Corrosion, or rusting, (kalawang in Tagalog), is the natural tendency of metals such as iron to return to its original stable state as iron oxide. It occurs through the electrochemical reaction with the environment such as water, or moisture in the air.
Most metal pipes such as steel pipes, contain iron (Fe). Before pipe was made, iron was sourced out through mining as an iron oxide (Fe+3). In this state, iron is in stable form. When it was manufactured into pipe, iron was converted into a very unstable or energized state as Fe0. Since iron in the metal pipe is now in unstable state, it will always try to go back to its original stable state by rusting itself or through the corrosion process. It is like a stretched rubber band that when released, will go back to its original length.
For corrosion to occur, the following factors should be present:
- Presence of anode site in the metal surface (the rough, dirty or uneven section)
- Presence of cathode site in the metal surface (the smooth or clean section)
- Presence of electrolyte such as water (to dissolve the metal ion)
- Presence of metallic conductor (the pipe itself) so the electrons can move from the anode to the cathode site. Through this electrochemical reaction process, the electric current will flow in the opposite direction, or from the cathode to the anode.
The Corrosion Process:
Anode (Oxidation Process)
At the anode, iron (Fe0) will corrode by oxidizing into ferrous ion (Fe+2), then eventually into ferric oxide (Fe+3), or rust (kalawang). This is the original state of iron when it was first mined and made into steel or iron pipes. When iron corrodes or oxidized into ferrous ion (Fe+2), it releases 2 electrons that move from the anode to the cathode site. The ferrous ion (Fe+2) dissolves in the water as soluble iron. Some of the ferrous ion (Fe+2) that eventually was converted into ferric oxide (Fe+3) stayed in the metal surface as rust (or kalawang), or insoluble iron.
Cathode (Reduction Process)
At the cathode, the two (2) electrons will be accepted by the oxygen in the water and convert it into hydroxyl ion (OH–). The hydrogen ion (H+) will also take the electron to convert it into hydrogen gas.