Study On Correlation Between Ion-selectivity Of Iron Sulfides And H₂S/CO₂ Corrosion Behavior

In oil and gas production industry,H₂S corrosion is widely concerned by researchers because it can cause serious economic losses and threaten personal safety.H2S corrosion is more complex than other kinds of corrosion.This is because many iron sulfides with different crystal structures can form on steels with the changing environments.The corrosion film composed by these iron sulfides has significant different influence on the corrosion behavior of steels.Ion-selectivity of the corrosion products can explain the effect of these corrosion behaviors through discussing the ion migration.However,studies on ion-selectivity of H₂S corrosion products are still unclear because the crystal structure of iron sulfides has not been taken into consideration.If the crystal structure of iron sulfide is used as a bridge,the relationship between the ion-selectivity of different iron sulfides and H₂S corrosion behaviors can be built.It is very helpful to understand the complex and variable corrosion rate and local corrosion behavior of H₂S corrosion.Besides,the effect of alloy element can be also analyzed through ion-selectivity.The above-mentioned problems were studied in this paper.The results and conclusions are listed below.Firstly,H₂S corrosion behaviors of P110 steel in different environments were studied.The relationship of iron sulfide crystal structure,ion-selectivity,corrosion rate,and corrosion behavior was built.The results indicate that the iron sulfide in low temperature and low H₂S pressure environment is anion-selective cubic-Fe S which is truncated-octahedron shape.In low temperature and higher H₂S pressure environment,cation-selective troilite is formed and it is rod-like.In high temperature and low H₂S pressure environment,both of the outer hexagonal plate pyrrhotite and the inner siderite are anion-selectivity.When the H₂S pressure rises in high temperature environment,the outer of the corrosion scale is cation-selective pyrite which is cubic shape.The middle and the inner are anion-selective pyrrhotite and siderite.The corrosion rates and the structure of the corrosion scales show that cation-selective troilite and pyrite can reduce corrosion rate and restrain localized corrosion.But the anion-selective cubic-Fe S and pyrrhotite do not reduce corrosion rate and lead to localized corrosion.Secondly,the ion-selectivity mechanism of different iron sulfides was studied.TEM was used to analyze the crystal structure and exposed surface of iron sulfides.Terminations of the exposed surfaces were confirmed by building crystal atom stacking models.First principle calculation was used to calculate the adsorbing energy of ions on crystal surface and the ion-selective mechanism of the iron sulfides was revealed.The results show that（1）the crystal atom stacking model of cubic-Fe S is truncated-octahedron.The termination of its exposed surface is（110）-Fe-S.（2）The crystal atom stacking model of troilite is the combination of hexagonal prism and hexagonal pyramid.The termination of its exposed surface is（100）-Fe（Ⅱ）.（3）The crystal atom stacking model of pyrrhotite is a hexagonal plate which owns equal opposite sides and inequal adjacent sides.The termination of its exposed surface is（001）-Fe.（4）The crystal atom stacking model of pyrite is a cube.The termination of its exposed surface is（001）-S（II）.The adsorption energy indicates that anions can adsorb on cubic-Fe S and pyrrhotite surface and they present anion-selectivity.Cations can adsorb on troilite and pyrite surface and they show cation-selectivity.Thirdly,the corrosion behaviors of high-Mn steels with different alloy element were analyzed through ion-selectivity in high temperature and low H₂S pressure environment.Corrosion rates were tested.The morphologies and structures of the corrosion scales and the corrosion scale/substrate interface were analyzed.The relationship of ion-selectivity and crystal structure was also discussed,and the effects of Cr,Ni,and Al on high-Mn steels were summarized.The results show that in high temperature and low H₂S pressure environment,Al or Cr can enrich at the corrosion scale/substrate interface and reduce the general corrosion rate.However,Cl^-migrated through anion-selective corrosion scale and accumulated at the interface.In local areas,the protective scale was destroyed and pitting occurred.The alloying of Ni did not change the crystal structure and the ion-selectivity of pyrrhotite.Consequently,Ni bringed little effect on the corrosion behavior of high-Mn steel.In addition,the original corrosion products in high temperature and low H₂S pressure environment were pyrrhotite.Finally,the effects of ion-selectivity on corrosion behavior of high-Mn steels were proved by studying the corrosion behaviors of steels with different alloy element content.The results show that（1）the content of Ni did not affect the corrosion behavior of high-Mn steels.（2）High-Mn steels with low Cr content owned better corrosion resistance in high temperature and low H₂S pressure environment.But when temperature decreased or H₂S pressure arised,the corrosion was aggravated because the corrosion transfers into H₂S controlling.（3）The corrosion resistance cannot be improved by adding Cr and Al together.It is because the protective scale turned to sulfides which offered no protection.