The erosion-corrosion behaviors of the full-scale Ni-P and Ni-Cu-NiW internal coating tubing

In the present work, a self-built device called the “full scale tubular goods erosion-corrosion test system” was used to investigate the erosion-corrosion behavior of Ni-P and Ni-Cu-NiW internal coating tubing in a simulated operating condition of a water injection well in the Xinjiang oil field, China. The tubing, with a total length of 10 m, was tested under accelerated conditions. The erosion-corrosion performance was evaluated using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The results showed that the Ni-P coating experienced severe corrosion, including internal cracking and peeling. The corrosion failure of the Ni-P coating was mainly attributed to the presence of defects such as pores and the penetration and self-catalysis of chloride ions. On the other hand, the Ni-Cu-NiW coating exhibited excellent corrosion resistance due to its three-layer composite structure and the addition of tungsten (W), which enhanced its resistance to chloride ion penetration.


Introduction
The development of oil fields such as Daqing and Xinjiang in China has entered the mid to late stage, and the failure of the water injection well strings is becoming increasingly serious, particularly in the tubing [1][2][3] .Due to the coexistence of gas, water, oil, and solid in water injection wells, multiphase flow corrosive media, and high-temperature and high-pressure service environments, the failure of water injection well strings is occurred frequently.To extend the service life of water injection well tubing, metal coatings [4][5][6] and organic coatings [7,8] are usually coated on the surface of the tubing to isolate corrosive media from the tubing.However, due to improper coating selection or preparation process, coating failure leads to reduced service life of tubing occurs frequently [5,[8][9][10][11][12][13][14] .
The development of Xinjiang oilfield has entered a high water content stage, and the water injection string is in a complex corrosive environment with high content of dissolved oxygen, corrosive gas medium CO2 and chloride [5,[9][10][11][12][13][14][15] .To solve the corrosion problem of water injection tubing, tungsten alloy coating, nickel phosphorus coating and organic coating was adopted to protect the tubing.However, the service life of some coatings fails to meet the 5-year requirement.
In the present work, samples of nickel phosphorus-coated and tungsten alloy-coated tubing for water injection wells were randomly selected from oil field warehouses.The "full-scale tubular goods erosion-corrosion test system" was employed to study the erosion-corrosion behaviors of Ni-P and Ni-Cu-NiW internal coating tubing with a total length of 10 m under gas-liquid two-phase flow conditions.The liquid is the simulation formation water of a water injection well in the Xinjiang oil field of China.To simulate the severe downhole corrosion environment, an accelerated test was conducted with conditions including a partial pressure of carbon dioxide of 0.2 MPa, a gas velocity of 3 m/s, and a temperature of 80°C.After 720 h testing, the corrosion morphologies and corrosion mechanism were studied.The findings of this study are expected to provide valuable technical support for the application of internal coating tubing in high-production water injection wells.

A apparatus
The full-scale tubular goods erosion-corrosion test system was built by Tubular Goods Research institute of China National Petroleum Corporation, the schematic of the facility is shown in Fig. 1.The test specimen was connected to the test system using flanges at both ends.The test solution, carbon dioxide, and nitrogen gases were injected through the "Liquid in" and "Gas in" tubes.A heat preservation layer was designed to stabilize the experimental temperature.

Method and conditions
The total length of the N80 tubings was 10 m, with a specification of Ф73.045.51mm.The thickness of the Ni-P and Ni-Cu-NiW internal coating are 18μm and 12μm, respectively.The full-scale Ni-P and Ni-Cu-NiW internal coating tubing was connected through flanges.Both plating surfaces had a silver color.The compositions of the specimens are shown in Table 1.The experiment aimed to investigate the erosion-corrosion resistance of Ni-P and Ni-Cu-NiW internal coating tubing under a liquid flow for 720 hours, with high temperature (80 o C), high production (3  10 5 m 3 gas per day) with a gas velocity of 3 m/s, 1 MPa inner pressure, and 0.2 MPa partial pressure of carbon dioxide.The liquid used was a simulated formation water from a water injection well in the Xinjiang oil field, and the compounds are shown in Table 2.
To investigate the causes and mechanisms of erosion corrosion of the specimens, the following analyses were conducted: scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) analysis of the micro-morphologies and compositions of the corrosion products.

Corrosion morphologies of the Ni-P and Ni-Cu-NiW internal coating tubing
The N80 tubing was sliced along the direction of fluid flow, and parts of it are shown in Fig. From Figure 4, a scale layer can be seen on the surface of the Ni-P coating, with the thickness of the scale layer being approximately 12 μm (Fig. 3b and Fig. 3c).The EDS analysis results indicate that the main components of the scale layer are C, O, Fe, and Ca, with small amounts of Na, Si, Al, and S. Based on the percentage content ratio of each element atom, the main components of the scale layer are determined to be FeCO3 and CaCO3, with a small amount of sedimentary salts.From Fig. 4b and Fig. 4c of the different part of the Ni-P coating, it can be seen that microcracks are visible inside the Ni-P coating, and some microcracks have extended to the interface between the coating and the N80 substrate.Local peeling of the coating can be observed.There are a large number of corrosion products at the interface between the Ni-P coating and the substrate.EDS analysis results show that these corrosion products are mainly composed of Fe, C, and O; The corrosion products near the substrate interface contain up to 10.77 wt.% of Cl -.From Figure 4, a scale layer is visible on the surface of the Ni-Cu-NiW coating, with the thickness being approximately 8 μm (Fig. 4b).Similar to the Ni-P coating, the main components of the scale layer on the Ni-Cu-NiW surface are C, O, Fe, and Ca.The main components of the scale layer are also FeCO3 and CaCO3, with a small amount of deposited salt.No cracks are found within the Ni-Cu-NiW coating, and it appears to be dense internally.Local inclusions are present at the interface between the Ni-Cu-NiW coating and the substrate.EDS analysis shows that these interface inclusions mainly consist of C, O, and Si, with a small amount of Fe.These inclusions are likely residues from the sandblasting treatment before electroplating the N80 tubing.In very few areas, surface damage of the Ni-Cu-NiW coating can be observed, which is consistent with the presence of brownish-red corrosion products in local areas of the coating surface found during macroscopic morphology analysis.EDS analysis indicates that the corrosion products on the surface of the coating mainly consist of Fe, C, and O, with a small amount of Ca.It is worth noting that in the damaged area of the coating surface, the corrosion products at the interface between the coating and the substrate only contain C, Cl, and Fe elements, with no O element being detected.The Cl -content is as high as 15.16wt.%.
Interestingly, the thickness of the scale layer on the surface of the Ni-P and Ni-Cu-NiW coatings is approximately 12 μm and 8 μm, respectively.This suggests that the surface of the Ni-P coating may be rougher than that of the Ni-Cu-NiW coating, making scaling more likely to occur during erosion corrosion testing.

Discussion
In this study, the experimental temperature was 80°C, and the composition of the nickel-phosphorus coating met the general corrosion resistance requirements of the GB/T 13913-2008 (ISO 4257:2003 Metallic coatings-Autocatalytic (Electroless) nickel-phosphorus alloy coatings-Specification and test methods) standard.However, extensive corrosion was observed on the inner surface of the tubing where the nickel-phosphorus coating was applied.The coating exhibited numerous cracks and even peeled off in some areas.This corrosion was primarily caused by chloride ions.Intensive studies indicated that chloride ions are prone to aggregation at points with strong adsorption such as grain boundaries [16][17][18] .The research indicates that chloride ions are easily adsorbed at the grain boundaries and other defects of the nickel-phosphorus coating, disrupting the dynamic equilibrium of Ni=Ni + +e -.As a result, the following autocatalytic reaction takes place: Ni 2+ +2Cl − ↔ NiCl2, leading to the formation of NiCl2 [16] .This reaction accelerates pitting corrosion, which penetrates into the interior of the coating.When the coating is damaged, corrosive media such as dissolved oxygen react directly with the substrate of the tubing, generating iron oxides as corrosion products.As the accumulation of corrosion products reaches a certain level, the nickel-phosphorus coating begins to peel off.The exposed N80 tubing substrate, in contact with the remaining non-peeled nickel-phosphorus coating, forms a corrosion couple, accelerating local corrosion in the peeled area of the coating.
In addition, there are inevitably pores in the chemical plating coating [19] , and the pores on the surface of the coating will become the preferred corrosion source for corrosion media such as Cl -, causing local corrosion.Numerous studies have shown that nickel phosphorus coatings cannot resist chloride ion penetration during long-term service, ultimately leading to pitting corrosion of components [20][21][22] .In addition, there are a large number of scale layers on the inner surface of nickel phosphorus plated tubings, and the formation of the scale layer is related to the presence of a large amount of Ca 2+ plasma in the working environment.The scale layer of nickel phosphorus plated tubing forms the condition of Crevice corrosion, thus inducing corrosion under scale.In addition, CO2 corrosion often manifests as comprehensive corrosion and localized corrosion beneath the sediment.Therefore, the scaling on the inner surface of the tubing exacerbates CO2 corrosion.
Additionally, chemical plating coatings inevitably contain pores [19] , and these surface pores act as preferential sites for localized corrosion by chloride ions and other corrosive media.Numerous studies have demonstrated that nickel-phosphorus coatings cannot effectively resist chloride ion penetration during long-term service, ultimately leading to pitting corrosion of components [20][21][22] .Furthermore, the presence of a large amount of Ca 2+ plasma in the working environment contributes to the formation of scale layers on the inner surface of nickel-phosphorus plated tubing.These scale layers create the conditions for crevice corrosion, which induces corrosion under the scale.Moreover, CO2 corrosion often manifests as comprehensive corrosion and localized corrosion beneath sediments.Therefore, the scaling on the inner surface of the tubing exacerbates CO2 corrosion.
Once the nickel-phosphorus coating is damaged, the tubing substrate is subject to synergistic corrosion from CO2 and O2.Research has shown that the synergy between CO2 and O2 [23] results in the formation of an iron oxide layer on the outer surface, followed by iron carbonate, iron oxide, and hydroxyl iron oxide layers.Near the metal substrate, an iron oxide layer is present.As the corrosion products at the interface between the nickel-phosphorus coating and the substrate increase, the adhesion of the coating decreases.Additionally, the nickel-phosphorus coating itself exhibits significant internal stress [24] , which further contributes to internal cracking.Over time, the nickelphosphorus coating inevitably experiences extensive cracking and peeling.
However, the Ni-Cu-NiW coating possesses a three-layer composite structure, which helps prevent the formation of penetrating pores.Research has indicated that the multilayer approach can improve the electrochemical corrosion resistance of the coating.By increasing the number of layers, the corrosion mode can be changed from single longitudinal corrosion to mixed longitudinal and transverse corrosion, thereby enhancing the corrosion resistance of the coating [20] .The incorporation of tungsten in the surface layer of the coating provides a sealing effect, improving its resistance to chloride ion penetration [20] .Additionally, copper has a standard electrode potential of 0.337V, while nickel has a standard electrode potential of -0.250V.Hence, when the surface nickel layer is damaged, the copper plating layer in the middle, acting as the cathode, will not undergo rapid corrosion.(1) In this research study, a full-scale tubular goods erosion-corrosion test system was utilized to investigate the erosion-corrosion behaviors of full-scale Ni-P and Ni-Cu-NiW internal coating tubing.The Ni-P coating exhibited severe corrosion, including internal cracking and peeling.On the other hand, the Ni-Cu-NiW coating demonstrated good corrosion resistance, with only a few localized areas experiencing pitting corrosion.

Conclusions
(2) The corrosion failure of the Ni-P coating can be attributed to the presence of defects such as pores, which facilitate the penetration and self-catalysis of chloride ions.In contrast, the Ni-Cu-NiW coating possesses excellent corrosion resistance due to its three-layer composite structure and the inclusion of tungsten (W) element.These factors enhance the coating's ability to resist chloride ion penetration.

Fig. 1
Fig.1 Schematic of full scale tubular goods erosion-corrosion test system

Fig. 2
Fig.2 The macro-morphology of (a) the Ni-P internal coating tubing and (b) the Ni-Cu-NiW internal coating tubing 3.2.Characterization by SEM and EDS Scanning electron microscope was used to observe the surface and cross section corrosion micro morphology of Ni-P and Ni-Cu-NiW coatings.Meanwhile, EDS was used to analyse the composition of corrosion products.The analysis results are shown in Fig.3 and Fig.4, and the composition is presented in Tab.3 and Tab.4.From Figure4, a scale layer can be seen on the surface of the Ni-P coating, with the thickness of the scale layer being approximately 12 μm (Fig.3band Fig.3c).The EDS analysis results indicate that the main components of the scale layer are C, O, Fe, and Ca, with small amounts of Na, Si, Al, and S. Based on the percentage content ratio of each element atom, the main components of the scale layer are determined to be FeCO3 and CaCO3, with a small amount of sedimentary salts.From Fig.4band Fig.4cof the different part of the Ni-P coating, it can be seen that microcracks are visible inside the Ni-P coating, and some microcracks have extended to the interface between the coating and the N80 substrate.Local peeling of the coating can be observed.There are a large number of corrosion products at the interface between the Ni-P coating and the substrate.EDS analysis results show that

Fig. 3
Fig.3 SEM image at the bottom part of the Ni-P coating tubing (a) surface image; (b) and (c) cross sectional image

Fig. 4
Fig.4 SEM image at the bottom part of the Ni-Cu-NiW coating tubing (a) surface image; (b) and (c) cross sectional image

Table 1
Composition of the Ni-P and Ni-Cu-NiW coating (mass fraction, %)

Table 2
Compounds of the simulation formation water

Table 3 Elements
of corrosion products on the Ni-P coating tubing by EDS(at.%)

Table 4 Elements
of corrosion products on the Ni-Cu-NiW coating tubing by EDS(at.%)