Effects of Cr, Al, and Mo on the Mechanical Properties and CO2 Corrosion Resistance of Cr-Al Alloyed Oil Well Tubular Steel

CO2 corrosion is one of the major failure mechanisms affecting oil well tubular steels during oil and gas extraction. Although high-Cr stainless steels exhibit excellent corrosion resistance, their high cost limits practical applications. Therefore, developing economical low-Cr steels with balanced mechanical properties and CO2 corrosion resistance is of great significance. In this study, Cr-Al alloyed steels were designed for oil well tubular applications, and the effects of alloy composition on microstructure, mechanical properties, and CO2 corrosion behavior were systematically investigated through microstructural characterization, mechanical testing, corrosion simulation, and electrochemical measurements. The results demonstrate that alloy composition strongly regulates microstructural evolution and mechanical properties. Increasing Al content promotes prior austenite grain and martensite packet coarsening, while excessive Cr addition without Mo addition accelerates carbide coarsening and aggregation, resulting in reduced strength. Ti addition results in two types of Ti-containing precipitates, including micrometer-scale TiN particles formed during steelmaking and nanoscale TiC precipitates formed during subsequent solid-state transformation, which contribute to microstructural stabilization. Cr-Al alloying significantly improves CO2 corrosion resistance by modifying the composition and structure of corrosion product films. For the 6.5Cr0.6Al and 6.5Cr0.6Al0Mo alloys, the corrosion rates decrease to 0.114 and 0.143 mm·a−1, respectively, approximately 7–9% of that of N80 steel. The improved corrosion resistance is associated with the formation of Cr- and Al-enriched corrosion product films containing FeCO3, Ca-containing carbonate/aluminate phases, and Al/Cr-containing hydroxide-related species. This study provides insights into the development of Cr-Al-alloyed oil well tubular steels through optimized alloying strategies, reduced reliance on costly alloying elements, and improved mechanical properties and CO2 corrosion resistance.