HIC of Line Pipe Steels.pdf

CORROSION SCIENCE SECTION

Hydrogen-Induced Crackingof Line Pipe Steels Used in Sour Service

R.W. Revie, V.S. Sastri, M. Elboujdaini, R.R. Ramsingh, and Y. Lafrenière*

ABSTRACT

Laboratory studies on a sour-service grade of line pipe steelshowed the steel to be highly resistant to hydrogen-inducedcracking even at a pH of 1.1, as evidenced by ultrasonicC-scan patterns obtained before and after exposure to acidicsolution saturated with hydrogen sulfide. H concentration atthe inside surface of the pipe increased with decreasing pHof the medium. The activation energy for H diffusion into theline pipe steel was 21 kJ/mol.

KEY WORDS: hydrogen, hydrogen-induced cracking,hydrogen sulfide cracking test, inhibitors, line pipe steels,stepwise cracking

INTRODUCTION

Line pipe steels used in sour service are prone tohydrogen-induced cracking (HIC) depending onmetallurgical and environmental factors.1 Themetallurgical factors consist of alloying elements,microstructure, strength, segregation, and shape ofnon-metallic inclusions. Some environmental factorswhich influence HIC, also known as stepwise cracking(SWC), are the partial pressures of hydrogen sulfide(H

2S) and carbon dioxide, temperature, pH of the

medium, and aggressive ions, such as chloride.Many failures of sour gas line pipes have occurred

around the world as a result of HIC.2,3 Considerable

CORROSION–Vol. 49, No. 7

Submitted for publication July 1992; in revised form, December 1992.* Energy, Mines, & Resources Canada, CANMET, 555 Booth St., Ottawa,

Ont., K1A 0G1, Canada.

0010-9312/93/00012NACE Inter

effort has been expended by steel line pipe producers,users, and research organizations to understand theHIC mechanism, to develop a laboratory test method toidentify and quantify material susceptibility to HIC, andto produce steels with greater HIC resistance.

NACE International has developed a standard test,TM-02-84, for line pipe steels.4 The method describesprocedures for evaluating the resistance of pipelinesteels to SWC induced by H absorption from aqueoussulfide corrosion. The test is applicable to line pipe withwall thicknesses of 5 mm to 30 mm. The procedureconsists of exposing unstressed coupons to syntheticseawater saturated with H

2S at ambient temperature

and pressure at a pH in the range 4.8 to 5.4 for 96 h.The samples are sectioned, polished metallographic-ally, and etched, if necessary, so that cracks can bedistinguished from small inclusions, laminations,scratches, or discontinuities. Crack sensitivity ratio(CSR), crack length ratio (CLR), and crack thicknessratio (CTR) are calculated for each section. Theaverage is determined for each coupon.

In recent work by Canada’s Centre for Mineral andEnergy Technology in collaboration with the CanadianStandards Association (CSA) Sour Service TaskForce, two parameters were determined experimentallyfor each of 19 line pipe steels: (1) threshold Hconcentration (C

thH) or the concentration of diffusible H

in the steel above which cracking occurs and (2)threshold pH (pH

th) or the pH below which cracking

occurs. These parameters were determined inH

2S-saturated saline buffered solutions to correlate

diffusible H concentration with HIC occurrence. Data

5311/$5.00+$0.50/0national


TABLE 1Chemical Composition of Line Pipe Steel

C Si Mn P S Al Cr Ni Cu Nb Ti N Mo Ca

0.080 0.41 0.81 <0.03 0.0016 0.13 0.027 0.015 o.o11 0.023 0.019 0.0058 <0.01 0.0040

TABLE 2Test Solution

pH Compostion

1.1 5% NaCl, 0.1 N HCI3.1 5% NaCl, 14.12g/L potassium hydrogen phthalate, 1.125 g/L HCI3.7 5% NaCl, 18.72g/L potassium hydrogen phthalate, 1.125 g/L HCI4.3 5% NaCl, 0.75% sodium acetate, 0.5% acetic acid

on Cth

H and pHth of the steels showed C

thH values

greater than 1.0 mL H2/100 g of steel indicated good

HIC resistance. Data showed pHth values could be

used to rank the steels with respect to HIC resistance.Recently, Hay carried out laboratory and field

tests on line pipe steels to measure the internal surfaceconcentration of H (C

0H) and to quantify HIC resistance

in terms of CTR and CLR.5

The present work involved laboratory measure-ment of H flux in a specific line pipe steel used in sourservice.

EXPERIMENTAL

Sample PreparationChemical composition of the steel is given in

Table 1. Coupons 100 mm long by 20 mm wide bywall thickness were prepared to NACE StandardTM-02-84.

HIC Test Procedure Total immersion tests (all side charging) of three

coupons of line pipe steel, each weighing ~100 g, werecarried out in a solution containing 5 wt% sodiumchloride (NaCl) saturated with H

2S at pH values of 1.1,

3.1, 3.7, and 4.3 obtained using HCl, potassiumhydrogen phthalate, HCl and sodium acetate, andacetic acid for 96 h to determine C

thH and pH

th values.

Compositions of the buffer solutions used to obtain pHvalues are listed in Table 2. At the end of 96 h,diffusible H in the coupons was determined by theJapanese standard method involving the displacementof glycerol by H at 45°C for 72 h.6

Three small samples weighing about 3 g eachalso were used in total immersion tests, and theirH content was analyzed by the hot extraction methodat 1,100°C using the Leybold-Hereaus apparatus. Gasvolumes reported were converted to standardtemperature, 273 K.

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Measurement of H FluxH flux was measured in one-sided charging

experiments using pipe sections 150 mm long filledwith solution. The pipe sections were fitted withpolycarbonate covers on the top and bottom. The topcover had openings for gas inlet and outlet, as well asfor the temperature probe and heater. The solutionswere deaerated with nitrogen overnight, saturated withH

2S and transferred into the pipe. H flux through the

steel was measured in the presence and absence of250 ppm of a commercial inhibitor used in sour servicein each of the following solutions at 60°C:

— NACE TM-02-84, synthetic seawater (ASTMD1141 E1-90) without heavy metal ions, pH range 4.8to 5.4 after saturating with H

2S;

— NACE TM-01-77, 5 wt% NaCl, 0.5 wt% aceticacid, initial pH of 2.8, final pH (before purging of H

2S

from solution) in the range of 2.8 to 3.5;7

— CSA medium of pH 3.5 saturated with H2S;

— And field process water, pH 7.0, saturatedwith H

2S.

Design, installation, and operation of the Hmeasuring probe used was described by Hay(Figure 1.)8,9 The probe contained a nickel/nickel oxideelectrode taken from a nickel-cadmium battery. Thesteel pipe was the second electrode. An oxidizingpotential was applied at the exit side of the metalsurface. As the H atoms diffused to the exit side, theyoxidized to H ions. This resulted in a measurablecurrent density. All paint and corrosion scale wereremoved from an area up to 80 mm from the center ofthe monitoring location. The area to be monitored(32 mm diam) was polished with no. 320 emery paperto remove pits and scratches. Methanol was used toclean the surface, masking tape was placed on thearea to be monitored, and epoxy was applied with afine brush on the prepared surface. The final thicknessof the film approximated 0.3 mm. After the film was dry,no. 320 emery paper was used to smooth the ridges,

CORROSION–JULY 1993


FIGURE 1. Schematic of H monitoring probe.

the masking tape was removed, and the monitoringarea was degreased with a solvent. The H-measuringprobe was attached to the pipe with a clamp.

Diffusion CoefficientsThe diffusion coefficient for H in the steel had to be

determined to calculate the H concentration at theinside wall of the pipe (C

0H). The experimental setup

and the procedure for determining the diffusioncoefficient were the same as used by Devanathan.10

The permeation cell was composed of two flasksconnected by glass joints which were grooved to takeO-rings. A clamp held the joints with the samplemembrane between them. The exposed specimenarea was 6 cm2. A 0.1 N sodium hydroxide (NaOH)solution was used for the anodic side, and a 0.1 Nsulfuric acid (H

2SO

4) solution (pH 2) containing

10 mg/L arsenic trioxide (As2O

3) was used for the

cathodic side. Both solutions were deoxygenated toreduce background current from reduction of oxygen.

A specimen was cut from a pipe wall andmachined to 40 mm (1.55 in) diam with a thicknessof 0.5 mm. The specimen was ground with SiCgrinding paper to 1200 grit. The specimen surfacewas polished with g-Al

2O

3 to 0.3 mm and rinsed in

acetone.


Prior to permeation measurement, the specimenwas plated with palladium on the anodic side toeliminate surface effects.

The entry side of the sample membrane(specimen) was exposed to H by cathodic polarization.On the exit side, the amount of H that permeated wasmeasured as a function of time. H was measuredelectrochemically by maintaining the exit side of thespecimen at a constant anodic potential of 250 mV

SCE.

At this potential, any H that reached the exit side wasionized. Under such conditions, the only anodicreaction which could take place was the oxidation ofatomic H to H ion. A potentiostat supplied the currentnecessary for H ionization without changing thepotential of the sample. The potentiostatic current (i)was a direct measure of the amount of H leaving theexit side of the specimen. This current was recorded asa function of time using a computer.

Buffered saline solution saturated with H2S was

not used for determining the diffusion coefficientbecause of formation of iron sulfide film on the internalpipe surface and its effect on the diffusion of H.Diffusion coefficients at 25, 40, 60, and 70°C weredetermined, and the activation energy was calculatedso that the diffusion coefficient at other desiredtemperatures could be calculated.

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TABLE 3Diffusible Hydrogen in AM-2 Data

pH of Weight of Diffusible Hot Extraction Immersion Steel Coupon Hydrogen Content STP Method* Solution (g) (mL/100 g, 25 °C) (mL/100 g) µmol/cm 3 (mL [STP]/100g)

4.8 102.98 1.3101.88 1.9 1.5 5.2 2.7101.64 1.8 1.6**

3.7 100.16 1.398.93 1.8 1.5 5.0 2.7

102.08 1.7 1.6**

3.1 100.48 1.1101.63 1.3101.72 2.2 1.5** 1.4 4.8 3.0

1.1 100.78 2.2101.44 1.9 1.8 6.3 3.3102.65 2.0 2.0**

* Leybold-Hereaus; ** Average value.

Glycerol Method (45 °C for 72 h)

Determination of CracksAn ultrasonic C-scan system operating at 5 MHz

and equipped with a modified C-scan recorder wasused to locate cracks in specimens. UltrasonicC-scans were obtained on line pipe test samplesbefore and after sample exposure to the HIC testmedia (Table 2).

RESULTS AND DISCUSSION

The data on diffusible H content obtained fromtotal immersion tests at different pH values arepresented in Table 3. The trend in H content movingfrom pH 1.1 to 4.3 was the same although absolutevalues obtained by the two methods of analysis —glycerol displacement and hot extraction — weredifferent. The glycerol displacement method formeasuring diffusible H involved placing calibratedcollector burettes on a flat polymethyl methacrylatestand in a glass container filled with glycerol andimmersed in a constant temperature bath at 45°C. Acoupon that had been washed, rinsed, dried andcharged in H

2S was placed in the collector tube. The

amount of diffusible H was the volume of H evolved per100 g of steel after 72 h.

The hot extraction method involved heating thesample at 1,100°C in a stream of high purity nitrogen.H was detected by the thermal conductivity method.This method measured total (diffusible and bound) H.

The H content of the samples was highest at pH1.1 and showed a decrease as pH increased to 4.3.

Ultrasonic C-scan measurements showed noevidence of cracking under severe test conditions.

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Because of the extremely high resistance of thissteel to HIC, it was not possible to define C

thH and pH

th.

H concentration at the inside surface of the pipe,C

0H, was determined using the H measuring probe

attached to the outside surface and by recording thechange in voltage as a function of time. A resistor(1 MΩ) was used to obtain the permeation current(Figure 2).The C

0H values were calculated from the

observed maximum of the permeation current using:5

peak C0H = IL

DF (1)

where 1 was the permeation current density, L was thewall thickness of the steel pipe, D was the diffusioncoefficient of H in the steel at the operatingtemperature, and F was the Faraday constant. Asexpected, the inhibitor addition resulted in reducedpermeation current density.

The diffusion coefficient was calculated by:11

I(t1)I(t2)

= t2t1

0.5 x e –L2

4D 1

t1 – 1

t2 (2)

where I(t1) and I(t

2) were the current densities at tines t

1

and t2, respectively. Four pairs of values of I(t

n) and t

n

were taken from the permeation curves [I(t1) = 5% I

max,

I(t2) = 10% I

max, I(t

3) = 15% I

max and I(t

4) = 20% I

max] and

were used to calculate six values of D, from which anaverage value of D was calculated.

The values of C0

H determined in the four mediastudied, in absence and presence of an inhibitor, are

CORROSION–JULY 1993


TABLE 4Hydrogen Concentration at the Inner Pipe Wall

Solution Without With(Temperature 60 °C) D x 10–6 cm2/s Inhibitor Inhibitor

Field water 4.8 5.0 ND(A)

TM-02-84 4.8 8.4 3CSA 4.8 10.0 3

TM-01-77 4.8 13.0 ND

(A)Not determined.

COH (µmol/cm 3)

FIGURE 2. H permeation curve in TM-02-84 medium.

FIGURE 3. Temperature dependence of diffusion coefficientfor H in linepipe steel.

presented in Table 4. The C0H values calculated from

the diffusion coefficient obtained by theelectrochemical charging method increased from thefield process water, through TM-02-84 and CSA to TM-01-77 – paralleling the decrease in pH. The C

0H values

obtained in the presence of the inhibitor were lowerthan the C

0H values obtained in absence of the

inhibitor. The C0H value for the pipe sample containing

H2S-saturated field process water was 5 µmol/cm3,

which was less than the threshold concentration(C

thH> 13) since ultrasonic C-scan patterns obtained

before and after exposure indicated no cracking.The diffusion coefficients for H in pipe samples

containing dilute H2SO

4 (pH 2.0) and 10 mg/L of As

2O

3

were measured at different temperatures (Figure 3).From the slope of the Arrhenius plot, the activationenergy for the diffusion of H in the line pipe steel wasfound to be 21 kJ/mol, which was similar to the values(e.g., 20 kJ/mol) quoted in literature.5 The temperature-dependence data of diffusion coefficients for H couldbe used to obtain the values at other temperatures.

CONCLUSIONS

Samples of a line pipe steel intended for use in sourservice showed no evidence of HIC when examinedusing ultrasonic C-scan after immersion in H

2S-

saturated saline solutions in the pH range between 1.1and 4.3. The experimental data showed the line pipesteel to be resistant to HIC even at pH as low as 1.1. The H concentration at the inside pipe surface (C

0H)

increased with the decreasing pH in both the presenceand absence of the inhibitor. The activation energy fordiffusion of H into the steel was 21 kJ/mol.

ACKNOWLEDGMENT

The author acknowledges partial funding for thiswork by the CSA Sour Gas Task Force.


REFERENCES

1. G.J. Biefer, MP 21, 6 (1982): p. 19.2. A. Ikeda, T. Kaneke, I. HashimotoI., M.Takeyama, Y. Sumitomo, T.

Yamura, Proc. Symp. on Effect of Hydrogen Sulfide on Steel, 22ndAnnual Conf. of Metallurgists, August 22-24,1983 (Edmonton, Canada:Canadian Institute of Mining and Metallurgy [CIM], 1983), p. 1-71.

3. W. Bruckhoff, “Rupture of a Sour Gas Line Due to Stress-Oriented,Hydrogen-Induced Cracking,” CORROSION/85, paper no. 389;(Houston, TX: NACE,1985).

4. NACE Standard TM-02-84, “Test Method – Evaluation of Pipeline Steelsfor Resistance to Stepwise Cracking,” (Houston, TX: NACE, 1984).

5. M.G. Hay, “Sour Gas Line pipes – The Need for Hydrogen-InducedCracking Resistance,” 39th Annual Meeting of the Petroleum Society ofCIM, paper no. 88-39-115 (Montreal, Canada: CIM, 1988).

6. Japanese Industrial Standard JIS Z 31 13-1975, “Method forMeasurement of Hydrogen from Deposited Metal,” (Toyko, Japan:Japanese Standards Assoc., 1975).

7. NACE Standard TM-01-77, “Test Method – Laboratory Testing of Metalsfor Resistance to Sulfide Stress Cracking in H

2S Environments,”

(Houston, TX: NACE, 1990)8. M.G. Hay, “An Electrochemical Device for Monitoring Hydrogen Diffusing

Through Steel,” Proc. Hydrogen Sulfide Symp., Conference ofMetallurgists, (Montreal, Canada: CIM, 1983).

9. ASTM Standard F-11-13-87, “Standard Test Method for ElectrochemicalMeasurement of Diffusible Hydrogen in Steels (Barnacle Electrode),”(Philadelphia, PA: ASTM, 1984).

10. M.A. Devanathan, Z. Stachurski, Proc. of the Royal Society A 270(London, U.K.: Royal Soc., 1962), p. 90.

11. J. McBreen, L. Nanis, W. Beck, J. Electrochem. Soc. 113, 11 (1966):p. 1218.

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HIC of Line Pipe Steels.pdf