Martensitic stainless steel is widely used in demanding environments, such as oil well pipelines, due to its superior mechanical properties and corrosion resistance. However, its corrosion resistance is influenced by several key factors, including alloy composition, microstructure, and external environmental conditions. To achieve optimal strength and corrosion resistance in oilfield applications, it is critical to refine the alloy design and manufacturing processes of martensitic stainless steel. This paper provides a comprehensive analysis of the primary factors influencing the corrosion resistance of martensitic stainless steel, with a focus on material and environmental considerations. It also examines the associated mechanisms to offer theoretical guidance for material design and engineering applications.
The corrosion resistance of martensitic stainless steel oil well pipes is primarily determined by their microstructure, which is influenced by alloy composition and manufacturing processes.
The development of 13Cr martensitic stainless steel oil well pipes began with UNS S42000. To meet the demands of oil field service, enhanced variants such as improved 13Cr, SUP13Cr, 15Cr, and 17Cr were developed through alloy modifications. The material’s strength grade has increased from 80 to 125, leading to improved high-temperature corrosion resistance. Alloy design is critical to enhancing material performance, especially through the inclusion of key elements such as C, Cr, Ni, Mo, Cu, and N. A primary goal of alloy design for martensitic stainless steel oil well pipes is to ensure the material achieves a fully martensitic structure through the quenching and tempering process, which is essential for achieving both high strength and corrosion resistance. The Schaeffler diagram is a useful tool for predicting the material’s room temperature structure based on Cr and Ni equivalents. Although the Schaeffler diagram is designed for predicting post-weld cooling outcomes, it remains a useful reference for alloy design despite differences from the quenching process used in seamless steel pipes. It helps estimate the material's metallographic structure boundaries, ensuring the material does not retain austenite or ferrite at room temperature, which could hinder hot processing and compromise strength.
Carbon is a cost-effective element for enhancing strength; however, it negatively impacts the CO2 corrosion resistance of martensitic stainless steel. Except for API Spec 5CT L80, other martensitic stainless steel grades are typically designed with ultra-low carbon content (with a maximum carbon mass fraction of 0.03%).
Masamura et al. investigated the effect of carbon content on the corrosion resistance of 13Cr. Their findings showed that the primary secondary-phase precipitates in 13Cr are M23C6-type carbides, which reduce the protective effect of chromium and accelerate material corrosion in low pH and H2S-containing environments at room temperature. Masamura et al. also introduced an effective chromium index, Ceff, to predict the uniform corrosion rate based on the Cr/Fe ratio of M23C6, approximately 2.25. Zhao et al. examined the impact of M23C6-type carbides on the pitting corrosion resistance of 13Cr stainless steel. They found that M23C6 carbides cause partial chromium depletion, forming a passive layer with low chromium content and poor stability, such as Cr(OH)3 and CrO3. This unstable passive layer dissolves easily, creating a transmission path between the corrosive solution and the matrix. Furthermore, the Cr-depleted region around M23C6 has a lower potential than the matrix, making it a preferential site for pitting corrosion initiation.
Although nitrogen is not a primary alloying element in martensitic stainless steel, it can influence production costs. The purity of the heat treatment atmosphere can significantly affect nitrogen behavior in martensitic stainless steel. The effect of nitrogen on corrosion resistance remains debated. Masamura et al. proposed that nitrogen forms CrN-type precipitates in chromium-containing martensitic stainless steel, reducing effective chromium content and weakening the material's resistance to CO2 corrosion. In contrast, Leda et al. suggested that nitrogen refines austenite grains and reduces the chromium content in M23C6 and IM23C6-type precipitates, thereby enhancing the material's resistance to CO2 corrosion.
Chromium plays a crucial role in enhancing corrosion resistance and promoting the formation of a protective passivation layer, primarily Cr(OH)₃, which significantly improves resistance to CO₂ corrosion. Xu et al. investigated the effect of chromium content on CO2 corrosion resistance by analyzing seven different materials with chromium concentrations ranging from 1% to 13%. The results, shown in Figure 12, reveal that the surface passivation layer in 1Cr steel consists primarily of FeCO3. In 3Cr to 6.5Cr steels, the passivation layer contains both Cr(OH)3 and FeCO3, with the FeCO3 content decreasing as the chromium content increases. In steels with 10Cr to 13Cr, the passivation layer is predominantly composed of Cr(OH)3. These variations in passivation layer composition lead to differences in CO2 corrosion resistance.
Figure 12 Effect of Cr content on corrosion rate
Nickel serves as an austenite stabilizer, aiding in the formation of a fully martensitic structure and contributing to material strength. Wen et al. found that nickel content no longer enhances strength once it exceeds 2%. Furthermore, increasing nickel content does not affect the breakdown potential of 13Cr in a 3% NaCl solution, suggesting that nickel does not directly improve the material’s corrosion resistance. However, Lei et al. found that in SUP13Cr, a high nickel content promotes the formation of reversed austenite around Cr-rich carbides at martensite lath boundaries during tempering at 620°C. This transformation improves impact toughness, reduces chromium depletion, stabilizes the passivation film, and enhances pitting corrosion resistance.
Molybdenum is an important element for enhancing the corrosion resistance of materials. First, molybdenum improves the material's resistance to pitting corrosion. Amaya et al. found that molybdenum significantly reduces both the critical passivation current density (J) and the reactivation current density (J), promoting the formation of a stable surface passivation film and enhancing pitting corrosion resistance (Figure 13). Second, molybdenum enhances the resistance of martensitic stainless steel to H2S stress corrosion cracking (SCC) and broadens its applicable service conditions. Amaya et al. proposed that in environments containing both CO₂ and H₂S, molybdenum accumulates as a sulfide in the outer layer of the passivation film, while chromium oxide forms in the inner layer. This dual-layer structure impedes H2S diffusion through the outer layer, thereby improving the material's SCC resistance.
Figure 13 Effect of Mo content on uniform corrosion and pitting corrosion resistance of 13Cr
Figure 14 Effect of Mo Content on H2S Stress Corrosion Resistance of 17Cr-5Ni-2.5Cu Steel
Niobium is an important alloying element used for grain refinement. In martensitic stainless steel alloy design, niobium is primarily used to mitigate the detrimental effects of carbon and nitrogen on corrosion resistance.
Copper is added to increase the effective chromium content in the material. During the development of 15Cr steel, 1% copper was incorporated to reduce Cr-rich carbide precipitation at grain boundaries, thereby enhancing pitting resistance.
The microstructure of chromium-containing martensitic stainless steel used in oil well pipes primarily consists of a martensitic matrix with a limited amount of retained austenite. Optimizing alloy design and heat treatment refines the material's microstructure, enhancing both its corrosion resistance and mechanical properties. Microstructural features such as martensite, precipitates, reverse austenite, and δ ferrite influence the material's performance in different ways.
Deformation and heat treatment affect the grain size, grain boundaries, and dislocation density of martensitic stainless steel, which have complex effects on its corrosion behavior. Zhao et al. compared 2Cr13 and S13CrSS and found that fine grains with high dislocation density increased the number of nucleation sites for passive film formation. Additionally, the higher chromium and molybdenum content in S13CrSS leads to a higher concentration of Cr- and Mo-based compounds within the passive film. These factors contribute to the formation of a denser and more stable passive film, thereby enhancing its corrosion resistance, as shown in Figure 15.
Figure 15 The influence of microstructure and material composition on the formation dynamics of passive film (the blue shaded area is the dislocation entanglement)
Chromium carbides at grain boundaries reduce the effective chromium content, diminishing the material’s corrosion resistance.
Zhang et al. found that reverse austenite in martensitic stainless steel forms during the tempering process.
Improper composition or solidification conditions may result in δ ferrite formation in martensitic stainless steel. To eliminate δ ferrite, high-temperature heat treatment (1,050–1,200°C) is required to dissolve δ ferrite by precipitating (Fe,Cr) carbides at the austenite/ferrite interface, facilitated by chromium and nickel diffusion. However, excessive heat treatment may reduce material strength or increase grain size, which can compromise performance.
Figure 16: Effect of ferrite on the impact properties of martensitic stainless steel
Figure 16: Effect of Reverse Austenite Content on the SSC Resistance of Improved 13Cr (5% NaCl, pH = 4, 10% H₂S + 90% CO₂, with loading stress at 90% of actual yield strength)
The impact of δ ferrite on material properties continues to be debated. Carrouge et al. suggested that δ ferrite affects the material's toughness. Their experiments showed that, assuming the same martensite grain size, increasing the proportion of δ ferrite to 14% resulted in a 50°C increase in the ductile-brittle transition temperature. Conversely, reducing the δ ferrite content from 14% to 2% aligns the ductile-brittle transition temperature with that of the tempered base material. Schafer et al. proposed that the low strength and good plasticity of δ ferrite can mitigate the adverse effects of dendritic carbides on ductility and toughness. Wang et al. suggested that impact cracks initiate in the δ ferrite within the ductile-brittle transition temperature range of low-carbon 13Cr-4Ni martensitic stainless steel. When the crack extends to the boundary between the δ ferrite and the matrix, it triggers brittle fracture of the tempered martensitic matrix, significantly reducing crack propagation energy. In the upper temperature range, the δ ferrite phase exhibits good plasticity, which counteracts the detrimental effects of its low strength during impact, as shown in Figure 17.
Figure 17 Effect of delta ferrite on impact energy of 13Cr-4Ni martensitic stainless steel
δ Ferrite adversely affects the corrosion resistance of martensitic steel. Hara et al. proposed that chromium carbides or nitrides precipitate near the δ ferrite, creating a chromium-depleted zone. The corrosion potential of the chromium-depleted zone decreases below the active potential, promoting SSC. However, the occurrence of SSC is not influenced by the shape of the δ ferrite. Bhambri et al. also identified the δ ferrite network at the grain boundaries of the original 13Cr austenitic structure as the primary cause of cyclic stress-induced intergranular fractures of 13Cr in NaCl solution.
In conclusion, the corrosion resistance of martensitic stainless steel is influenced by both material and environmental factors. In terms of material factors, the careful selection of alloying elements (such as C, Cr, Ni, and Mo) and the optimization of microstructure (including martensite, reverse austenite, and δ ferrite) through heat treatment or processing techniques are critical to improving corrosion resistance. As for environmental factors, external conditions such as temperature, CO₂ partial pressure, H₂S partial pressure, pH, and Cl⁻ concentration significantly affect corrosion behavior. Future research should focus on developing cost-effective alloys, investigating in-situ corrosion mechanisms, and establishing industrial evaluation standards to support the application of martensitic stainless steel in more complex environments, thereby providing a solid scientific foundation for improved performance.
