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Robert answered on
Dec 25 2021
Review article of scale formation of iron in Hydrogen sulfide
1. H2S Co
osion :
Co
osion is the spontaneous degradation of active metal using environmental condition like presence of gases and moisture. In some cases, moisture is not required for the process and the phenomenon can be progressed using co
osive gases only. These co
osive gases can be SO2, CO2 or H2S. Such co
osion is characterized as DRY CORROSION or OXIDATIVE CORROSION.
The oil and gas industry has received considerable attention from researchers because of rapid co
osion caused by co
osive gases. Unfortunately, oil mining and transportation have become increasingly difficult because of equipment damage caused by co
osive media.
and H2S
Hydrogen sulfide is a colorless, flammable and extremely hazardous gas with a strong odor like of rotten eggs. Hydrogen sulfide is generally dangerous to equipment when moisture is present, therefore, it is also sometimes refe
ed as SOUR CORROSION, can lead to rapid and extensive damage to metals.
This type of co
osion is one of the major problems faced by the oil and gas industry, especially in wells. Hydrogen sulfide gas causes severe co
osion of metal, in piping especially. In order to minimize the rate of co
osion, amines solutions are used for the removal of H2S gas or CO2 gas.
Co
osion due to H2S is mainly electrochemical in nature [1]. The products of dissociation of H2S gas are destructive and can catalyze the electrochemical reaction, especially the dissolution of Fe.
2. Mechanism of H2S
The internal co
osion of iron pipes with hydrogen sulfide gas is a significant problem associated with gas pipelines and natural fuel supply plant.
The mechanism involves ELECTROCHEMICAL PROCESS where redox reaction goes on. It also involves the formation of FeS phases with different composition.
The overall reaction between H2S and Fe results in the formation of iron sulfide film:
Fe + H2S (
FeSx + H2
Researchers examined the uniform H2S co
osion of steel in order to predict the rate of co
osion over time at pH 5.0–5.5. They reported that the co
osion rate increased with an increase in H2S gas concentration but the degree of the co
osion rate is lower after 24 hours compared to 1 hour because of the build-up of a protective iron sulfide film [2].
Few studied the effect of H2S gas concentration level at room temperature and reported an increase in co
osion rate with increasing gas concentration [3].
The mechanisms for H2S co
osion of ca
on steel proposed in the literature are based on the formation of Mackinawite film. The proposed mechanisms are given in Figure 1. The feasibility of the mechanisms given in Figure 1 was evaluated using computational chemistry [4].
3. Scale formation
Scales are defined as hard and adherent precipitates formed on the inner wall of system where co
osion is taking place due to high temperature and high pressure. They are very difficult to be removed as they are rigid in their nature and could not be even taken off by hammer or chisel. Scale formation is enhanced by the presence of several ions as well.
Surface scale formation in the inner wall of pipeline is one of the major factor governing the co
osion rate. The development of scale depends generally on the KINETICS of scale formation. In pure CO2 co
osion, the straight forward iron ca
onate is precipitated out but in case of H2S co
osion, many types of iron sulfide is formed such as amorphous fe
ous sulfide, mackinawite, cubic fe
ous, smythite, pyrite, among which mackinawite is considered to form first on the steel surface by a direct surface reaction [5].
The poorly known mechanism of H2S co
osion makes it difficult to quantify the
kinetics of iron sulfide scale formation.
3.1. Iron sulfide scale formation
Iron sulfide is a multifaceted scale in the oilfield industry. In fewer amounts, it can enhance the protective film formed with co
osion inhibiting compounds. In gas lines, under the appropriate conditions, it can form protective layers by itself. Its effect can be compared with that of calcium ca
onate scale as well.
Iron sulfide scales are almost everywhere when hydrogen sulfide is produced frequently the result of tubular co
osion in the presence of H2S. There are various mechanisms that can lead to the formation of iron sulfide. However, all of these mechanisms require sources of hydrogen sulfide and iron. Iron sulfide scale is present in oil and gas producing wells, water injection and supply wells.
Scale formation can take place effectively and at higher amount when hydrogen sulfide is formed from sulfate reducing bacteria, thermal decomposition of sulfate, or being introduced into the well as in gas lift operations. Iron is the source from iron made pipelines.
Combination of hydrogen sulfide and iron will cause formation of various iron sulfide species. The factors on which ratio of iron to sulfide species depends are temperature, pressure, pH, and hydrogen sulfide concentration. This ratio plays a key role in determining the best method to remove iron sulfide scales. Hydrochloric acid can be used to dissolve iron sulfide species that contain iron and sulfur at a molar ratio close to unity. Non-acid formulae or any other specy can be used to remove iron sulfide scale, however, their capacity to dissolve iron sulfide depends on the molar ratio of iron to sulfide [6].
When too much iron sulfide is formed it no longer provides its protective service. It may crack and portions of it may flake off. The remaining deposits may provide the perfect environment for crevice or under-deposit co
osion. The adherent deposits may interfere with the operation of pumps, valves and other equipment [7].
3.2. FeS phases in H2S co
osion
The structure and composition of the protective FeS film depends greatly on the concentration of co
osive gas as well. The protective...