Hot-salt stress-corrosion of titanium

Effect of plasma electrolytic oxidation on the hot salt corrosion fatigue behavior of the TC17 titanium alloy

“…Consequently, this salt will inevitably attach to engine-compressor parts in-service. The supersonic wind-tunnel experiment conducted by Lockheed showed that 80% of the salt deposits on the sample surface were retained after 15-20 h of testing at 600-700°C, [3] which confirmed that airflow during aircraft flight could not eliminate the influence of the salt attached to the blade surface in the simulated marine environment. The coupling effect of salt deposition, high temperature, and alternating loads can easily lead to hot salt corrosion fatigue (HSCF) damage to titanium alloy engine blades/ disks, which will affect the safety, reliability, and service life of the engine.…”

AgCl-induced hot salt stress corrosion cracking in a titanium alloy

“…When the passive oxide film is broken down, this exposes the underlying alloy to the salt, which always competes with a repassivation process if oxygen is accessible. In the case of NaCl-associated HSSCC, it has been proposed [9,12,38] that TiO 2 can be consumed through the reaction with NaCl and moisture at elevated temperature, forming sodium titanates and gaseous HCl. A localized HCl-rich environment can retard oxide repassivation [39], especially at lower oxygen concentrations underneath the salt deposits, which can enable attack of the base alloy and thence to crack initiation under applied stress.…”

“…Hydrogen is suggested to concentrate at the crack tip owing to the stress field and/or the existence of dislocation traps in the crack tip plastic zone [11], retarding the diffusion of H into the bulk. Hydrogen embrittlement is widely proposed as the cracking mechanism in chloride-induced HSSCC [4,9,12,17,41]. Brittle titanium hydrides were also detected by XRD and TEM in a previous study on HSSCC of Ti-6246 induced by NaCl [12], which suggested precipitation of titanium hydrides occurs during cooling owing to the decreasing hydrogen solubility in bulk materials.…”

“…However, since the 1960s there has been a recurring concern around hot salt stress corrosion cracking (HSSCC) susceptibility in Ti alloys when exposed to halides, particularly chlorides [4,5,6,7,8]. Bauer first reported the failure of a Ti-6Al-4V turbine blade due to HSSCC attack by NaCl residues from fingerprints during laboratory creep testing at 220 °C [9]. Subsequent investigations found that other chloride-containing metal salts, such as MgCl 2 , CaCl 2 and KCl, found in seawater could cause a similar effect [4,5].…”

“…A number of reaction models or mechanisms have been pro-posed and it is widely agreed that the presence of moisture and/or oxygen is critical for cracking [4,9,11]. Initially the protective oxide layer can be ruptured mechanically or consumed through its reactions with salts in the presence of moisture and/or oxygen, subsequently forming HCl or Cl 2 .…”