Investigations Of The Phase Transition Of Corrosion Product On Steel In H₂S Environments And Their Influence Mechanism On The Corrosion Of Steel

Hydrogen sulfide（H₂S）corrosion is one of the most serious problems in the development of oil and gas fields,which can lead to uniform corrosion,pitting and hydrogen damage.In Fe-H₂S-H₂O environment,the surface of steel form complex corrosion products film（polycrystalline iron-sulfide compounds）,its morphology and crystal structures would change with the environmental factors,and different types of products have a very different effect the follow-up corrosion process.However,there are still a number of basic scientific problems to explore in the study of corrosion products,such as the growth mechanism of corrosion products,phase transition of corrosion products in wet H₂S environment and the essential reasons for the difference protection for the steel substrates.Therefore,this paper explores the above elementary problems,the main contents and conclusions are as follows.Firstly,the nucleation and growth process of corrosion products were studied.It was discovered for the first time that（1）The grain boundaries corrode at the initiation stage and that corrosion products initially form on both sides of the grain boundary and then accumulate.The depth of the grain boundary is between 100-150nm and the depth of the grain is only 50 nm.（2）Troilite which formed at low temperature is developed from beam-or hexagonal wire-shaped grains by electrostatic interactions along a certain lattice plane.The large single beam-shaped troilite has a growth pattern along the c axis.Superstructure pyrrhotite,which formed at high temperature,is developed from hexagonal layer,based on the nucleus of the new flaky crystal.（3）Polymorphic iron-sulfide compounds have a considerable difference in morphology:Amorphous FeS has an irregular morphology;Mackinawite appears with a flake shape.Cubic FeS has a perfect/truncated octahedral shape,and pyrite is framboid-shaped.The corresponding crystal structure and micro-morphology of the corrosion products are provided,and the three-dimensional models of them were generated.The mechanism of crystal structure evolution was explored by high resolution transmission electron microscopy（HRTEM）.It was discovered for the first time that（1）metastable crystals such as mackinawite and cubic FeS were more sensitive to high energy electron beams,deformed or even dissipated.The stability of the troilite is better,but the continuous radiation also causes the rearrangement of the atoms.（2）The evolution order of corrosion products formed in the low temperature Fe-H₂S-H₂O environment in:mackinawite+a small amount of cubic FeS?troilite+small amount of greigite?troilite+small amount of pyrite.（3）In the Fe-H₂S-H₂O system,both mackinawite and cubic FeS can transform into greigite,which is a transitional crystal that is metastable than cubic FeS.The effect of corrosion products film on the corrosion resistance of carbon steel was investigated by electrochemical method.The experimental results demonstrate that:（1）Corrosion resistance definitely improved with the increase in thickness of corrosion product films.（2）Mackinawite cannot effectively prevent the hydrogen atoms from entering into steel,while the troilite and pyrrhotite have an inhibitory effect on the anodic dissolution of the steel and the permeation of hydrogen.（3）During the process of transformation of mackinawite to pyrrhotite,the semiconducting characteristics changed from p-type to n-type,resulting in selective adsorption and migration of ions at the interface.The effect of suspended amorphous iron sulphide（FeS）particulate on the corrosion behaviour of carbon steel in aqueous H₂S environment was investigated via corrosion tests and electrochemical techniques.FeS was demonstrated to be cathodic compared to steel in the liquid environment.FeS forms preferential cathodic sites with lower overpotential favoring hydrogen evolution.This leads to microgalvanic cells that are formed between FeS and steel resulting in selective dissolution of steel and the influence in the kinetics by galvanic effect.Adsorbed FeS accumulated and combined with adjacent FeS particles to form huge crystals of about 100μm.The formation of larger corrosion products contributed much to the sharp rise in corrosion rate.The interaction of H₂S and FeS particulate resulted in serious corrosion on steel and changed the transformation of corrosion products.