Is the Corrosion Resistance Degradation of Nickel-based Alloys Synchronous with Mechanical Properties Degradation in Hydrochloric Acid Environments?
Corrosion resistance performance: Nickel-based alloys (e.g., Hastelloy C276, Inconel 625) form a dense passive film (mainly composed of Cr₂O₃, MoO₃) on the surface in low-to-medium concentration hydrochloric acid at room temperature. This passive film can effectively inhibit the infiltration of Cl⁻ ions, resulting in low corrosion rates and negligible corrosion resistance degradation.
Mechanical properties: Since the corrosion is limited to the surface and does not penetrate into the matrix, there is almost no change in key mechanical properties such as tensile strength, yield strength, and elongation. At this stage, mechanical properties degradation lags far behind any potential corrosion resistance changes.
Corrosion resistance performance: When the hydrochloric acid concentration increases (e.g., >10%) or the temperature rises (e.g., >60°C), the passive film is damaged or cannot be repaired in time. Cl⁻ ions will induce localized corrosion, such as pitting corrosion and crevice corrosion. The corrosion rate increases significantly, and the corrosion resistance degrades sharply.
Mechanical properties: With the expansion of local corrosion pits, stress concentration occurs at the pit tips. However, the overall matrix structure remains intact, so the decrease in mechanical properties is relatively slight. Only when pitting corrosion develops into intergranular corrosion will the mechanical properties start to show a noticeable downward trend. At this stage, corrosion resistance degradation is still ahead of mechanical properties degradation.
Corrosion resistance performance: Under high-concentration, high-temperature hydrochloric acid conditions, localized corrosion further develops into uniform corrosion or intergranular corrosion, and the alloy surface is severely corroded. The corrosion resistance is almost completely lost.
Mechanical properties: The corrosion damage penetrates deep into the alloy matrix, leading to a reduction in the effective load-bearing area of the material. Meanwhile, intergranular corrosion weakens the bonding force between grains. As a result, the tensile strength, yield strength, and toughness of the alloy drop sharply, and the material may even fail prematurely under load. At this stage, the two types of degradation tend to be synchronized, but the initiation of corrosion resistance degradation is still earlier than that of mechanical properties degradation.
Alloy composition: Alloys with high Mo and W content (e.g., Hastelloy C276) have stronger resistance to Cl⁻ ion corrosion, which can delay the occurrence of localized corrosion and reduce the asynchrony between corrosion resistance and mechanical properties degradation.
Environmental factors: Higher hydrochloric acid concentration and temperature will accelerate the damage of the passive film, shorten the time interval between the two types of degradation, and make them tend to be synchronized faster.
