An excerpt taken from a submission written for Fuel & Energy
Abstract— This article describes the effect from gaseous corrosion on equipment as well as on the processed product. It specifically includes the driving force of corrosion and its outcome from the gaseous contamination along with the cause by main source. Hence, corrosion by vapors and its generation is also discussed with parameters to overcome
Full article can be accessed here: Gaseous Corrosion & Its Effects: Full Article
I. CORROSION IN OIL & GAS INDUSTRY
Precisely, in petroleum industry the most common gases/vapors that cause corrosion to the equipment or transmission lines are those which come along with it naturally during extraction or some of them are naturally present in the liquid state in it and upon changing its state into gas/vapor, become corrosive in nature. This thing is common,
whether, there is an oil field or gas field, and the contaminants almost remain the same with some exceptions. Although, there are some additives that is added during the refining or purification process which may also form some by-products that should be removed as they are corrosive in nature also. Detailed explanation will be provided in the upcoming sections. The most common corrosion causing gases/vapors are: SOx, NOx, H2S, and CO2.
II. MAIN TYPES OF CORROSION PRESENT
There are basically three main types of corrosion present in the gas/oil industry on the basis of the causing agent. This should not be confused with equipment corrosion. The types are as under:
Sweet corrosion is mainly caused by Carbon dioxide (CO2), as it causes to form Carbonic Acid (H2CO3) and acts as an active proton donor causing acidic effect to propagate during the processing of gas/oil. Along with it also contaminates the piping system as well as the process equipment, forming deadspots.
Sour corrosion is mainly caused by Sulphur and its relevant compounds. So far, the biggest source is H2S which presents naturally in natural gas and crude. While during processing, it can cause to generate SOx which later may transform to Sulphuric Acid which acts as a strong dehydrating as well a corrosive agent.
Oxygen Based Corrosion
While Oxygen is itself not so corrosive in nature but being an oxidizing agent, it contributes in increasing corrosion population. Bacterial growth in pipe due to excess deposition of moisture propagates more in the presence of Oxygen that is aerobic environment. Detail will be discussed shortly.
III. CONTAMINANTS PRESENT IN NATURAL GAS
Natural gas and its constituents that is other gaseous fuels like Butane and Propane are not itself contaminant whether in raw form or refined form. The two main undesirable content are water vapor and hydrogen sulfide (H2S) which is corrosive in nature. Water condensate is corrosive for the process plant equipment but its severity becomes high when it dissolves acid in it, although, it is not as severe as H2S and other sulfur based compounds. Carbon dioxide is also objectionable in natural gas due to its acid forming nature and also because it lowers the heating value of the natural gas. Hence, proper removal is necessary.
IV. HOW WATER VAPOR & H2S IS REMOVED?
The simplified process depicts that, in industry, there is a compression system that compresses the gas followed by a cooling system to remove the water content. Glycols are also widely used for its high affinity with water and chemical stability. Activated Alumina, Silica Gel and concentrated solutions of Sodium thiocyanate is widely used in this regard.
Monoethanolamine is widely used as a solvent to remove Hydrogen Sulfide followed by various scrubbing process.
V. WHAT WILL HAPPEN IF WATER VAPORS REMAIN IN NATURAL GAS?
If water present in most fuel gas is not removed, unduly high corrosion chances will occur in the transmission lines and trouble may also result in the form of Hydrates which can
cause line stoppage of various plant equipment. Freezing of valves and regulators in cold weather can also cause difficulty. The worst among this would be the formation of MIC** whose main cause of formation is excessive moisture environment but requires aerobic conditions to proceed its population.
VI. **MICRO-BIOLOGICALLY INFLUENCED CORROSION (MIC)
It comes under the shadow of Oxygen based corrosion. It is actually bacterial population that forms in the transmission/pipelines. It is mentioned here because its cause of formation is gas and its outcome is in the form of corrosive gas too. In aerobic environment, the cultural growth leads to the formation of acidic gases like HCl and it also cause to metabolize Sulphur and its gases to accelerate attack on steel.
VII. ADDITION OF ODORANT IN NATURAL GAS?
Usually, Sulphur based liquid or Merceptans is used as an odorant in the natural gas to identify leakages or to identify the gas. Under standard condition this isn’t corrosive but on distribution system this can cause corrosion. Hence non-sulfur based additive like Methethyl pyrazine is encouraged to use although, Dimethyl Sulfide and Ethan thiol is still used in this regard.