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Sunday, 28 February 2016

CORROSION IN OIL AND GAS INDUSTRY: SOUR CORROSION

We have already discussed the corrosion engineering and also we have seen corrosion failure case study, today we will focus over the very important topic “corrosion in oil industries” i.e. sour corrosion engineering. 

Corrosion is basically the depreciation of materials due to electrochemical reactions with its environment. Corrosion is occasionally also used to the deterioration of concrete, plastics and wood, but we normally used corrosion for metals only.

Figure 1: Corrosion phenomena

Corrosion is basically a normal and also a natural procedure. We must understand that corrosion could not be completely prevented, but yes we can reduce the effect of corrosion i.e. corrosion could be reduced or controlled by suitable selection of material, coatings, design and sometimes by altering the environment. Different types of non-metallic and metallic coatings are used regularly to assure the metal parts free from corrosion effect. 

Corrosion consist the reaction between a metallic material and its environment and is a natural Procedure in a sense that metal is trying to revert to the chemically combined state wherein it will be almost always found in the earth’s crust. 

Therefore corrosion is a process that might be normally assumed to occur, it should not be regarded as impending, we must understand that controlling and prevention of corrosion is quite possible by different types of process that we will discuss very soon in this literature review report on corrosion. 

Corrosion consist the various reactions and all reactions causing the corrosion effect will be in the category of electrochemical reactions. So we could not avoid the corrosion to be present but we can control the corrosion by controlling the reasons that produces the corrosion so we may say corrosion controlling depends on the skills of the design engineer because it is the section of corrosion prevention specialist. 

There are various types and reasons of corrosion. If we consider the corrosion phenomenon in oil and gas industries, corrosion is normally divided in four basic types and these are as mentioned here.
  1. Sweet Corrosion
  2. O2 Corrosion
  3. Electrochemical corrosion
  4. Sour corrosion
We have to only review here sour engineering

Sour Corrosion

Sour corrosion is basically termed as the depreciation of metal, when metal comes in contact with hydrogen sulphide and moisture. Even if we look the various researches and lab report, we will note that hydrogen sulphide (H2S) is not corrosive in nature by itself but when it will come in contact with water or moisture i.e. presence of moisture or water will act as the media for causing and forming the sour corrosion.

Let us see the general form of equation of sour corrosion which could be explained as following.

Figure 2: Sour Corrosion
Sour corrosion could be seen by observing the black deposition i.e. deposition of iron sulphide over the surface of steel. Iron sulphide will act as cathode and steel will act as anode and corrosion occur at anode side i.e. surface of steel and therefore corrosion mechanism will be faster. 

There will be action of deep pitting over the surface of equipment suffering with sour corrosion. Hydrogen which will be released in above equation will cause embrittle the structure of equipment i.e. steel.

Availability of CO2 and O2 will aggravate the mechanism of corrosion. If we consider the sour corrosion in the case of oil industries, the severity of sour corrosion will be quite dependent over the content of sulphur in blend of oil applied.

Let us see the various oil units, where case of sour corrosion has been observed and these oil units are as listed below here.
  1. CDU Lower reflux circuits
  2. Amine column
  3. Catalytic crackers
  4. Diesel drier overhead accumulators
  5. Flame knockout drums
  6. Fuel gas mixing drum
  7. Gas compressor
  8. Compressor suction drums
  9. Hot wells
  10. Oil pre-saturators & absorbers
  11. High pressure sour gas separators
  12. Hydro-cracker reactor effluent air coolers
  13. Low pressure sour gas separators
  14. Vacuum off gas knockout drum
  15. Off gas knockout pots
  16. VBU flare knockout drum
  17. Sour flare lines
  18. On site flare knockout drum
  19. Recycle gas knockout drum
  20. Sour water strippers
 
The rusting effect on simple mild steel is the best and most common type of corrosion effect and overall assembles to a higher portion of the whole cost associated to corrosion. 

We have seen various corrosion effects over simple mild steel where the complete metal surface will be exposed and attacked by electrochemical reactions and causing corrosion and such activity of corrosion will lead the failure of the engineering component and will destroy the complete machineries after little time  if care not given to reduce the effect of corrosion.

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