Surface films formed during H 2 S corrosion of pipeline steels and the effect on hydrogen permeation

Transcription

1 Surface films formed during H 2 S corrosion of pipeline steels and the effect on hydrogen permeation Frank Cheng Department of Mechanical Engineering University of Calgary June 1, 2017

2 Pipelines in sour service H 2 S corrosion of pipelines Pipeline cracking (i.e., sulfide stress cracking or sulfide stress corrosion cracking) Actually, H 2 S corrosion occurs via complex, multi-stepped processes. The nature of sour cracking is hydrogeninduced cracking (HIC).

3 H 2 S corrosion products Fe 1-X S Hexagonal Fe 1-X S Cubic FeS 2

4 H 2 S corrosion products The H 2 S corrosion products are quite complex. Their composition, structure and morphology depend on many factors such as H 2 S content, solution ph, fluid flow, temperature, exposure time, etc. The corrosion products usually govern the corrosion mechanism and define the rate-limiting step of corrosion reaction kinetics. The H 2 S corrosion products affect the hydrogen evolution and permeation, as well as the susceptibility of steels to hydrogeninduced cracking.

5 What is the actual H 2 S corrosion reaction? Different people may propose their own reaction mechanisms, and thus the different corrosion products. Characterize the actually generated corrosion products, and determine the corrosion mechanism specific to the actual environmental conditions.

6 Experimental setup for H 2 S corrosion testing X52 steel H 2 S gas Chloride solution with 0.01 M HCl to adjust ph NaHCO 3 Platinum plate X52 steel coupons

7 XRD spectra of H 2 S corrosion products ph 3.5 ph 4.5 Similar compositional features at the two ph values. At low H 2 S concentrations such as 0.2 mm, crystalline iron sulfide (FeS) only. At high H 2 S concentrations such as 2 and 20 mm, the mackinawite (tetragonal FeS) is also generated.

8 XRD spectra of H 2 S corrosion products (cont.) ph 5.5 Different compositional feature at the low H 2 S concentration (0.2 mm), where amorphous FeS is generated. At the H 2 S concentration of 2 mm, crystalline iron sulfide (FeS) is formed. At the higher H 2 S concentrations of 20 mm, both crystalline FeS and mackinawite are cogenerated. One kind of stoichiometric FeS with three phases, i.e., amorphous, crystalline and mackinawite (i.e., tetragonal FeS), are formed on the steel surface in H 2 S environments, subject to varied solution ph and H 2 S concentrations.

9 Proposed H 2 S corrosion mechanism of pipeline steel Steel corrosion in aqueous H 2 S occurring by sequential chemisorption and anodic discharge of two consecutive electron transfers. at high solution ph, such as ph 5.5 at low solution ph, such as ph 3.5 The formation of crystalline FeS and mackinawite becomes increasingly difficult with increasing ph and decreasing H 2 S concentration, resulting in the formation of amorphous FeS at high ph (i.e., ph 5.5) and low H 2 S concentration (i.e., 0.2 mm).

10 Morphology of corrosion product films * Generally, the film thickness increases with the H 2 S concentration, and the outer layer of the film becomes porous and loose. * The structure of the film is more compact with the increasing solution ph.

11 Protective property of the surface films The corrosion rate of the steel reduces with elevated solution ph and reduced H 2 S concentration in the solution. The compact crystalline FeS generated at low ph and low H 2 S concentration is more protective for steel corrosion. Mackinawite generated at high H 2 S concentration is somewhat protective as the film thickness increases. The amorphous FeS, which is generated at high ph and low H 2 S concentration, is not protective.

12 Also, the cracking in sour service Sour cracking, either named sulfide stress cracking (SSC) or sulfide stress corrosion cracking (SSCC), is a hydrogeninduced cracking (HIC) in nature. During H 2 S corrosion, the cathodic reaction is usually the hydrogen evolution, but 2H + + 2e H 2 >90% of adsorbed H are combined to form H 2, and <10% is possible to enter steels.

13 Poisonous effect of sulfides on H recombination Poisonous effect to promote hydrogen entry into steels Inhibit the H-H recombination reaction. Preferential adsorption of the poisonous agents, such as sulfides, on the steel surface. Favorable to generate adsorbed hydrogen atoms on the steel surface, increasing the possibility for hydrogen entry. Direct evidences to support the proposed concepts are not sufficient.

14 Hydrogen permeation testing on filmed steel H 2 S gas Cl - solution Pipe steel pre-filmed during H 2 S corrosion

15 Hydrogen permeation current curves (ph 3.5) µa/cm µa/cm 2 Current density (µa/cm 2 ) µa/cm 2 2 No film, c 1 H2S =20 mm With film, c H2S =20 mm Time (s) The blocking effect of the sulfide films on hydrogen permeation is obvious. The hydrogen permeation current increases with the increasing H 2 S concentration. For a strong blocking effect, the film formed at a high c H2S is preferred.

16 Hydrogen permeation current curves (c H2S 20 mm) 2.9 µa/cm µa/cm µa/cm 2 The blocking effect of the sulfide films on hydrogen permeation is obvious. The hydrogen permeation current decreases with the elevated solution ph. The film formed at a low solution ph possesses a large blocking effect.

17 Sulfide films effective to blocking of hydrogen permeation Sulfide films formed at high c H2S and low solution ph are effective to block hydrogen permeation. Sulfide films formed at these two conditions are primarily the mixture of crystalline FeS and mackinawite, where the mackinawite generates from the crystalline FeS. The mackinawite is effective to block hydrogen permeation into the steel.

18 Ion selectivity of sulfide film The net surface charge of the film affects directly the hydrogen permeation. The linear relationship between the cell potential difference, E, and the ratio of solution concentrations indicates the net charge carried by the film. A negative slope indicates a net positive charge, and the film is anion-selective. A positive slope is associated with a net negative charge, and the film is cationselective.

19 Ionic selectivity of sulfide film E (mv) Blank membrane ph=3.5, 0.2 mm H 2 S ph=3.5, 2 mm H 2 S ph=3.5, 20 mm H 2 S ph=4.5, 20 mm H 2 S ph=5.5, 20 mm H 2 S log(c 2 /c 1 ) The sulfide film formed on the steel in this work, i.e., mackinawite and crystalline FeS, carries a net positive charge, and are anion-selective.

20 Surface charge and hydrogen permeation The net positive charge on the film surface would attract anion ions, such as HS - and S 2-, and repeal cation ions such as H +, in the solution. In environments with a high c H2S, more HS - and S 2- ions diffuse from the solution to react with Fe 2+ to form corrosion product films. The H + ions are repelled from the steel surface for reductive reaction. Thus, the hydrogen evolution and permeation are inhibited. The increased film thickness may also contribute to the blocking effect.

21 Conclusions The sulfide films formed on steel during H 2 S corrosion are complex, and must be characterized under specific environmental conditions. Three phases, i.e., amorphous, crystalline and mackinawite, are formed on the steel under this testing condition. The compact crystalline FeS is more protective for steel corrosion. Mackinawite is somewhat protective as the film thickness increases. The amorphous FeS is not protective. The sulfide film composing of mackinawite and crystalline FeS is effective to block the hydrogen permeation, which is associated with its anion selectivity.

22 Thank You! Contact: Prof. Frank Cheng, +1(403) ,