Microstructure , Mechanical Properties and Corrosion of Ceramic-lined Composite Steel Pipe Prepared by Centrifugal-SHS Process

A ceramic-lined composite steel pipe (CLCSP) was fabricated with N80 steel tube as substrate by self-propagating high-temperature synthesis and centrifugal casting technique. The constituent phases and microstructure of the ceramic layer were analyzed, and the mechanical properties of CLCSP were measured, including hardness, crashing strength, compression-shear strength and tensile performance. The wear and corrosion behaviors between CLCSP and N80 steel tube were also compared. The results show that ceramic layer consists of columnar dendrites of Al2O3 and spinel-like structure FeAl2O4, CLCSP has a much higher hardness, squeezing deformation resistance, wear and corrosion resistance as compared with N80 steel tube.


Introduction
The self-propagating high-temperature synthesis in a centrifugal force field (Centrifugal SHS process) is a new technology for preparation of ceramic-lined composite steel pipes [1][2][3][4].When thermit mixture of Fe 2 O 3 +Al is filled in a steel pipe and ignited to combust, molten products of Al 2 O 3 and Fe produced by thermit reaction can be separated in layer by virtue of their specific gravity difference under centrifugal force.After cooling, a ceramic-lined composite steel pipe is produced with an Al 2 O 3 ceramic layer and an intermediate Fe-rich layer on the inner surface of steel pipe.This type of CLCSP has pronounced advantages, such as long life cycle, light weight, low cost, and high resistance to abrasion, corrosion, erosion, and heat, as compared with other surface techniques such as nitriding, electroless Ni-P coatings, boriding and hard Cr coatings [5][6][7][8].Therefore, it has been recommended for industrial applications to the transport of coal cinder, mineral powder, limestone flour, an oil-water mixture [9][10][11].
N80 steel tube is widely used in oilfield due to its low cost and proper mechanical properties.However, it always faces two problems including high corrosion in an acid environment containing CO 2 , H 2 S and heavy wear due to partial-wear between the tube and sucker rod in the oil well [12 13].Previous studies reported that a thick ceramic layer of Al 2 O 3 could be coated on the inner surfaces of steel pipes using the centrifugal-SHS process, and made them get better performance of resistance to corrosion and abrasion [14,15].Therefore, using centrifugal SHS process to apply a ceramic-lining to N80 steel tube is (Z.Wen) expected to improve effectively the corrosion and wear resistance of N80 steel tube in the oilfield.However, up to date, the microstructure and conventional mechanical properties of centrifugal-SHS ceramic-lined composite N80 steel tube are still absent, such as crushing strength, compression-shear strength, tensile strength.In addition, the wear and corrosion proprieties also need to be made a comparative study between CLCSP and N80 steel tube before practical application in an oil well.Thus, in the present study, centrifugal-SHS ceramic-lined composite N80 steel tube was produced using thermit mixture of Fe 2 O 3 +Al, the microstructure and mechanical properties such as hardness, crushing strength and compression-shear strength was investigated.The wear and corrosion properties were also studied between CLCSP and N80 steel tube.

Experimental procedure 2.1. Preparation of ceramic-lined composite N80 steel pipe
The substrate material was an N80 steel with chemical composition as listed in Tab.I.The testing pipes of 400 mm in length were cut from 10 meter long N80 steel oil pipe with 73.02 mm diameter and 5.51 mm wall thickness.The pure powders, Fe 2 O 3 (≥99%, 80-140 μm) and Al (≥99.5%, 80-140 μm) were used as the thermit reactants.The chemical mole ratio of the Al and F 2 O 3 powders conformed to the following chemical reaction: Prior to preparation of centrifugal-SHS CLCSP, the thermit reactants were mixed for 1 h with a barrel mixer and then dried thoroughly at 100 o C for 2 h.The N80 steel pipe was loaded with the thermit mixture and placed horizontally on the SHS-centrifugal apparatus.The thermit mixture was ignited at one end of the pipe by a tungsten filament when the centrifuge apparatus reached the required rotational velocity.Upon the completion of the thermit reaction, the centrifuge apparatus was allowed to run for 10 more min until the prepared CLCSP cooled down.

Microstructure observation
The phase constituents in ceramic layer were identified by means of a Ragaku D/MAX 200PC diffractometer using Cu k α radiation.The cross-sectional microstructures at different locations of the ceramic layer were observed under a LEXT-OLS3000 confocal scanning laser microscope.

Mechanical properties
The mechanical properties of the centrifugal-SHS CLCSP were investigated through hardness test, squeezing test, compression shear test and tensile strength test.The Vickers hardness of ceramic layer was measured using a load of 500 gf for a dwell period of 15 s.Squeezing test represents the crushing strength of the ceramic layer.The crushing strength of CLCSP was measured using MTS810 material test system.In the crushing test, a piece of pipe with 50 mm length was compressed in the radial direction, the corresponding schematic diagram and photograph of the compression test are shown in Fig. 1 (a) and (b), respectively.The crushing strength (σ F ) was calculated using formula described by Odawara and Ikeuchi [16]: where L, t and D are the lengths, thickness and an average diameter of the test pipe, respectively, F is the turning point load deviated from a straight line at the compressed loaddisplacement curve, and k is a cross section factor:  The compression-shear strength of the centrifugal-SHS ceramic-lined composite pipe was measured using MTS810 material test system.In the crushing test, a piece of pipe of 10 mm length was compressed on the ceramic layer in the axial direction of the pipe, the corresponding schematic diagram and photograph of the compression-shear test are shown in Fig. 2(a) and (b).The compression-shear strength (σ s ) was calculated by the following formula [16]: Tensile strength represents the fracture strength of the ceramic layer.The tensile strength of CLCSPs was measured using WAW-1000D universal tensile testing machine.Fig. 3 is the photograph of the tensile test of CLCP.The fracture strength of ceramic layer was determined by step-by-step method within a certain load range.

Wear and corrosion tests
Dry sliding wear performance was investigated between the ceramic layer and N80 steel on a pin-on-disc type wear machine.The applied load was 50 N, the sliding speed was 0.8 m/s, and the sliding distance was 1508 m.The counterface discs had a diameter of 70 mm and were made of middle carbon chromium steel with a Vickers hardness of 570 HV.Pins of 5 mm in diameter and 12 mm in height were machined from N80 steel pipe and gravityseparation SHS ceramic layer that had the same composition as a ceramic layer of CLCSP but much thicker.The volumetric losses of ceramic and N80 steel pins were plotted as a function of sliding distance.
The corrosion immersion test was performed in oilfield water from an oil well in Jilin Oilfield of China with a total mineralization of 2.1×10 4 mg/L whose composition is listed in Tab.II.To evaluate the effect of high content of H 2 S solution on the N80 steel pipe and ceramic-lined composite steel pipe, a saturated H 2 S solution was used in which gas was pumped into the oil field water until saturation.The content of H 2 S in the oil field water was about 0.1 mol/L.The pipes containing corrosion solution were tightly sealed on both ends, and corrosion immersion was chosen a long period of 240 days.Fig. 4 is the sealed CLCSP and N80 steel pipe filled with corrosion solution.The corrosion solution was replaced by unused one for every 30 days.The corrosion products were examined using XRD analysis, and the corrosion morphologies were analyzed using scanning electron microscope (SEM).

Microstructure of ceramic layer
The prepared CLCSP was examined and the dimensions of the ceramic layer and Fericher are listed in Tab.III.The cross-sectional microstructures of the ceramic layer were observed in three locations including top, middle, and bottom, as well as the corresponding 3D morphology maps, as shown in Fig. 6.The top microstructure consists of long columnar dendrites of Al 2 O 3 with an average width of about 7.1μm along the radial direction of the CLCSP and spinel-like structure FeAl 2 O 4 distributed at dendrite boundaries (Fig. 6a).The preferred growth orientation of columnar dendrites of Al 2 O 3 is apparently is (110) according to the XRD analysis, i.e. along the radial direction of the CLCSP.In addition, a lot of pores are also found in the top microstructure, as shown in the 3D morphology map (Fig. 6b).In the middle location, the characteristic of columnar dendrites is still pronounced (Fig. 6c), but the number of pores decrease greatly (Fig. 6d).In the bottom location, a few of spheres of Fe is embed in Al 2 O 3 ceramic layer (Fig. 6e), and the original mechanical bonding interface is separated and a wide gap is formed between ceramic layer and Fe-rich layer during the preparation of metallographic sample, and a lot of pores are formed on both sides (Fig. 6f).The columnar dendrites grow through the entire ceramic layer during solidification.Based on the microstructural observations and the melting temperatures of Al 2 O 3 (2030 o C), FeAl 2 O 4 (1780 o C) and Fe (1538 o C), the microstructure of Al 2 O 3 ceramic layer can be presumed to be formed as follows: During the solidification of the thermit melt, the Al 2 O 3 lies at the interior surface layer followed by Fe-rich melt and steel substrate pipe due to separation from centrifugal force.Al 2 O 3 nucleates first due to its high melting temperature, and grows towards the rapid heat releasing direction i.e. along the radius of the pipe, forming the long columnar dendrites.When the temperature of the ceramic layer decreases to about 1750 o C, FeAl 2 O 4 eutectic phase begins to form at the boundaries of Al 2 O 3 dendrites with reference to Al 2 O 3 -FeO binary diagram.Therefore, the ceramic layer completely formed firstly, then the Fe-rich melt solidified at about 1538 o C, forming the Fe-rich layer between the ceramic layer and steel substrate pipe.The cooling rate is rather high during the centrifugal SHS process, a great amount of gas can not emit in time from the viscous Al 2 O 3 melt before solidification of the ceramic layer, which is why a large number of pores are observed in the top region of the ceramic layer.The high cooling rate also introduce a large compressive stress is produced in ceramic layer because the steel pipe and Fe-rich layer have a higher coefficient of thermal expansion than the ceramic layer.Wang and Yang [17] analyzed the temperature distribution and transformation during the cooling process of the CLCSP preparation with finite element numerical simulation; they found a large compressive stress of 152.22 MPa produced in the Al 2 O 3 ceramic layer.Consequently, it is considered that the original gap formed between the ceramic layer and the Fe-rich layer is very narrow, and it apparently helps to relieve the impact shock on the ceramic layer from the external loading on the outer surface of the CLCSP.

Mechanical properties
The cross-sectional hardness distribution of CLCSP is presented in Fig. 7.The hardness is only around HV600 at depth of 0.2 mm from surface owing to existence of a lot of pores, and increases to a high level of HV1130-1423 when the depth ranges from 0.3 to 1.9 mm, then fall to the lowest level of HV230-500 in the Fe-rich layer region, and finally increases again to HV500-580, which corresponds to the hardness of steel substrate pipe.The squeezing behavior of CLCSP was investigated by comparing with N80 pipe and ceramic-layer-peeled Centrifugal SHS pipe, as shown in Fig. 8.
It is observed that after a small displacement of elastic deformation, two horizontal stages occur in succession on the crashing curve of N80 steel pipe, then the curve rapidly rises linearly with increasing displacement and finally presents a high plateau.The first horizontal stage corresponds to the yield deformation at the top and bottom locations of the steel pipe; the second one corresponds to the yield deformation at left and right corners of steel pipe due to stress concentration.The ceramic-layer-peeled Centrifugal SHS pipe and CLCSP present similar radial crushing load vs. displacement curves, the curve of ceramic-layer-peeled Centrifugal SHS pipe almost overlap with that of N80 steel pipe within the most displacement, while the curve of CLCSP is apparently higher than the two others.This implies that Fe-rich layer cannot improve the squeezing deformation resistance due to its low strength, whereas ceramic layer enhances significantly the squeezing deformation resistance.In addition, it is found that the ceramic layer begins to crack firstly at the top and bottom locations when the curve turns into the rising stage, and finally cracks at left and right locations when the curve suddenly turns into a rapid drop from the climbing state.Therefore, the crashing load of CLCP is determined from the curve, as listed in Tab.IV, and the calculated crushing strength is 328.1 MPa.The compression shear load vs. displacement curve of CLCP was measured using MTS810 material test system, as shown in Fig. 9.The load demonstrates a linear relationship with ceramic layer displacement until 0.8 mm, then the load maintains a constant even though the displacement still increases, indicating that the mechanical bonding between the ceramic layer and the Fe-rich layer is completely destroyed after displacement of 0.8 mm.Therefore, the compression shear load of CLCSP are determined from the curve, as listed in Tab.IV, and the calculated crushing strength is 25.6 MPa.

Tab. IV
The tensile tests of CLCSP were carried out on a universal material testing machine to determine the fracture load and strength of ceramic layer under uniaxial tension load condition.The criterion for determining the fracture of the ceramic layer is the occurrence of cracking on locations of the inner surface near end or at the middle of the CLCSP.The cracking of ceramic layer cannot be observed instantly due to the usage of clamping apparatus with inner thread, therefore, a large tensile load like 500 kN is firstly applied to the tensile specimen, it is verified to exceed the fracture load of the ceramic layer, then a small load of 400 kN that is lower the fracture load of ceramic layer is used to identify the ceramic layer being intact.Finally, the applied tensile load is increased with a load step of 10-15 kN from 400 N until the cracking of ceramic layer.The fracture load of the ceramic layer is determined to be about 455 kN, and the ceramic layer cracks at the middle of the CLCSP.The calculated fracture strength of ceramic layer is 269 MPa using Eq.( 5), given that the load is uniformly applied on the cross-section area of the CLCSP.

S P k
where σ k is the fracture strength of ceramic layer, P k is the fracture load, S is the cross-section area of CLCSP.The stress level of 269 MPa can bear a total of 45.7 tons oil CLCSPs, which can reach about 3500 m depth and enough for the conventional oil well.

Wear and corrosion properties of CLCSP
The comparison of wear resistance between the ceramic layer and N80 steel is shown in Fig. 10.It notes that with increasing sliding distance, the volumetric loss of N80 steel increases significantly with a high slope, the average wear rate is 39.2x10 -13 m 3 m -1 , whereas the volumetric loss of ceramic layer increases modestly with a low slope, the average wear rate is only 3.6x10 -13 m 3 m -1 .The ceramic layer has superior wear resistance to N80 steel owing to its high hardness of about twice of N80 steel.Fig. 10.The volumetric loss against sliding distance for the ceramic layer and N80 steel.
The corrosion resistance of N80 steel pipe and CLCSP was compared after immersion in oilfield water for 240 days.The inner surface of N80 steel pipe was found covered by a black corrosion product film, and the originally smooth surface became rather rough with pits, as shown in Fig. 11a.Most of them have the depth of several hundred micrometers as shown in the 3D image of Fig. 11b.However, a very few of pits even have a depth more than half thickness of the steel pipe, i.e. reached a depth of 2.7 mm, as shown in Fig. 11c and Fig. 11d.Corrosion pits could penetrate the steel pipe if corrosion continues for a much longer time.The XRD pattern of the black corrosion film formed on inner surface of N80 steel pipe only shows two constituent phases, they are FeS and α-Fe phase, as shown in Fig. 12. FeS is the typical corrosion product in the H 2 S-containing environment.The pH value is 6.8 for oilfield water.In general, corrosion of steel in H 2 S containing solution with a pH similar to the studied solution involves the following reactions: Fe→Fe 2+ +2e (anodic reaction) (6) H 2 S→HS -+H + (7) HS -+e →H +S 2-(cathodic reactions) (8) Fe 2+ +HS -→Fe x S y + H + (general reaction) (9) The types of the iron sulfide (Fe x S y ), which are formed on the surface of the steel during H 2 S corrosion, are a key factor controlling the H 2 S corrosion [18].The tetragonal mackinawite (FeS (1-x) , x=0.0057-0.064) was found in the corrosion product film during the experimental work, which is the less protective iron sulfide film.The feature of corrosion morphology of N80 steel is consistent with the corrosion pitting.The corrosion solution had low fluidity in the pits; therefore a lock-out corrosion battery gradually formed which could induce the autocatalytic effect of corrosion reaction in the pits and enhance the corrosion, and finally led to the corrosion perforation.This corrosion form is the most typical corrosion failure of N80 oil tube in the oilfield water.In contrast, the corrosion morphology of the inner surface of the ceramic layer is shown in Fig. 13.It notes that most area is smooth and intact after corrosion test (Fig. 13 (a)).It is ascribed to the fact that there are strong bonds between Al and O elements, and Al 2 O 3 has acid-alkaline resistance.This renders ceramic layer a high corrosion resistance in the oilfield water.However, localized areas of the surface of the ceramic layer were eroded (Fig. 13b), that is, a few corrosion grooves of 0.5 mm length and holes of 20 μm depth were found.The corrosion sites are located at the boundaries of Al 2 O 3 dendrites at which FeAl 2 O 4 phase is rich.It was reported that FeAl 2 O 4 phase has an inferior corrosion resistance to Al 2 O 3 , and is easy to be eroded in the ceramic layer [19].
The cracking of the ceramic layer is ineviTab.sometimes due to the fragile nature of Al 2 O 3 and thermal stress produced during centrifugal-SHS preparation.The cracks help corrosion solution leak in, therefore the Fe-rich layer can be eroded.Corrosion test of another piece of CLCSP with a big crack was performed under the same corrosion condition.The corrosion of Fe-rich layer beneath a big crack of the ceramic layer was examined, as shown in Fig. 14.The brown corrosion products can be observed forming on the both sides of the big crack (Fig. 14a).After the ceramic layer was peeled off, the Fe-rich layer was found eroded (Fig. 14b), but not as badly as N80 steel eroded.This is due to that the Fe-rich layer contains less C element and has a higher corrosion resistance.In addition, a corrosion product film containing FeS also formed on the Fe-rich layer just beneath the crack; it more or less protected the Fe-rich layer from being eroded heavily, even though it cracked up possibly due to the dry examination environment (Fig. 14c).The oil well water first electrochemically reacted with Fe-rich layer forming FeS product when it went through the crack, then it defused along the interface between ceramic and Fe-rich layer and formed a corrosion product film of oxide with a flack structure (Fig. 14d), since the content of S 2-decreased and content of O 2 increased.O 2 originated from air blocked in the pores in the Fe-rich layer during fabrication of CLCSP.Therefore, it can be concluded from the corrosion results that CLCSP has a superior corrosion resistance to the N80 steel tube because the ceramic layer serves as the first corrosion resistant protection, and the Fe-rich layer acts as the second barrier to corrosion even though the micro or large-sized cracks may exist.

Fig. 1 .
Schematic diagram (a) and photograph (b) of the compression test for CLCSP.

Fig. 2 .
Schematic diagram (a) and photograph (b) of the compression-shear test for CLCSP.

Fe
Tab. III Thickness of ceramic layer and Fe-rich layer for CLCSP.CLCSP diameter (mm) Ceramic layer thickness ( mm) constituent phases in the ceramic layer were examined by XRD diffractometer.Al 2 O 3 and FeAl 2 O 4 phases were founded from the XRD pattern of the ceramic layer shown in Fig. 5.

Fig. 12 .
Fig. 12. XRD pattern of the corroded inner surface of N80 steel pipe.

Fig. 13 .
Fig. 13.Optical microphotograph (a) and SEM image (b) of the corroded surface of the ceramic layer.

Fig. 14 .
Fig. 14. corrosion of CLCSP with a long crack: (a) ceramic layer surface, (b) Fe-rich layer, (c) FeS product film on the Fe-rich layer, (d) magnification photograph of corrosion product

1 .
CLCSP was fabricated by centrifugal-SHS technique.The microstructure of ceramic layer consists of long columnar dendrites of Al 2 O 3 and spinel-like structure FeAl 2 O 4 distributed at dendrite boundaries.2. The ceramic layer has a hardness of HV1130-HV1423, about as twice high as the steel substrate, and enhances significantly the squeezing deformation resistance as compared with an N80 steel pipe.

3 .
Tensile test of CLCSP shows the fracture strength of 269 MPa for the ceramic layer.The ceramic layer has a better wear resistance than the N80 steel, only a tenth of wear rate of N80 steel under given dry sliding wear condition.4. The ceramic layer exhibits a better corrosion resistance than the N80 steel in oilfield water; the corrosion sites are located at dendrite boundaries of Al 2 O 3 where the FeAl 2 O 4 phase is distributed.