Physiological environments are severe corrosive conditions which allow the metallic biomaterials to be oxidized by the surroundings including the cell activities. Corrosion reactions in biological conditions and their effects on cellular responses including immunological responses are reviewed. On the other hand, metal ions play essential roles in cellular activities by regulating the gene expression, enzyme activities and also the intracellular signaling pathways. Cellular activities mediated by metal ions are briefly reviewed. Finally, the hierarchical anisotropic architecture of bone is introduced. Bone tissue can realize its mechanical function because of the anisotropic microstructure which is derived from cellular activities in response to the physical and chemical environment; the establishment of biomaterials which realize the appropriate anisotropy of regenerated bone is imperative for the medical innovation which enables the functional tissue regeneration.
Two-step thermal oxidation of Ti and Ti alloys was proposed for improving their bone compatibility; this is a surface-treatment process involving the formation of an anatase-containing TiO2 layer. The photocatalytic activity of the TiO2 layer formed was thus evaluated. The two-step thermal oxidation process consisted of: 1) treatment in a CO atmosphere and 2) treatment in air; the anatase fraction in the TiO2 layer could be controlled by changing process parameters. An increase in the anatase fraction resulted in photo-induced superhydrophilicity and an increase in the decomposition rate of methylene blue for the TiO2 layer.
Shape memory alloys have become widely applied for biomedical devices such as stents and coils. At present, NiTi is practically and widely used, and Ni-free Ti-based alloys is expected to replace NiTi from the viewpoint of further safety to human body, since the Ti-based alloys do not contain toxic elements such as Ni. This paper summarizes corrosion behavior of NiTi and Ni-free Ti-based biomedical shape memory alloys reported after 1990s. Based on the data in the literature, although the corrosion resistance of both NiTi and Ni-free Ti-base shape memory alloys is judged to be sufficient for biomedical applications, the corrosion resistance of Ni-free Ti-based shape memory alloys is better than that of NiTi. Therefore, from the viewpoint of corrosion, further development of Ni-free Ti-based shape memory alloys is desired.
Titanium has been selected as the materials for several dental devices because of its excellent mechanical properties, biocompatibility, development of casting process and corrosion resistance to chloride solutions.Besides, fluoride is well utilized as an anticariogenic agent showing the reduction of demineralization, the enhancement of remineralization, the interference of pellicle and plaque formation, the inhibition of microbial growth and metabolism, and others. However, Ti has a nature to dissolve in aqueous fluoride solutions, so that there have been some reports regarding corrosion problem of the titanium dental devices. One of the countermeasure to the problem is to develop a new titanium alloy resistant to corrosion in the fluoride solutions. Therefore, the paper aims to introducing the overview of corrosion behavior of Ti in fluoride solutions and some recent trials to development of new Ti alloys corrosion-resistant to the fluoride solution.
It has been reported that titanium carbonitrides could be a initiation site of pitting corrosion even at high corrosion resistance ferritic stainless steels such as Ti added stainless steels. However, it is hardly found any observations from the viewpoint of pre- and post- pitting corrosion related with the inclusion. Therefore, the pitting corrosion behavior in the vicinity of titanium carbonitride has not fully understood yet. The objective in this study was to clarify the inititation behavior of pitting at titanium carbonitride by means of in the test solution AFM observation under a potentiostatic condition. As a result, it was found that pitting corrosion initiated at a severe crevice which formed by polishing or rolling at titanium carbonitride/matrix boundary, with a size of several tens of nm in depth and several hundreds of nm in width. And then, the pitting corrosion grew toward depth direction of matrix side with time.
This study was carried out to clarify the effect of temperature and time of wetness on corrosion of carbon steel under insulation. The specimen used was carbon steel SS400. In this test, the specimen was covered with calcium silicate insulation and heated in a temperature range from 40 to 200°C by an electric heater. Corrosion test was conducted by pouring deionized water of 25 to 50 ml into the insulation. The time of wetness on the steel surface under the insulation was evaluated using the ACM sensor. The results of the corrosion test showed that corrosion was not found at temperatures above 100°C. On the other hand, corrosion occurred locally at temperatures between 40~90°C and the corrosion depth became deeper with decreasing steel surface temperature. The time of wetness of the steel surface measured with the ACM sensor increased with decreasing steel surface temperature. This indicates that the increase in time of wetness on the steel surface has more significant effect on corrosion of the steel under insulation than the increase in the corrosion reactivity with increasing temperature.
Immersion tests of copper tubes in 100 mg/L formic acid solutions with different concentrations of dissolved oxygen for up to 84 days under room temperature were carried out to investigate the effect of dissolved oxygen on ant's nest corrosion of copper. The ant’s nest corrosion occurred on the copper tubes immersed in formic acid solution with 8.4 and 14.3ppm dissolved oxygen while the corrosion was not observed on the copper tubes immersed in formic acid solution with 2.1 ppm dissolved oxygen. The weight losses of specimens immersed in the formic acid solution with 2.1ppm dissolved oxygen were especially low compared to the weight losses of specimens immersed in the formic acid solution with 8.4 and 14.3 ppm dissolved oxygen. The reduction of dissolved oxygen in the middle of the immersion test inhibited the increase of the weight loss of specimens. Therefore, the dissolved oxygen contributes to an initiation and a propagation of ant’s nest corrosion of copper.
In order to evaluate the influence of boundary length (BL) between graphite nodule and matrix on the corrosion rate of spheroidal graphite cast iron, the corrosion test was performed in a sulfuric acid solution at pH 2. The presence of graphite nodule was considered to influence the corrosion at the boundary between matrix and graphite nodule. The contiguous length between graphite nodule and matrix per unit area was defined as BL. The corrosion potential and rate were measured. The surface morphology of the specimens was observed. Anodic dissolution of the matrix in the vicinity of the graphite nodules took place preferentially. As a result, the graphite nodules were detached from the matrix. The graphite nodules worked as cathode. However, no influence of BL on corrosion rate was seen.
“ACTE” (Accelerated corrosion test for electrical appliances) is an accelerated corrosion and consists of salt deposition process and cyclic process in which absolute humidity is constant. This accelerated corrosion test is useful for sufficient evaluation of surface-treated steel. In this study, improvement of “ACTE” was performed to evaluate atmospheric corrosion for aluminum alloys. In this improved “ACTE” for Al alloys (“Al-ACTE”), temperature/relative humidity of wet or dry conditions were selected as 20°C/95%RH or 35°C/40%RH respectively. These conditions were derived from measurements of actual environments and were decided to avoid the formation of passive film in oxidation conditions. Exposure time of cyclic wet/dry process was selected as 4 hours for a cycle. “Al-ACTE” for ADC12 and A1100 was compared with exposure tests of Miyakojima-shielded and Chyoshi-shielded from the view point of depth of pitting. The acceleration ratio of “Al-ACTE” against the atmospheric exposure tests was calculated. It was found that the depth of pitting of A1100 was more affected by the amount of deposited-salt than that of ADC12. This dependence in Al-ACTE showed the same tendency as the atmospheric exposure tests. “Al-ACTE” makes it possible to appropriately evaluate the corrosion resistance on aluminum alloys by considering the amount of deposited salt.