Material properties of Japanese and western wooden nails

The corrosion characteristics of Japanese (J) and Western (W) nails with and without coating the Fe₃O₄ layer on the nail surface were examined experimentally to understand their mechanical properties and corrodibility. In this study, (i) one of the historical J-nails made by the forging process and (ii) commonly employed commercial W-nails created by extruding were used. Although the grain size and strain value differed depending on the nail and its location(sharpened tip, body, and flattened head), the microstructure of both nails consisted of ferrite and cementite structures. A high internal strain was created in the W-nail due to the nail creation by the extruding process. In particular, a high strain was obtained in the sharpened tip and flattened head areas of the W-nail compared to its body area, caused by severe plastic deformation. A high strain was also obtained in the sharpened tip area of the J-nail owing to the severe forging process. Such a high strain was related to the high hardness. The high hardness of the sharpened tip area of the J-nail (220 HV) was slightly lower than that of the W-nail's tip area. The hardness of the J-nail decreased non-linearly to its head area, which was affected by a low strain and largely grown grains. The corrosion occurred on the J- and W-nail surfaces, where lepidocrocite (γ-FeOOH) was detected due to the weak-acidity oxidation, i.e., acid rain. The corrosion extent was different depending on the nail. Although the corrosion was detected for the J-nail after exposure for 3 months, the corrosion occurred on W-nail after exposure to the atmosphere for a week. Different corrosion extents between J- and W-nails were affected by their microstructural characteristics and strain levels, i.e., different surface energy. The corrosion resistance of both nails was improved by coating the Fe₃O₄ layer on the nail surface, where the corrosion extent for both J- and W-nails was similar due to the corrosion protection by the Fe₃O₄ layer. The corrosion characteristics directly affected their mechanical properties: tensile strength and ductility. This work systematically analyzed details of the mechanical properties of the nails after corrosion.