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Zhou, et al.
phase may be a future choice. Bioactive ceramics can form a dense layer on the metal surface, thus reducing
simultaneously improve the mechanical and biological the degradation rate. The texture also influences the
performance of biodegradable metals. Nevertheless, to corrosion behavior of metal matrix [141] . When the crystal
the best of our knowledge, studies regarding LPBF of plane (0002) is the major plane parallel to the surface,
Zn-based composites containing bioactive ceramics are the textured AZ31 samples show superior corrosion
currently unavailable. Besides, the fatigue behavior of resistance [142] . Similar results are found in the extruded
degradable Zn implants is also very important because AZ31 samples with preferred grain orientation [143] .
the implants in the load-bearing parts need to bear cyclic Therefore, a strong texture with a preferred growth
loads in human body. Li et al. [134,135] showed that LPBF- direction (0001) presents on laser melted Zn samples,
processed porous Zn shows higher fatigue strength than which can cause preferential corrosion for other growth
porous Mg alloy under similar structural. The surface directions. In this case, heterogeneous corrosion formation
roughness and the micropores in the LPBF-processed Zn can lead to the increase of corrosion rate.
parts may be used as the nucleation site of the crack. The The elemental composition and precipitates also
forming defects can be used as the starting point of the influence the degradation of Zn implants. The influence
crack and to shorten its fatigue life. The biodegradation of alloying elements on the degradation rate in LPBF-
behavior will undoubtedly reduce the fatigue life of processed Zn is mainly attributed to two aspects, including
porous biodegradable metal. However, it was proven that the grain size and the galvanic corrosion between the
the fatigue life of LPBF-processed porous Zn is better precipitated second phase and matrix [144,145] . The fine-
than that tested in air, which may be due to the good grained structure may cause a decreased corrosion
combination between the formed degradation products rate due to the formation of corrosion products in the
and Zn matrix. passivation environment, and the specific situation is
related to the passivation properties of metal surface [146] .
4. Biodegradation behavior For example, Yang et al. processed Zn-Mg implant
[33]
Usually, bone implants are required a mechanical and found that Mg element could enhance the anti-
support with at least 3 months and preferably completed corrosion ability of Zn implant. The enhancement of Zn-
absorption within 2 years [136,137] . At present, the Mg corrosion resistance was mainly due to the increase
degradation behavior of Zn has been reported in a large in grain boundary density because of grain refinement,
thereby enhancing the passivation of the surface film.
number of literatures [138] . Its degradation is closely related
to microstructure, chemical composition, and porous It is widely accepted that the secondary phase can act
structure. Li et al. [139] studied the degradation behavior as a galvanic cathode, which accelerates the corrosion
[145]
of porous Zn scaffolds fabricated by LPBF. The results rate . The specific effect depends on the amount and
the distribution of the secondary phases. Shuai et al.
[116]
showed that their degradation rate was moderate, which revealed the degradation behavior of LPBF prepared Zn-
lost 7.8% and 3.6% of volume after dynamic and static
immersion in simulated body fluid (SBF) for 4 weeks, Al-xSn (x = 0, 0.5, 1, 2, 3) alloy and found that Zn-Al-
respectively. Qin et al. prepared bulk pure Zn by LPBF 3Sn alloy exhibited the highest degradation rate among
[82]
and found that the corrosion rate was lower than that of all the alloys. In general, in Zn-Al alloy, there were
only Zn enriched phase and Zn-Al eutectic phase. With
cast Zn. This is due to fine-grained structure caused by the the increase of Sn content, the presence of Sn-enriched
rapid cooling rate and the passivation effect of corrosion
products. phase further enhanced the galvanic corrosion in the
Considering the fine grains and homogeneous Zn-Al-Sn alloy. Therefore, the Zn-Al-3Sn alloy has the
[66]
microstructure obtained by laser rapid solidification, fastest degradation rate. Yang et al. investigated the
degradation behavior of LPBF-processed Zn-Nd alloy.
LPBF-processed Zn indeed presents some unique After alloying with REs, the diameter of the capacitive
degradation behavior. Theoretically, grain refinement
leads to an increase of grain boundaries, which means high arc and impedance moduli at low frequency gradually
increases to maximum, reflecting a high charge transfer
degradation reaction of sample surface. The influence of resistance. In general, during the corrosion process,
grain size on corrosion rate (i ) is described by [140] :
1 corr the basic Zn chloride with good compactness is easily
− 9
i corr = L 1 + 2 .( ) .d 2 exp (− L 8 .A n 2 ) (4) affected by carbonate ion to form basic Zn carbonate with
. However, the ion transfer could be
amorphous state
[147]
Where, both the L and L are constants depending effectively cut off by REs, which maintains the stability
1
2
on the material composition. The and A are the average of basic Zn chloride, thus decreasing corrosion rate [148] .
n
grain size and the grain size distribution, respectively. It should be noted that current studies regarding on the
Based on the Equation 4, the decrease of grain size leads degradation behavior of laser melted Zn-based implants
to more uniform degradation products, which can easily are mainly focused on in vitro tests. It is well accepted that
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