Detailed tests and characterizations were used to investigate the corrosion degradation behaviors of Ti6Al4V alloys in simulated marine environments. These alloys suffered from very slight pitting and a miniscule weight loss of 0.018 mg/cm² during the 50 cycle salt spray exposure but experienced significant oxygen erosion in the high-temperature oxidation test, resulting in a high weight gain of 2.657 mg/cm² at 400 h. The oxidation and degradation reactions simultaneously occurred during the high-temperature hot salt test. The chlorine (Cl₂) induced by the eutectic reaction of the mixed salts accelerated the degradation of the substrate and led to a higher weight gain of 4.265 mg/cm² at 400 h. In contrast, this alloy suffered from severe corrosion damage during the high-temperature hot salt–water vapor synergy test. The degradation of TiO₂, Al₂O₃, and V₂O₅ was aggravated by the synergistic action of chlorine salt and water. The reaction forming hydrochloric acid (HCl) further degraded the matrix metal and consequently led to a high weight loss of 16.358 mg/cm² at 400 h. These current findings provide a comprehensive understanding for the degradation mechanisms of Ti alloys in these specific marine environments.

This study investigates the corrosion damage mechanisms of Ti₂AlNb-based alloys under high temperature, salt spray and coupled high temperature-salt spray conditions. This alloy was analysed in detail from macroscopic to microscopic by means of microscale detection (XRD, SEM and EDS). The results indicated that Ti₂AlNb-based alloy surface oxide layer is dense and complete, and the thickness is only 3 µm after oxidation at 650 °C for 400 h. Compared to the original sample, the production of the passivation film resulted in almost no damage to Ti₂AlNb-based alloy after 50 cycles of salt spray testing at room temperature. The tests showed that Ti₂AlNb alloy shows good erosion resistance at 650 °C and in salt spray. However, this alloy had an oxide layer thickness of up to 30 µm and obvious corrosion pits on the surface after 50 cycles of corrosion under alternating high temperature-salt spray conditions. The Cl₂ produced by the mixed salt eutectic reaction acted as a catalytic carrier to accelerate the volatilisation of the chloride inside the oxide layer and the re-oxidation of the substrate. In addition, the growth of unprotected corrosion products (Na₂TiO₃, NaNbO₃ and AlNbO₄) altered the internal structure of the oxide layer, destroying the surface densification and causing severe damage to the alloy surface.

The effect of a chromium (Cr) transition layer on the hot-salt corrosion behavior of an AlSi alloy coating was studied. Hot-salt corrosion experiments were performed at 650 °C and corrosion kinetic curves were plotted. The weight gain of the AlSi-coated samples increased to 0.89 mg/cm² at 100 h and then decreased steadily to 0.77 mg/cm² at 200 h. The weight of the AlSi-coated samples with the addition of a Cr transition layer increased immediately to 0.79 mg/cm² at 20 h and then gradually increased to 0.85 mg/cm² at 200 h. This Cr diffusion promoted the preferential creation of an Al₂O₃ layer, which effectively hindered the upward diffusion of Fe and also resulted in the production of a Cr₂O₃-SiO₂ layer, which impeded the multi-scale salt mixture’s penetration. The Cr diffusion also caused a notable seal-healing effect, which healed the micro-pores. These oxidation and degradation reactions were considerably repressed by the high barrier properties of these oxide layers and the dense surface, resulting in the increased hot-salt corrosion resistance of the AlSi alloy coating. The current findings provide a feasible strategy for the design of a diffusion barrier layer of a thermal protective coating on martensitic stainless steel.