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Surface films formed during H2S corrosion of pipeline steels and the effect
on hydrogen permeation
Frank Cheng Department of Mechanical Engineering
University of Calgary
June 1, 2017
Pipelines in sour service
H2S corrosion of pipelines Pipeline cracking (i.e., sulfide stress cracking or sulfide stress corrosion cracking)
Actually, H2S corrosion occurs via complex, multi-stepped processes.
The nature of sour cracking is hydrogen-induced cracking (HIC).
H2S corrosion products
Fe1-XS
Hexagonal Fe1-XS
Cubi
c Fe
S 2
H2S corrosion products
The H2S corrosion products are quite complex. Their composition, structure and morphology depend on many factors such as H2S 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 H2S corrosion products affect the hydrogen evolution and permeation, as well as the susceptibility of steels to hydrogen-induced cracking.
What is the actual H2S 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.
Experimental setup for H2S corrosion testing
X52 steel
Chloride solution with 0.01 M HCl to adjust pH
H2S gas
X52 steel coupons
Platinum plate
NaHCO3
XRD spectra of H2S corrosion products
pH 3.5 pH 4.5
Similar compositional features at the two pH values. At low H2S concentrations
such as 0.2 mM, crystalline iron sulfide (FeS) only.
At high H2S concentrations such as 2 and 20 mM, the mackinawite (tetragonal FeS) is also generated.
XRD spectra of H2S corrosion products (cont.)
pH 5.5
Different compositional feature at the low H2S concentration (0.2 mM), where amorphous FeS is generated. At the H2S concentration of 2 mM, crystalline iron sulfide (FeS) is formed. At the higher H2S concentrations of 20 mM, both crystalline FeS and mackinawite are co-generated.
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 H2S
environments, subject to varied solution pH
and H2S concentrations.
Proposed H2S corrosion mechanism of pipeline steel
The formation of crystalline FeS and mackinawite becomes increasingly difficult with increasing pH and decreasing H2S concentration, resulting in the formation of amorphous FeS at high pH (i.e., pH 5.5) and low H2S concentration (i.e., 0.2 mM).
Steel corrosion in aqueous H2S 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
Morphology of corrosion product films
* Generally, the film thickness increases with the H2S 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.
Protective property of the surface films
The corrosion rate of the steel reduces with elevated solution pH and reduced H2S concentration in the solution.
The compact crystalline FeS generated at low pH and low H2S concentration is more protective for steel corrosion.
Mackinawite generated at high H2S concentration is somewhat protective as the film thickness increases.
The amorphous FeS, which is generated at high pH and low H2S concentration, is not protective.
Also, the cracking in sour service
Sour cracking, either named sulfide stress cracking (SSC) or sulfide stress corrosion cracking (SSCC), is a hydrogen-induced cracking (HIC) in nature.
During H2S corrosion, the
cathodic reaction is usually the hydrogen evolution, but
2H+ + 2e → H2 >90% of adsorbed H are combined to form H2,
and <10% is possible to enter steels.
“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.
Hydrogen permeation testing on filmed steel
Pipe steel pre-filmed during H2S corrosion
H2S gas
Cl- solution
Hydrogen permeation current curves (pH 3.5)
0 2000 4000 6000 8000 100000
1
2
3
4
5
6
7
8
No film, cH2S=20 mM With film, cH2S=20 mM
Cur
rent
den
sity
(µA/
cm2 )
Time (s)
The blocking effect of the sulfide films on hydrogen permeation is obvious. The hydrogen permeation current increases with the increasing H2S concentration. For a strong blocking effect, the film formed at a high cH2S is preferred.
2.9 µA/cm2
2.7 µA/cm2
0.5 µA/cm2
Hydrogen permeation current curves (cH2S 20 mM)
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.
2.9 µA/cm2 2.4 µA/cm2
0.3 µA/cm2
Sulfide films effective to blocking of hydrogen permeation
Sulfide films formed at high cH2S 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.
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 cation-selective.
Ionic selectivity of sulfide film
-2 -1 0 1 2-30
-20
-10
0
10
20
30
40 Blank membrane pH=3.5, 0.2 mM H2S pH=3.5, 2 mM H2S pH=3.5, 20 mM H2S pH=4.5, 20 mM H2S pH=5.5, 20 mM H2S
E (m
V)
log(c2/c1)
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.
Surface charge and hydrogen permeation
The net positive charge on the film surface would attract anion ions, such as HS- and S2-, and repeal cation ions such as H+, in the solution.
In environments with a high cH2S, more HS- and S2- ions diffuse from the solution to react with Fe2+ 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.
Conclusions
The sulfide films formed on steel during H2S 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.
