THE SIGNIFICANCE OF THE H₂S/H₂ RATIO IN
HIGH-TEMPERATURE SULFIDIC CORROSION
― THE COUPER-GORMAN CURVES REVISITED ―
Oene Roorda
March 5, 2023
1.
Introduction
High-Temperature Sulfidic Corrosion is non-aqueous corrosion that occurs in a temperature range of
approximately 200-400°C (400-750°F). It results from the reaction of iron with sulfur compounds that
forms a sulfide scale.
Two different types of sulfidizing services occur in refineries:
•
Sulfidation in hydrogen-free environments; Hydrogen-free services are defined as those in which
hydrogen is not intentionally added as part of the process.
•
Sulfidation in H₂S/H₂ environments
A good overview of sulfidic corrosion is provided by Rebak (Rebak, 2011), (API RP 939-C, 2019), and
(NACE International Publication 34103, 2014).
Corrosion rates in H₂-free environments can be determined with the modified McConomy curves
(Gutzeit, 1986). Corrosion rates in a H₂S/H₂ environment can be determined with the Couper-Gorman
curves (Couper & Gorman, 1971). However, the effect of the hydrogen concentration is not reflected in
these curves. In API RP 939-C (API RP 939-C, 2019) and other literature (Niccolls, Stankiewicz,
McLaughlin, & Yamamoto, 2008), suggestions for modified experience-based corrosion curves are made
for low and high hydrogen content without quantifying hydrogen concentration or H₂S/H₂ ratio.
We will look closer at the Couper Gorman curves, the underlying thermodynamics of the hightemperature sulfidic reaction, and the H₂S/H₂ ratio by exploring historical literature that formed the
basis of the Couper Gorman curves. This review leads to the introduction of the Modified CouperGorman curves, which are valid for all H₂S/H₂ ratios.
2.
The Couper-Gorman Curves
In H₂S/H₂ -containing streams, the Couper-Gorman curves (Figure 1) are often used to predict corrosion
rates of carbon steel, low alloy steels, and 12Cr stainless steels, all of which are iron-based, for naphtha
and gasoil desulfurizers. These curves are based on an industry survey conducted by the NACE
THE SIGNIFICANCE OF THE H₂S/H₂ RATIO IN HIGH-TEMPERATURE SULFIDIC CORROSION
Oene Roorda
C: +1-403-585-5648
oene.roorda@hghouston.com
Page 1 of 11
Committee on Refining Industry Corrosion in 1971. Of note is the region where 'no corrosion' is
predicted; this region corresponds to a combination of temperature and H₂S concentrations where a FeS
scale is not thermodynamically stable (Couper & Gorman, 1971) (API RP 939-C, 2019). Line segment A-B
delineates the boundary of the 'no corrosion' region (see Figure 1).
The Couper-Gorman curves are based on an industry survey that includes the effect of H₂ on the overall
corrosion rate; however, the H₂ partial pressures used for these curves are not documented but
generally believed to be relatively high (NACE International Publication 34103, 2014).
Figure 1 – Couper-Gorman curves for carbon steel in
naphtha diluent (Couper & Gorman, 1971)
THE SIGNIFICANCE OF THE H₂S/H₂ RATIO IN HIGH-TEMPERATURE SULFIDIC CORROSION
Oene Roorda
C: +1-403-585-5648
oene.roorda@hghouston.com
Page 2 of 11
3.
The Origin of the Couper-Gorman Curves
The origin or inspiration for the Couper Gorman curves can be found in the isokinetic curves for the
reaction of steel with H₂S /H₂ mixtures by Dravnieks and Samans (Dravnieks & Samans, 1958) (Figure 2).
In the isokinetic curves, the molar ratio of H₂S/H₂ is plotted as a function of temperature (1/T in °K).
Figure 2 – Isokinetic curves and phase existence
limits for the reaction of steel with H₂S mixtures at
atmospheric pressure, according to Dravnieks and
Samans (Dravnieks & Samans, 1958)
It will be demonstrated in the following that line segment A-B of the isokinetic curves (Figure 2)
corresponds with line segment A-B in the Couper-Gorman curves (Figure 1).
THE SIGNIFICANCE OF THE H₂S/H₂ RATIO IN HIGH-TEMPERATURE SULFIDIC CORROSION
Oene Roorda
C: +1-403-585-5648
oene.roorda@hghouston.com
Page 3 of 11
The isokinetic curves are based on a thermodynamic study by Rosenqvist (Rosenqvist, 1954), who
developed a pseudo Fe-S phase diagram (Figure 4). The phase transition between Fe and FeS in this
diagram (line segment A-B in Figure 4) corresponds with line segment A-B in the isokinetic curves.
Figure 3 – Pseudo Fe-S phase diagram according to
Rosenqvist (Rosenqvist, 1954) Plot of H₂S/H₂ versus 1/T for
the system iron-sulphur. (Compositions of alloys are in wt.% of sulphur)
THE SIGNIFICANCE OF THE H₂S/H₂ RATIO IN HIGH-TEMPERATURE SULFIDIC CORROSION
Oene Roorda
C: +1-403-585-5648
oene.roorda@hghouston.com
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The equation of the phase transition line (line segment A-B) is provided by Rosenqvist:
𝑙𝑙𝑙𝑙𝑙𝑙 �
In which,
𝐻𝐻2 𝑆𝑆
−3100
�=
+ 0.179
(𝑇𝑇 + 273.15)
𝐻𝐻2
[1]
T = temperature (°C)
H₂S and H₂, in mol%
4.
Verification of the 'No Corrosion' Boundary Line
The 'no corrosion' boundary line of the Couper-Gorman curves was measured, curve fitted, and plotted
together with the phase transition line of the Rosenqvist pseudo Fe-S phase diagram (Figure 4).
Figure 4 – Comparison of the 'no corrosion' boundary (Couper &
Gorman, 1971)) and the phase transition line (equation [1])
(Rosenqvist, 1954)).
Figure 4 shows that the 'no corrosion 'boundary of the Couper-Gorman curves is shifted downward by a
factor of 100 compared with the phase transition line in Rosenqvists pseudo Fe-S phase diagram. The
two lines would overlap if the y1-axis (H₂S, mol%) were plotted as H₂S, mole fraction. This means that
THE SIGNIFICANCE OF THE H₂S/H₂ RATIO IN HIGH-TEMPERATURE SULFIDIC CORROSION
Oene Roorda
C: +1-403-585-5648
oene.roorda@hghouston.com
Page 5 of 11
the y-axis (H₂S, mol%) in the Couper-Gorman diagram is directly proportional to the H₂S/H₂ ratio
through:
1 𝑚𝑚𝑚𝑚𝑚𝑚% 𝐻𝐻2 𝑆𝑆 ∝
𝐻𝐻2 𝑆𝑆
𝑥𝑥 0.01
𝐻𝐻2
[2]
In other words, the Couper-Gorman diagram is valid for process conditions where the H₂ concentration
is very high, close to 100 mol% (say 90-95%). At lower H₂ concentrations, the corrosion rate will be
higher than shown in the Couper-Gorman diagram.
The above suggests that for all H₂S/H₂ ratios, high-temperature sulfidic corrosion rates can be
conveniently determined with the Couper-Gorman diagram by using the H₂S/H₂ ratio in % rather than
mol% H₂S (Figure 5).
Figure 5 – Modified Couper-Gorman curves.
THE SIGNIFICANCE OF THE H₂S/H₂ RATIO IN HIGH-TEMPERATURE SULFIDIC CORROSION
Oene Roorda
C: +1-403-585-5648
oene.roorda@hghouston.com
Page 6 of 11
The Couper-Gorman equation, obtained through data correlation (Couper & Gorman, 1971), was
reconfirgured to plot the corrosion rate (in mpy) versus H₂S/H₂ ratio in % (Figure 6). This graph is similar
to the experience-based corrosion curves for high hydrogen content published in (API RP 939-C,
2019)(Figure B.11). Figure B.11 is shown in Figure 9.
Figure 6 – Reconfirgured Modified Couper-Gorman curves; corrosion rate (in
mpy) versus H₂S/H₂ ratio in % based on Modified Couper-Gorman curves
(Figure 5).
5.
Comparison Reconfigured Modified Couper-Gorman Curves with
Literature Data
5.1.
Iso-Corrosion Curves for 0-5Cr Steels (Backensto & Sjoberg, 1959)
Backensto and Sjoberg published the iso-corrosion curves for 0-5Cr steels (Backensto & Sjoberg, 1959).
These curves plot the H₂ (vol%) as a function of temperature (°F) (Figure 9). Most of the data was
obtained below 1000°F at a hydrogen pressure of 450-500 psig.
THE SIGNIFICANCE OF THE H₂S/H₂ RATIO IN HIGH-TEMPERATURE SULFIDIC CORROSION
Oene Roorda
C: +1-403-585-5648
oene.roorda@hghouston.com
Page 7 of 11
Figure 7 – Iso-Corrosion Curves for 0-5Cr steels; effect of
temperature and H₂S concentration on corrosion rate. Graphs are
from laboratory data at pH2 = 450-500 psig (< 1000°F) (Backensto &
Sjoberg, 1959)
The iso-corrosion curves were digitized and replotted as H₂S/H₂ ratio (%) versus corrosion rate (mpy)
(Figure 8) to allow easy comparison with the reconfigured Modified Couper-Gorman curves (Figure 8). A
comparison shows that there is no good correlation between both data.
THE SIGNIFICANCE OF THE H₂S/H₂ RATIO IN HIGH-TEMPERATURE SULFIDIC CORROSION
Oene Roorda
C: +1-403-585-5648
oene.roorda@hghouston.com
Page 8 of 11
Figure 8 – Iso-Corrosion Curves for 0-5Cr steels (Figure 7), replotted as H₂S/H₂
x 100 (%) versus corrosion rate (mpy); for 500-700°F and pH2 = 475 psia.
5.2.
API RP 939-C Figure B.11
Alternative curves for the Couper-Gorman curves that some companies use are published in (API RP
939-C, 2019), Figure B.11. These are experience-based corrosion curves for high hydrogen content,
plotted as the partial pressure of H₂S (pH2S) versus corrosion rate (mpy) (Figure 9).
A direct comparison between the reconfigured Modified Couper-Gorman curves and API RP 939-C,
Figure B.11 cannot be made because the hydrogen content for which the data was obtained is not
known more precisely than the hydrogen content is high. The reference provided for the data is "Data
from NACE Publication 59-10, 1958", which are the iso-corrosion curves (Figure 7) (Backensto & Sjoberg,
1959). However, as is clear from comparing Figure 9 with Figure 7 and Figure 8, the corrosion rates from
API RP 939-C Figure B.11 are much higher. Hence, "Data from NACE Publication 59-10, 1958" was not
the source for API RP 939-C Figure B.11.
The origin of API RP 939-C Figure B.11 seems to be (Niccolls, Stankiewicz, McLaughlin, & Yamamoto,
2008). Niccolls et al reference field data from (Couper & Dravnieks, 1962) as the source for this chart.
THE SIGNIFICANCE OF THE H₂S/H₂ RATIO IN HIGH-TEMPERATURE SULFIDIC CORROSION
Oene Roorda
C: +1-403-585-5648
oene.roorda@hghouston.com
Page 9 of 11
Figure 9 – Corrosion Rate in H₂S/High H₂ Partial Pressure—All Vapor
(Figure B.11 from (API RP 939-C, 2019))
6.
Conclusions
•
For all H₂S/H₂ ratios, high-temperature sulfidic corrosion rates can be determined with Modified
Couper Gorman curves which use H₂S/H₂ x 100 (%) rather than H₂S (mol%) (more verification
with field data is required).
•
There is no good correlation between the reconfigured Modified Couper Gorman curves and the
iso-corrosion curves for 0-5Cr steel (Backensto & Sjoberg, 1959).
•
A direct comparison between the reconfigured Modified Couper-Gorman curves and API RP 939C, Figure B.11 cannot be made because the hydrogen content used in Figure B.11 is not known.
•
The source data for API RP 939-C (API RP 939-C, 2019), Figure B.11 (and by association Figure
B.12), is not ˝Data from NACE Publication 59-10, 1958˝ (Backensto & Sjoberg, 1959), but rather
collected field data from (Couper & Dravnieks, 1962).
THE SIGNIFICANCE OF THE H₂S/H₂ RATIO IN HIGH-TEMPERATURE SULFIDIC CORROSION
Oene Roorda
C: +1-403-585-5648
oene.roorda@hghouston.com
Page 10 of 11
7.
References
API RP 939-C. (2019). Guidelines for Avoiding Sulfidation (Sulfidic) Corrosion Failures in Oil Refineries.
Washington, D.C., USA: American Petroleum Institute.
Backensto, E. B., & Sjoberg, J. W. (1959). Iso-Corrosion Rate Curves—For High Temperature HydrogenHydrogen Sulfide—A Contribution to the Work of NACE Technical Group Committee T-8.
Corrosion, 3(15), 41-43.
Couper, A. S., & Dravnieks, A. (1962). High Temperature Corrosion by Catalytically Formed Hydrogen
Sulfide. Corrosion, 18(8), 291-298.
Couper, A. S., & Gorman, J. W. (1971). Computer Correlations to Estimate High-Temperature H2S
Corrosion in Refinery Streams. Materials Performance, 10(1).
Dravnieks, A., & Samans, C. H. (1958). Kinetics of Reaction of Steel with Hydrogen Sulfide-Hydrogen
Mixtures. Journal of the Electrochemical Society, 105(4, April), 183-191.
Gutzeit, J. (1986). High Temperature Sulfidic Corrosion of Steels. In B. J. Moniz, & W. I. Pollock, Process
Industries Corrosion (pp. 367-372). Houston, TX, USA: National Association of Corrosion
Engineers.
NACE International Publication 34103. (2014). Overview of Sulfidation (Sulfidic) Corrosion in Petroleum
Refining Hydroprocessing Units. Houston, TX, USA: NACE International.
Niccolls, E. H., Stankiewicz, J. M., McLaughlin, J. E., & Yamamoto, K. (2008). Paper 2365 - High
Temperature Sulfidation Corrosion in Refining. 17th International Corrosion Congress. Las Vegas,
NV, USA: NACE International.
Rebak, R. B. (2011). Sulfidic Corrosion in Refineries – a Review. Corrosion Reviews(29), 123-133.
Rosenqvist, T. (1954). A Thermodynamic Study of the Iron, Cobalt and Nickel Sulfides. Journal of the Iron
and Steel Institute, 37-57.
THE SIGNIFICANCE OF THE H₂S/H₂ RATIO IN HIGH-TEMPERATURE SULFIDIC CORROSION
Oene Roorda
C: +1-403-585-5648
oene.roorda@hghouston.com
Page 11 of 11