Haruo Tomari

Improvement of corrosion resistance of titanium by co-implantation

Abstract A number of investigations have successfully represented the beneficial effects of ion implantation in improving the corrosion resistance of metals. In this paper, we review studies on improving the corrosion resistance of titanium by co-implantation with metal ions. These studies have demonstrated that co-implantation is a possible process for attaining excellent corrosion protection of titanium against aggressive environments. The electrochemical behavior of titanium implanted with tantalum, molybdenum, nickel ions, or combinations of two of these ions, have been investigated in boiling sulfuric acid solution. The polarization curves of tantalum-implanted titanium are similar to that of unimplanted titanium, showing the active region followed by passivation. However, anodic polarization current densities are considerably decreased in the active and passive regions, and tantalum implantation suppresses the dissolution of titanium. The corrosion potentials of titanium implanted with nickel or molybdenum are shifted in the noble direction with an increase in fluence, and reside in the passive region of titanium at higher fluences. No significant change is observed in the passive current densities. Nickel and molybdenum implantation preferentially promotes the passivation of titanium in such an aggressive environment. These effects are diminished in a relatively short-term immersion. Titanium implanted with a combination of molybdenum and tantalum ions, or a combination of tantalum and nickel ions, exhibits stable passivation behavior with low passive current densities. The corrosion rate estimated from the polarization curves is approximately 0.1 mm year−1 for co-implantation with tantalum and nickel ions. Co-implantation provides long-term protection for immersion. The outstanding corrosion resistance of titanium is achieved by the complementary effect of co-implantation with these metal ions.

The effect of dry passivation treatments on the corrosion resistance, moisture release and structure of the surface oxide film on electropolished stainless steel

Abstract The objective of the present study is to investigate the effect of dry passivation treatments on corrosion resistance, moisture release behavior and structure of oxide film on electro-polished Type 316L stainless steel. The corrosion resistance was evaluated by measuring the incubation time until H 2 evolution occurred in HCl solution and the linear polarization resistance in boiling 0.005M-Na 2 SO 4 solution. The moisture release behavior was investigated by measuring the volume of H 2 O desorption with ultra pure N 2 gas purging. The structure of surface oxide film was analyzed using AES, XPS, LRS etc. The corrosion resistance and the moisture release resistance were remarkably improved by the oxidation treatments in oxygen atmospheres controlled with a dew point of below −100°C at temperatures from 400°C to 450°C, compared with as-electropolished surface film. At the treatment temperature over 500°C, those properties changed for the worse. From the results of surface analyses, it was clarified that a fine and amorphous oxide film containing enriched Fe and Cr was formed at the oxidation temperature of 400∼450°C, whereas oxide films crystallized and became coarse at oxidation temperature over 500°C.

Outward transport of substrate atoms during dynamic ion mixing

Abstract The transport of deposited atoms and substrate atoms during dynamic ion mixing was investigated by Auger electron spectroscopy (AES). Mild steel substrates were treated by dynamic ion mixing, in which Ne, Ar or Kr ion implantation and Ti deposition were carried out simultaneously. Before the treatment, Ta markers were embedded in the surface region of the steel by Ta ion implantation in order to confirm the location of the original substrate surface. The distance of outward transport of Fe atoms (ΔFe) gradually exceeds that of the inward transport of Ti atoms (ΔTi) as the ion/atom arrival ratio (i.e. the ratio of impacting ions to condensing atoms) increases. The same Δ Fe Δ Ti ratio is attained with a lower ion/atom arrival ratio when the ion mass is larger and ion energy is lower, i.e. when the sputter yield of the ion implantation is larger. The outward transport of substrate atoms is thought to be caused by the reaction of the sputtered Fe atoms the depositing Ti atoms during the process.

Boronizing of steel by outward transport of Fe atoms during dynamic ion mixing

Abstract Mild steel substrates were treated by B-ion implantation or dynamic ion mixing in which B-ion implantation and Ti deposition were carried out simultaneously. Before the treatments, some substrates were implanted with C ions to confirm the location of the original substrate surface. The compositional depth profiles of the surface regions were obtained by using Auger electron spectroscopy (AES). For the B-ion implantation, an accumulation of the implanted B species takes place followed by a growth of the B layer as the B fluence is increased. The Fe matrix is separated during the process with outward transport of Fe atoms caused by the growth of the B layer. For the dynamic ion mixing, the outward transport of Fe atoms is enhanced during the process which is thought to be caused by not only the growth of the B layer but also the reaction between the sputtered Fe atoms and the depositing Ti atoms at steel surface. The TiFe reaction causes an incorporation of Fe atoms into the growing B layer resulting in the formation of a several μm thick boronized layer.

Effect of structural change of α-SiC film synthesized by dynamic ion mixing on surface hardness and friction coefficient

Abstract Mild steel specimens were treated by dynamic ion mixing in which Si-deposition and 40 keV C-ion implantation were carried out simultaneously. A mixed layer of Si phase and α-SiC phase is synthesized when the Si-deposition rate is 0.10 nm/s and the C-ion current density is 20 μA/cm 2 . The Si phase transforms to α-SiC phase, which grows to form a monolithic α-SiC layer as the C-ion current density increases up to 30 μA/cm 2 . With a further increase in the C-ion current density or a decrease in the Si-deposition rate, the implanted C species accumulate to form a graphite phase, resulting in the growth of a graphite layer. The friction coefficient during a reciprocal motion wear test decreases and the surface hardness increases as the C-ion current density increases from 20 μA/cm 2 to 30 μA/cm 2 , i.e. as the α-SiC phase grows to form the monolithic α-SiC layer. The friction coefficient increases and the surface hardness decreases with a further increase in the C-ion current density or a decrease in the Si-deposition rate, i.e. as the graphite layer grows.

Isothermal Oxidation of Intermetallic TiAl Implanted with Tantalum Ion

The influence of tantalum ion implantation on the oxidation behavior of the intermetallic γ −TiAl has been investigated under isothermal conditions. The implanted TiAl were oxidized at 1273K in a slowly flowing air up to 24 hours. The oxidation reaction was followed thermogravimetrically and the oxide scales were analyzed mainly by using scanning electron microscopy with energy dispersive x-ray analysis. It has been found that tantalum ion implantation leads to a reduction in thermal oxidation of TiAl. The oxidation rate was decreased by nearly a factor of three at fluences above 3×1016 ions/cm2. The oxide scale exhibits layered structure, which is constituted of an external rutile layer and an internal layer mixed with rutile and alumina. Tantalum ion implantation enhanced segregation of the alumina enriched layer, which is protective.