Corrosion Resistance of Metals Processed by High-Pressure Torsion
High-pressure torsion (HPT) is a well-known process that brings multiple advantages compared to alternative techniques.
Metallic grain refinement is a large attractive industry due to its superior effectiveness compared to other strengthening mechanisms to enhance materials and meet increasing demands for higher-strength, lower-speciﬁc density, higher wear resistance, better corrosion resistance and greater heat-resistant properties. One approach for the refinement of metals is by processing the metal through high-pressure torsion (HPT) applications. HPT is a well-known process that brings multiple advantages, such as simpler processing and better effectiveness in terms of consolidating metallic particles compared to other alternative techniques, such as equal channel angular pressing.
This article reviews the effects of high-pressure torsion on the corrosion resistance of metals such as titanium, aluminum and magnesium.
High-Pressure Torsion and Titanium
Titanium is widely used in high-performance applications such as biomedical equipment, marine infrastructure, aerospace structures, and chemical and petrochemical plants and devices. Furthermore, titanium has excellent corrosion resistance and biocompatibility, as well as a superior strength-to-weight ratio and high heat transfer capability compared to other metals. Therefore, there is a strong incentive to examine and enhance titanium performance through HPT.
A study was performed on commercially pure titanium (CP Ti) under high-pressure torsion. A pressure of 6.0 GPa was applied with several torsional revolutions to CP Ti samples. Then, their corrosion properties were examined by electrochemical impedance spectroscopy and potentiodynamic polarization tests.
CP Ti specimens were annealed at 700 °C (1292 °F) for two hours to release the residual stress introduced during the production stage. In the annealed condition, the microstructure was equiaxed with a grain size of 10.5 μm. After that, the specimens were sliced from the annealed billet with a diameter of 10 mm and their surfaces ground with silicon carbide abrasive papers to a ﬁnal thickness of 0.8 mm.
Quasi-constrained HPT processing was used and there was some restricted outﬂow of material around the periphery of the sample during the processing operation. The processing was conducted at room temperature with an apparatus having two anvils with circular ﬂat-bottom depressions at the centers.
The processed titanium specimens were wet ground with 120, 800 and 1200 grit silicon carbide abrasive papers, polished with 6 and 1 micrometer diamond pastes and then degreased with distilled water and acetone followed by air drying. The disk specimens were placed in a Teﬂon sample holder with a 6 mm in diameter window exposed to the solution. Electrochemical measurements were performed in a 3.5 % NaCl solution at room temperature.
After immersion into the 3.5 % NaCl test solution, the Ti disk specimens were held for one hour to achieve a steady open-circuit potential (OCP). Electrochemical impedance spectroscopy (EIS) was performed at OCP over a frequency range from 100 kHz to 0.01 Hz using a sinusoidal AC voltage amplitude of ±10 mV. Subsequently, potentiodynamic polarization was carried out in the range of -0.2 to +1.8 V. To maintain a high statistical accuracy, all electrochemical measurements were repeated at least three times.
Results, Review & Discussion
The corrosion resistance of CP Ti after HPT processing is lower than that of the annealed coarse-grained unprocessed specimen, but the corrosion susceptibility of the HPT-processed specimens decreases with increasing torsional strain.
High Pressure Torsion and Aluminum
Aluminum is unique in that its attraction of oxygen allows it to form a protective layer of oxide on any surface exposed to air. It is used in transportation, building and construction, for electrical applications, and in machinery and equipment, among other things.
In a study of the corrosion fatigue fracture properties of SiC whisker/aluminum matrix composite, tension-torsion loading of a SiC whisker-reinforced A6061 aluminium alloy fabricated by a squeeze casting process is undertaken. Then, tests were conducted on both the composite and its unreinforced matrix material, A6061-T6, under out-of-phase load-controlled conditions with a constant value of the combined stress ratio, α=τ_ /σ_ in a 3.5% NaCl solution at free corrosion potential.
The aluminum materials tested included a silicon carbide whisker-reinforced aluminium alloy and its non-reinforced matrix material. The composite was fabricated through a squeeze casting process. The composition of the matrix material in mass percentage is as follows: 0.54 Si, 0.23 Fe, 0.24 Cu, 0.02 Mn, 1.00 Mg, 0.19 Cr, 0.01 Zn, 0.02 Ti and the rest Al.
The corrosive environment consisted of a 3.5% NaCl solution. The temperature of the solution was kept at 25 °C (77 °F) by a thermoregulator. Using a computer-controlled electro-hydraulic tension–torsion fatigue testing machine, the solution was circulated between the corrosion test cell and a solution reservoir by a vane pump made of synthetic resin.
Discussion: Corrosion Fatigue Strength, Crack & Corrosion Pit
The fatigue strength of both composite and matrix in a 3.5% NaCl solution is reduced compared to that in air. The fatigue strength in air is determined by either the maximum principal stress or the von Mises equivalent stress depending on the combined stress ratio. However, the corrosion fatigue strength of both the matrix and composite is determined by the maximum principal stress, regardless of the combined stress ratio.
The mechanism of corrosion fatigue crack initiation of the matrix is completely diﬀerent from that of the composite. For the matrix material, hydrogen embrittlement causes a brittle, ﬂat facet to form, which then progresses to a corrosion fatigue fracture. For the composite material, on the other hand, a crack is nucleated at the bottom of a corrosion pit, which forms on the specimen surface, leading to ﬁnal failure.
Magnesium is used in lightweight metals and alloys, especially in the aircraft, automotive and electronics industries. When exposed to air, it tends to be protected by a thin layers of oxide.
Commercially pure as-cast Mg disks, having diameters of 10 mm and thicknesses of 0.8 mm, were processed by HPT by compressing and then deforming between two anvils under an applied pressure of 6.0 GPa at room temperature to totals of either one or ﬁve complete turns. This processing was conducted under quasi-constrained conditions in which there is a small outﬂow of material around the periphery of the disk during the processing operation. A preparation was conducted directly on the cast Mg at room temperature with a process of one or five turns without the introduction of any cracking.
The HPT was conducted at warmer temperatures. The microhardness of the samples was measured to estimate the changes in the microstructure and strength attributed to the HPT processing. A scanning electron microscope equipped with an electron backscatter diffraction device was used to observe the microstructures of the samples.
Results, Review & Discussion
The HPT processing reﬁned the grain structure and resulted in much reﬁned equiaxed grains. Smaller grains of micrometers in size are apparent at the center of the sample disk. Measurements showed that the grain size was effectively reﬁned and the basal texture was intense even after processing through only one turn. The microstructure became more homogeneous throughout the disks by increasing the HPT processing to 5 turns.
Processing by HPT produces a very signiﬁcant grain reﬁnement in the cast pure Mg with a grain size reduction at the edge of the disk. The yield stress was also enhanced to about seven times in comparison with the cast Mg. The results suggest that the corrosion resistance was not signiﬁcantly improved by using HPT but more experiments are needed to fully characterize the corrosion resistance.