this oxide is formed, the metal changes from an active metal (near the anodic end of the galvanic chart ) to a passive metal (further down the galvanic list). The anode will be consumed in any galvanic corrosion while the cathode will be protected.The further apart two metals are on the galvanic chart, the more aggressive the galvanic (like a battery) action will be between them. Here is where the problem originates. Not all SS is passivated (passive). SS forms a passive coating in free Oxygen. If free Oxygen is not present this coating will not be formed. While water contains oxygen, it is in solution and is not free to join with the Chromium to passivate the SS. Thus SS will corrode under water. This corrosion is called Oxygen Deprivation Corrosion. It can occur on SS anywhere Oxygen is absent.
Examples where Oxygen Deprivation Corrosion can occur.
Under water, Under gaskets, Under o-rings, under paint, Under insulation, Under bolt heads.
Also, corrosion of any metal is not caused by Chlorinated water. This Chlorine (a non-ionizing molecule) is a dissolved gas added to the water. All corrosion is the result of galvanic reaction. In order to take place, it requires an electrolyte to allow ion transfer. Nearly all free water contains some salt (Sodium Chloride). Salt, when dissolved in water, releases many Chloride ions which makes an excellent electrolyte. Hence the greater the concentration of salt, the more rapid the corrosion. I guess I'm starting to ramble. I hope someone finds this interesting. -Mark.
Back on topic: How does stainless steel corrode? First lets discuss how SS avoids corrosion. To avoid corrosion, SS forms a coating of Chromium Oxide on its surface. This is done in free Oxygen very rapidly. When this oxide is formed, the metal changes from an active metal (near the anodic end of the galvanic chart ) to a passive metal (further down the galvanic list). The anode will be consumed in any galvanic corrosion while the cathode will be protected.The further apart two metals are on the galvanic chart, the more aggressive the galvanic (like a battery) action will be between them. Here is where the problem originates. Not all SS is passivated (passive). SS forms a passive coating in free Oxygen. If free Oxygen is not present this coating will not be formed. While water contains oxygen, it is in solution and is not free to join with the Chromium to passivate the SS. Thus SS will corrode under water. This corrosion is called Oxygen Deprivation Corrosion. It can occur on SS anywhere Oxygen is absent.
Examples where Oxygen Deprivation Corrosion can occur.
Under water, Under gaskets, Under o-rings, under paint, Under insulation, Under bolt heads.
Also, corrosion of any metal is not caused by Chlorinated water. This Chlorine (a non-ionizing molecule) is a dissolved gas added to the water. All corrosion is the result of galvanic reaction. In order to take place, it requires an electrolyte to allow ion transfer. Nearly all free water contains some salt (Sodium Chloride). Salt, when dissolved in water, releases many Chloride ions which makes an excellent electrolyte. Hence the greater the concentration of salt, the more rapid the corrosion. I guess I'm starting to ramble. I hope someone finds this interesting. -Mark.
