Effects of Iron Content on Microstructure and Crevice Corrosion of Grade-2 Titanium

Abstract

The effects of iron content on the microstructure and crevice corrosion of Grade-2 titanium (Ti-2) were studied using a galvanic coupling technique combined with optical microscopy and secondary ion mass spectrometry (SIMS) imaging. This study reveals that iron content has a significant effect on the microstructure and crevice corrosion behavior of Ti-2. The grain size decreased significantly with increasing iron content. For a Ti-2 material with medium iron content, crevice corrosion readily initiated and the metal exhibited extensive intergranular attack that could be associated with the more reactive iron-stabilized β-phase within the α-phase matrix, as revealed by SIMS imaging. By contrast, Ti-2 materials with low and high iron contents showed suppressed crevice attack. The smaller surface area of available grain boundaries in Ti-2 of low iron content could account for this limited attack. For the material with high iron content, SIMS imaging suggests that some Ti x Fe intermetallic particles were formed. These particles may act as proton reduction catalysts and enhance crevice corrosion resistance via cathodic modification.

Crevice Corrosion of Grade-2 Titanium in Saline Solutions at Different Temperatures and Oxygen Concentrations

“…A previous study on a different Ti-2 (containing 0.107 wt% Fe) conducted in 0.27 mol/L NaCl solution also showed the percentage of damage caused by the consumption of O 2 was in the range 20% to 50%. 12 This percentage did not change consistently with [Cl − ] up to 5.0 mol/L, indicating that the electrolyte concentration is not a key feature in the coupling to external crevice support. 15 While it is possible that extension of this experiment ( Figure 5) to longer times could have allowed further active propagation events, this seems unlikely given the progressive decrease in E p -E c and the accompanying decrease in I c .…”

“…12 Repassivation/stifling was enhanced for the highest and lowest Fe-contents, while penetration was deep and widespread at the intermediate Fe level. Microstructurally, decreasing the Fe content leads to an increase in α-Ti grain size.…”

“…This is the expected signature for the initiation of crevice corrosion and has been discussed in detail previously. 5,9,12 Here, it can be appreciated by comparing E c to E p . E p increased steadily to more positive values as the passive oxide on the planar electrode grew and became more protective.…”

“…However, an increase leads to the formation of Ti x Fe intermetallic precipitates, inevitably on the grain boundaries. 10,12 Electrochemically, both Fe-containing β-phase and Ti x Fe particles may catalyze H + reduction, 5,12 thereby assisting crevice stifling by enhancing cathodic modification. Since the Ti-2 used in this study contains Fe levels at the high end of this range (0.091 wt%) and abundant Ti x Fe intermetallic precipitates are present (Figures 3 and 4), it is likely that crevice stifling was promoted by enhanced cathodic kinetics.…”

“…When present in the α-Ti hexagonal close packed structure, it is detrimental. 12 However, when segregated into secondary Ti x Fe precipitates, this phase can act as a catalytic site for H + reduction within the crevice, thereby leading to repassivation via the well-known cathodic modification effect. 13,14 In this paper, we have studied the influence of O 2 on crevice propagation of a commercially available Grade-2 Ti (Ti-2) at temperatures of 120…”