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B-type carbonate apatite (CAp) coatings were formed on as-cast and T4-treated Mg

The influences of Zn content and heat treatment on microstructure and corrosion behavior of Mg-xZn (x = 1 wt%, 3 wt%, 5 wt%, and 7 wt%) alloys were studied. (α-Mg+MgZn) eutectic cells and Zn-segregated regions were formed in the as-cast alloys. The Zn-rich phases acted as microcathodes in galvanic corrosion. Volume fraction of the Zn-rich phases increased with Zn content of the as-cast alloys, leading to a decrease in corrosion resistance. The corrosion rate of the as-cast alloys increased four times with an increase of the volume fraction of eutectic cell from 0.07 vol% of Mg-1Zn alloy to 2.18 vol% of Mg-5Zn alloy. The corrosion rate of Mg-7Zn alloy with 2.87 vol% eutectic cells was two times higher than that of Mg-5Zn alloy. The Zn-rich phases dissolved by the T4 treatment and only the T4-treated Mg-7Zn alloy obviously showed eutectic cells of 1.73 vol%. The polarization resistance (Rp) of the T4-treated Mg-1, 3, and 5Zn alloys was 2 to 10 times higher than that of the as-cast alloys. The T4-treated Mg-7Zn showed similar Rp to the as-cast Mg-5Zn alloy. Consequently, the volume fraction of Zn-rich phases dominated the corrosion resistance of Mg-xZn alloys.

Hydroxyapatite (HAp) and octacalcium phosphate (OCP) coatings were formed on a Mg-3Al-Zn (AZ31) alloy with a chemical solution deposition method using a Ca-EDTA solution at various pH levels. The adhesion strength of the coatings was examined using the pull-off method. The microstructures of HAp and OCP coatings were measured X-ray diffraction (XRD). The morphology and composition of the surface and cross section of the samples before and after the adhesion test were characterized using scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), and a 3D profilometer. The results showed that plate-like OCP crystals grew from a continuous OCP layer on the surface of the AZ31 substrate in the case of a pH 6.3 coating solution. At pH values of 7.5 and 8.6, the HAp coating showed a two-layer structure with a HAp rod-like outer layer and a HAp continuous inner layer. Regardless of the pH of the coating solutions, a very thin Mg (OH)2 intermediate layer was formed between OCP or HAp coating and substrate. The highest adhesion strength of the coatings was 6.7±0.5 MPa, achieved at a coating solution pH value of 7.5. A part of Mg (OH)2 layer remained on the substrate, indicating that the delamination occurred in the Mg (OH)2 intermediate layer. The primary particles in the inner layer formed at pH 7.5 was smaller than those at pH 8.6. This result indicates that the initial corrosion of substrate AZ31 at pH 7.5 was more rapidly than that at pH 8.6, presumably leading to the formation of mixed layer of Mg (OH)2 and calcium phosphate. Further investigation is necessary to understand the better adhesion strength at pH 7.5 than that at pH 8.6. This good adhesion could be due to the flawless and rod-like uniform crystal, which had the densest and finest structure on the surface.

Calcium phosphate (Ca-P) surface coating is a simple but effective way to enhance both corrosion resistance and biocompatibility of ZK60 magnesium alloy. However, cell compatibility on different Ca-P layers coated on ZK60 alloy has seldom been investigated. In this study, the effects of type, morphology and corrosion protection of several Ca-P coatings formed at pH 6.5, 7.8 and 10.2 on cell behavior were examined by using an osteoblastic cell line MC3T3-E1. Furthermore, in vivo behavior in rabbits of the alloy coated with the optimum Ca-P layer was also studied. It was found that the surface factors governed the cell morphology and density. The coating morphology plays a dominant role in these surface factors. The sample coated at pH 7.8 showed the best cellular biocompatibility, suggesting that the hydroxyapatite (HAp) layer formed at pH 7.8 was the optimum coating. In rabbits, this optimum coating enhanced remarkably the corrosion resistance of the alloy. During implantation, the outermost crystals of the HAp coating were shortened and thinned due to the dissolution of HAp caused by the body fluid of the rabbits. It is indicated that ZK60 alloy coated at pH 7.8 can be applied as a biodegradable implant.

High-strength steels are known to be susceptible to hydrogen embrittlement. Even small amounts of hydrogen which enter the high-strength steels under corrosion conditions could cause the hydrogen embrittlement. In order to elucidate the mechanism of hydrogen entry in corrosive environments, and to prevent the corrosion-induced hydrogen embrittlement, the simultaneous analysis of the corrosion behavior and the hydrogen entry is required. Some techniques, such as scanning Kelvin probe force microscope, have been reported for visualizing the hydrogen distribution in metals with a high spatial resolution. However, it is still challenging to visualize the hydrogen distribution of an observation area of several tens of millimeters with high spatial and temporal resolutions. In this study, we succeeded the visualization of the distribution of hydrogen entering a pure Fe sheet immersed in a NaCl aqueous solution using a conductive polymer. A pure Fe sheet (99.5 %) was used as a specimen. The Fe sheet was electrically polished in a mixed solution of perchloric acid and acetic acid. The thickness of the Fe sheet was 0.4 mm after the polishing. In order to visualize the hydrogen distribution in the Fe sheet, a polyaniline (PANI) layer was formed on one side of the Fe sheet. It was reported that the PANI layer changes its color owing to the reaction with atomic state hydrogen in Fe sheets.1 To enhance the sensitivity of the PANI layer to hydrogen, a Ni interfacial layer was formed between the PANI layer and the Fe sheet. The PANI layer was formed on the Ni interfacial layer through electrolytic polymerization at a constant voltage of 1 V in a 0.5 M H2SO4-0.5 M aniline aqueous solution. On the other side (bare Fe side) of the Fe sheet, an electrode area of 1 cm × 1 cm was prepared using a resin. Only the bare Fe side was immersed in a 0.1 M NaCl aqueous solution. In addition to the open-circuit potential measurements, the corrosion behavior was observed in situ using a digital camera during the corrosion test. The PANI layer was exposed to air without contacting the NaCl solution. During the corrosion test, the color of the PANI layer was observed in situ using another digital camera. It was confirmed that the small amount of hydrogen entering the Fe sheet due to corrosion can be visualized using the PANI layer. The distribution of hydrogen that entered the Fe sheet was analyzed based on the color distribution of the PANI layer. The in situ simultaneous observation of the corrosion behavior and the hydrogen entry revealed that the hydrogen entry was promoted on the dissolved area of the Fe sheet in the NaCl solution. Reference H. Kakinuma, S. Ajito, T. Hojo, M. Koyama, E. Akiyama, Adv. Mater. Interfaces, 2101984 (2022).

Hydroxyapatite (HAp) and octacalcium phosphate (OCP) layers were formed on Mg- 4mass% Y- 3mass% rare earth (WE43) alloy by a chemical solution deposition method at various pH values of pH 5.5, 6.2, 7.5, and 8.6. Adhesion strength of HAp and OCP layers was evaluated before and after immersing in a medium for 14 days by a pull-off test. The corrosion resistance of these coatings was measured by polarization tests performed in a simulated body fluid (SBF). XRD analysis demonstrated that HAp coating layers were formed at pH 7.5 and 8.6, while OCP coating layers were formed at pH 5.5 and 6.2. Adhesion test results showed that the as-coated pH7.5-HAp layer had the highest adhesion strength of 8.6 MPa, which was attributed to the very dense structure of the coating layer. The as-coated pH8.6-HAp layer showed the adhesion strength of 6.5 MPa. The adhesion strength of the as-coated pH5.5- and pH6.2-OCP layers was 3.9 and 7.1 MPa, respectively, that was governed by the thick and fragile property of the layers. After immersing in the medium for 14 days, the adhesion strength of pH7.5- and pH8.6-specimens decreased to 5.8 and 5.6 MPa, respectively. The pitting corrosion and formation of Mg(OH)2 under the HAp layers were responsible for the decrease of adhesion strength. The polarization tests in SBF at 37 °C showed that the corrosion current density decreased with the HAp and OCP coatings, indicating the improvement of the corrosion resistance of WE43 alloy. The HAp coatings improved the corrosion resistance more efficiently than the OCP coatings.

The synergistic effects of plastic deformation and Cl− on corrosion initiation of steel rebars was examined by applying rapid tensile strain and cyclic stress to the steel in a simulated concrete pore solution with various concentrations of Cl−. In a rapid straining electrode test, the passive film of the steel was broken under plastic deformation and the anodic current increased sharply with the onset of slip step formation, whereas it was hardly broken under elastic deformation. Localized corrosion was initiated on the slip steps when the Cl− to OH− concentration ratio ([Cl−]/[OH−]) of the test solution was 0.4 or more. Localized corrosion also occurred under cyclic stress at [Cl−]/[OH−] ≥ 0.4. Experimental results demonstrated that the combination of slip step formation due to local stress concentration and the inhibition of repassivation by Cl− initiated corrosion under a significantly small [Cl−]/[OH−] ratio.

Hydroxyapatite (HAp) coatings were formed on as-cast and T4-treated Mg-xZn (x = 1, 5, and 7 wt%) alloys by a chemical solution deposition method. The uniformity of HAp coatings showed an enhancement with finer particle size and lower volume fraction of (α-Mg + MgZn) eutectic cells and Zn-segregated regions. The Mg 1Zn sample showed a defect-free HAp coating, while the HAp coating of the Mg 7Zn sample showed cracks preferentially over the eutectic cells causing micro-galvanic corrosion with the matrix. Initially, the corrosion rate of the HAp-coated alloys was governed by the uniformity of HAp coating, and eventually it was dominated by the corrosion resistance of substrate alloys after the HAp coatings irreversibly broken. Ultimately, the volume fraction and size of (α-Mg + MgZn) eutectic cells and Zn-segregated regions governed the corrosion behavior of the HAp-coated Mg Zn alloys. The HAp-coated Mg 1Zn alloy showed 2–4 times lower corrosion rate than other alloys after 1 day and 4–10 times after 14 days of immersion. The 14-day average corrosion rate of the as-cast Mg-1, 5, and 7Zn alloys decreased by 45%, 5% and 15% with the HAp coatings, respectively. The polarization resistance (Rp) of the as-cast Mg-1, 5, and 7 Zn alloys increased by 10, 7.5 and 5 times with the HAp coatings, respectively. Consequently, the HAp coating effectively improved the corrosion resistance of Mg Zn alloys with low volume fraction of (α-Mg + MgZn) eutectic cells.

The effect of the anionic polymer sodium polyacrylate (SPA) on the healing behavior of a hydroxyapatite (HAp) coating formed on Mg-3 mass% Al-1 mass% Zn (AZ31) alloy was examined. Static immersion ...

Biomedical Mg alloy is promising for its widespread use clinically. In vitro and in vivo studies showed that the degradation products of biomedical Mg alloy were composed of O, P, Ca, Mg and other alloying elements. However, little is known about the metabolism of the degradation products. In this study, the in vitro macrophage phagocytosis of the degradation products of a biomedical Mg-Nd-Zn-Zr alloy was directly observed. This result affirms the necessity to investigate the long-term fate of Mg alloy degradation products in physiological environments. Besides, an electrochemical method was proposed to prepare enough amount of degradation products in vitro efficiently.

Various coatings have been developed for biodegradable Mg alloys to control the degradation speed and to improve the bone conductivity. In this study, hydroxyapatite (HAp) coatings were formed on pure Mg, Mg-0.8mass% Ca (MgCa), Mg-4mass% Y-3mass% rare earth (RE) (WE43), Mg-3mass% RE-1mass% Y (EW31) and Mg-4mass% RE (RE4) alloy rods with a chemical solution deposition method. The HAp-coated and uncoated Mg/Mg alloy rods were implanted in the femurs of rats for 3–6 months, and the corrosion suppression and bone formation abilities of the HAp coating were examined using a scanning electron microscope. The corrosion rate of WE43 was suppressed by 1/3 with the HAp coating for 6 months, and the corrosion product showed very slow dissolution. The effect of the HAp coating for pure Mg and MgCa disappeared in 1–2 months with the thinning of the rods accompanying with the obvious dissolution of the corrosion products. The effect of the HAp coating for EW31 and RE4 was not stable due to the expansion and collapse of the corrosion products. The bone formation was enhanced on the HAp layers. Eventually, the HAp coating basically suppressed the corrosion initiation and corrosion progress of Mg substrates. The magnitude of the suppression effect depended mainly on the chemical and physical stability of the corrosion products.

For applications of biodegradable load-bearing implants, we incorporated 10 or 20 vol% &beta, tricalcium phosphate (&beta, TCP) into Mg-based composites through reaction sintering in the spark plasma sintering process. We previously reported that the evolved microstructure enhanced mechanical properties before degradation and modified in vitro degradation behaviors. In this study, immersion tests in physiological saline and subsequent compression tests in the air were conducted to investigate the effects of degradation on mechanical integrity. In the immersion tests, Mg/&beta, TCP composites showed no visible disintegration of sintered particles due to interfacial strength enhanced by reaction sintering. Local corrosion was observed in the Mg matrix adjacent to the reaction products. In addition, Mg/10% &beta, TCP showed dense degradation products of Mg(OH)2 compared with Mg and Mg/20% &beta, TCP. Those degradation behaviors resulted in reducing the effective load transfer from the Mg matrix to the reaction products as reinforcement. The yield strength decreased by 18.1% for Mg/10% &beta, TCP and 70.9% for Mg/20% &beta, TCP after six days of immersion. These results can give a broad view of designing spark plasma sintered Mg/bioceramic composites with the consideration of mechanical integrity.

The role of mill scale on the corrosion behavior of steel rebars in mortar was studied using Hyperbaric-Oxygen Accelerated Corrosion Tests (HOACT) and polarization measurements. Defects of mill scale deteriorated passivity of rebars when mortar and saturated Ca(OH)2 solution contained chloride. Localized corrosion beneath mill scale with defects was responsible for the acceleration of depassivation. SEM observations and Raman spectroscopy measurements revealed that a Fe3O4 layer initially formed under the mill scale and further corrosion resulted in migration of Fe ions through the defects and formation of a rust layer mainly composed of α- and γ-FeOOH on the mill scale.

Hydroxyapatite (HAp) and octacalcium phosphate (OCP) coatings were formed on AZ31 alloy and pure Mg, and the surfaces were scratched to expose the metal surface. The specimens were immersed in a cell-culture medium or 0.9% NaCl. Release of Mg ions was not accelerated by scratching. In both solutions, a corrosion product layer containing Mg and P was formed in the scratches; i.e., the coatings self-healed. The corrosion morphology of HAp-coated AZ31 was examined by periodic medium replacement or using a gelatine-added medium. The former enhanced calcium phosphate precipitation; the latter increased corrosion localization.

The anticorrosion ability of ultrathin coatings with bio-inspired organic polymers is demonstrated. We prepared a series of catechol-containing poly(alkyl methacrylate)s by free radical polymerization. These copolymers were spin-coated on various corrosion susceptible metal/alloy substrates of magnesium, aluminum, copper and iron without any harsh pretreatment. Several key factors like molecular structure, composition ratio and processing conditions were wisely tailored to afford a transparent, firm and sub-micron polymer coating on those substrates. Corrosion resistance of the polymer-coated substrates was thoroughly investigated by immersion tests in salt-water and acidic solutions, polarization tests, and visual inspection. Formation of an anomalous dense layer of ca. 5 nm thickness adjacent to the metal surface and a remarkable effect of thermal treatment were clearly observed by neutron reflectivity measurements, leading to a highly protective ability against foreign molecules e.g. water or corrosive ions. A key molecular design for anticorrosive polymer coating was revealed to be a combination of strong and versatile binding ability of catechol units and defect free polymer layers formed on the metal substrates in the presence of hydrophobic alkyl chains.

Mg matrix in situ composites were fabricated from Mg and ZnO powder by a spark plasma sintering method. The composition and microstructure of the sintered samples were characterized. Corrosion properties of fabricated composites were evaluated by immersion and by electrochemical tests using Hanks’ solution. The results showed that the formation of in situ products improved significantly the corrosion resistance of the fabricated composites compared with pure Mg; Mg-10 wt % ZnO composites especially exhibited the lowest corrosion rate. In addition, an energy-dispersive X-ray (EDX) analysis showed that calcium phosphate formed as a corrosion product on the surface of Mg-10 wt % ZnO composites, while Mg(OH)_2 appeared as a corrosion product on the surface of Mg-20 wt % ZnO composite. The findings suggested Mg-10 wt % ZnO composite as a potential candidate for temporary implant application.

Uniform hydroxyapatite (HAp) and octacalcium phosphate (OCP) with and without monetite layers were formed on Mg-3mass% Al-1mass% Zn (AZ31) alloy by a chemical solution deposition method at various pH values from pH 5.5 to 8.6. Adhesion strength of as-coated HAp and OCP layers was evaluated by a pull-off test, and that of HAp layers after immersing in a medium for 14 days was also evaluated. The as-coated pH 7.5-HAp layer showed higher adhesion strength of 6.7 MPa than 3.8 MPa of the as-coated pH 8.6-HAp layer, which was attributed to the former denser structure than the latter. The adhesion strength of the as-coated pH 5.5- and pH 6.2-OCP (+monetite) layers was both 4.2 MPa that was governed by the thick and fragile property of the layers. After immersing in a medium, the adhesion strength of pH 7.5- and pH 8.6-specimens decreased to 4.9 and 3.1 MPa, respectively. The pH 8.6-specimen showed a uniform Mg(OH)2 corrosion layer and corrosion pits; while the pH 7.5-specimen showed mostly corrosion pits. The pitting caused the decrease of adhesion strength of the HAp layers and the formation of Mg(OH)2 layer also weakens the adhesion. The adhesion strength of HAp and OCP layers was governed by the density and thickness of the layer, and was weaken by pitting and formation of Mg(OH)2 corrosion product layer.

Corrosion of reinforcing steel rebar is a main cause for losing the long-term reliability of concrete structures. When the rebar is corroded, the rust induces cracks and detachment of the cover concrete. On the rebars, the mill scale which is a thick oxide film is formed when the rebar is hot rolled. The role of mill scale on corrosion behavior of rebars in concrete has not been clarified, yet. In this study, the corrosion behavior of rebars with mill scale in concrete was studied using a new accelerated corrosion test we developed. Commercial rebars with 19 mm diameter, which have mill scale of several tens of micrometers in thickness, were used in this study. The corrosion behavior of rebars with and without mill scale in mortar was investigated. To remove the mill scale on the rebar surface, electrolytic polishing was carried out. The rebars with or without mill scale were embedded in mortar with 5.5 mm cover thickness and the sample was cured for 28 days. In order to prompt breakdown of passive film on the rebars, 2.06 M NaCl solution was mixed to the mortar. Using these samples, Hyperbaric-oxygen accelerated corrosion test (HOACT) was carried out [1]. In the HOACT, pressurized oxygen gas is supplied to the sample, and the oxygen reduction reaction on the steel surface is enhanced, which leads to the corrosion acceleration of the steel. In this study, the mortar-covered rebar samples were located in a high pressure gas chamber, and pressurized oxygen gas of 0.6 MPa was supplied. The periods of HOACT were 14, 28 and 56 days. After finishing the HOACT, the samples were embedded in resin, and then the cross section of the sample was observed using an optical microscope. The rust composition was analyzed using Raman spectroscopy measurement. The corrosion of the rebar with mill scale was more significant than that without mill scale. In the case of the sample with mill scale after 14-day HOACT, a rust layer was formed between the steel surface and mill scale, whereas no obvious corrosion was observed on the rebar without mill scale. To investigate the effect of the mill scale on the corrosion resistance of the rebar, anodic polarization measurements of the rebars with and without mill scale were carried out in a saturated Ca(OH)2 solution with Cl-. The pitting potential of the rebar with mill scale was more noble than that of the sample without mill scale. However, when the mill scale was scratched the pitting potential was significantly shifted to the less noble direction and it was below that of the rebar without mill scale. These results show that the mill scale improves the corrosion resistance of the rebar in mortar, whereas the corrosion resistance of the rebar becomes even worth than that of the rebar without mill scale when the mill scale contains defects. After 56-day HOACT of the sample with mill scale, two rust layers were observed above and beneath the mill scale. Raman spectroscopy revealed that the rust layer above the mill scale was mainly composed of a- and g-FeOOH, and the layer just beneath the mill scale was mainly composed of Fe3O4. The rust formed on the sample with mill scale during 14-day HOACT was monolayer just above the steel surface and it was composed only of Fe3O4. It is speculated that the Fe3O4 layer grows under the mill scale at the early stage of corrosion of a rebar. [1] K. Doi, S. Hiromoto, H. Katayama and E. Akiyama, Journal of the Electrochemical Society, 165(9), C582-C589 (2018).

Zr-based bulk metallic glasses (BMG) show high corrosion resistance in vitro and higher strength and lower Young's modulus than crystalline alloys with the similar composition. This study aimed to perform an in vivo evaluation of Zr65Al7.5Ni10Cu17.5 BMG. Osteotomy of the femur was done in rats and stabilized with intramedullary nails made of Zr65Al7.5Ni10Cu17.5 BMG, Ti-6Al-4V alloy, or 316L stainless steel. Systemic and local effects of each type of nail were evaluated by measuring the levels of Cu and Ni in the blood and the surrounding soft tissue. Changes of the surface of each nail were examined by scanning electron microscopy (SEM). Healing of the osteotomy was evaluated by peripheral quantitative computed tomography and mechanical testing. No increase of Cu and Ni levels was recognized. Surface of the BMG showed no noticeable change, while Ti-6Al-4V alloy showed Ca and P deposition and 316L stainless steel showed surface irregularities and pitting by SEM observation. The stress strain index, maximum torque, torsional stiffness, and energy absorption values were larger for the BMG than those for Ti-6Al-4V alloy, although there was no significant difference. The Zr-based BMG can promote osteotomy healing as fast as Ti-6Al-4V alloy, with the possible advantage of the Zr-based BMG that bone bonding is less likely, allowing easier nail removal compared with Ti-6Al-4V alloy. The Zr-based BMG is promising for the use in osteosynthetic devices that are eventually removed.

Hydroxyapatite (HAp) and octacalcium phosphate (OCP) coatings deposited on a pure magnesium (Mg) substrate from an aqueous solution containing C10H12N2O8Na2Ca (Ca-EDTA) and KH2PO4 were precisely analyzed using X-ray photoelectron spectroscopy (XPS) and high-resolution X-ray diffractometry (XRD) to clarify their chemical and crystallographic characteristics. The calcium phosphate coatings comprised single-phase HAp or OCP containing small amounts of sodium (Na). The intermediate layer formed underneath the coatings consisted of Mg(OH)2 mixed with calcium phosphate. The atomic ratios of calcium to phosphorus ([Ca]/[P]) in the HAp and OCP coatings were notably smaller than those of stoichiometric compounds. The expansion along the a-axis compared to that of stoichiometric HAp was observed in XRD results. Also, we did not detect Na compounds in the coating. Therefore, it is presumed that Na existed as a substitute of Ca in the OCP and HAp structures.

Calcium phosphate (CaP) was formed on glutamic acid-modified nickel-free high-nitrogen stainless steel (HNS) by a chemical solution deposition method in neutral/alkaline solution. Modification of glutamic acid on the surface of HNS was performed using trisuccinimidyl citrate (TSC) as a linker. The glutamic acid-derived carboxyl groups introduced on HNS initiated rapid nucleation of CaP during each treatment at pH 7.3 or 8.9. X-ray diffraction analysis and Fourier transform infrared spectra showed each CaP deposited on glutamic acid-immobilized HNS after 24 hrs was identified as low-crystallinity calcium-deficient carbonate-containing hydroxyapatite (HAp). A significant difference in the microstructure between the two pH values was observed; HAp deposited at pH 7.3 was composed of plate-like crystals, whereas that at pH 8.9 was chestnut-like crystals. Therefore, the pH value of the solution played an effective role in controlling the microstructure of HAp deposited on glutamic acid-immobilized HNS.

The corrosion resistance and adhesiveness of a hydroxyapatite (HAp)-coated AZ31 magnesium alloy rolled plate were evaluated by a salt spray test (SST: JIS Z 2371) and a cross-cut test (CCT: JIS K 5600), respectively. The length of the HAp coating solution treatment was 4 or 6h. A HAp coating consisting of an inner continuous layer and an outer porous layer uniformly covered the surface regardless of the treatment period. After the SST for 7 days, the HAp-coated surfaces showed no significant delamination but several visible pits. The HAp coating in the non-corroded area retained its original morphology after the SST by a SEM observation. The rating number (RN) for the HAp-coated specimens was over 9, which was much higher than RN 4 observed for the chemically polished specimen covered almost entirely with filiform corrosion. The RN of HAp-coated specimen increased slightly with a longer treatment period. The adhesiveness of the HAp coating was not influenced by the SSTaccording to the CCT. After removing the corrosion products and HAp coating from the non-corroded area, small pits with a diameter of a few hundred micrometers appeared; however, general corrosion did not take place underneath the HAp coating. The HAp coating shows high corrosion resistance and good adhesiveness to the AZ31 substrate. [doi:10.2320/matertrans.M2011346]

Hydroxyapatite (HAp) coatings were uniformly formed on pure Mg by a hydrothermal treatment using a C 10 H 12 N 2 O 8 Na 2 Ca (Ca-EDTA) solution. The growth mechanism of the HAp coating was investigated with XRD, SEM and TEM. At the initial stage, dome-shape HAp precipitates were formed on the Mg. Subsequently, the precipitates grew and the coating became a dual-layer consisting of an inner dense HAp layer and outer course layer consisting of rod-like HAp crystals. The protectiveness of the coatings with different treatment times was investigated by a polarization test in a 3.5 wt.% NaCl solution. The corrosion current density decreased with the growth of the HAp coating.

The fatigue strength of commercial SUS316L stainless steel was studied in PBS(−) (phosphate buffer solution) with different dissolved oxygen contents and CPBS (a 0.9 mass% NaCl-containing citric phosphate buffer solution) with different pH levels, i.e., 5.0, 6.0, and 7.5. The results obtained are as follows. (1) The fatigue behaviour in PBS(−) was hardly affected by the dissolved oxygen content. (2) The fatigue strength in a high-cycle region was much lower in CPBS of pH 7.5 than in PBS(−) of pH 7.5. (3) The fatigue strength in CPBS decreased with decreasing pH from 7.5 to 5.0.