Abstract
This work is focused on the corrosion behaviour of weldments in X80 pipeline steel. In order to clarify their susceptibility to soil corrosion, general corrosion and stress corrosion crack experiments were carried out in a simulated soil solution. Based on these results, the heat affected zone (HAZ) demonstrated higher corrosion susceptibility than the base metal and the weld metal. This was due to both the higher intrinsic corrosion rate of the HAZ and the galvanic effects between the HAZ and the other regions of the weldment. Additionally, in slow strain rate testing, corrosion damage in the HAZ resulted in a significant decrease in fracture area reduction and time to failure and an increase in the appearance of brittle failure on the fracture surface compared with the base metal. Elemental segregation and coarse grains appear to be two factors that lead to the high corrosion of HAZ.
Introduction
Long distance transport of hydrocarbons in the petroleum industry is generally required because of the distances between production and consumption. For instance, natural gas in Russia needs to be transported to most European countries for the greater benefit of the whole European economy. Similarly, gas and oil in western China must be piped to the more industrialised eastern region. Therefore, the China government has initiated the west to east gas pipeline programme in 2002 and the Sichuan gas to east transmission pipeline programme in 2007.
X80 grade steel has been widely used for the installation of low cost, high pressure gas transmission systems over the last decades.1 One of the most important factors in safe pipeline operation lies in the properties of the weldments. In aggressive environments, the base (or parent) metal (BM), the weld metal (WM) and the heat affected zone (HAZ) may present different corrosion behaviours.2 – 4
Soil corrosion is a very important factor in the failure of pipelines. It causes damage by surface corrosion and stress corrosion cracking phenomena, where the soil aggressiveness appears5,6 even when protective coatings and cathodic protection are applied. Field failures have even been found under disbonded protective coatings.7,8 In order to understand such failures, a simulated soil solution (e.g. NS4) has become an important research medium to evaluate the soil corrosion of pipelines.9 – 11
In view of the frequent application of X80 pipe fabricated by spiral welding, X80 weldments from a field pipeline were used to investigate the corrosion sensitivity in NS4 solution. In this study, immersion corrosion and slow strain rate testing (SSRT) experiments were carried out to evaluate the preferential corrosion location in welded X-80 pipeline steel.
Experimental
Welded X80 steel was cut from the section of the raw materials for the China west to east gas pipeline of overall diameter 1219 mm.
The metallurgical structures of BM, WM and HAZ were observed by an XJL-03 model metallographic microscope after polishing and etching in alcohol solution dissolved with 3% nitric acid (nital etch). The elemental compositions of BM and HAZ were respectively detected using a SPECTROTEST model spectrograph. An HVS-1000 model microscopic hardness meter was adapted to measure the microhardness of each region of the weldment. The measured Knoop hardness values were obtained by persistent loading of 0·98 N (100 g load) on the sample for 10 s.
The soil corrosion test was operated in a simulated soil solution (NS4) with composition: NaHCO3: 0·483 g L−1, MgSO4.7H2O: 0·131 g L−1, CaCl2: 0·127 g L−1 and KCl: 0·122 g L−1, which was aerated in all experiments. Each sample of BM, WM and HAZ was polished using SiC paper from 200 to 800 grit in sequence and then cleaned using acetone and alcohol. The weight and dimension of the samples were measured. They were immersed in the NS4 solution at a temperature of 45°C. After 168 h of immersion in the NS4 solution, the corrosion products were removed from the surface by chemical solvent containing 10% hydrochloric acid and 0·2% inhibitor. Every sample was weighted again. The corrosion rate was calculated by weight loss
Potentiodynamic polarisation was performed using an Autolab PGSTAT302N potentiostat in NS4 solution at 45°C with a saturated calomel reference electrode and platinum auxiliary electrode. The potential scan rate was fixed at 0·83 mV s−1.
The method of SSRT was utilised to investigate the stress corrosion cracking of samples prepared, as shown in Fig. 1. The experiments were performed in NS4 solution at 45°C at a strain rate of 2×10−6 mm s−1. It has been suggested as the highest susceptibility of pipeline steel to stress corrosion crack (SCC) in soil solution at this strain rate.12 After failure, the samples were observed using a JSM-6360LV model scanning electron microscope (SEM) to obtain the microstructure of the fracture surface. Corrosion products were characterised by a Rigaku D/Max X-ray diffractometer with a Cu Kα X-ray source. The scanning range of 2θ started from 5 to 75°.
Schematic diagrams of SSRT samples
All the presented data in this work were average values calculated by the results of three replicate samples.
Results and discussion
Materials microstructure
Figure 2 shows the metallurgical structures of WM, BM and HAZ. Fine sized upper bainite and quasi-polygonal ferrite (quasi-PF) were found in the BM, with some acicular ferrite (AF), presumably transformed from the quasi-PF. This microstructure was consistent with X-80 steels developed for high strength and good toughness. Martensite–austenite (MA) was generally present at grain boundaries with the AF. In the WM, columnar crystals were composed of PF with AF and traces of pearlite. This region had a grain size close to the BM.
Metallograph of X80 weldment
In comparison with the other regions, coarse grain was observed in the HAZ. These appeared to be composed of transgranular lath shaped ferrite and island-like MA with precipitate of carbides or other substances along the gain boundaries and between ferrite laths.
General corrosion susceptibility
The corrosion rates of WM, BM and HAZ in NS4 are listed in Table 1. The corrosion rate of HAZ was triple that of BM, while that of the WM was only slightly faster than BM. Thus, the HAZ exhibits the weakest corrosion resistance among the three regions and is most susceptible to soil corrosion.
Corrosion rates of different regions
Moreover, the corrosion is related to the grain size. From the surface morphology of samples after removing the corrosion scale, as pictured in Fig. 3, the corrosion appears to follow the grain morphology observed before corrosion, that is, the outline of the original grain boundaries is maintained after corrosion. It might be therefore postulated that carbide precipitates at grain boundaries play the role of cathodes. Thus, control of segregation on grain boundaries is probably important to enhance the corrosion resistance.
Morphology of samples after removing corrosion products
The corrosion rate of the HAZ is determined by many factors including the presence of anodic or cathodic phases and elemental segregation within relatively coarse grains. Previous work has shown that within the HAZ, the formation of MA and segregation of C, Ti, Nb, Mn, Cr, Mo and S are prevalent.13 – 15 Element analyses of the BM and HAZ (in wt-%) regions were determined as the results in Table 2. Enrichment of C and Mn compared with BM is clearly evident in the X80 HAZ.
Compositions of steel
Further investigation on the electrochemical performance was carried out with the results shown in Fig. 4. The corrosion current, determined by Tafel extrapolation from polarisation curves, as shown in Table 1, for the HAZ metal was greater than in the other two, and its corrosion potential was ∼100 mV more negative; respectively for BM, WM and HAZ, the potentials were −560, −570 and −660 mV. Therefore, a significant galvanic effect between the relatively anodic HAZ and the other two regions in the X80 weldment is possible.2,16,17
Polarisation curves of different regions
The X80 weldment sample, including BM, WM, and HAZ, was immersed in NS4 solution under the same conditions. The photomacrograph of the sample after corrosion is shown in Fig. 5. Relatively little corrosion product covered the HAZ surface compared with the BM and WM regions. The galvanic effect is remarkable and is likely to lead to increasing corrosion susceptibility within the HAZ. Figure 6 shows the X-ray diffraction of the corrosion products. The corrosion products were composed of FeOOH and Fe3O4 and are typical products of oxygen consumption type of corrosion.
Photomacrograph of X80 weldment after corrosion
X-ray diffraction pattern of corrosion products
The primary reactions are written as
Stress corrosion crack susceptibility
The SSRT has been widely used to evaluate the susceptibility to SCC of various materials. 18 18,19 Figure 7 shows the typical stress–strain curves of X80 steel and X80 weldment in NS4 solution during SSRT. The crack parameters calculated from the experiments are listed in Table 3. The time to fracture of the weldment was shortened with respect to X80 steel.
Stress–strain curves in NS4 solution
Crack parameters of X80 and X80 weldment
The susceptibility to SCC may be estimated by fracture area reduction calculated by equation (8)
The fracture area reduction decreased from 64 to 22% after welding. These data proved that the SCC susceptibility of X80 was limited, whereas welding greatly increased the hazard of SCC in soil. The time to failure of the weldment decreased to 68% of the BM.
From Fig. 8, a large reduction in area is evident near the facture in X80 steel, while the X80 weldment had a much smaller reduction in area, which implies embrittlement of the weldment. As labelled in the picture, the crack site was located within the HAZ. The microhardness of BM, WM and HAZ was measured, and their results are listed in Table 4. The highest value was found in HAZ, which can be attributed to the presence of MA and carbide precipitates. The other two regions presented similar hardness. According to the general corrosion results, more rapid dissolution in HAZ than in BM and WM may lead to stress concentration in this region. Zhang et al. reported that dissolution current increased with applied stress.20 Thus, the dissolution in HAZ is likely to be accelerated with increasing time because of the increasing stress concentration. The microcracks generally appear to initiate from MA and precipitates.13 The higher hardness phases lead to the lower density of microcracks, while the coarse grains show a smaller tendency for crack arrest,21,22 which results in the rapid development of the main crack. As a result, the crack may propagate to failure after relatively limited plastic deformation, i.e. the material is locally more brittle.
Photomacrograph of cracked sample
Microhardness of X80 weldment
The SEM images of the fracture are shown in Fig. 9. Dense and deep dimples were distributed all over the fracture of the BM, and ductile rupture was the dominant failure mode. However, the fracture surface of the X80 weldment appeared smoother with fewer and more shallow ductile evident; this was more characteristic of locally limited ductility that might be induced by premature local cracking during straining.
Images (SEM) of fracture of tensile specimens
These results illuminate that welding strengthens the HAZ and increases the brittleness simultaneously. In contrast to the base X80 steel, it promotes SCC susceptibility in simulated soil solutions.
Conclusions
The general corrosion and the stress corrosion of welded X80 pipeline steel were studied in this work focussing on soil corrosion susceptibility. The experimental results revealed the following conclusions:
The corrosion rates varied between different regions within the X80 weldment. Specifically, the corrosion rate of the HAZ was higher than the base and WMs. A significant galvanic effect between the HAZ and the other two regions was supported by polarisation curve and immersion experiments.
Based on the results of SSRT, the HAZ showed limited ductility compared with the rest of the weldment in simulated soil solution. It is postulated that this effect in the HAZ is driven by preferential anodic dissolution with the local hardness and coarse grains enhancing the effects of stress concentration and crack propagation.
Footnotes
Acknowledgements
This work was financially supported by the Southwest Petroleum University Foundation (grant no. 2010XJZ178). The help of Dr S. Tang is acknowledged.