AFM Study of the Corrosion of Pipeline Steel in Organic Compounds Extracted from Soil

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1 / The Electrochemical Society AFM Study of the Corrosion of Pipeline Steel in Organic Compounds Extracted from Soil K.A. Yasakau a, C.R. Weber b, L.M. Rodrigues b, M. Zheludkevich a, M.G. S. Ferreira a and L.F.P. Dick b, c a University of Aveiro, CICECO, Department of Ceramics and Glass Engineering, , Aveiro, Portugal, b Department of Metallurgy and c Electron Microscopy Center, Federal University of Rio Grande do Sul, Av. Bento Gonçalves, 9500/75, , Porto Alegre, Brazil Aiming to simulate the external corrosion of pipelines, the first stages of the inclusions attack and nucleation of pits were studied by AFM on the API 5LX56 steel in pure 10 mm Na 2 SO 4 solutions with additions of humic and fulvic acids, both extracted from a Brazilian peat soil. For this, selected inclusions were first marked with micro indentations, analyzed in the SEM in the backscattering mode and chemically mapped with EDS. Pit formation was then investigated by scanning Kelvin probe force microscopy (SKPFM). The SKPFM evidenced that HA added to sulfate solutions promotes the fast dissolution of the sulfides of complex inclusions, resulting in pit nucleation, while FA additions slow this attack, acting as a natural corrosion inhibitor. Introduction Different complex inclusions are precursor sites for pitting corrosion of stainless steels (1-8). For these materials, the corrosion studies were always performed in very aggressive media. The first step leading to pit initiation is the dissolution of MnS inclusions and the adsorption of the sulfur species released into the solution on the surrounding passive film (9). Corrosion behavior of carbon steels exposed to dilute sulfate media and sulfate containing humic and fulvic acids was investigated in previous works (10, 11). Now, the subject for investigation is the localized corrosion activity of inclusions during the short exposure to humic substances solutions i Humic substances are organic compounds with very high molecular weights, ranging from a few hundreds to 10 5 Daltons (12). They are classified according to their phdependent solubility in water. The humic acid (HA) is the fraction which is not soluble in ph values lower than 2, while fulvic acid (FA) is soluble in the whole ph range (12). In this work, we studied by AFM techniques the behavior of complex inclusions on the localized corrosion of the pipeline steel API 5LX56 in dilute sulfate media with additions of humic and fulvic acids. The aim was to simulate the external corrosion of the underground pipeline steels. The atomic force microscopy (AFM) has been used in the study of the surface morphology of metallic samples and different coatings. More recently, the surface

2 108 morphology and corrosion resistance of different materials, as galvanized steel (13) and sol-gel treated AA2024-T3 alloy were studied by AFM (14). AFM had been also used coupled with scanning Kelvin probe (SKPFM) for studying the inhibiting action of organic substances. The inhibiting effect of several organic compounds, as quinaldic acid, salicylaldoxime, 8-hydroxyquinoline (15), 2,4-triazole, 3- amino-1,2,4-triazole, benzotriazole and 2-mercaptobenzothiazole (16) on the corrosion of AA2024-T3 alloy in neutral chloride solution was investigated via SKPFM. In this case, the inhibiting action is the consequence of suppression of dissolution of Mg, Al and Cu from the corrosion active intermetallic zones (15). Inclusions in AA2024-T3 alloy were also studied by SKPFM, the corrosion behavior of a population of inclusions and the interactions between individual inclusions. The results showed that Al-Cu-Fe-Mn precipitates were cathodic in nature and interacted strongly with their surroundings. The size and location of the cathodic regions proved to be critical issues. Accelerated matrix dissolution took place for exposed areas containing a large area fraction of Al-Cu-Fe-Mn inclusions. The location of the cathodic sites had a strong influence on the local chemistry of the environment and controlled the corrosion behavior of the surface (17). Experimental Samples with a geometric area 1cm 2 were cut from the API 5LX56 pipeline steel, obtained from TRANSPETRO. The nominal composition and chemical analysis by optical emission are presented in the Table 1. The API 5LX56 steel contained rounded inclusions. These modified inclusions are usually obtained by final additions of Al, Si, and Ca to the steel melt, according previous paper (10). These samples were polished with mineral oil to avoid H 2 0-contact, down to 0.2 with diamond paste and rinsed with alcohol. The API 5LX56 inclusions were first marked with Vickers micro indentations and were then microscopically analyzed. TABLE 1. Nominal Composition and Chemical Analysis (wt %) of the API 5LX56 steel. Nominal Composition (maximum values) C Mn S P Chemical Analysis C Mn S P Si Al Cr Mo Ni Ca Cu Nb Ti V W Fe The corrosion attacks were carried out at 25ºC in aerated pure 10mM Na 2 SO 4 solutions, or with additions of 3 gl -1 humic acid (HA) or 7 gl -1 fulvic acid (FA), according to the solubility of these substances for a ph value of 7. A drop of solution was placed in a zone with marked inclusion for 2 or 10 seconds. Therafter, the drop was removed by fast rinsing with under a stream of 2-propanol and dried in a flow of air.

3 109 Raman spectra of the samples were obtained using a Dilor-Jobin Yvon Spectrophotometer attached to an Olympus BX60M optical microscope. The laser power on the samples was 12 mw, using the nm line of a He-Ne laser. All spectra were recorded using a pinhole with a diameter of 600 µm, resulting from 1 accumulation of 50s without filters. Pure HS samples were prepared as pellets containing 100mg of HA or FA. The microscopy analyses were performed with a JEOL JSM-5800 scanning electron microscope (SEM) provided with an energy dispersive x-ray spectrometer (EDS) and by using minerals standards and measurement of the probe current for a higher analytical accuracy. The inclusions were analyzed before and after of the attack using SEM in the mode backscattering electron imaging (BEI) and secondary electron imaging (SEI), while EDS was performed using standards. The initial morphology of the inclusions was analyzed in a SPM J3 atomic force microscope Atomic Force Microscope. An equipment Digital Instruments NanoScope III system with the Extender electronic module was used for AFM and SKPFM measurements. In SKPFM mode AFM was operating in interleave mode with two pass scans. The first scan acquired the topography in tapping mode, while during the second scan piezoelectric actuator was switched off, tip was lifted on 100nm from the surface and AC amplitude 1000 mv was applied between the tip and sample to induce electrostatic perturbations of cantilever. Using the nulling technique, the Volta potential difference was measured along the line scan. For all experiments, silicone probes covered by Cr-Pt layers were used. The Volta potential map and topography of the inclusion were collected before and after corrosive exposure. Results and Discussion The Figure 1 shows a backscattering electron image of the inclusion of the API 5LX56 steel. The marked points in the Figure 1 correspond to the Table 2 which shows detailed EDS analysis about the chemical composition before and after attack during 2s in pure Na 2 SO 4 solution. The composition of inclusion can be described as complex calcium aluminosilicates. There can be seen sulfur containing compounds at the boundary between the iron-matrix and inclusion. After a 2s exposure in 10 mm Na 2 SO 4, the Fe content on the sulfide region always decreases, probably because Fe is dissolved from a Fe-Mn-containing inclusion. The inclusions can be described as complex calcium aluminosilicates surrounded by a sulfide layer Figure 1. SEM-view (BEI) of the inclusion before attack.

4 110 TABLE 2. Composition of inclusion in at % by EDS. Before attack O Mg Al Si S Ca Mn Fe After 2s in 10mM Na 2 SO 4 O Mg Al Si S Ca Mn Fe Fig. 2 shows the morphology of the inclusion before (a, a 1 ) and after (b, b 1 ) a short time exposure (2s in 10mM Na 2 SO 4 ) by SEM in SEI-mode and AFM. Analysis of the topography of AFM images shows a deformation ring in the border of the inclusion, which can be associated with different hardness and polishing rate of the inclusion in such places. The formation of small amounts of corrosion products can also be observed after 2s of attack. Fig. 3 shows the SEM-view of the inclusion of the API 5LX56 steel in the BEI-mode before attack. EDS-analysis showed the higher content of oxides in the inclusions center and of S compounds in its outer region. As previous, the marked points in the Fig. 3 correspond to the Table 3, which shows EDS measurements before and after a 10s attack in pure Na 2 SO 4 solution. After this exposure time, the Fe-content continuously decreases, probably because to its selective dissolution.

5 µm (a 1 ) Figure 2. SEM view (SEI) and AFM of inclusion before attack (a, a 1 ) and after 2s in 10mM Na 2 SO 4 (b, b 1 ) Figure 3. SEM-view (BEI) of the inclusion before attack.

6 112 TABLE 3. Composition of inclusion in at % by EDS. Before attack O Mg Al Si S Ca Mn Fe After 10s in 10mM Na 2 SO 4 O Mg Al Si S Ca Mn Fe The Figure 4 shows the morphology of the inclusion before (a, a 1 ) and after (b, b 1 ) 10s time exposure in 10mM Na 2 SO 4 by SEM in mode SEI and AFM. In AFM photos is possible to see the formation of small amount of corrosion products after attack in 10s.

113 2. 1µm Figure 4. SEM view (SEI) and AFM of inclusion before attack (a, a 1 ) and after 10s in 10mM Na 2 SO 4 (b, b 1 ).

of attack (b, b 1, b 2 ) in Na 2 SO 4 solution containing HA respectively.

The absence of corrosion products may be a result of the short immersion time or due to the influence of rinsing with 2-propanol

7 µm Figure 4. SEM view (SEI) and AFM of inclusion before attack (a, a 1 ) and after 10s in 10mM Na 2 SO 4 (b, b 1 ). Figure 5 shows SEM images of the inclusion, Volta potential maps and Volta potential profiles before (a, a 1, a 2 ) and after 2s of attack (b, b 1, b 2 ) in Na 2 SO 4 solution containing HA respectively. Notice the dissolution of the zone that is close to the Fe matrix. The absence of corrosion products may be a result of the short immersion time or due to the influence of rinsing with 2-propanol after the immersion test. It should be noticed the modification of the Volta potential map after immersion. It seems that a small layer of inclusion have been dissolved or modified, which resulted in the increase of Volta potential over the inclusion.

114 Volta Potential, mv 30 before attack 20 (a 10 2 ) 0-10 -20-30 -40-50 0 5 10 15 20 25 30 X, µm Volta potential, mv 30 20 10 0-10 -20-30 2s exposure in Na 2 SO 4 / HA

Figure 6 shows SEM images of the inclusion, Volta potential maps and profiles, before (a, a 1, a 2 ) and after 10s of attack (b, b 1, b 2 ) in a HA-Na 2 SO 4 solution,

8 114 Volta Potential, mv 30 before attack 20 (a 10 2 ) X, µm Volta potential, mv s exposure in Na 2 SO 4 / HA (b 2 ) X, µm Figure 5. Topography (a, b), Volta potential map (a 1, b 1 ) and Volta potential difference across the profile (a 2, b 2 ), before (a, a 1, a 2 ) and after 2s in 10mM Na 2 SO 4 with HA addition (b, b 1, b 2 ). Figure 6 shows SEM images of the inclusion, Volta potential maps and profiles, before (a, a 1, a 2 ) and after 10s of attack (b, b 1, b 2 ) in a HA-Na 2 SO 4 solution, respectively. After a10s attack, it is possible to observe huge precipitates of corrosion products covering the whole inclusion. That indicates a strong dissolution of Fe caused by the presence of HA in the solution. The Volta potential map after corrosion exposure shows a strong potential increase in a zone of deposit and in an outer zone that is most probably associated with the corrosion products layer deposited around the inclusion.

115 (a 1 ) (b 1 ) Volta potential, mv 40 20 0-20 -40 (a 2 ) before attack Volta Potential, mv -60 0 5 10 15-40 20 0 10 20 30 40

Raman spectra measured over the corrosion products covering the pit or inside the partially dissolved inclusion (Figure 7) show

In the case of HA additions, the Raman peaks identified in a previous work (11) for pure HA (broad peaks around 1300 and 1570 cm

9 115 (a 1 ) (b 1 ) Volta potential, mv (a 2 ) before attack Volta Potential, mv X, µm X, µm (b 2 ) 10s exposure in Na 2 SO 4 / HA Figure 6. Topography (a, b), Volta potential map (a 1, b 1 ) and Volta potential difference across the profile (a 2, b 2 ), before (a, a 1, a 2 ) and after 10s in 10mM Na 2 SO 4 with HA addition (b, b 1, b 2 ). Raman spectra measured over the corrosion products covering the pit or inside the partially dissolved inclusion (Figure 7) show some of the characteristic peaks of hematite (α-fe 2 O 3 ) at 220, 280, 500, 600, 650 and 1300 cm -1 (18, 19). In the case of HA additions, the Raman peaks identified in a previous work (11) for pure HA (broad peaks around 1300 and 1570 cm -1 ) are also observed for corrosion products localized on the border of the inclusion. Thus, the HA attack to the region of the inclusion-matrix interface and metal surface around inclusions is followed by the formation of insoluble organic Fe compounds.

10 116 Raman Intensity (c) 1 2 Raman Wavenumber (cm -1 ) Intensity (d) Wavenumber (cm -1 ) Figure 7. SEM-view (BEI) of the inclusion attacked 10s in solution containing HA, Raman spectra of pure HA (presented previously in ref. 10) and Raman measured in the points 1 (c) and 2 (d) of the photo. Figure 8 shows SEM images of the inclusion, Volta potential maps and Volta potential profiles before (a, a1, a2) and after 10s of attack (b, b1, b2) respectively in Na 2 SO 4 solution containing FA. A localized attack of the Fe matrix around the inclusion can be observed. However, there is not much corrosion products after the immersion test, which can testify for the less reactive nature of FA. The Volta potential map shows some changes over the all surface of scan. There are some new features appeared on the map that can be associated with the action of FA on the surface of iron matrix. The whole Volta potential image doesn t show contrast and can be related with the presence of a small layer of the deposits on the whole surface.

10 15 20 25 30 X, µm 5 0-5 -10-15 -20-25 12µm (b 2 ) -102.

Topography (a, b), Volta potential map (a 1, b 1 ) and Volta potential difference across

11 117 (a 1 ) (b 1 ) mv Volta potential, mv before attack (a 2 ) Volta Potential, mv X, µm µm (b 2 ) mv 2s exposure in Na s SO 4 / FA X, µm Figure 8. Topography (a, b), Volta potential map (a 1, b 1 ) and Volta potential difference across the profile (a 2, b 2 ), before (a, a 1, a 2 ) and after 10s in 10mM Na 2 SO 4 with FA addition (b, b 1, b 2 ). Raman spectra measured over the corrosion products covering the pit (Figure 9) show also some of the characteristic peaks of hematite (α-fe 2 O 3 ) at around 1300 cm -1, but in this case, the previously identified Raman peak (11) for pure FA at ca cm -1 is absent.

118 1 Raman Intensity 2 Raman Intensity (c) Wavenumber (cm -1 ) Figure 9. SEM-view (BEI) of the inclusion attacked 10s in solution containing FA, Raman measured in the points 1 and 2 (c) of the photo.

12 118 1 Raman Intensity 2 Raman Intensity (c) Wavenumber (cm -1 ) Figure 9. SEM-view (BEI) of the inclusion attacked 10s in solution containing FA, Raman measured in the points 1 and 2 (c) of the photo. Conclusions Attack of complex inclusions in simulated soil solution (organic matter): The outer region of complex inclusions in API steels contains sulfides surrounding the SiO 2.MgO.CaO.Al 2 O 3 -inclusions core. The outer sulfide region, after a short exposure, has a lower Fe content: Fe is selectively dissolved from a Fe-Mn-containing inclusion. Fe-matrix surrounding inclusions, which is deformed by sample preparation dissolves first, starting the nucleation of pits. Inclusion attack is enhanced by humic acid, and by a less extent, by fulvic acid. Before exposure, inclusions have a lower Volta potential compared to Fe. After exposure, Volta potential increases more strongly on the inclusion. Raman spectra show hematite and organic substance response on corroded area, indicating that the corrosion products contain complex Fe-carboxilates. Acknowledgments The authors acknowledge the financial support of CNPq and CAPES-GRICES (Contract Nº. 154/06). Two of them (L. M. R. and K. A. Y.) also gratefully acknowledge personal CAPES Grant BEX5089/06-9 and FCT Grant SFRH/BD/25469/2005..

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