Corrosion Damage Mechanism of Ti6Al4V Alloy in Alternating High and Low Temperature Marine Environment

The frequent alternating high and low temperature service pattern of aero engines in the marine environment is an important factor that reduces their service life. The work aims to investigate the corrosion damage failure process of Ti6Al4V alloy in alternating high and low temperature marine service environment and reveal in detail the corrosion damage and degradation mechanisms of the alloy under the synergistic effects of high temperature oxidation, deposited salt films and frequent start-up operating modes. High temperature oxidation, alternating high temperature oxidation-salt spray and alternating high temperature oxidation-seawater immersion tests were employed to evaluate the corrosion behaviors of Ti6Al4V alloy. The phase composition of the test sample surface was characterized by X-ray diffractometry (XRD). The microstructure and local elemental distribution of the test samples were characterized by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Corrosion product composition and compound valence were characterized by X-ray photoelectron spectroscopy (XPS). An in-suit dense and uniform (TiO2+Al2O3) oxide layer was formed on the surface of Ti6Al4V alloy after oxidation at 650 ℃ for 400 h. The rate of sample weight gain gradually decreased over time during the test and was only 2.657 mg/cm2 at the end of the test. Ti6Al4V alloy showed significant cracking and peeling on the surface of the sample after 50 cycles of alternating high temperature oxidation and salt spray test. Severe corrosion damage occurred on the alloy surface caused by the transformation of the liquid salt film in the salt spray environment into a solid salt film adhered to the sample surface in the high temperature environment, which resulted in the laminar separation of the oxide layer. Corrosion damage to the Ti6Al4V alloy was further accelerated during 50 cycles of high temperature oxidation-seawater immersion test, reflected by a rapid growth of the oxide film and an aggravated delamination failure. Although corrosion product shedding was more severe in the high temperature oxidation - seawater immersion environment, the sample was in a constant state of weight gain throughout the test, with a final weight gain up to 5.167 mg/cm2. The test results indicated that the oxide layer on the Ti6Al4V alloy showed a slow and uniform growth trend during the high temperature oxidation process, indicating that the Ti6Al4V alloy had excellent oxidation resistance in the 650 ℃ environment, but experienced a rapid growth and structural damage during the alternating high and low temperature working mode. As a very strong depassivating agent, Cl‒ not only affected the formation of passivation films on the surface of the sample, but also significantly affected the corrosion rate and the degree of corrosion of the metal. A higher concentration of Cl‒ resulted in a more severe degradation damage. In addition, the solid salt film exacerbated the cracking of the oxide layer on the surface of the alloy, which provided a channel for the infiltration of Cl‒. As the denseness and integrity of the oxide layer on the surface of the sample are destroyed, the mixed salt gradually invades the interior of the oxide layer and intensifies the volatilisation of the chloride, which indicates the poor corrosion resistance of the Ti6Al4V alloy in alternating high and low temperature environment. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.