Initial corrosion behavior of non-nitriding and nitriding 38CrMoAl steel in salt spray environment

In order to study the initial corrosion characteristics and electrochemical corrosion behavior of as-received and nitrided 38CrMoAl steel in salt spray environment. The salt spray test was performed on the as-received and nitrided 38CrMoAl steel specimens to simulate its initial corrosion behavior in the marine atmosphere, and the surface morphology, corrosion rate, FT-IR, and FT-IR of the specimens after different corrosion cycles Potential polarization, electrochemical impedance and micro-area electrochemical analysis and research. In the salt spray test, the as-received specimens gradually developed from pitting corrosion to uniform corrosion. The nitrided parts always showed local corrosion characteristics, and the corrosion rate of the nitrided parts was less than that of the as-received parts. The corrosion products of as-received and nitrided specimens contain Fe3O4, α-FeOOH, γ-FeOOH and δ-FeOOH, and the content of γ-FeOOH in the corrosion product layer of nitrided parts is relatively low. The self-corrosion current density of as-received parts showed an increasing-decreasing-increasing trend, and the capacitance arc radius showed a decreasing-increasing-decreasing trend. When corroded for 6 days, the self-corrosion current density of as-received parts was 147.83 µA/cm2, the corrosion reaction resistance is 2103.6 Ω·cm2. At this time, the corrosion rate of the specimen is the lowest, and the dense corrosion product layer hinders the diffusion process of the corrosion solution and oxygen into the metal matrix. The self-corrosion current density of nitrided parts showed an increase-decrease trend, and the capacitance arc radius showed a decrease-increasing trend. After 12 days of salt spray corrosion, the self-corrosion current density of nitrided parts was 35.76 µA/cm2, The corrosion reaction resistance is 3021.5 Ω·cm2, and the corrosion product layer of the nitrided part appears to inhibit the corrosion rate later. In the early stage of salt spray corrosion, the SKP potential difference on the surface of the as-received piece increased rapidly, and obvious cathode areas and anode areas were formed on the surface of the test piece. Subsequently, the potential difference remained stable, and the surface corrosion of the test piece developed toward uniform corrosion. The surface potential of the nitrided test piece is more dispersed and the corrosion is more uneven. The protective performance of the corrosion product layer of as-received parts increases with the increase in thickness, but as the corrosion time increases, the corrosion product covering layer continues to thicken, and the corrosion product cracks due to the increase of internal stress, which reduces the protection performance of the corrosion product layer. The main protective role of the nitrided parts in the early stage of salt spray corrosion is the surface nitriding layer. As the corrosion progresses, the nitriding layer is gradually eroded and even cracks appear, and the corrosion product layer gradually plays a leading role. © 2021, Chongqing Wujiu Periodicals Press. All rights reserved.