why we need to conduct corrosion monitoring?
Corrosion monitoring is an activity to check corrosion happened in an equipment. before we conduct a corrosion monitoring, we must understand the basic of corrosion and corrosion type. Corrosion is the destruction or degradation of a metal due to a redox reaction between a metal and various substances in the environment that produce undesirable compounds. The most common example of corrosion is iron corrosion.
In the event of corrosion, the metal undergoes oxidation, while oxygen (air) is reduced. The metal rust is generally an oxide or carbonate. The chemical rust formula is Fe2O3.nH2O, a solid brown-red substance. Corrosion is an electrochemical process. it means we must understand an electrical side and chemical side. In iron corrosion, certain parts of the iron act as the anode, in which iron oxidizes.
Fe(s) <–> Fe2+(aq) + 2e
The freed electrons in the anode flow to another part of the iron acting as a cathode, in which the oxygen is reduced.
O2(g) + 4H+(aq) + 4e <–> 2H2O(l)
O2(g) + 2H2O(l) + 4e <–> 4OH–(aq)
The iron ions (II) formed at the anode subsequently oxidized to form iron (III) ions which then form the hydrated oxide compound, which is iron rust. As to which part of the iron acts as the anode and which part acts as a cathode, it depends on various factors, such as impurities, or the density of the metal.
Corrosion can also be interpreted as an attack that damages the metal because the metal reacts chemically or electrochemically with the environment. There is another definition that corrosion is the opposite of a metal extraction process of its mineral ore. For example, iron ore minerals in the wild are in the form of the iron oxide or iron sulfide compounds, after extracting and treating, an iron will be produced for the manufacture of steel or alloy steel. During use, the steel will react with the environment causing corrosion (back to the iron oxide compound).
Corrosion monitoring is used to measure how severe the corrosion is. the corrosion rate is used as a parameter to measure the corrosion severe. therefore the corrosion monitoring is similar to corrosion rate monitoring. the highest corrosion rate will result in equipment damage, oil spill, and safety or environment consequence.
what is the best practice to measure the corrosion rate or corrosion monitoring?
in practical, we use a corrosion coupon installed in the pipeline system. after 30 days, we take the corrosion coupon and bring it to the lab to measure the corrosion rate happened in this period. the corrosion rate data will be recorded by asset integrity software and used by asset integrity team to make a recommendation or pipeline repair program.