Oxygen Corrosion Mechanism
Steel’s oxygen corrosion under near neutral pH is cathodic reaction controlled, in which oxygen diffusion rate is the rate determination step for the reduction of oxygen. Corrosion in oxygenated fluids increases with the velocity of fluid because a greater amount of oxygen is made available to metal surface for further reaction. Therefore,
Usually is utilised with Cobalt salt catalyst. The catalyst cobalt(II) can be deactivated by precipitation and chelating agent (EDTA), which is commonly utilised to control hardness. Other readily oxidisable species e.g. alcohol, phenol, amine etc may disrupt the sulphite scavenger process.
is practical only at elevate temperature. It is cancergeneous substance which has been phased out in most countries. ppm level of Cu(II) is required as a catalyst. However, Cu(II) will plate out on steel surface and cause severe pitting corrosion.
is used in boiler for high purity water.
Amine (or Imadazoline) salt of sulfurous acid
Acting as both filming amine and oxygen scavenger. In practical, basic amine inhibitor and sulphite are added separately to form such compound in situ. It may also be formulate with alcohol solvent for easy field handling.
Oxygen scavenger in sour gas
- Excessive Co(II) as catalyst
- Fe(II), Mn(II) as alternative catalyst
- Tertiary Butyl Hydroperoxide (TBHP) as an initiator
- Sodium Hydrosulphite (dithionite)
Oxygen corrosion Inhibition
Filming amine used as an acidic corrosion inhibitor (low concentration) has little effect on oxygen corrosion control. They normally impact the adsorption and electron exchange steps in corrosion electrochemical process, which is not a reaction rate determining step for oxygen corrosion. Only thick film of filming amine inhibitor have such diffusion control capability (in percentage range), which is not practical in normal production. The example will be packer fluid, in which very high concentration of corrosion inhibitor may potentially be utilised.
A known effective passivating inhibitor (an anodic inhibitor) for oxygen corrosion, has been banned in most countries due to its toxicity.
Another anodic inhibitor for oxygen corrosion control in boilers and heat exchanger.
Inorganic barriers, e.g. Zn(II)-phosphate, silicate, Ca(II)-bicarbonate
Inorganic barrier for cathodic reactions, is widely used in portable water system.
Aminomethylene phsophonate (AMP)
Similar to inorganic polyphosphate, but works well in sour system.
Works well in sour system. It is primary supresing cathodic reaction, therefore, pitting attack is not serious even used in lower concentration. 15 ppm is reported working well in the presence of hydrocarbon. For brine system , it may require 1000 ppm level to render the inhibition.
For the system with oxygen corrosion risks, the preference is usually in the following orders:
- Preventing the oxygen ingression.
- Oxygen stripping and scavenging.
- Materials upgrade including coating and lining.
- Corrosion inhibition.