IMPACT OF RESIDUAL STRESS ON THE SUSCEPTIBILITY OF CORROSION OF LONGITUDINAL WELDS IN API 5L X60 MICROALLOYED STEEL PIPE

V.M. Ventura-Sobrevilla; F.A: Reyes-Valdés

Corporación Mexicana de Investigación en Materiales Ciencia y Tecnología #790, Saltillo 400

Saltillo, Coahuila México 25290

D.I. Martinez-Delgado

Universidad Autónoma de Nuevo León Facultad de Ingeniería Mecánica y Eléctrica Av. Universidad S/N, Ciudad Universitaria

San Nicolás de los Garza, N.L. México 66450

F. Almeraya-Calderón

Centro de Investigación en Materiales Avanzados S.C. Miguel de Cervantes #120 Chiahuahua, Mexico 31190

SUMMARY This article presents the results obtained from experiments done to measure the impact that residual stress generated during the phase of longitudinal welding and cold expansion has on API 51 X60 carbon steel line pipe during the manufacturing phase. The electro-chemical potentiodynamic technique was used to determine the behavior of different areas of the joint in filler metal as well and in the area affected by the heat and the adjacent metal base. We also designed special equipment for this purpose, since we have to measure the entire ring, seeing that if we divide it, the stress relaxes and disappears. The curves obtained present an increasing relationship as to the velocity of corrosion when the residual stress increases. Results are analyzed and discussed. Key words.- Residual Stress, welding, potentiodynamic curves,

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Paper No.

4048

INTRODUCTION The pipe designed for sour gas service is susceptible to corrosion. A combination of chemical attack and residual stress are present in the pipe. 1 Residual stress present in the pipe is generated mainly through the process of submerged arc welding, in which two lines are applied, one exterior and one interior where axial direction stresses are bigger than hoop direction. 2 After, the pipe is cold expanded, reestablishing the linearity and reducing stress. 3 This project is divided in two phases, first the susceptibility to corrosion in general and second the susceptibility to hydrogen damage in accordance with the permeation of this species. The residual stress distribution thermally generated during the welding process, have variation during the cooling, from high to cero of the thermal gradient. When the temperature gradient is higher, the compressive stresses are generated in regions near the weld, and tension in the away one, because the expansion of the weld zone has restriction by the cold metal surrounding. 4 After that the weld had been cooling and the temperature gradient is cero, the tension residual stress had been generate in the weld area, and compression residual stress in the away zones, by means of contractions by cooling, 4 see Figure 1.

Change in stresses during welding

Transversal distance from welding direction

Str

esse

s

Melted regionCooled region

Figure 1. Change in stresses during welding

EXPERIMENTAL PROCEDURE For this experiment we used API grade line pipe with three different levels of mechanic expansion. Material used. The chemical composition is shown in Table 1. Mechanical properties of the base metal are shown in Table 2.

TABLE 1 CHEMICAL COMPOSITION OF BASE METAL USED TO THE PROJECT.

API 5L X60 Chemical Composition C S Mn P Si Cr Ni Mo Cu V Nb Ti .20 .015 1.19 .007 .43 .06 .03 .05 .06 .008 .029 .010

Chemical Composition of Filler Metal C S Mn P Si Cr Ni Mo Cu V Nb Ti .18 .013 2.55 .026 .903 .01 .02 .494 .46 .003 .038 .003

0 Weld bead

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TABLE 2. MECHANICAL PROPERTIES OF BASE MATERIAL STUDIED

API 5L X60 Tensile Strength psi Min. Yield Point psi Hardness (HRB) 81,221 (560 MPa) 60,045 (414 MPa) 86

The microstructure of the base metal and of the welding joint is shown in figures 2 and 3 respectively.

Figure 2.- Base metal microstructure shown Perlite islands in a Ferrite matrix. 100X, etched with

Nital reactive.

Figure 3.- Weld zone microstructure shown integranular Ferrite, acicular Ferrite and lower

Bainite. 100X. etched with Nital reactive Three welded joints were studied, all with different cold expansion and, therefore, with different residual stress. See Table 3.

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TABLE 3 RELATION OF SAMPLES EVALUATED, PERCENT OF EXPANSION AND RESIDUAL

STRESS. % of expansion Residual stress psi

.6 7592.07 ( 52.34 MPa)

.8 6316.67 (43.55 MPa) 1 6052.18 (41.72 MPa)

In order to calculate the residual stress, the ring slitting was applied, by means of the equation 12

S =Et(1/Do – 1/Df) (1)

Where : S, is the circumferential residual stress, psi E, is the Young Module t, thickness, in Do, original mid-wall diameter Df, final mid-wall diameter Test procedures. Two one-centimeter squared areas in each ring included the base metal and welding zone, were evaluated. These areas were polished with silicon carbide to grid 600 and rinsed with acetone. 5 The rest of the ring was coated with elastomeric paint that ensured the isolation of the test portion of the ring. Figure 4 shows the preparation..

Figure 4. Preparation of the sample

An acrylic cell was designed that measured 33” X 33” X 14” with holes for connecting the electrodes.6 The acrylic guarantees that the test will be isolated and it is a good material since it does not react with the electrolyte. In order to perform the potentiodynamic test, three electrodes array was used, a graphite bar as an auxiliary6 8 and a calomel electrode as a reference one 7 8, see figure 5. The electrolyte used was brine with H2S. The composition of the electrolyte 9 is shown in Table 4.

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TABLE 4 COMPOSITION OF THE BRINE USED IN THE STUDY

Reactive Gr.

NaCl 10,7 CaCl2 0,3392 MgCl2-6H2O 0,2068 Na2SO3 0,0004449 Distilled water 85,16 Acetic acid 0,17 Sodium sulfide 0,353

In the reference electrode we used a saline bridge composed of sodium chloride to a 3.5% weight. Three tests per area were developed of each ring, which were performed in an oxygen-free environment obtained through the introduction of nitrogen gas into the acrylic cell and then maintaining this condition with a constant flow in the cell. As seen in figure 5, the acrylic cell is hermetically sealed and the electrolyte only comes into contact with the exposed surface of the ring. A visual method for the evaluation of the presence of oxygen in the cell during the test is the presence of oxygen bubbles on the working electrode. See figure 5.

Figure 5.- Test equipment installed

Results and discussion Van Boven 10,11 has suggested that residual tension stress acts as anodic zones while the residual compression stress acts as cathode zones. The polarization curves obtained in the metal base, figure 6, show a lower cathode curve and an active anodic curve which suggests a relationship with the presence of tension stress. The potential of corrosion varies with respect to the percentages of expansion; inversely proportional.

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Figure 6.- Potential dynamic curves in the base metal at different levels of circumferential tension stress.

In figure 6 we can see the results obtained in the evaluation of the line of welding, suggesting a relationship with respect to the level of mechanical expansion (residual stress). 1% is the level with less circumferential tension stress that sho ws less potential of corrosion. We can see at the potentiodynamics curves of the welding zone (Figure 7), that the anodic curves are in activation state, it shown a higher corrosion density in the high more expansion areas. This was corroborating by visua l inspection, the base metal shown more corrosion evidence than the weld zone. Figure 8 shows the total potentiodynamic curves; we can see that at the same percent of expansion, the base metal shown high corrosion density than the weld zone. The Table 5 shows the results of the test.

Figure 7. Potentiodynamic curve in the weld at different levels of residual stress

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Figure 8.- Potentiodynamic curves in the base metal and weld.

TABLE 5

FINAL RESULTS

% Expansion Residual Stresses

(psi) Zone

Io (Amp/cm2)

Eo

(Volts)

Weld 0.00072932 -0.68861 1% 6052.18

Base Metal 0.0010907 -0.61623

Weld 0.0010508 -0.60819 0.80% 6316.67

Base Metal 0.0013647 -0.60094

Weld 0.0020735 -0.60278 0.60% 7592.07

Base Metal 0.0027338 -0.57933

In figure 9 we graphically show the Io for the base metal and the welded area with respect to the different percentages of expansion described in Table 5. The differences between the base metal and the line of welding can be observed as well as the change with respect to the percentage of expansion.

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Current Density

00.0005

0.001

0.0015

0.0020.0025

0.003

1% 0.80% 0.60%

% Expansion

Io (

Am

p/cm

2)

Weld

Base Metal

Figure 9.- Results of the current density between the base metal and the line

The base metal showed more corrosion susceptibility than the weld bead, we assume that the reason is the Mo content of the filler metal. The next step of the project will evaluated the corrosion susceptibility at more than 1% of mechanical expansion, to know the maximum expansion recommended for this kind and size of pipes.

CONCLUSIONS

• The study of the effect of residual stress and its relationship to the rate of corrosion can be analyzed using the potentiodynamic technique, conserving the circumferential residual stress in the work electrode.

• The designed test equipment is not adequate to evaluate the corrosion susceptibility in relation to

axial residual stress. • The corrosion susceptibility increase in a directly proportional relation with the tension residual

stress.

• The base metal at given conditions of expansion showed a greater rate of corrosion, with respect to the weld zone, this for the Mo content of the filler metal.

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REFERENCES

1 P.F. Timmins "Solutions to hydrogen attack in steels"p 112-113

2 Michael law, Thomas Guaëpel-Herold, Vladimir Luzin, Graham Bowie"Neutron residual stress measurements in line pipe"

3 B. Arellano Ayala "Efecto del proceso de expansión en soldadura por arco sumergido con aplicación de 2 y 3 arcos en la fabricación de tubería para uso en ambiente amargo" p 8-21

4 Sindo Kou, “Welding Metallurgy” p 122-130

5 Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens (ASTM G1-03, 2003)

6 Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements(ASTM G5-94, 1994)

7 Standard Practice for Conventions Aplicable to Electrochemical Measurements in Corrosion Testing (ASTM G3-89, 1989)

8 B. Vargas Arista "Analisis del comportamiento mecanico, microestructural y corrosion del envejecimiento artificial en la uniones soldadas de tubo de acero API 5L grado X 52" p 54- 134

9 Nace 1D-182 wheel test method used for evaluation of film-peristent Corrosion Inhibitors for oilfield applications 1995

10 G. Van Boven, W Chen, R. Rogge "The role of residual stress in neutral pH stress corrosion cracking of pipeline steels. Part I: Pitting and cracking occurrence.

11 G. Van Boven, W Chen, R. Rogge "The role of residual stress in neutral pH stress corrosion cracking of pipeline steels. Part II: Crack dormancy

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