Effect of Hot Isostatic Pressing Treatment on the Microstructure and Corrosion Properties of As-cast TA15 Titanium Alloy

WANG Fang; LIU Zhenhua; WU Ruirui

doi:10.16490/j.cnki.issn.1001-3660.2026.14.005

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Surface Technology ›› 2026, Vol. 55 ›› Issue (14) : 55-64. DOI: 10.16490/j.cnki.issn.1001-3660.2026.14.005

Effect of Hot Isostatic Pressing Treatment on the Microstructure and Corrosion Properties of As-cast TA15 Titanium Alloy

WANG Fang^1a,2, LIU Zhenhua^1a, WU Ruirui^1b,*

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Abstract

This study systematically investigated the effects of hot isostatic pressing (HIP) conducted at 920 ℃ and 123 MPa for 2 h on the microstructural evolution and electrochemical corrosion properties of as-cast TA15 (Ti-6Al-2Zr-1Mo-1V) titanium alloy, with a focus on revealing the intrinsic relationship between microstructural modification and corrosion resistance improvement. A combination of X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM) was employed for qualitative and quantitative analysis on the microstructure of as-cast and HIP-treated specimens. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were carried out in sodium chloride solutions with mass fractions of 3.5%, 5% and 7.5%, as well as hydrochloric acid solutions with molar concentrations of 1 mol/L, 2 mol/L and 3 mol/L, to evaluate the corrosion behavior of the alloy in chloride-containing neutral and acidic corrosive environments.
Microstructural characterization results indicate that the HIP treatment does not induce new phase transformation. Both the as-cast and HIP-treated alloys consist of dominant α-Ti phase and a small amount of β-Ti phase. HIP treatment effectively eliminates inherent casting defects such as micropores and shrinkage porosity inside the as-cast alloy and remarkably improves the material densification. The as-cast TA15 alloy exhibits a typical coarse Widmanstätten structure, which is characterized by oversized prior β grains, thick lamellar α phases and elemental segregation. After HIP treatment, the original structure is largely transformed into refined and uniformly distributed basket-weave microstructure. The lamellar α phases are obviously shortened and arranged in a staggered manner, while the β phase presents a dispersed distribution. Compositional segregation and micro residual stress are effectively relieved, and the uniformity of grain size and crystal orientation is enhanced.
Electrochemical test results provide sufficient data to verify the enhancement of corrosion resistance via HIP treatment. Potentiodynamic polarization tests show that the corrosion potential shifts slightly positively and the corrosion current density decreases for HIP-treated specimens compared with the as-cast ones. Calculation of corrosion rate demonstrates that the corrosion rates of HIP-treated TA15 alloy are only 0.018 μm/a in 3.5wt.% NaCl solution and 0.028 μm/a in 1 mol/L HCl solution, which are lower than 0.051 μm/a and 0.104 μm/a of the as-cast counterparts. The corrosion rate of both groups rises gradually with the increase of chloride ion concentration in the two media, while HIP-treated specimens always possess a lower corrosion rate at the same concentration. EIS data are fitted with the corresponding equivalent circuit. The results reveal that HIP-treated alloy has larger capacitive arc radius, higher film resistance and charge transfer resistance in all test media. Specifically, the charge transfer resistance of the HIP-treated specimen reaches 245 200 Ω·cm² in 3.5wt.% NaCl solution, in contrast to 126 650 Ω·cm² of the as-cast specimen. In 1 mol/L HCl solution, the charge transfer resistance is 125 620 Ω·cm² for the HIP-treated sample and 99 400 Ω·cm² for the as-cast sample. Metallographic observation after corrosion further confirms that numerous large and deep pitting pits form at phase boundaries and defect areas of the as-cast alloy, whereas only a small number of shallow pits emerge on HIP-treated specimens with the overall microstructure well preserved.
Based on the above experimental results, this paper illustrates the novel mechanism for corrosion resistance improvement of the alloy by HIP treatment. The elimination of microdefects and homogenization of microstructure after HIP treatment not only remove the preferential nucleation sites for pitting corrosion, but also weaken the micro-galvanic corrosion effect caused by inhomogeneous composition and phase distribution. The refined basket-weave microstructure facilitates the formation of continuous, dense and stable TiO₂-based passive film on the alloy surface. This passive film can effectively hinder the adsorption and penetration of chloride ions, and restrain the breakdown of passive film as well as the anodic dissolution of titanium matrix. This study clarifies the regulation mechanism of HIP process on the microstructure and corrosion properties of as-cast TA15 titanium alloy, and provides solid experimental data and theoretical basis for the popularization and application of TA15 alloy in harsh chloride-containing corrosive service conditions such as marine engineering and chemical equipment.

Key words

TA15 titanium alloy / electrochemical corrosion / acid solution corrosion / hot isostatic pressing (HIP) technology / microstructural evolution / basket-weave structure

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WANG Fang, LIU Zhenhua, WU Ruirui. Effect of Hot Isostatic Pressing Treatment on the Microstructure and Corrosion Properties of As-cast TA15 Titanium Alloy[J]. Surface Technology. 2026, 55(14): 55-64

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