Corrosion Fatigue Behavior of Carbon Steel in Drilling Fluids

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Corrosion fatigue of carbon steel (CS) in drilling fluids was studied using a self-made rotary bending corrosion fatigue testing apparatus under simulated drilling ...
CORROSION ENGINEERING SECTION

Corrosion Fatigue Behavior of Carbon Steel in Drilling Fluids F. Chaoyang and Z. Jiashen*

ABSTRACT Corrosion fatigue of carbon steel (CS) in drilling fluids was studied using a self-made rotary bending corrosion fatigue testing apparatus under simulated drilling conditions. Mechanisms of the effects of cyclic stress, chloride (Cl–), sulfide (S2–), and pH of drilling fluids on corrosion fatigue of CS as well as the inhibiting action of the imidazoline inhibitor and oxygen (O2) scavenger sodium sulfite (Na2SO3) on corrosion fatigue were studied. Results showed Cl– and S2– promoted corrosion fatigue crack initiation and growth. Fatigue life was lengthened after reducing subjected stress, increasing the pH of the drilling fluids, or adding the inhibitor and O2 scavenger. KEY WORDS: drilling fluid, fatigue, imidazoline, inhibitor, oxygen scavenger, pH, stress, type 45 carbon steel

EXPERIMENTAL

Material and Testing Mud The material studied was type 45 CS (UNS G10450).(1) Table 1 shows the chemical composition and mechanical properties of type 45 CS. Table 2 shows the testing polymer mud composition.

Corrosion Weight-Loss Test

INTRODUCTION During drilling, the drill pipe revolves in aggressive drilling mud in deep wells. Under cyclic stresses such as bending stress and torsion and the corrosive actions of dissolved oxygen, hydrogen sulfide (H2S), and salts, drill pipe corrosion fatigue failure can cause enormous losses.1-2 Submitted for publication October 1996; in revised form, July 1997. * Huazhong University of Science and Technology, Wuhan, P.R. China, 430074. (1) UNS numbers are listed in Metals and Alloys in the Unified Numbering Systems, published by the Society of Automotive Engineers (SAE) and cosponsored by ASTM.

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In the present work, actual working conditions of drill pipe were simulated to study mechanisms of the effects of chloride (Cl–), sulfide (S2–), and pH on corrosion fatigue of carbon steel (CS) in drilling fluids, along with the action of a corrosion inhibitor, using a self-made rotary bending corrosion fatigue apparatus.

A wheel test method was used to measure the dynamic corrosion rate of CS. The rotation rate was 28 cpm, and the testing period was 16 h at 80°C ± 2°C. Each bottle contained 500 mL (0.132 gal) drilling mud and two specimens. The size of each specimen was ␾3 x 150 mm (␾0.118 x 5.906 in.) (diameter and length). The corrosion rate was calculated by weight-loss methods.

Electrochemical Measurement The tests were conducted in a three-electrode electrochemical cell. The working electrode (cylinder of an area of 1 cm2 [0.155 in.2]) was masked with epoxy resin. Platinum and saturated calomel elec-

0010-9312/98/000143/$5.00+$0.50/0 © 1998, NACE International

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TABLE 1 Chemical Composition and Mechanical Properties of Type 45 CS C

Si

Mn

S

P

0.43

0.23

0.66

0.014

0.002

Yield Strength (MPa)

Tensile Strength (MPa)

Elongation (%)

Reduction in Area (%)

Rockwell Hardness (HRC)

621

365

252

53.7

23.6

Chemical composition (wt%)

Mechanical properties

TABLE 2 Composition of Testing Mud (wt%) Na-bentonite

Hydrolysate of Polyacrylonitrile

Polyanionic Cellulose Derivative

NaCl

Water

~ 4.0

~ 0.2

~ 0.4

~ 1.0

Residential

trodes (SCE) were used as counter and reference electrodes, respectively. Polarization curves were obtained using the dynamic potential scanning method.

Corrosion Fatigue Test A cantilever-type rotary bending corrosion fatigue testing apparatus was used to simulate drill pipe movement in flowing mud. The fatigue test sample was a sub-size simulated drill pipe, the load frequency was 1.3 Hz, and the stress ratio was –1. The stress was calculated using:3

carried out using a two-compartment cell separated by a steel membrane that was 0.01-cm (0.0039-in.) thick and abraded to an 800-grade emery finish.5 The test environment was set in one of the compartments, and 0.2 mol/L sodium hydroxide (NaOH) solution was placed in the other, which was the current measuring compartment. The current required to oxidize the hydrogen permeating through the specimen was measured and used, after subtracting the background passive current, to calculate the steady-state hydrogen permeation current density (iH), the hydrogen permeation coefficient (DH), and hydrogen atom concentration on the surface (C0).

M W

(1)

RESULTS AND DISCUSSION

M = PL

(2)

Effect of Cyclic Stress on Corrosion Fatigue of Type 45 CS

σ=

The fatigue fracture surface was observed using a scanning electron microscope (SEM). Prior to observation, the specimen was cleaned in 10% hydrochloric acid (HCl) + 1% acidic inhibitor for 10 min to remove the corrosion products (iron oxide or sulfide) and then dipped in acetone for 15 min.

The experimental stress/cyclic number (S/N) curve of type 45 CS in drilling fluid is shown in Figure 1. Fatigue life decreased with increasing cyclic stress. For stress changes of ~ 4 times, from 100 MPa to 378 MPa, the corrosion fatigue life of CS decreased 30 times (from 3.1 x 105 to 1.0 x 104 cycles). Compared to life in air (no crack fatigue initiation after 106 cycles under stress of 142 MPa), the fatigue life of CS was shortened greatly in the presence of a corrosive media. Both mechanical and environmental factors played an important role in the process of crack initiation and propagation, and the mechanical factor was more prominent. In reality, increasing the gap of the well ring adequately or using drilling fluids with good lubricity could lengthen the life of drilling pipe by reducing the applied stress.

Hydrogen Permeation Experiment

Effect of Cl– on Corrosion Fatigue of Type 45 CS

The hydrogen permeation rate in CS was measured using a potentiostat.4 These experiments were

The corrosion fatigue life of type 45 CS in solutions of different sodium chloride (NaCl) concen-

W=

π 32

3

D 1–

d

4

D

(3)

where M is the force moment, W is the bending coefficient, P is the weight of load, L is the length of the force arm, D is the outside diameter of the test sample, and d is the inside diameter.

Fatigue Fracture Analysis

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trations (CNaCl) are shown in Figure 2 (under cyclic stress of 142 MPa). Results showed that increasing CNaCl decreased corrosion fatigue life little by little. Fatigue life changed little until CNaCl reached 3%. There were many corrosion spots on the outside of the sample, and the pits were remarkable for CNaCl > 3%. Pitting and cleavage steps existed on the fracture surface, and the steps in 3% NaCl solution were sharpest. Crack propagated from the outside to the inside and disappeared in the static fracture zone. SEM results indicated obvious corrosion pits and brittle fatigue striations. The fractures in NaCl were intergranular quasicleavage (Figure 3). Corrosion rates of CS measured using the wheel test (Table 3) demonstrated that the corrosion rate varied with CNaCl. The corrosion rate was maximum in 3% NaCl solution and minimum in saturated solution (23%). Oxygen content in the solutions was reduced linearly with increasing CNaCl. The corrosion action of CS in salt solution was dominated by the content of Cl– and oxygen. The effect of oxygen was essential. All of the corrosion patterns in salt solution were pitting corrosion. Breakdown potentials (Eb) of type 45 CS in solutions of different NaCl concentrations were measured using the electrochemical polarization curve method. There was a linear correlation between Eb and the common logarithm of CNaCl (Figure 4):

Eb = –167 × logCNaCl – 659

FIGURE 1. Stress life relationship of type 45 CS.

(4)

Equation (4) indicates that the higher CNaCl was, the easier it was to pit type 45 CS in aqueous solution. With increasing CNaCl, pits occurred and grew more rapidly on the surface of CS.6 Under the action of cyclic stress, the active dissolving of electrochemical self-catalysis through anodic dissolving along the stress concentration zone, the crack occurred and propagated to failure. The action of oxygen depolarization dominated crack propagation.7

FIGURE 2. Effect of CNaCl on fatigue life of type 45 CS.

Effect of pH of the Drilling Fluid on Corrosion Fatigue The pH values of the drilling fluid were adjusted to 5.5, 8.5, 9.5, and 11, respectively, using 10% HCl and 1 mol/L NaOH. Figure 5 shows the fatigue life of CS in various pH drilling fluids (␴ = 142 MPa). In blank drilling fluid (pH = 7.5), the CS corrosion fatigue life was longer than in salt solution. At pH 5.5, the corrosion fatigue life was reduced to about half. With higher pH, the fatigue life increased correspondingly. At pH 11, life increased by > 100%. SEM examinations (Figures 6[a] through [c]) showed pits and irregular fatigue striations on the fracture surface in blank drilling fluid (pH = 7.5). The crack showed intergranular rupture. At pH 5.5,

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FIGURE 3. SEM micrograph of type 45 CS corrosion fatigue fracture in 3% NaCl solution (500x).

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TABLE 3 Effect of CNaCl on Corrosion Rate of CS CNaCl (%)

0

0.1

1

3

5

10

23

Corrosion rate (g/m2-h) Oxygen content (mg/L) Corrosion pattern

1.56 9.64 Pit

1.59 9.4 Pit

1.71 9.22 Pit

1.90 8.59 Pit

1.66 7.93 Pit

1.48 6.57 Pit

0.87 1.67 Pit

(a)

FIGURE 4. Effect of CNaCl on Eb of type 45 CS.

(b)

FIGURE 5. Effect of pH on corrosion fatigue life of type 45 CS.

there were obvious hydrogen-induced cavities and crack propagation along cavities. At pH 11, the crack fracture surface exhibited quasicleavage fracture. The corrosion rate of CS depended upon pH of the drilling fluid. The higher the pH of the drilling fluid was, the weaker the corrosion of the CS. As shown in Figure 7, the polarization curves moved to the left, and corrosion potentials moved positively

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(c) FIGURE 6. SEM micrograph of type 45 CS fracture in drilling fluid (500x): (a) blank (pH = 7.5), (b) pH = 5.5, and (c) pH = 11.

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with increasing pH. For the range of testing, the pH of the drilling fluid mainly altered the anodic behavior of the CS. In low-pH drilling fluid, corrosion of the CS was very serious because of the combined action of hydrogen and oxygen depolarization. In addition, the action of the oxygen concentration cell and a local acidic catalysis effect accelerated corrosion. Hydrogen atoms penetrated into the surface of metal and created cavities to promote crack initiation and growth. Under the action of strong local corrosion, the crack propagated rapidly to reduce fatigue life greatly. High-pH drilling fluid showed only oxygen depolarization, and oxygen transmission was obstructed by the poor mobility of the drilling fluid, which slowed the action of corrosion and further slowed crack initiation and growth.

FIGURE 7. Polarization curves of type 45 CS in various pH drilling fluids.

Effect of Sulfide on Corrosion Fatigue Sodium sulfide (Na2S) was added to the drilling fluid, and the pH of fluid adjusted the same as the blank by using 10% HCl to study the effect of S2– on corrosion fatigue of CS (␴ = 142 MPa). Figure 8 shows that corrosion fatigue life was shortened quickly with addition of Na2S. Fatigue life decreased to one-quarter when the concentration of Na2S (CNa S) was 1,000 mg/L. The SEM fracture micrograph showed that the fracture with quite a few cavities that gradually propagated into cracks was an intergranular brittle one (Figure 9). Table 4 shows that, after adding Na2S, DH and C0 both increased. As S2– could inhibit hydrogen atoms from combining to hydrogen molecules, hydrogen atoms penetrated into the metal matrix and remained within the grains to form cavities which, under the action of cyclic stress, caused crack initiation and propagation.8 2

FIGURE 8. Effect of S2– on corrosion fatigue life of type 45 CS.

Inhibiting Effect of Inhibitor on Corrosion Fatigue The effect of the corrosion inhibitor (designated DPI-2) and the O2 scavenger sodium sulfite (Na2SO3) on the corrosion fatigue life of CS in drilling fluid is shown in Figure 10 (␴ = 142 MPa). The addition of inhibitor had a fairly good restraining action on fatigue cracking of CS in drilling fluid. Inhibitor DPI-2 is an adsorption type of imidazoline inhibitor of low concentration and gives only definite protection to the incomplete adsorption film on the surface of metal. At a high concentration, the inhibitor film is complete and can inhibit the anodic dissolution of metal so that it can suppress crack initiation and propagation. Compared with the blank drilling fluid (Figure 6[a]), the pits on the fracture surface were decreased appreciably with the inhibitor added. The fracture in the inhibiting mud was a quasicleavage one (Figure 11).

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FIGURE 9. SEM fracture micrograph of type 45 CS in drilling fluid with 1,000 mg/L Na2S (500x).

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TABLE 4 Effect of Na2S on Hydrogen Permeation of CS in Drilling fluid CNa2S (mg/L)

iH (A/cm2)

t1/2 (s)

DH (cm2/s)

C0 (mol/cm3)

0 50 100 1,000

5.5 x 10–6 1.1 x 10–4 2.7 x 10–4 4.2 x 10–4

600 135 71.3 48.8

9.2 x 10–8 4.1 x 10–7 7.8 x 10–7 1.1 x 10–6

1.3 x 10–5 5.4 x 10–5 7.2 x 10–5 7.6 x 10–5

FIGURE 10. Effect of inhibitor on corrosion fatigue life of type 45 CS in drilling fluid.

FIGURE 11. SEM fracture micrograph of type 45 CS in drilling fluid with 2,000 mg/L inhibitor (500x).

CONCLUSIONS ❖ The fatigue life of CS depended upon the load stress in actual drilling. The life of the drill pipe could be prolonged by increasing the gap of the well ring adequately or by using drilling fluids of good lubricity to reduce the working stress. ❖ In NaCl solution, there was a linear correlation between Eb of type 45 CS and the percentage of NaCl concentration: Eb = –167 x log CNaCl – 659. Corrosion fatigue failure of CS was accelerated by Cl–-induced

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pitting. O2 played an important role in the process of crack initiation and propagation. ❖ Corrosion fatigue life of CS depended greatly upon the pH of drilling fluid. Increasing pH of the system lengthened the corrosion fatigue life of the material. ❖ S2– was detrimental to the corrosion fatigue of CS in drilling fluid, because it accelerated hydrogen permeation to promote hydrogen-induced cracking. ❖ The imidazoline inhibitor and the O2 scavenger markedly suppressed corrosion fatigue failure of CS in drilling fluid. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

N. Huaiying, Natural Gas Industry 2, 3 (1982): p. 56. H. Mingqu, H. Chengzu, Oilfield Chem. 5, 1 (1988): p. 43. Z. Jiashen, Z. Jingmao, Corrosion 49, 3 (1993): p. 256. S. Shizhe, Electrochemical Methods on Corrosion (Beijing, China: Chemical Industry Publishing House, 1988), p. 46. M.A.V. Davanathan, Z.O.J. Stachurski, J. Electrochem. Soc. 111 (1969): p. 619. Z. Xiangyang, K. Wei, Acta Metall. Sinica 28, 8B (1992): p. 356. L. Mingqi, S. Yue, T. Zhishen, J. Chin. Soc. Corros. Prot. 4, 1 (1984): p. 61. F. Zhizhong, P. Fangming, et al., J. Huazhong Univ. Sci. Tech. 23, Supp. 2 (1995): p. 148.

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