A Novel Martensitic Stainless Steel Material for CO₂ Corrosion Environment

Abstract

The novel martensitic stainless steel 13CrU was developed based on 13CrS by adding trace alloying elements, such as Mo, Ni, and Ta. This study compares the mechanical and corrosion resistance properties of the two martensitic stainless steels to assess the effect of these alloying elements on 13CrU’s performance. Experimental results show that increasing Mo by 0.67 wt% and Ni by 1.13 wt% improves the yield strength of 13CrU by 11.6% compared to 13CrS. The addition of Ta enhances the corrosion resistance of 13CrU beyond that of 13CrS. Overall, the addition of trace alloying elements significantly improves the mechanical properties of 13CrU and enhances its resistance to CO₂ corrosion.

1. Introduction

In recent years, the limitations of conventional 13Cr stainless steel in oil extraction have driven the development of advanced materials to meet the increasing demands of extreme environments. Super martensitic stainless steel, 13CrS, has gained significant attention due to its superior strength, low-temperature toughness, and enhanced corrosion resistance compared to traditional 13Cr stainless steel. This material is developed by incorporating alloying elements, such as nickel (Ni), molybdenum (Mo), and copper (Cu) into standard API 5CT [1] 13% Cr stainless steel [2,3,4]. Table 1 illustrates how these elements improve the mechanical properties and corrosion resistance of 13CrS: Reduced carbon content (~0.03 wt%) suppresses Cr carbide precipitation, preserving corrosion resistance [5,6]; nickel (5 wt%) facilitates the formation of single-phase martensite, enhancing toughness [7]; mcroalloying elements (e.g., Ti, Nb, V) promote a fine dispersion of carbide particles and high-density dislocation structures, reducing stress corrosion sensitivity [8,9].

However, the development of high-temperature, high-pressure ultra-deep wells, and highly corrosive oil fields presents significant challenges for 13CrS stainless steel. Failures such as corrosion perforation, cracking, and fracture have been reported in domestic oil fields (Figure 1), highlighting the material’s limitations in addressing the demands of modern oil extraction environments.

This study introduces a novel martensitic stainless steel composition designed to overcome these limitations. Unlike conventional 13CrS materials, this research proposes increasing levels of critical alloying elements, such as Cr, Ni, and Mo, as well as incorporating trace microalloying elements like tantalum (Ta) and tungsten (W). The innovative aspect of this composition lies in its ability to achieve a more uniform microstructure, enhanced mechanical properties, and improved corrosion resistance under high-temperature, high-pressure, and highly corrosive conditions [10].

Additionally, this work systematically investigates the effect of optimized heat treatment processes on structural uniformity, dislocation density, and carbide dispersion, which are critical for improving performance. This dual focus on alloy composition and process optimization distinguishes the study from prior research. Furthermore, from an industrial perspective, the proposed material offers long-term cost-effectiveness by reducing equipment failure rates and maintenance costs, making it highly suitable for demanding oil field applications.

Through comprehensive experimental evaluation, this research aims to provide actionable insights into the development of next-generation martensitic stainless steels, supporting safer and more efficient oil extraction operations.

Table 1. Chemical elements and their functions in stainless steel materials [11,12,13,14,15].

Element	Effect on Material Properties
Cr	The addition of Cr elements can enhance the inertness of materials. Therefore, the higher the Cr content, the stronger the corrosion resistance of the material, especially suitable for CO2 corrosion environments. As the Cr content increases, the hardness of the steel improves; however, its impact toughness may decrease.
Ni	The primary function of Ni is to provide the material with good ductility. When the Ni content exceeds 20%, it can endow the material with good resistance to stress corrosion cracking. Excessive addition of nickel may reduce hardness and wear resistance, affect high-temperature strength, promote stress corrosion cracking, and impair weldability, among other effects.
Mo	The addition of Mo elements is beneficial for enhancing the inertness of materials, preventing pitting and crevice corrosion in the material. Excessive or improper addition of molybdenum may lead to reduced toughness, increased brittleness, and deterioration in hot working and machining performance.
Ti	The addition of Ti elements is advantageous in preventing the precipitation of chromium carbides, thereby reducing the sensitivity to intergranular corrosion. Excessive addition or improper process control may result in reduced weldability, microstructural inhomogeneity, decreased corrosion resistance, reduced toughness, and increased processing difficulties.
Mn	Mn helps to prevent the occurrence of S and increase the strength of the material. Excessive manganese may lead to reduced corrosion resistance, decreased toughness, deteriorated hot workability, exacerbated sensitization, increased susceptibility to temper embrittlement, and impaired weldability.
Nb	The addition of Nb elements is similar to Ti in its effect. Excessive addition may lead to increased costs, embrittlement, poor weldability, reduced machinability, grain coarsening, and decreased oxidation resistance, among other issues.
Cu	It can improve the material’s corrosion resistance in acidic environments. Excessive copper may result in poor weldability, increased processing difficulties, enhanced low-temperature brittleness, reduced pitting corrosion resistance, and greater oxidation tendency, among other issues.
Al	The addition of Al elements can improve the material’s resistance to sulfur. Excessive or improper addition of aluminum may reduce mechanical properties, machinability, corrosion resistance, and high-temperature strength, increase welding difficulty, and promote embrittlement tendency.
C	Reducing the carbon (C) content can decrease the precipitation of chromium carbides and enhance the resistance against intergranular corrosion. High carbon content, on the other hand, can improve the alloy’s creep resistance and enhance its mechanical properties at high temperatures. However, as the carbon content increases, the corrosion rate of the alloy also increases, leading to a decrease in corrosion resistance.
S	The addition of S elements makes the material sensitive to pitting corrosion.
P	An appropriate amount of phosphorus can enhance the strength and hardness of steel; however, excessive phosphorus increases brittleness. Phosphorus affects grain growth and phase transformations, potentially leading to precipitate formation, which can impact the overall performance of the material.
V	An appropriate amount of vanadium can significantly increase the strength, hardness, and wear resistance of alloys while reducing the risk of brittle fracture. Vanadium refines the grain structure and promotes a uniform phase distribution, thereby enhancing overall mechanical properties.

Table 1. Chemical elements and their functions in stainless steel materials [11,12,13,14,15].

Element	Effect on Material Properties
Cr	The addition of Cr elements can enhance the inertness of materials. Therefore, the higher the Cr content, the stronger the corrosion resistance of the material, especially suitable for CO2 corrosion environments. As the Cr content increases, the hardness of the steel improves; however, its impact toughness may decrease.
Ni	The primary function of Ni is to provide the material with good ductility. When the Ni content exceeds 20%, it can endow the material with good resistance to stress corrosion cracking. Excessive addition of nickel may reduce hardness and wear resistance, affect high-temperature strength, promote stress corrosion cracking, and impair weldability, among other effects.
Mo	The addition of Mo elements is beneficial for enhancing the inertness of materials, preventing pitting and crevice corrosion in the material. Excessive or improper addition of molybdenum may lead to reduced toughness, increased brittleness, and deterioration in hot working and machining performance.
Ti	The addition of Ti elements is advantageous in preventing the precipitation of chromium carbides, thereby reducing the sensitivity to intergranular corrosion. Excessive addition or improper process control may result in reduced weldability, microstructural inhomogeneity, decreased corrosion resistance, reduced toughness, and increased processing difficulties.
Mn	Mn helps to prevent the occurrence of S and increase the strength of the material. Excessive manganese may lead to reduced corrosion resistance, decreased toughness, deteriorated hot workability, exacerbated sensitization, increased susceptibility to temper embrittlement, and impaired weldability.
Nb	The addition of Nb elements is similar to Ti in its effect. Excessive addition may lead to increased costs, embrittlement, poor weldability, reduced machinability, grain coarsening, and decreased oxidation resistance, among other issues.
Cu	It can improve the material’s corrosion resistance in acidic environments. Excessive copper may result in poor weldability, increased processing difficulties, enhanced low-temperature brittleness, reduced pitting corrosion resistance, and greater oxidation tendency, among other issues.
Al	The addition of Al elements can improve the material’s resistance to sulfur. Excessive or improper addition of aluminum may reduce mechanical properties, machinability, corrosion resistance, and high-temperature strength, increase welding difficulty, and promote embrittlement tendency.
C	Reducing the carbon (C) content can decrease the precipitation of chromium carbides and enhance the resistance against intergranular corrosion. High carbon content, on the other hand, can improve the alloy’s creep resistance and enhance its mechanical properties at high temperatures. However, as the carbon content increases, the corrosion rate of the alloy also increases, leading to a decrease in corrosion resistance.
S	The addition of S elements makes the material sensitive to pitting corrosion.
P	An appropriate amount of phosphorus can enhance the strength and hardness of steel; however, excessive phosphorus increases brittleness. Phosphorus affects grain growth and phase transformations, potentially leading to precipitate formation, which can impact the overall performance of the material.
V	An appropriate amount of vanadium can significantly increase the strength, hardness, and wear resistance of alloys while reducing the risk of brittle fracture. Vanadium refines the grain structure and promotes a uniform phase distribution, thereby enhancing overall mechanical properties.

Key alloying elements are elaborated below for their roles and effects:

Molybdenum (Mo): The addition of molybdenum improves the corrosion resistance of martensitic stainless steel, particularly enhancing its resistance to pitting and crevice corrosion caused by chlorides. This effect is especially pronounced in high-temperature and acidic environments. Molybdenum also enhances the mechanical properties of steel at elevated temperatures by reducing grain coarsening, thereby improving both high-temperature strength and oxidation resistance. Furthermore, molybdenum promotes the transformation of martensite, enabling the alloy to achieve higher hardness under the same heat treatment conditions while maintaining good toughness.

Nickel (Ni): Nickel improves the toughness and ductility of stainless steel, particularly in low-temperature environments. It also enhances weldability and cold workability. Additionally, nickel stabilizes a certain amount of austenite, which helps refine the microstructure and improve the mechanical properties of the alloy during heat treatment. Nickel further boosts the corrosion resistance of stainless steel, especially in oxidizing media, enhancing its oxidation resistance and acid resistance.

Tantalum (Ta): Tantalum significantly increases the stability and creep resistance of steel at high temperatures. In martensitic stainless steel designed for high-temperature applications, tantalum improves oxidation resistance and high-temperature strength. It forms solid solutions in steel, enhancing solid–solution strengthening and thereby improving hardness and tensile strength. Tantalum also contributes to the formation of fine carbides or nitrides, which reinforce grain boundaries and enhance the comprehensive mechanical properties of the steel.

2. Experimental Materials and Methods

2.1. Experimental Material

The material used in this experiment is a novel martensitic stainless steel, designated as 13CrU. It was developed from super martensitic stainless steel through adjustments in the contents of trace alloying elements (Mo, Ni, and Ta). Chemical composition analyses of 13CrS and 13CrU were conducted using an HCS-140 high-frequency infrared sulfur and carbon analyzer. A SIGMA 500 high-resolution field emission scanning electron microscope (SEM) (Shanghai, China) was used to analyze the cross-sectional morphology of the surface oxide layer on 13CrS and 13CrU materials. The imaging parameters for the scanning electron microscope (SEM) include an acceleration voltage of 20 kV. Additionally, X-ray photoelectron spectroscopy (XPS) was employed to determine the composition of the surface oxide layer on 13CrU. The X-ray photoelectron spectroscopy (XPS) analysis was conducted with an energy of 150 eV.

2.1.1. Chemical Composition

The chemical compositions of 13CrS and 13CrU are shown in

Table 2. Chemical composition test results (wt%).

	Experimental Material	13CrS	13CrU	API
Element Composition
C		0.02	0.016	-
Si		0.24	0.24	-
Mn		0.41	0.3	-
P		0.016	0.011	≤0.03
S		0.001	0.001	≤0.03
Cr		12.76	12.85	-
Mo		2.16	2.83	-
Ni		5.29	6.42	-
Ti		0.016	0.01	-
Nb		0.03	0.03	-
Cu		0.05	0.16	-
V		0.012	0.04	-
Al		0.032	0.04	-
Ta		N.A.	0.03	-

. A comparison of the experimental results with the allowable values in the API 5CT standard indicates that the contents of harmful elements, such as S and P, are controlled within the standard’s permissible range, and the chemical compositions meet the specified requirements. Compared to 13CrS, the content of trace alloying elements, such as Mo, Ni, and Ta in 13CrU, is significantly higher, which is beneficial for enhancing the material’s mechanical and corrosion resistance properties.

2.1.2. Surface Oxide Composition and Structure

Surface Oxide Composition

The composition of the surface oxide layer on 13CrU was determined using X-ray photoelectron spectroscopy (XPS), and the full XPS spectrum is shown in Figure 2. As seen in the figure, the oxide layer contains elements such as Cu, Ni, Fe, Mn, Cr, O, V, N, C, Mo, S, P, Si, and Ta. To precisely identify the components within the oxide layer, narrow-scan (high-resolution) XPS spectra were obtained for several elements, and peak fitting analysis was conducted to determine their specific forms. Figure 3 presents the Ta4f narrow spectrum, revealing that Ta is primarily present as Ta₂O₅ in the oxide layer. According to the literature, Ta₂O₅ exhibits excellent corrosion resistance. Figure 3 shows the Fe2p XPS narrow spectrum, indicating that Fe exists in the forms of Fe₂O₃ and Fe₃O₄ within the oxide layer. Figure 3 presents the Cr2p XPS narrow spectrum, demonstrating that Cr is present as Cr₂O₃. Figure 3 shows the Mo3d XPS narrow spectrum, with peak fitting results indicating that Mo exists as MoO₃ and MoO₂. These findings suggest that the primary components of the oxide layer are Fe₂O₃, Fe₃O₄, Ta₂O₅, Cr₂O₃, MoO₃, and MoO₂.

Surface Oxide Structure

The cross-sectional morphology of the surface oxide layers on the 13CrS and 13CrU samples is shown in Figure 4 and Figure 5. As observed in the figures, the oxide layer on the 13CrS sample contains distinct transverse and longitudinal cracks, with the cracks extending toward the substrate surface and reaching lengths of approximately 5 μm. The oxide layer on the 13CrU sample also shows noticeable transverse cracks; however, longitudinal cracks are less prominent, with minimal extension toward the substrate. Longitudinal cracks are sources of stress corrosion cracking, which can propagate under cyclic loading and lead to reduced strength in the tubing. This phenomenon has been identified as one of the primary causes of failure in 13CrS stainless steel tubing in a domestic oil field.

2.2. Experimental Device and Principle

Following the GB/T 13298-2015 standard [16], Methods for Metallographic Inspection of Microstructure, metallographic test samples were cut from the new tubing. The sample dimensions were 10 × 10 × 6.45 mm. The samples were sequentially polished with sandpaper of increasing grit size up to 2000 grit, followed by diamond powder polishing. A 5% nitric acid alcohol solution was used to etch the 13CrS sample, while a 5% KCl electrolyte was used to etch the 13CrU sample. Metallographic analysis of non-metallic inclusions, grain size, and microstructure was performed using an Axio Scope.A1 research-grade upright metallographic microscope (Carl Zeiss, Oberkochen, Germany).

In accordance with the GB/T 228.1-2010 standard [17], Metallic Materials—Tensile Testing, and the ISO 11960-2014 standard [18], Petroleum and Natural Gas Industries—Steel Pipes for Use as Casing or Tubing for Wells, flat tensile test specimens were cut from the new tubing. Three specimens were prepared, and tensile testing was conducted using an MTS-810 tensile testing machine (MTS Systems Corp., Eden Prairie, MN, USA) at a test temperature of 25 ± 1 °C in an air environment. The tensile test speed was controlled at 50 mm/min.

Following the GB/T 19748-2005 standard [19], Steel–Charpy V-notch Pendulum Impact Test—Instrumented Test Method, and the GB/T 229-2007 standard [20], Metallic Materials—Charpy Pendulum Impact Test Method, impact test specimens were cut from the new tubing. The specimens were prepared with dimensions of 55 mm × 10 mm × 5 mm, with a Charpy V-notch. The Charpy impact test machine was used to assess the impact toughness of the material’s longitudinal section at room and low temperatures. Before testing, specimens were soaked and cleaned in anhydrous ethanol and then dried in an oven. The test environments included 25 °C, −20 °C, and −45 °C, with the impact hammer energy set to 2.75 J for low-temperature tests and 5.5 J for room temperature tests.

Following the GB/T 231.1-2002 standard [21], Metallic Materials—Brinell Hardness Test, impact test specimens were cut from the new tubing and prepared as rings with a height of 10 mm. The upper and lower radial surfaces were ground and polished to ensure a surface roughness of less than 1.6 μm. At room temperature, hardness testing was conducted using a Brinell hardness tester (MC010-HBE-3000) (Shanghai, China). Four regions (A, B, C, and D) were selected along the radial direction, and in each region, measurements were taken at three locations: near the outer wall, at the middle position, and near the inner wall, as shown in Figure 6. Using a square-based pyramid diamond indenter, the Brinell hardness value was calculated as the average pressure over the indentation area.

The anodic polarization curves of 13CrS and 13CrU materials in a 3% NaCl solution were measured using a CorrTest electrochemical workstation in a three-electrode system. The electrochemical test specimens were small cylindrical shapes with dimensions of Φ11.3 × 3 mm. Wires were soldered to the specimens by brazing (e.g., tin soldering) or spot welding, and the electrodes for electrochemical measurement were embedded in PEEK with an exposed working area of 1 cm². The non-test surfaces and wires were sealed with insulating materials, such as epoxy resin, vinyl resin, or a mixture of paraffin and rosin. Prior to use, the working surfaces of the test specimens were sequentially polished with 400#, 600#, 800#, 1000#, and 1200# sandpaper, rinsed with distilled water, and degreased with acetone.

A saturated calomel electrode (SCE) was used as the reference electrode, and a large-area graphite inert electrode served as the auxiliary electrode. Before the electrochemical test, N₂ gas was purged for 1 h to remove oxygen. During the experiment, CO₂ gas was bubbled into the solution to reach saturation and continuously supplied to maintain CO₂ saturation. After connecting the electrodes, the specimen was allowed to stabilize in the solution for 10 min before polarization began. Starting from the open circuit potential, polarization was conducted at a scan rate of 20 mV/min to obtain the dynamic polarization curve of the working electrode. The pitting potential was measured according to the GB/T 17899-1999 standard [22], Method for Measuring the Pitting Potential of Stainless Steel. The anodic potential corresponding to a current density of 100 μA/cm² was defined as the pitting potential of the material.

The corrosion resistance of 13CrS and 13CrU materials was evaluated using a Cortest high-temperature high-pressure circulating autoclave. Specimens were prepared according to the GB10124-88 Standard [23], Laboratory Immersion Test Methods for Uniform Corrosion of Metallic Materials, with dimensions of 40 mm × 10 mm × 3 mm. Before the experiment, each specimen was sequentially polished with 400#, 600#, and 1000# sandpaper to remove machining marks, and geometric dimensions were measured with a vernier caliper. The specimens were then rinsed with anhydrous ethanol, air-dried, and weighed on a balance after drying.

In the weight-loss experiment, three parallel specimens were suspended on a PEEK holder without contacting each other and fully immersed in the test solution. Prior to testing, a 20wt% NaCl solution was purged with CO₂ to remove oxygen. All autoclave lines were flushed and vacuumed with high-pressure CO₂, and then the prepared CO₂-saturated solution was transferred to the sealed autoclave containing the specimens to ensure no residual dissolved oxygen. Once sealed, CO₂ was used to pressurize the autoclave to the specified pressure, and the solution was heated to the required temperature. The corrosion rate in the weight-loss experiment was calculated using the following formula.

(1) Calculation of corrosion rate:

$$V_{\mathrm{corr}}={\displaystyle \frac{365000\Delta G}{\rho St}}$$

where V_corr is the average corrosion rate, mm/a; ΔG is the weight loss of sample, g; ρ is the density of the material; t is the test time, d; and S is the specimen’s surface area, mm².

(2) Determination of corrosion degree:

In the oil and gas industry, the NACE standard is used to classify the degree of CO₂ corrosion. The NACE RP-0775-05 standard [24] provides detailed specifications for assessing corrosion severity, as shown in

Table 3. NACE requirements for corrosion degree.

Sort	Uniform Corrosion Rate (mm/a)	Pitting Rate (mm/a)
Mild corrosion	<0.025	<0.127
Moderate corrosion	0.025~0.125	0.127~0.201
Severe corrosion	0.126~0.254	0.202~0.381
Very severe corrosion	>0.254	>0.381

.

3. Experimental Result

3.1. Metallographic Structure

The metallographic test results are shown in

Table 4. Metallographic tissue test results.

	Test Item	Nonmetal Inclusion	Metallographic Structure	The Diameter of the Average Grain Section/µm	Grain Size
Materials
13CrS		D1 fine series (Lv. 0.5)	Tempered martensite with retained austenite and a small amount of granular ferrite.	22.15	Lv. 8
13CrU		D1 fine series (Lv. 0.5)	Tempered sorbite and a very small amount of δ ferrite	46.38	Lv. 6

, with microstructures illustrated in Figure 7. The non-metallic inclusions are annular oxide-type inclusions, with both 13CrS and 13CrU samples classified as D1 fine series. The results indicate that 13CrS has a grain size of grade 8, with a microstructure consisting of sorbite with retained martensitic orientation and a small amount of granular ferrite, containing approximately 3% δ-ferrite. For 13CrU, the grain size is grade 6, and the microstructure comprises tempered sorbite with a very small amount of δ-ferrite, with δ-ferrite content around 1%.

3.2. Tensile Mechanical Properties

As shown in the tensile mechanical properties results (Table 5), the yield strength (Rp0.2) of 13CrS is 859 MPa, the tensile strength is 944 MPa, and the elongation at fracture is 22.11%. For 13CrU, the yield strength (Rp0.2) is 959 MPa, the tensile strength is 1021 MPa, and the elongation at fracture is 22.43%. The stress–strain curves in Figure 8 also indicate a noticeable improvement in the tensile mechanical properties of 13CrU compared to 13CrS.

The comparison of the two martensitic stainless steels demonstrates that increasing trace elements can enhance mechanical properties. As reported in the literature [25], an increase in Mo content contributes to improved tensile strength. With a 0.67 wt% increase in Mo content, the yield strength of 13CrU increased by 11.6% compared to 13CrS. Studies also indicate [26] that in ultra-low carbon 13Cr martensitic stainless steels, an increase in Ni content can improve yield strength. In this study, a 1.13 wt% increase in Ni content resulted in a significant enhancement of the material’s overall mechanical properties.

Table 5. Experimental results of tensile mechanical properties.

Materials	ID	Yield Strength Rp0.2 (MPa)		Experimental Bar (%)	Tensile Strength (MPa)		Experimental Bar (%)	Yield Ratio		Experimental Bar (%)	Elongation at Break δ (%)		Experimental Bar (%)
		Actual Measurement	Average		Actual Measurement	Average	/	Actual Measurement	Average	/	Actual Measurement	Average	/
13CrS	1#	869.68	859	1.2	952.69	944	0.9	0.91	0.91	0.0	24.35	22.11	10.1
	2#	841.80		−2.0	930.87		−1.4	0.90		−1.1	22.06		−0.2
	3#	867.32		1.0	951.05		0.7	0.91		0.0	19.92		−9.9
13CrU	1#	979.53	959	2.1	1023.10	1021	0.2	0.96	0.97	−1.0	22.34	22.43	−0.4
	2#	950.53		−0.9	1020.80		0.0	0.97		6.6	22.76		1.5
	3#	945.56		−1.4	1020.12		−0.1	0.97		6.6	22.19		−1.1
ISO 13680		758~965		/	≥793		/	—		/	≥12.5		/
SY/T 5525-2009 [27]		≥689		/	≥931		/	—		/	≥13		/

Table 5. Experimental results of tensile mechanical properties.

Materials	ID	Yield Strength Rp0.2 (MPa)		Experimental Bar (%)	Tensile Strength (MPa)		Experimental Bar (%)	Yield Ratio		Experimental Bar (%)	Elongation at Break δ (%)		Experimental Bar (%)
		Actual Measurement	Average		Actual Measurement	Average	/	Actual Measurement	Average	/	Actual Measurement	Average	/
13CrS	1#	869.68	859	1.2	952.69	944	0.9	0.91	0.91	0.0	24.35	22.11	10.1
	2#	841.80		−2.0	930.87		−1.4	0.90		−1.1	22.06		−0.2
	3#	867.32		1.0	951.05		0.7	0.91		0.0	19.92		−9.9
13CrU	1#	979.53	959	2.1	1023.10	1021	0.2	0.96	0.97	−1.0	22.34	22.43	−0.4
	2#	950.53		−0.9	1020.80		0.0	0.97		6.6	22.76		1.5
	3#	945.56		−1.4	1020.12		−0.1	0.97		6.6	22.19		−1.1
ISO 13680		758~965		/	≥793		/	—		/	≥12.5		/
SY/T 5525-2009 [27]		≥689		/	≥931		/	—		/	≥13		/

3.3. Impact Fracture Toughness

The Charpy impact test results for 13CrS and 13CrU materials are shown in

Table 6. Results of 13CrS material oscillographic impact experiment (10 mm × 5 mm × 55 mm).

Temperature (°C)	ID	Impact Work/Wt (J)	Total Displacement/St (mm)	Maximum Force/Fm (kN)	Yield Force/Fgy (KN)	Crack Initiation Work/Wm (J)	Crack Propagation Work/Wa (J)
25	1	197.34	24.96	24.90	19.04	52.12	145.22
	2	201.58	17.47	24.46	19.18	62.54	139.04
	3	199.04	25.15	24.76	19.20	64.60	134.44
	Average	199.32	22.53	24.70	19.14	59.76	139.56
−20	1	191.42	24.21	24.16	18.46	50.56	140.86
	2	195.54	16.95	23.72	18.60	60.66	134.86
	3	193.06	24.40	24.02	18.62	62.66	130.40
	Average	193.34	21.85	23.96	18.56	57.96	135.38
−45	1	157.88	19.97	19.92	15.24	41.70	116.18
	2	161.26	13.98	19.56	15.34	50.04	111.24
	3	159.24	20.12	19.80	15.36	51.68	107.56
	Average	159.46	18.02	19.76	15.32	47.80	111.66

and

Table 7. Results of 13CrU material oscillographic impact experiment (10 mm × 5 mm × 55 mm).

Temperature (°C)	ID	Impact Work/Wt (J)	Total Displacement/St (mm)	Maximum Force/Fm (kN)	Yield Force/Fgy (KN)	Crack Initiation Work/Wm (J)	Crack Propagation Work/Wa (J)
25	1	296.63	30.58	23.34	17.88	59.29	237.34
	2	286.90	31.28	23.79	17.03	58.44	228.46
	3	278.66	29.71	23.32	18.51	62.57	216.09
	Average	287.40	30.52	23.48	17.81	60.10	227.30
−20	1	276.00	29.57	24.67	18.43	61.81	214.19
	2	294.90	30.93	24.05	18.03	59.80	235.10
	3	243.88	26.43	24.83	18.68	62.89	180.99
	Average	271.59	28.98	24.52	18.38	61.50	210.09
−45	1	223.81	27.51	25.11	19.10	73.95	149.86
	2	188.20	24.29	25.39	20.32	42.88	145.32
	3	212.62	26.13	23.85	18.15	70.25	142.37
	Average	208.21	25.98	24.78	19.19	62.36	145.85

, with the oscillographic impact curves illustrated in Figure 9. The results indicate that the impact toughness of 13CrU is significantly higher than that of 13CrS. At room temperature (25 °C), the crack propagation energy for 13CrU is 227.30 J, compared to 139.56 J for 13CrS. At −20 °C, the crack propagation energy for 13CrU is 210.09 J, while 13CrS registers 135.38 J. At −45 °C, 13CrU exhibits a crack propagation energy of 145.85 J, in contrast to 111.66 J for 13CrS. Crack propagation energy reflects the material’s resistance to cracking, demonstrating that 13CrU has superior crack resistance, compared to 13CrS.

The results of the low-temperature impact test also indicate that both 13CrS and 13CrU maintain relatively high impact energies across a broad temperature range, with minimal ductile-to-brittle transition. Only at −45 °C does the material exhibit some degree of low-temperature brittleness (Figure 9). The ductile-to-brittle transition temperature is associated with the material’s microstructure; grain refinement improves toughness, and tempered sorbite structures offer optimal impact energy and ductile-to-brittle transition temperature [28]. As shown in the metallographic structures (Table 6), both 13CrS and 13CrU meet these criteria.

3.4. HB

The Brinell hardness test results for 13CrS and 13CrU materials are shown in

Table 8. HB test result.

		Quadrant	Brinell Hardness (HB) Test Value
	Test Location		Ⅰ	Ⅱ	Ⅲ	Ⅳ	Average
Materials
13CrS	Close to outer wall		266	270	265	260	265
	Intermediate position		268	266	269	263	267
	Close to wall		268	268	265	261	266
13CrU	Close to outer wall		294	328	336	366	331
	Intermediate position		298	324	343	362	332
	Close to wall		301	324	350	356	333

. As indicated in Table 8, the HB value of 13CrU is higher than that of 13CrS, primarily due to differences in δ-ferrite content. δ-ferrite, also known as high-temperature ferrite, is relatively uncommon at room temperature but remains present in 13CrS steel. Since δ-ferrite is brittle, its presence negatively affects the hardness of the tubing. According to the ISO 13680 standard [29], the δ-ferrite phase content in 13CrS stainless steel should be less than 5%. Additionally, some studies [30] suggest that the δ-ferrite content should be below 1.5%.

3.5. Pitting Resistance

Metals will only undergo pitting corrosion when the potential exceeds a specific threshold, known as the critical pitting potential. A higher critical pitting potential indicates better corrosion resistance of the material [31]. The critical pitting potential can be obtained by measuring the anodic polarization curve, as shown in Figure 10, where E_b represents the critical pitting potential or pitting potential. When the potential significantly exceeds the critical pitting potential, the polarization curve is reversed (the potential is gradually reduced) until the passivation potential, E_rp, is reached, as illustrated in Figure 10.

When E > E_b, pitting corrosion initiates and progresses rapidly. In the range E_rp > E > E_b, no new pitting occurs, though existing pits may continue to propagate. When E < E_rp, pitting does not occur. The closer E_rp and E_b are, the stronger the material’s passivation film repair capability.

A stable corrosion potential indicates the formation of a stable passive layer with good protective performance. In contrast, a fluctuating corrosion potential may suggest the rupture or localized dissolution of the passive layer, increasing the risk of localized corrosion. A uniform and dense passive layer can stabilize the electrochemical state of the metal surface and raise the corrosion potential (more positive). Conversely, if the passive layer has defects or undergoes localized dissolution, the corrosion potential may fluctuate or even decrease (become more negative). When the passive layer is compromised—such as by localized dissolution induced by chloride ions—the corrosion potential typically drops rapidly, indicating exposure of the substrate and transition to an active corrosion state. If the passive layer can self-repair, facilitated by elements like Mo or Ta that promote secondary passivation, the corrosion potential will return to a stable value.

Figure 10 shows the anodic polarization curves of 13CrU and 13CrS steels in a 3% NaCl solution. The figure indicates that the pitting potential of 13CrU is slightly lower than that of 13CrS, suggesting that the pitting resistance of 13CrU in a 3% NaCl solution is marginally lower than that of 13CrS. However, the difference between the pitting potential and the protection potential for 13CrU is significantly smaller than that for 13CrS, indicating that the passivation repair capability of 13CrU is much stronger than that of 13CrS.

Figure 11 shows the anodic polarization curves of 13CrU and 13CrS steels in a formate annular protection fluid. In this fluid, there is no significant difference between the curves of 13CrU and 13CrS, except that the current density within a specific region of the passivation zone is lower for 13CrU than for 13CrS.

Based on the test results, there is no significant difference in pitting resistance between 13CrU and 13CrS in the annular protection fluid; however, 13CrU demonstrates a stronger ability to repair its surface passivation film. A material’s pitting resistance is influenced by internal factors, such as alloying elements and microstructure, as well as external factors, such as solution composition and temperature. As shown in the chemical compositions of the two materials (Table 2), 13CrU has a higher Cu content than 13CrS. The presence of Cu can cause Cr to segregate at grain boundaries, leading to intergranular corrosion [32], which is one reason for the slightly lower pitting resistance of 13CrU compared to 13CrS. Molybdenum in the passive film exists primarily as MoO₃ and MoO₄²⁻, both of which exhibit high chemical stability. In particular, MoO₃ enhances the passive film’s resistance to pitting and crevice corrosion. Molybdenum can adsorb at corrosion defects and form a protective secondary passive film, thereby preventing further corrosion propagation. In the cathodic regions of pitting or crevice corrosion, molybdenum ions reduce the cathodic reaction rate, weakening the electrochemical driving force of corrosion.

Tantalum, in oxidizing environments, preferentially forms a dense layer of Ta₂O₅, an oxide with exceptional chemical stability and resistance to dissolution. The formation of Ta₂O₅ effectively blocks the penetration of corrosive agents, such as Cl⁻, through the passive film. Tantalum oxides have extremely low ionic conductivity, significantly reducing the migration of metal cations (e.g., Fe²⁺, Cr³⁺) from the substrate to the surface of the passive film, thereby slowing localized film dissolution.

3.6. CO₂ Corrosion Test

The weight-loss corrosion test results (

Table 9. Corrosion test results of 13CrS and 13CrU materials in 1 MPa CO₂ 20% NaCl for 336 h.

Experimental Temperature, °C	Materials	Corrosion Rate, mm/a
150	13CrS	0.16
	13CrU	0.02
180	13CrS	0.52
	13CrU	0.22
220	13CrS	1.95
	13CrU	1.45

) indicate that the CO₂ corrosion resistance of 13CrU is superior to that of 13CrS. This is attributed to the stronger passivation repair capability of 13CrU, as well as the composition of its surface oxide layer, which contains a small amount of Ta₂O₅. Additionally, previous studies have shown that the addition of Mo to stainless steel enhances passivation, thereby improving corrosion resistance [33]. Molybdenum dynamically participates in the repair process of the passive film in corrosive environments, promoting secondary passivation in locally dissolved regions, thereby extending the service life of the passive film. Ni has also been reported to enhance corrosion resistance by reducing the dissolution of Fe and Cr, thus stabilizing the surface passivation film [34]. Furthermore, Ta improves alloy performance by influencing microstructure and phase transformation kinetics, forming a dense passivation film on the metal surface [35]. When molybdenum and tantalum coexist in martensitic stainless steel, they exhibit a synergistic effect that further enhances the corrosion resistance of the passive film. Together, molybdenum and tantalum contribute to the formation of the passive film, potentially generating more stable composite oxides (e.g., Mo-doped Ta₂O₅), significantly improving the film’s chemical stability. The presence of tantalum promotes a more uniform distribution of molybdenum oxides within the passive film, reducing the likelihood of localized corrosion and thereby increasing the durability of the passive film.

4. Analysis and Discussion

The content of alloying elements significantly affects the overall performance of stainless steel by contributing in the following ways:

(1)

Improving the microstructure of stainless steel, enhancing structural stability, and reducing or eliminating inhomogeneity;

(2)

Promoting the formation and stability of the stainless steel passivation film;

(3)

Increasing the electrode potential of stainless steel;

(4)

Balancing or mitigating the adverse effects of carbon on corrosion resistance;

(5)

Strengthening the stainless steel matrix, thus improving its mechanical properties;

(6)

Enhancing the cold and hot workability of the material.

This study primarily examines the effects of Mo, Ni, and Ta on the overall performance of two martensitic stainless steel materials.

The main role of Mo in stainless steel is solid solution strengthening and age-hardening, particularly through tempering hardening, which enhances corrosion resistance. The most notable effects of Mo include improved tempering stability, enhanced secondary hardening, and increased strength and crack resistance. The mechanism by which Mo improves tempering stability involves the formation of fine Mo₂C or Fe₂Mo phases, which strengthen the secondary hardening effect [29,30].

Studies have shown that the strength of steel increases with Mo content, with an accelerated rate of strength increase when the Mo content exceeds 2%. However, when the Mo content is above 4%, the rate of yield strength increase slows. This is due to Mo’s role in promoting the formation of intermetallic phases and the δ-ferrite phase in the microstructure [29,30,31]. Adding Mo to stainless steel also enhances passivation, thereby improving corrosion resistance. Appropriate Mo additions lead to the formation of insoluble Mo compounds on the material surface, which reduces chloride ion accumulation and suppresses the nucleation and growth of pitting, thereby lowering the risk of pitting corrosion. The combination of Mo and Cr in stainless steel provides superior pitting resistance [32,33,34].

Ni improves both corrosion resistance and toughness, facilitates hot rolling, and allows for higher Cr content, while reducing the tendency to form high-temperature δ-ferrite. It also helps lower the ductile-to-brittle transition temperature [35]. In the comparative performance experiments on the two martensitic stainless steel materials in this study, the δ-ferrite content in 13CrU was controlled at approximately 1%, resulting in significantly higher hardness compared to 13CrS. Additionally, 13CrU exhibited excellent impact toughness at low temperatures.

Studies have shown that in ultra-low-carbon 13Cr martensitic stainless steel, a Ni content of 2% increases the yield strength by approximately 1.5 times compared to steels without Ni. When Ni content ranges from 2% to 6%, yield strength remains stable, but adding more than 6% leads to a slight decrease in yield strength, suggesting that Ni content should be kept below 6% [36]. In this study, an increase of 1.13 wt% Ni resulted in a significant enhancement of the material’s overall mechanical properties. However, to meet corrosion resistance requirements in certain media environments, stainless steel requires a Ni content of around 27%, meaning that Ni alone is not typically used as the sole alloying element in stainless steel. In alloyed stainless steels, Ni reduces the dissolution of Fe and Cr, thereby stabilizing the passivation film. The effect of Ni on localized corrosion is also influenced by the Cr and Mo content in the steel.

Ta’s unique properties make it a critical material in corrosive environments. It enhances alloy performance by improving microstructure and phase transformation kinetics; its formation of tantalum carbides, solid solutions, and intermetallic precipitates provides higher strength, phase stability, and deformation capacity [37]. Ta readily forms a dense passivation film on metal surfaces. As shown in the CO₂ corrosion test results, both materials exhibit low corrosion rates in harsh environments below 180 °C, with only a slight increase in corrosion rate at 200 °C. The corrosion resistance of 13CrU is superior to that of 13CrS, which is attributed to the trace amount of Ta in its composition. Analysis of the surface oxide layer of 13CrU reveals a dense Ta₂O₅ layer. The literature indicates that this protective film can withstand strong acids and bases, maintaining structural stability up to 190–250 °C before altering with environmental changes [38]. This contributes to the slightly increased corrosion rate observed in CO₂ testing at 220 °C.

While the addition of individual elements can enhance material performance, it may also introduce some adverse effects. The interactions between different alloying elements, such as Mo, Ni, and Ta, can mitigate these negative effects and improve the overall properties of the material. Therefore, careful consideration should be given to the mass ratio of these elements to minimize undesirable factors, such as the formation of brittle phases.

By optimizing the contents of Mo, Ni, and Ta, the 13CrU steel alloy is designed with a typical tempered sorbite microstructure (Figure 7), containing a very small amount of δ-ferrite (1%), resulting in a significant increase in hardness. The increased Mo and Ni content lowers the martensitic transformation temperature, refining the lath martensite and thereby enhancing the material’s overall mechanical properties, including tensile strength and impact toughness.

Furthermore, the addition of Mo and Ni promotes the formation of fine precipitates along martensitic lath boundaries and grain boundaries, such as Mo carbides and σ-phase, which inhibit the formation of coarse precipitates (Fe, Cr)₂₃C₆ at grain boundaries. This refinement improves the material’s pitting resistance. Additionally, the presence of Ta significantly enhances the material’s resistance to CO₂ corrosion.

5. Conclusions

This study highlights the significant advancements achieved in developing a novel martensitic stainless steel, 13CrU, with superior properties, compared to the widely used 13CrS. The key findings are summarized as follows:

Performance Enhancement: The novel 13CrU stainless steel exhibits remarkable improvements in tensile mechanical properties, impact toughness, hardness, and CO2 corrosion resistance. These advancements address the limitations of 13CrS in high-temperature, high-pressure, and highly corrosive environments.

Alloying Element Contributions: The increased contents of Mo, Ni, and the introduction of Ta significantly enhance both the mechanical properties and corrosion resistance. Specifically, a 0.67 wt% increase in Mo and a 1.13 wt% increase in Ni improved the yield strength of 13CrU by 11.6% over 13CrS. The addition of Ta further elevates the corrosion resistance, making 13CrU more suitable for extreme operating conditions.

Optimized Element Ratios: While individual alloying elements contribute to performance enhancement, the optimal balance of Mo, Ni, and Ta is critical. This study recommends maintaining Mo between 2% and 4% and Ni below 6.0% to achieve the best trade-off between strength, toughness, and corrosion resistance.

Future Prospects: The novel martensitic stainless steel 13CrU demonstrates significant industrial application potential, particularly in oil extraction and similar extreme environments. Future research will focus on evaluating the scalability of this alloy, including its production cost, resource availability, and environmental impact. Further optimization of alloying element ratios and the integration of advanced fabrication processes will be explored to achieve a balance between performance improvement and cost control.