In vitro corrosion and biocompatibility of binary magnesium alloys
Xuenan Gu a ,Yufeng Zheng a ,b ,*,Yan Cheng b ,Shengping Zhong b ,Tingfei Xi b
a
State Key Laboratory for Turbulence and Complex System and Department of Advanced Materials and Nanotechnology,College of Engineering,Peking University,Beijing 100871,China b
Center for Biomedical Materials and Tissue Engineering,Academy for Advanced Interdisciplinary Studies,Peking University,Beijing 100871,China
a r t i c l e i n f o
Article history:
Received 2September 2008Accepted 21October 2008
Available online 9November 2008Keywords:
Magnesium alloy Corrosion In vitro
Cytotoxicity
Hemocompatibility
a b s t r a c t
As bioabsorbable materials,magnesium alloys are expected to be totally degraded in the body and their biocorrosion products not deleterious to the surrounding tissues.It’s critical that the alloying elements are carefully selected in consideration of their cytotoxicity and hemocompatibility.In the present study,nine alloying elements Al,Ag,In,Mn,Si,Sn,Y,Zn and Zr were added into magnesium individually to fabricate binary Mg–1X (wt.%)alloys.Pure magnesium was used as control.Their mechanical properties,corrosion properties and in vitro biocompatibilities (cytotoxicity and hemocompatibility)were evaluated by SEM,XRD,tensile test,immersion test,electrochemical corrosion test,cell culture and platelet adhesion test.The results showed that the addition of alloying elements could in?uence the strength and corrosion resistance of Mg.The cytotoxicity tests indicated that Mg–1Al,Mg–1Sn and Mg–1Zn alloy extracts showed no signi?cant reduced cell viability to ?broblasts (L-929and NIH3T3)and osteoblasts (MC3T3-E1);Mg–1Al and Mg–1Zn alloy extracts indicated no negative effect on viabilities of blood vessel related cells,ECV304and VSMC.It was found that hemolysis and the amount of adhered platelets decreased after alloying for all Mg–1X alloys as compared to the pure magnesium control.The rela-tionship between the corrosion products and the in vitro biocompatibility had been discussed and the suitable alloying elements for the biomedical applications associated with bone and blood vessel had been proposed.
ó2008Elsevier Ltd.All rights reserved.
1.Introduction
There is a growing interest in using corrodible alloys in a number of critical medical applications,after decades of designing strategies to minimize the corrosion of metallic biomaterials [1].Degradation or decomposition,most often due to corrosion of the materials in the body ?uid,has been demonstrated in metals and alloys that may have potential orthopedic and cardiovascular applications.Examples are pure metals such as Fe [2–6],Mg [7–12],and W [13,14],and alloys such as Fe–Mn [15],AE21[16],AZ21[12],AZ31[9,17],AZ63[18],AZ91[17,19],AZ91D [20],ZE41[11],WE43[17,21–23],LAE442[17,20],Mg–Mn–Zn [24],and Mg–Ca [12,25]).
Metal ions released from corrodible alloys to the surrounding tissues,may cause biological responses in short term or prolonged periods.The toxicity of a metallic material is governed not only by its composition and toxicity of the component elements but also by its corrosion and wear resistance [26].In a saline environment,magnesium-based alloys would be degraded to magnesium
chloride,oxide,sulphate or phosphate [3].It is believed that the release of magnesium ions from corroding magnesium alloys should not cause toxicity (local or systemic),and may even have bene?cial effects on some of the structures,including cells,in the relevant local tissue [1].Yet most of the currently investigated magnesium-based alloys [9,11,12,17–22]for biomedical applications,as listed in Table 1,were originally designed for industrial applications (in pursuit of high strength and corrosion resistance)without taking the biocompatibility into considerations during composition design.For example,the absorbable metal stent (AMS,Biotronik GmbH),currently under clinical investigation,was reported to be made of an alloy containing magnesium,zirconium,yttrium and lanthanides [3].Since the exact composition of AMS stent materials was not disclosed,its potential toxicity and degradation products could not be completely determined,therefore we relied on the biocompati-bility testing carried out by the manufacturers [3].
In the design of degradable biomaterials,elements with potential toxicological problems should be ideally avoided if possible,and these elements should be only used in minimal,acceptable amounts if they cannot be excluded from the design [27].Therefore,the toxic elements should be identi?ed and excluded from the current and future generation of magnesium alloys.As documented in the ISO 3116:2007[28]and EN 1753:1997
*Corresponding author.Department of Advanced Materials and Nanotechnology,College of Engineering,Peking University,No.5Yi-He-Yuan Street,Beijing 100871,China.Tel./fax:t861062767411.
E-mail address:yfzheng@https://www.doczj.com/doc/ec6413826.html, (Y.
Zheng).Contents lists available at ScienceDirect
Biomaterials
journal homepage:https://www.doczj.com/doc/ec6413826.html,/loca
te/biomaterials
0142-9612/$–see front matter ó2008Elsevier Ltd.All rights reserved.doi:10.1016/j.biomaterials.2008.10.021
Biomaterials 30(2009)484–498
[29]?les,the main alloying elements for the industrial magnesium alloys are Al,Zn,Mn,Si,RE,Zr,Ag and Y.In the present study,Mg–1X binary alloys were prepared by adding elements of Al,Ag,Mn,Si,Y,Zn and Zr separately into magnesium in 1%(wt),and the speci?c cellular responses (bone and blood vessel related)to the added metal ions together with the magnesium ion were evaluated.The results may help screening the elements that have minimal detri-mental effect on cellular response,and help understand the cyto-toxicity of the current multi-component magnesium alloys (listed in Table 1).It may also provide guidance on the future selection of the preferable alloying elements for bone-or vascular-related applications.Furthermore,it may also direct the future composi-tion design of the biodegradable ternary/multiple magnesium alloy systems.
Besides the established industrial compositions,some novel composition designs were proposed and investigated as biomedical magnesium alloys.The recent development of Mg–1Ca alloy revealed that it had the acceptable biocompatibility as a new biodegradable bone implant materials [25].In the present study,metals In and Sn,commonly used in dental materials [34],having over 1wt.%solubility in magnesium,were selected as additives,to evaluate their feasibility as potential alloying elements in magne-sium alloy for biomedical applications.
2.Experimental procedure
2.1.Material preparation
Nine binary magnesium alloys were melted and cast by commercial pure Mg (99.95%)and commercial pure element powders (Al,Ag,In,Si,Sn,Zn and Zr)or Mg–Mn/Y master alloy under a mixed gas atmosphere of SF 6and CO 2using a mild steel crucible,with nominal 99wt.%of Mg and 1wt.%of alloying element (Al,Ag,In,Mn,Si,Sn,Y,Zn and Zr).The analyzed chemical compositions of the nominal Mg-1wt.%X alloy ingots by energy dispersive spectrum (EDS)were given in Table 2.The various Mg–1X alloy ingots were cut into 6mm thick plates and hot rolled to about 1.5mm thick sheets after heating to 400 C for 20min.The as-cast and as-rolled samples were further cut into square samples (10?10?2mm 3)for corrosion,cytotoxicity and hemocompatibility tests.Each sample was mechanically polished up to 2000grit,ultrasonically cleaned in acetone,absolute ethanol and distilled water,and then dried in open air.For cytotoxicity tests,samples were sterilized by ultraviolet-radiation for at least 2h.
2.2.Microstructural characterization
X-ray diffractometer (XRD,Rigaku DMAX 2400)using Cu K a radiation was employed for the identi?cation of the constituent phases in the as-cast and as-rolled Mg–1X alloys and their corrosion products after immersion.2.3.Tensile test
The tensile samples of Mg–1X alloys were machined according to ASTM-E8-04[35].The tensile tests were carried out at a displacement rate of 1mm/min in
a Instron 3365universal test machine.An average of at least three measurements was taken for each group.2.4.Electrochemical measurement
A three-electrode cell was used for electrochemical measurements.The counter-electrode was made of platinum and the reference electrode was saturated calomel electrode (SCE).The exposed area of the working electrode (pure Mg and Mg–1X alloy samples)to the solution was 1cm 2.All the measurements were carried out on an electro-chemical workstation (CHI660C,China)at the temperature of 37 C.The test milieu was simulated body ?uid (SBF)[36]and Hank’s solution [7].2.5.Immersion test
The immersion test was carried out according to ASTM-G31-72[37]in SBF [36]and Hank’s solution [7].Experimental samples (10?10?2mm 3)were immersed in 50ml solutions and the temperature was kept at 37 C by water bath.After different immersion times,the samples were removed from SBF and Hank’s solution,gently rinsed with distilled water,and dried at room temperature.Changes on the surface morphologies and microstructure of the samples before and after immersion were characterized by environmental scanning electron microscopy (ESEM,AMRAY-1910FE),equipped with energy-disperse spectrometer (EDS)attachment,and X-ray diffractmeter.The amount of hydrogen generated by pure Mg and Mg–1X alloys was measured in accordance to Ref.[38].The induc-tively coupled plasma atomic emission spectrometry (Leeman,Pro?le ICP-AES)was employed to measure the concentrations of magnesium and alloying element ions which had dissolved from the alloy plates.An average of three measurements was taken for each group.2.6.Cytotoxicity test
Murine ?broblast cells (L-929and NIH3T3),murine calvarial preosteoblasts (MC3T3-E1),human umbilical vein endothelial cells (ECV304)and rodent vascular smooth muscle cells (VSMC)were adopted to evaluate the cytotoxicity of pure Mg and Mg–1X alloys.L-929,NIH3T3,MC3T3-E1,VSMC and ECV304cells were cultured in Dulbecco’s modi?ed Eagle’s medium (DMEM),10%fetal bovine serum (FBS),100U ml à1penicillin and 100m g ml à1streptomycin at 37 C in a humidi?ed atmosphere of 5%CO 2.The cytotoxicity tests were carried out by indirect contact.Extracts were prepared using DMEM serum free medium as the extraction medium with the surface area of extraction medium ratio 1.25ml/cm 2in a humidi?ed atmosphere with 5%CO 2at 37 C for 72h.The supernatant ?uid was withdrawn and centrifuged to prepare the extraction medium,then refrigerated at 4 C before the cytotoxicity test.The control groups involved the use of DMEM medium as negative controls and 0.64%phenol DMEM medium as positive controls.Cells were incubated in 96-well cell culture plates at 5?103cells/100m l medium in each well and incubated for 24h to allow attachment.The medium was then replaced with 100m l of extracts.After incubating the cells in a humidi?ed atmosphere with 5%CO 2at 37 C for 2,4and 7days,respectively,the 96-well cell culture plates were observed under an optical microscope.After that,10m l MTT was added to each well.The samples were incubated with MTT for 4h at 37 C,then 100m l formazan solubilization solution (10%SDS in 0.01M HCl)was added in each well overnight in the incubator in a humidi?ed atmosphere.The spectrophotometrical absorbance of the samples was measured by microplate reader (Bio-RAD680)at 570nm with a reference wavelength of 630nm.The pH value and the concentrations of magnesium and alloying element ions in the extraction medium were also
measured.
Table 1
Table 2
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2.7.Hemolysis test
Healthy human blood from a volunteer containing sodium citrate (3.8wt.%)in the ratio of 9:1was taken and diluted with normal saline (4:5ratio by volume).Pure Mg and Mg–1X alloys samples (10?10?2mm 3)were dipped in a standard tube containing 10ml of normal saline that were previously incubated at 37 C for 30min.Then 0.2ml of diluted blood was added to this standard tube and the mixtures were incubated for 60min at 37 C.Similarly,normal saline solution was used as a negative control and deionized water as a positive control.After this period,all the tubes were centrifuged for 5min at 3000rpm and the supernatant was carefully removed and transferred to the cuvette for spectroscopic analysis at 545nm.In addition,the hemolysis was calculated using an ultraviolet spectro-photometer (UNIC-7200,China).The hemolysis was calculated based on the average of three replicates.hemolysis ?
OD etest TàOD enegative control T
OD epositive control TàOD enegative control T
?100
2.8.Platelet adhesion
Platelet-rich plasma (PRP)was prepared by centrifuging the whole blood for 10min at a rate of 1000rpm/min.The PRP was overlaid atop the experimental alloys plates and incubated at 37 C for 1h.The samples were rinsed with PBS to remove the nonadherent platelets.The adhered platelets were ?xed in 2.5%glutaraldehyde solutions for 1h at room temperature followed by dehydration in a gradient ethanol/distilled water mixture (50%,60%,70%,80%,90%,100%)for 10min each and dried in hexamethyldisilazane (HMDS)solution.The surface of platelet attached experimental alloy plates were observed by ESEM.Different ?elds were randomly counted and values were expressed as the average number of adhered platelets per mm 2of surface.
3.Results
3.1.Microstructures of binary Mg–1X alloys
The as-cast and as-rolled pure Mg and Mg–1X alloys were all characterized by XRD for their phase composition,and the results show that as-cast and as-rolled pure Mg and Mg–1X alloys,except for Mg–1Si,are composed of one single phase a (Mg),with the representative spectra of as-cast samples shown in Fig.1.This result is as expected since the solubility of elements Al,Ag,In,Mn,Sn,Y,Zn and Zr in Mg are 12.7wt.%,15.0wt.%,53.2wt.%, 2.2wt.%,14.5wt.%,12.4wt.%,6.2wt.%and 3.8wt.%respectively,according to
the binary phase diagrams [39].It can be easily understood since there is almost no solubility for Si in Mg [39],and 1wt.%Si will lead to the formation of Mg 2Si phase.The addition of 1wt.%Si is just for better comparison with other alloying elements.3.2.Mechanical properties of binary Mg–1X alloys
Fig.2shows the tensile properties of as-cast (Fig.2(a))and as-rolled (Fig.2(b))Mg–1X alloys,with pure Mg as control.It can be seen that the addition of various alloying elements into pure Mg displays two distinctive in?uencing patterns on the tensile strength of Mg–1X alloys.The separate addition of elements Al,Ag,In,Si,Sn,Zn or Zr improves the yield strength and ultimate tensile strength,while the addition of Mn or Y does not enhance the strength obviously but decreases the elongation.
The yield strength and ultimate strength are largely improved after hot rolling,especially for the addition of Al or Zn,as shown in Fig.2(b).However,the elongations of as-rolled Mg–1X alloys decrease,compared with that of the as-cast ones.3.3.Corrosion behavior of binary Mg–1X alloys
3.3.1.Immersion corrosion behavior of binary Mg–1X alloys
Fig.3illustrates the X-ray diffraction patterns of as-cast and as-rolled Mg–1X alloys after 500h immersion in SBF (the corre-sponding XRD patterns of samples in Hank’s solution were similar,and therefore omitted),with pure Mg as control.No data are available for as-cast Mg–1X (X ?Al,Si,Y)alloys and the as-rolled Mg–1Al alloy since these plate samples had been suffered severe corrosion and broken into tiny particles after 500h immersion.It can be seen that as-cast alloys samples show more broad Mg(OH)2peaks than that of the as-rolled ones.
Fig.4shows the hydrogen evolution tendency of as-cast and as-rolled Mg–1X alloy samples during immersion in both SBF and Hank’s solution,with pure Mg as control.It can be seen that (i)Mg–1X alloys exhibit faster corrosion rate in SBF than that in Hank’s solution,which may be attributed to the buffer in SBF [36,40];(ii)The as-rolled Mg–1X alloys display less hydrogen evolution in comparison to the as-cast ones immersed in both
SBF
Fig.1.X-ray diffraction patterns of as-cast pure Mg and Mg–1X alloy (X ?Al,Ag,In,Mn,Si,Sn,Y,Zn and Zr)samples at room temperature.
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and Hank’s solutions.(iii)For the as-cast Mg–1X alloys,the Mg–1Y and Mg–1Si indicate higher hydrogen evolution rate in SBF and Hank’s solution than that of pure Mg;for the as-rolled Mg–1X alloys,the Mg–1Al indicates higher hydrogen evolution rate in SBF and Hank’s solution than pure Mg while Mg–1Si indicates higher hydrogen evolution rate in Hank’s solution than pure Mg;(iv)The Mg–1Al and Mg–1Zn alloys show the lowest hydrogen evolution rate among the as-cast Mg–1X alloys,whereas Mg–1Mn and Mg–1Zn alloys show the lowest hydrogen evolution rate among the as-rolled Mg–1X alloys.Fig.5presents the released ion concentrations of magnesium and elements X in the SBF and Hank’s solution for as-cast and as-rolled Mg–1X alloys at different immersion durations,with pure Mg as control.It can be seen that (i)the dissolutions of magnesium and alloying elements progressed with the immersion time;(ii)The concentration of the released magnesium and alloying elements ions is always higher for the as-cast Mg–1X alloys than that of the as-rolled Mg–1X alloys in both SBF and Hank’s solutions,except for Mg–1Al alloy;(iii)The dissolution of Al,Si,Sn,Y is much higher than other tested alloying elements for as-cast Mg–1X
alloys
Fig.2.Tensile properties of (a)as-cast and (b)as-rolled pure Mg and Mg–1X alloy (X ?Al,Ag,In,Mn,Si,Sn,Y,Zn and Zr)samples at room temperature.
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whereas Al and Mn showed higher dissolution rate compared with other elements for as-rolled Mg–1X alloys.
3.3.2.Electrochemical corrosion behavior of binary Mg–1X alloys
Fig.6shows the electrochemical polarization curves of as-cast and as-rolled Mg–1X alloys in the SBF (similar results had been got for the samples tested in Hank’s solution,and therefore omitted),with pure Mg as control.The average electrochemical parameters and corrosion rates calculated for the present Mg–1X alloys according to ASTM [41]are listed in Table 3.It can be seen that (i)the addition of elements Al,Ag,Mn,Si or Y improve the corrosion potential of the resulted as-cast Mg alloys,while the addition of In,Sn or Zr decreases the corrosion potential of the resulted as-cast Mg alloys in SBF and Hank’s solution.Moreover,as-cast Mg–1Zn alloy exhibits positive corrosion potential than pure Mg in SBF,whereas it shows negative potential compared with pure Mg in
Hank’s
Fig.3.X-ray diffraction patterns of as-cast and as-rolled pure Mg and Mg–1X alloy (X ?Al,Ag,In,Mn,Si,Sn,Y,Zn and Zr)samples immersed in SBF for 500h.
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solution;(ii)The addition of elements Ag,Si or Y increase the current densities of the resulted as-cast Mg alloys in both SBF and Hank’s solution.There is no signi?cant differences between the current densities of as-cast pure Mg and Mg–1X (X ?Al,In,Mn,Sn,Zr)alloys.The addition of Zn improves the corrosion resistance of the resulted as-cast Mg–1Zn alloy;(iii)After hot rolling,the corrosion rates of experimental alloys decrease except for Mg–1Al alloy.The reduced corrosion resistance of as-rolled Mg–1Al alloy can be explained as the eutectic a phase may precipitate during the hot rolling process and accelerate the corrosion rate due to the different electrochemical behaviors of primary a and eutectic a [38,42].As-rolled Mg–1Mn alloy exhibits the lowest current densities in both SBF and Hank’s solution among the as-rolled Mg–1X alloys.
3.4.Cytotoxicity tests of binary Mg–1X alloys
Fig.7illustrates the magnesium and alloying element ion concentrations of the as-cast Mg–1X alloys extraction medium,with the as-cast pure Mg as control.It can be seen that the concentration of released Si and Sn ions are relatively high (1.8–2.0m g/ml),followed by Al,In and Zr ions (0.5–0.6m g/ml)and Ag,Mn,Y and Zn ions (0.1–0.2m g/ml).For the present study,the pH values of the extraction medium are 9.18?0.14(pure Mg),8.2?0.1(Mg–1Al),8.34?0.06(Mg–1Ag),8.49?0.17(Mg–1In),8.4?0.18(Mg–1Mn),8.3?0.12(Mg–1Si),8.82?0.07(Mg–1Sn),8.69?0.1(Mg–1Y),8.47?0.12(Mg–1Zn)and 8.58?0.05(Mg–1Zr).Fig.8shows the viability of murine ?broblast cells L-929(Fig.8(a)),and NIH3T3(Fig.8(b)),murine calvarial preosteoblasts MC3T3-E1(Fig.8(c)),human umbilical vein endothelial cells ECV304(Fig.8(d))and rodent vascular smooth muscle cells VSMC (Fig.8(e))expressed as a percentage of the viability of cells cultured in negative control after cultured in as-cast pure Mg and Mg–1X alloys extraction medium solutions for different periods (2,4and 7days).It can be seen that (i)for L-929?broblasts,the extracts of pure Mg,Mg–1Ag,Mg–1In,Mg–1Mn,Mg–1Si,Mg–1Y and Mg–1Zr alloys lead to signi?cantly reduced cell viability (p <0.05)in comparison with negative controls for 4days culture;(ii)The cell viability of NIH3T3?broblasts signi?cantly decreases (p <0.05)after cultured in pure Mg,Mg–1Ag,Mg–1Mn,Mg–1Y and Mg–1Zr alloys extracts and the extracts of other Mg–1X alloys show no toxicity to NIH3T3?broblasts for 7days culture;(iii)In the case of MC3T3-E1osteoblastic cells,the extracts from pure Mg,Mg–1Ag,Mg–1In and Mg–1Mn alloys show decreased cell viabilities (p <0.05)compare to the negative controls after 7days culture.Moreover,Mg–1Sn and Mg–1Y alloy extracts exhibit lower (p <0.05)cell viabilities in days 2and 4,but no differences in days 7;(iv)In either ECV304or VSMC models,pure Mg,Mg–1In,Mg–1Mn,Mg–1Y and Mg–1Zr alloys extracts show signi?cantly reduced values of cell viability,compared to the negative control (p <0.05)in 7days culture.Mg–1Ag,Mg–1Si and Mg–1Sn alloys extracts show decreased (p <0.05)cell viabilities in ECV304cell model,but no signi?cant differences in VSMC
model.
Fig.4.The hydrogen evolution volumes of as-cast and as-rolled pure Mg and Mg–1X alloy (X ?Al,Ag,In,Mn,Si,Sn,Y,Zn and Zr)samples immersed in SBF and Hank’s solution for 500h:(a)as-cast samples in SBF;(b)as-cast samples in Hank’s solution;(c)as-rolled samples in SBF;(d)as-rolled samples in Hank’s solution.
X.Gu et al./Biomaterials 30(2009)484–498489
3.5.Hemocompatibility of binary Mg–1X alloys
Fig.9shows the hemolysis of as-cast and as-rolled pure Mg and Mg–1X alloys samples.For the as-cast group,pure Mg,Mg–1Ag,Mg–1Zn and Mg–1Zr alloys induce over 30%of hemolysis when contact with whole blood (n ?3)for 60min.For the as-rolled group,the hemolysis of pure Mg,Mg–1Ag and Mg–1Zn alloys were reduced,whereas the hemolysis of Mg–1Al,Mg–1Sn and Mg–1Zr alloys increase.In addition,the hemolysis of Mg–1X (X ?In,Mn,Si,Y)alloys are equal to or lower than 5%.The pH value of the saline solution was also measured after the hemolysis tests.The results show that higher hemolysis occurred for the samples with
higher
Fig.5.Element concentrations in SBF and Hank’s solution at 3,10and 20d for the as-cast and as-rolled pure Mg and Mg–1X alloy (X ?Al,Ag,In,Mn,Si,Sn,Y,Zn and Zr)samples:(a)Magnesium concentration of as-cast samples in SBF;(b)Alloying element concentration of as-cast samples in SBF;(c)Magnesium concentration of as-cast samples in Hank’s solution;(d)Alloying element concentration of as-cast samples in Hank’s solution;(e)Magnesium concentration of as-rolled samples in SBF;(f)Alloying element concentration of as-rolled samples in SBF;(g)Magnesium concentration of as-rolled samples in Hank’s solution;(h)Alloying element concentration of as-rolled samples in Hank’s solution.
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pH values (around 9.5–10),especially for the samples (as-cast pure Mg and as-rolled Mg–1Sn alloy)with solution turning brown.
The morphologies of adhered human platelet were character-ized and the results showed that platelets present nearly round with only one or two short pseudopodia spreading on the surface of as-cast and as-rolled pure Mg and Mg–1X alloys,as typically shown by the case of pure Mg and Mg–1Sn alloy sample in Fig.10.
Fig.11illustrates the number of platelets adhered on as-cast and as-rolled pure Mg and Mg–1X alloys samples after incubation in PRP for 1h.It can be seen that (i)the number of platelets adhered on both the as-cast and as-rolled pure Mg are the highest among all samples;(ii)For the as-cast group,the addition of various alloying elements improve the platelet adhesion resistance to certain degrees compare to pure Mg control.Mg–1In and Mg–1Zr alloys samples shows higher amount of adhered platelets,followed by Mg–1Al,Mg–1Ag and Mg–1Sn alloys samples.The surface of Mg–1Mn,Mg–1Si,Mg–1Y and Mg–1Zn alloys samples show the lowest amount of adhered platelets;(iii)For the as-rolled group,the number of adhered platelets is reduced on the surface of as-cast pure Mg,Mg–1In,Mg–1Sn and Mg–1Zr alloys samples after hot rolling,whereas increased platelet adhesion is seen for the other as-rolled Mg–1X alloys samples.4.Discussion
4.1.Corrosion products of Mg–1X alloys and their in?uence on the in vitro cytotoxicity and hemocompatibility tests
Based on the above experimental results,it can be suggested that corrosion is the inherent response of magnesium alloys to the body ?uid,with the following corrosion products:released Mg 2tions,hydrogen bubbles,alkalization of solution by the OH àions,released alloying element metal ions and peeled-off particulates from the implant.(i)The evolved hydrogen bubbles from a corroding magnesium alloy implant can be accumulated in gas pockets next to the implant during in vivo tests,and cause separation of tissues and tissue layers [11,43].Yet for in vitro cytotoxicity tests,H 2will escape directly to the atmosphere during the extraction medium prepa-ration process and should not in?uence the in vitro cytotoxicity test results.(ii)Mg(OH)2precipitated from the solution during the corrosion of magnesium alloys due to saturation and localized alkalization.In addition,the second phases Mg 17Al 12in AZ91alloy [38],Mg 24Y 5and Mg 12Nd in WE43alloy [44],usually had higher corrosion potentials than a -Mg matrix and stable in corrosion
[44,45].The potential differences between magnesium matrix phase and the second phase led to microgalvanic corrosion [42]and the second phase might detach from the implant in the form of particulate corrosion products [38].For the in vitro cytotoxicity test,particles had been excluded after the centrifuge step during the preparation of the extract medium.Therefore only the ions (Mg 2t,X tand OH à)in the corrosion products play the key role to in?uence the in vitro cytotoxicity evaluation.
For the in vitro cytotoxicity tests,(i)it could be seen that the Mg 2tions concentrations of Mg–1X alloys extracts varied between 130and 210m g/ml (5.4–8.8m M /L),which was much lower than the IC 50of magnesium ions for MG-63osteoblasts [46].It was reasonable to believe the release of Mg 2tin extraction medium was not a dominant factor for the outcomes of Mg–1X alloy cytotoxicity test.(ii)The tested cells may have different ability to tolerate the changes of pH value.An increase of pH value by 0.39had no obvious in?uence on the viability of MG-63,but led to a reduced viability for macrophages in the study of cellular response to AZ91D alloy [47].Groos et al.[48]indicated that colony-forming ability of RT112cells was reduced signi?cantly at pH values greater than 8.4after 24h exposure.For the present Mg–1X alloys,the tolerant threshold of pH value for all the cell lines was around 8.8,since no signi?cant reduced cell viability for Mg–1Sn alloy extract (with the highest pH value of 8.82?0.07among Mg–1X alloys)was seen by all the cell lines except for ECV304,while signi?cantly reduced cell viability for pure Mg extracts (9.18?0.14)was seen for all the cell lines.Therefore,focus of the study was the in?uence of alloying elements on cells instead of pH value effect.(iii)The amount of ions released from an alloy may not be proportion to the composition of the alloy [49].For the present Mg–1X alloys,the concentration of the released alloying element ions in the extract,as shown in Fig.7,was not consistent with the nominal composition.The effect of the nine tested alloying elements for the Mg–1X alloys on their cytotoxicity will be discussed in detail below.
(1)Al .As shown in Fig.8,Mg–1Al alloy extracts had no signi?cant
cytotoxicity to all tested cell lines with Al concentration 20?7m M /L (Fig.7).As reported,IC 50s of Al(NO 3)3to MC3T3-E1and L-929cells were 2.92m M /L and 4.18m M /L,respectively [50],and no signi?cant differences were seen for MG-63cell viabilities until the Al concentration exceeded 1m M [46].In addition,no negative effect was observed for the MG-63and HBDC cellular response to AZ91D (9wt.%Al content)extraction medium [47]
.
Fig.5.Continued
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(2)Ag .Signi?cantly reduced cell viability for Mg–1Ag alloy
extract could be seen for all the cell lines except VSMC,with the concentration of Ag at 0.9?0.5m M /L.Similar effect of Ag ions,which induced serious toxicity on ?broblasts and osteoblasts at very low concentrations (10à6M /L),was also reported by Wataha et al.[50],Yamamoto et al.[51]and Schedle et al.[52].
(3)In .Mg–1In alloy extraction medium reduced cell viability for
MC3T3-E1,ECV304and VSMC cells but had no signi?cant toxicity effect on ?broblasts at a concentration of In 5?1.8m M /L.Wataha et al.[50]also reported nontoxic effect of In salts for Balb/c 3T3?broblasts at 0.435m M /L,whereas Schedle et al.[52]reported a serious toxicity of In salts on L-929?broblasts and gingival ?broblasts at 0.01m M /L.Similar phenomena were also found by Yamamoto et al.[51]that In ions showed a serious toxic effect on MC3T3-E1at a rather low concentration (2.03m M /L),but did not induce toxicity (>50%)at concentra-tions lower than 145m M /L for L-929?broblasts.(4)Mn .Mg–1Mn alloy extraction medium induced serious toxic
effect for all the cell lines used in this study,at the concen-tration of Mn 1.8?0.5m M /L.The poisonous effect of additive of Mn in magnesium alloys was also found on viability and proliferation of EC and SMC [53].Yamamoto et al.[51]and Hallab et al.[46]indicated that Mn ions induced toxicity (viability <50%)to L-929,MC3T3-E1and Balb/c 3T3cells at about 10à5M /L.
(5)Si .Mg–1Si alloy extract signi?cantly increased the osteoblastic
cell viability (Fig.8(c))but reduced the cell viabilities for ECV304and VSMC,with the Si concentration at 71?27m M /L.It was found that the supplementation of human osteoblast-like cells for 48h with the dissolution product of Zeolite A,a Si-containing compound,signi?cantly increased osteoblast proliferation and differentiation [54].In addition,Si in the form of orthosilicic acid at around physiological concentrations (10–20m M /L)signi?cantly increased osteoblastic differentiation in MG-63cells [55]and increased type I collagen synthesis in
the
Fig.6.Potentiodynamic polarization curves of (a)as-cast and (b)as-rolled pure Mg and Mg–1X alloy (X ?Al,Ag,In,Mn,Si,Sn,Y,Zn and Zr)samples immersed in SBF.
X.Gu et al./Biomaterials 30(2009)484–498
492
MG-63human osteosarcoma cell line,an immortalized clonal human bone marrow cell line(HCC1),skin?broblasts and bone marrow stromal cells[55].
(6)Sn.Mg–1Sn alloy extract did not have toxic effect for all cell lines
except for ECV304with the Sn concentration at15.8?7.8m M/L.
Yamamoto et al.[51]found that Sn4tdid not show toxic effects to both L-929and MC3T3-E1cells at high concentrations
(66.5m M/L and1.11m M/L,respectively),whereas Sn2tinduced
severe toxicity to L-929and MC3T3-E1at a rather low concentration(0.141m M/L and0.025m M/L,respectively). (7)Y.Except for MC3T3-E1cells,Mg–1Y alloy extract showed
decreased cell viability for L-929,NIH3T3,ECV304and VSMC cells at concentration of2.3?0.7m M/L for Y.The toxicity effect was noticed for the L-929cellular response to YCl3at10à4M/L, and the poisonous effect on MC3T3was also seen at similar concentrations[51].
(8)Zn.Mg–1Zn alloy extract showed similar or increased cell
viabilities for all the cell lines with the concentration of Zn at
2.6?1m M/L.As reported by Wataha et al.[50]and Schmalz
et al.[56],the IC50of Zn2tfor Balb/c3T3and L-929was28m M/L and25.5m M/L,whereas the addition of Zn had negative effects on viability and proliferation of EC and SMC[53].
(9)Zr.Mg–1Zr alloy extract signi?cantly reduced cell viability for
L-929,NIH3T3,ECV304and VSMC cells at a concentration of Zr
(6.9?3.3)m M/L.In contrast,Yamamoto et al.[51]indicated the
Zr ions,when lower than10à3M/L,did not induced toxicity (>50%)for L-929and MC3T3-E1.It was interesting to note AMS stent containing element Zr showed good in vivo biocompati-bility after clinical implantation[3].
For the hemocompatibility evaluations,(i)Li and Liu[57] reported that Mg2tdid not induce hemolysis even at concentration of10à3M/L.Among the tested Mg–1X alloys,the highest magne-sium concentration released within1h was0.57m M/L(calculated from the magnesium concentration of Mg–1Si alloy as shown in Fig.5(a)).It implied that the release of Mg2tions should not be
the Table3
Fig.7.Magnesium and alloying element concentrations of as-cast pure Mg and Mg–1X alloy(X?Al,Ag,In,Mn,Si,Sn,Y,Zn and Zr)samples in their extraction medium.
X.Gu et al./Biomaterials30(2009)484–498493
primary factor determining the hemolysis of Mg–1X alloys.(ii)Hydrogen,released during platelet adhesion test,may promote the platelet adhesion as the increased gas nuclei may affect the proteins and direct contact with platelet [58].(iii)Another factor in?uencing the hemolysis of Mg–1X alloys may be the large pH variation after 1h incubation in saline solution,since high pH coincided with high hemolysis percentage.For example,high corrosion rate could be seen for as-rolled Mg–1Al alloy which led to high pH,thus resulted in high hemolysis percentage,as shown in Fig.9.As reported in the study of hydrogen ion concentration of saponin hemolysis,the independent hemolytic effect of alkali had been found when the pH value was higher than 10[59].(iv)The release of alloying element ions within 1h was calculated from the results in Fig.5(b),(d).It was found that the concentrations of Al,Ag,In,Si,Sn,Y and Zn
were
Fig.8.Cell viability expressed as a percentage of the viability of cells in the control after 2,4and 7days of culture in as-cast pure Mg and Mg–1X alloy (X ?Al,Ag,In,Mn,Si,Sn,Y,Zn and Zr)extraction mediums:(a)L-929;(b)NIH3T3;(c)MC3T3-E1;(d)ECV304and (e)VSMC.
X.Gu et al./Biomaterials 30(2009)484–498
494
at 10à4M /L whereas those of Mn and Zr were at 10à5M /L.Given the low concentration of Mn,Zn and Al,these elements may not lead to hemolysis,due to the critical hemolytic concentration was found to be at 4?10à3M /L (Mn 2t),5?10à3M /L (Zn 2t)and 1?10à4M /L (Al 3t)[57],respectively.In the case of Mg–1Ag alloy,it was reported that the extent of hemolysis was greatest in erythrocytes incubated with Ag 2tand Hg 2t[60]and the critical hemolytic concentration of Hg 2twas 10à7M /L [57].This may explain why Mg–1Ag alloy induced high hemolysis percentage before and after hot rolling,the relatively higher Ag concentration at 10à5–10à4M /L level was shown in Fig.5(b),(d),(f)and (h).(v)In the present platelet study,increased adhesion of platelet was seen on the rough surface (e.g.as-cast Mg–1Sn alloy presented a more rough surface than that of as-rolled Mg–1Sn alloy).The microtexture could
result
Fig.9.Hemolysis percentage of as-cast and as-rolled pure Mg and Mg–1X alloy (X ?Al,Ag,In,Mn,Si,Sn,Y,Zn and Zr)
samples.
Fig.10.SEM images of platelets adhering to as-cast and as-rolled pure Mg and Mg–1Sn alloy samples,and platelet morphologies had been found for other Mg–1X alloy (X ?Al,Ag,In,Mn,Si,Y,Zn and Zr)samples similar to that of Mg–1Sn alloy samples.
X.Gu et al./Biomaterials 30(2009)484–498495
in increased surface area,which in turn could lead to increased plasma protein adsorption,and subsequent platelet adhesion and activation [61].
https://www.doczj.com/doc/ec6413826.html,ments on the in vitro cytotoxicity results of previously reported magnesium alloys from the viewpoint of corrosion products
In general,most of the investigated magnesium alloys were composed of two or more alloying elements (as listed in Table 1).While studying the effect of corrosion products in vitro biocom-patibility,we considered either the in?uence of individual alloying element metal ion,or the synergetic effects of Mg 2tand two or more alloying elements.On the one hand,our cytotoxicity results on the commonly used alloying elements by the binary Mg–1X alloy models may provide reference on the future multi-compo-nent magnesium alloys by combining these commonly used alloying elements.In this study,good cytocompatibility was seen for both Mg–1Al and Mg–1Zn alloys.The data could help under-stand the cell culture results of AZ system magnesium alloys,composed mainly of Mg,Al and Zn.For example,Pietak et al.[12]reported that AZ21substrates supported the adhesion,differenti-ation and growth of stromal cells towards an osteoblast-like phenotype with the subsequent production of a bone like matrix under in vitro conditions.Macrophages,osteoblasts and human bone derived cells were observed to adhere,proliferate and survive on the corroding surface of AZ91D in a cell culture system [47].On the other hand,the interactive effects need to be further examined by actual in vitro cell culture since it was reported that the combined toxicity of two metal cations did not necessarily exert their toxic effects independently [50].For example,the Mg–1Mn alloy indicated toxicity to all the cell lines in the present study,whereas Xu [24]found good in vivo biocompatibility and bone formation around Mg–Mn–Zn alloy implant.In addition,AMS stents (Mg with the addition of Y and rare earths)showed good compatibility to EC and SMC during in vitro and in vivo implanta-tion [3,53],even though the present Mg–1Y alloy extract induced signi?cant toxicity for blood vessel related cells (Fig.8(d),(e)).
4.3.Prospect on further composition design of biodegradable magnesium alloys used within bone and blood vessel
According to the ASTM nomenclature principles of magnesium alloys [30],22alloying elements (Al,Bi,Cu,Cd,RE,Fe,Th,Sr,Zr,Li,Mn,Ni,Pb,Ag,Cr,Si,Sn,Gd,Y,Ca,Sb,Zn)were documented for the engineering magnesium alloys,which could be classi?ed into ?ve groups from the material science and nutrition point of view.(i)Impurities (Fe,Ni,Cu).Fe,Ni and Cu were considered extremely
deleterious to corrosion because they had low solid-solubility limits in magnesium and provided active cathodic sites [38].(ii)Well-known toxic elements (Pb,Cd and Th).Pb was not only
nephrotoxic but also toxic to the hematopoietic and nervous systems [62].Cd exposure might result in effects at multiple sites,including the lungs,prostate,testes,heart,liver and kidneys [62].Cardiovascular effects of Cd,like hypertension,were also repeatedly described by investigators [62].The main hazard of exposure of Th was the effect of radioactivity [63].(iii)Nutrient elements in human (Ca,Cr,Mn,Zn,Sn,Si).Ca was an
essential macro-nutrient that had large minimal daily requirements,and also essential in chemical signaling with cells [64].Cr,Mn and Zn were considered to be essential micro-nutrients (unfortunately Cr was considered carcino-genic in humans Ref.[62]).Sn and Si had not yet been recog-nized by health authorities as essential to human nutrition,but believed to have some valid health bene?ts [65].
(iv)Nutrient elements found in plants and animal (Al,Bi,Li,Ag,Sr and
Zr).They were found in plant and animal tissue,yet their importance in human was still being determined.Aluminum was well-known as a neurotoxicant and its accumulation had been suggested to be an associated phenomenon in various neurological disorders as dementia,senile dementia and Alz-heimer disease [66].The toxicity of Bi appeared to be limited to medicinal applications where large amounts were admin-istered parentally or orally [63].Although Li salts had been used in the treatment of manic-depressive psychoses,it would induce toxicity as a result of the plasma Li concentration rising above the therapeutic range (plasma Li of 2m M /L
was
Fig.11.Number of platelets adhering on the surface of as-cast and as-rolled pure Mg and Mg–1X alloy (X ?Al,Ag,In,Mn,Si,Sn,Y,Zn and Zr)samples.
X.Gu et al./Biomaterials 30(2009)484–498
496
frequently associated with toxic symptoms;4mM/L might be fatal)[63].It was not quite elucidated whether Sr was essential for man,but the amount of0.3g Sr2tin bone possessed no danger[62].
(v)Others(Sb,Gd,Y and RE).Compounds of Sb,Gd,Y and rare earth elements were not biologically essential for human and were not found in the human body.There were controversies on the biological effects of rare earth element.On the one hand,it was reported that ionic RE easily form colloid in blood and the colloidal material was taken by phagocytic cells of the liver and spleen[67].In addition,rare earth ions induced hemolytic effect at a very low concentrations by inducing domain and pore formation of the erythrocytes membrane[68],ranging from3–17?10à7M/L[69],and the critical hemolytic concen-trations of La,Ce,Dy,Pr,which were used in WE43and LAE442
[17],were7.5?10à7M/L,9.5?10à7M/L,3?10à7M/L and
8?10à7M/L,respectively[69].Jha[70]indicated that there was signi?cant increase in the frequency of chromosomal aberrations in bone marrow cells after administration of Pr and Nd and severe hepatotoxicity had been detected after the administration of cerium and praseodymium[71].On the other hand,morality studies revealed that rare earth elements were not highly toxic(LD50values for iv-injected RE are10–100mg/ kg/bw and those of ip-injected RE are150–7000mg/kg/bw)
[67].Because RE have also been used directly in humans for
therapy for cancer and synovitis,more extensive studies, including chronic toxicity experiments,are required[67].
Based on the above discussions,the following elements were chosen to be primary candidates of alloying elements with magne-sium for the development of biodegradable magnesium alloys:Ca, Mn,Zn,Sn,Si,Al,Bi,Li,Sr,Zr,Sb,Y,RE.Besides the consideration of biocompatibility,another important factor needs to be considered was the total amount of additive elements implanted,since the biodegradable magnesium alloy implant would be an additional source of the metal ions,along with the normal diet.De?ciencies, excesses,or imbalances in the supply of inorganic elements could have an important deleterious in?uence on human health.
The biodegradable Mg alloy researches have been mostly focused on the bone and blood vessel related applications.From the above discussion,we can conjecture that(1)Al and Y might be good alloying elements for metallic magnesium alloy stent materials;(2) Ca,Zn,Sn,Si,Mn and Al might be good alloying elements for orthopedic Mg alloy implants.(3)More tests need to be done for their cytotoxicity and hemocompatibility before clinical usage for elements Zr,In and RE.
5.Conclusions
In this study,binary Mg-1wt.%X alloys(X?Al,Ag,In,Mn,Si,Sn, Y,Zn,Zr)were developed as experimental models to screen acceptable alloying element for biomedical magnesium alloy design.It was found the addition of Al,Si,Sn,Zn or Zr improved the strength of magnesium,and the addition of Al,In,Mn,Zn or Zr reduced the corrosion rate of as-cast Mg–X alloys in both SBF and the Hank’s solutions.However elements Si and Y had negative effect on magnesium corrosion properties.The cytotoxicity tests indicated that Mg–1Al,Mg–1Sn and Mg–1Zn alloy extracts showed no signi?cant reduced cell viability to?broblasts(L-929and NIH3T3);Mg–1Al,Mg–1Si,Mg–1Sn,Mg–1Y,Mg–1Zn and Mg–1Zr alloy extracts indicated no signi?cant toxicity to osteoblasts (MC3T3-E1);Mg–1Al and Mg–1Zn indicated no negative effect on viabilities of blood vessel related cells,ECV304and VSMC.In the hemolysis tests,Mg–1In,Mg–1Mn,Mg–1Si and Mg–1Y alloys induced less than5%hemolysis.The adhered platelets showed nearly round shape and slightly pseudopodia spreading,and the amount of adhered platelets was reduced after alloying for all Mg–1X alloys as compared to the pure magnesium control. Acknowledgement
This work was supported by the National Natural Science Foundation of China(No.30670560)and Program for New Century Excellent Talents in University(NCET-07-0033).The authors would like to thank associate professor Xinlin Li,Mr.Benli Wang,Guorui Ma and Bingbing Zhang(Harbin Engineering University,China)for their assistance in preparing the experimental as-cast and as-rolled materials,and thank Prof.Fuzhai Cui(Tsinghua University,China), Chun Yang(Beihang University,China)for providing VSMC and MC3T3-E1cells,and Zheng Yan(Peking University People’s Hospital,China),Shanshan Li,Enwei Zhang and Yanjie Bai(Peking University,China)for their assistance in the cytotoxicity tests and supplying blood samples.
Appendix
Figures with essential color discrimination.Many of the charts and graphs in this article are dif?cult to interpret in black and white.The full color images can be found in the on-line version,at doi10.1016/j.biomaterials.2008.10.021.
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钢结构防腐及防火 （1）Anticorrosion of steel structure 钢结构的防腐 The requirements of steel structure surface derusting, support equipment, frame beam column beam is Sa2.5 grade, Sa3 grade (rust and other components should be used in sandblasting), component cleaning should be immediately after spraying and coating primer matching. 钢结构的表面要求除锈，除锈等级框架梁柱、支撑、设备梁为Sa2.5级，其余构件Sa3级（除锈应采用喷砂），构件除锈后应立即喷涂与涂料相配套的底漆。 Anticorrosive coatings: 防腐涂料： Epoxy zinc rich primer 2, a total of 70um thick. 环氧富锌底漆2道，共70μm厚； Epoxy cloud iron intermediate paint 1, 70um thick. 环氧云铁中间漆1道，70μm厚； High chlorinated polyethylene or polyurethane finish 2~3 channel, total is of 60~100um. 高氯化聚乙烯或聚氨酯面漆2~3道，共60~100μm， The general steel plate by hot dip galvanized anti-corrosion coating; zinc coating thickness should be greater than 85um. 一般钢格板采用热浸锌防腐涂层，锌镀层的厚度应大于85μm。 Anticorrosion service life of steel structure: 钢结构防腐使用年限： All exposed steel structures shall be protected against corrosion besides fire protection. According to the general corrosion of steel structure anticorrosion Chemical Anticorrosion Coatings by atmospheric design, anti-corrosion treatment,
l Applications and Design l Engineering Analysis l Engineering Basics l Engineering Calculators l Engineering Materials Corrosion Definition - Corrosion and Galvanic Compatibility Corrosion and Galvanic Compatibility Knowledge Corrosion Testing Equipment Corrosion is the deterioration of a material due to interaction with its environment. It is the process in which metallic atoms leave the metal or form compounds in the presence of water and gases. Metal atoms are removed from a structural element until it fails, or oxides build up inside a pipe until it is plugged. All metals and alloys are subject to corrosion. Even the noble metals, such as gold, are subject to corrosive attack in some environments. The corrosion of metals is a natural process. Most metals are not thermodynamically stable in the metallic form; they want to corrode and revert to the more stable forms that are normally found in ores, such as oxides. Corrosion is of primary concern in engineering and design Corrosion occurs in every metal is subject to it. Even though this corrosion cannot be eliminated, it can be controlled. ? Copyright 2000 - 2009, by Engineers Edge, LLC All rights reserved. Disclaimer Engineering Solutions Industrial Project Management for critical applications https://www.doczj.com/doc/ec6413826.html, Corrosion damage?The specialist in surface treatment of stainless steel and mild steel! www.inox-ferro.nl Cortec Corporation Prevent, remove rust & corrosion VCI/VpCI film, emitters, paper, etc https://www.doczj.com/doc/ec6413826.html, Tankinetics Fiberglass Engineered FRP Equipment ASME RTP 1, Section X, tanks, pipe https://www.doczj.com/doc/ec6413826.html, Material Characterization Providing solutions for over 25 yrs SIMS, XPS, TOF-SIMS, SEM & more. https://www.doczj.com/doc/ec6413826.html,
不銹鋼常見腐蝕種類 1. 2.電流腐蝕（galvanic corrosion）或稱二金屬腐蝕（two-metal corrosion） 兩不同金屬在電解質溶液中接觸，當兩者的電位不同時，活性較大者將成為陽極，活性較小者將成為陰極，形成一個封閉回路，兩極間即有電流流動，造成電流腐蝕。電流腐蝕的大小，取決於兩不同金屬的電位差大小。 3.裂隙腐蝕（crevice corrosion） 裂隙腐蝕是發生在裂隙處的局部腐蝕，常見的裂隙處為搭接面（lap joint），止洩墊面（gasket）螺絲丁頭下，以及沈積物（deposit）下等。不論是金屬與金屬或金屬與非金屬接合面間隙，都可能發生裂隙腐蝕。 4.孔蝕（pitting） 孔蝕是局部的穿孔腐蝕，在金屬表面生成一個個或是許多密集的坑坑洞洞，深淺不一，使金屬表面看起來粗糙，但也只是一區一區的，並不是整個表面。 孔蝕的生成原因很多，最普通的一個是不清潔，金屬表面有灰塵、鐵銹、污垢等沈積物。 5.粒界腐蝕（intergranular corrosion） 晶粒邊界是液態金屬最後凝固的部分，其熔點最低，固體金屬熔解時，此部分也最先熔解。晶粒邊界也是高能量區，富有化學活性，所以金屬腐蝕時，也容易先由晶粒邊界開始。 6.選擇腐蝕（或稱分離腐蝕） 選擇固體合金中某一合金元素腐蝕。最常見的例子是黃銅（30﹪Zn＋70﹪Cu）因腐蝕而失去鋅，失去鋅的部位表面顯現出銅原有的紅色，肉眼即可辨別出紅色和黃色。所以也稱為失鋅（Dezincification）。 7.應力腐蝕（stress corrosion） 內有應力，外有J腐蝕媒體，聯合造成的金屬腐蝕，叫做應力腐蝕。應力腐蝕大多會發生裂紋，所以又稱為應力蝕裂（stress corrosion cracking，簡寫成SCC）。 應力腐蝕可能有兩種情況： (1) 應力促進的腐蝕（stress-accelerated corrosion ） (2) 應力蝕裂（SCC），是比較重要的一種情況。 8.沖蝕（erosion corrosion） 機件遇到流動的腐蝕流體（corrodent）所造成的腐蝕，叫做沖蝕。形成的要件有二，一是腐蝕媒體是流體（fluid），一是腐蝕媒體是流動的。腐蝕流體包括氣體，水溶液，有機溶液，和液態金屬。 與沖蝕有關的因素是： (1) 媒體的腐蝕性強弱。 (2) 流體中有無懸浮的固體顆粒，如泥漿（slury）。 (3) 流體的流動是穩定流（steady flow）或是亂流（turbulent flow），以及流速的大小。 9.其他腐蝕 腐蝕的種類很多有些少見的現象，是在無法觀察處漸漸進行，並非由顯著外力造成的物質敗壞，也可歸類於腐蝕。下面列出的就是此類。 (1)刃狀腐蝕（knife-line attack），簡寫為KLA (2)磨蝕（fretting corrosion） (3)熱變（thermal gradient） (4)絲狀腐蝕（filiform corrosion）
Pitting Corrosion G.S.Frankel,The Ohio State University Fig.1 Deep pits in a metal MANY ENGINEERING ALLOYS,such as stainless steels and aluminum alloys,are useful only because of passive ?lms,which are thin (nanometer-scale)oxide layers that form natu-rally on the metal surface and greatly reduce the rate of corrosion of the alloys.Such passive ?lms,however,are often susceptible to localized breakdown,resulting in accelerated dissolution of the underlying metal.If the attack initiates on an open surface,it is called pitting corrosion;at an occluded site,it is called crevice corrosion.These closely related forms of localized corro-sion can lead to accelerated failure of structural components by perforation or by acting as an initiation site for cracking.Figure 1shows an example of deep pits on a metal surface. It should be noted that,whereas localized dis-solution following breakdown of an otherwise protective passive ?lm is the most common and technologically important type of pitting corro-sion,pits can form under other conditions as well.For instance,pitting can occur during ac-tive dissolution if certain regions of the sample are more susceptible and dissolve faster than the rest of the surface.This section concentrates on the better-known and widely studied phenome-non of pitting corrosion of passive metals. Pitting corrosion is in?uenced by many dif-ferent parameters,including the environment,metal composition,potential,temperature,and surface condition.Important environmental pa-rameters include aggressive ion concentration,pH,and inhibitor concentration.Other phenom-enological aspects of localized corrosion include the stochastic nature of the processes and the stages of localized attack,including passive ?lm breakdown,metastable attack,stable growth,and perhaps eventual arrest. Phenomenology of Pitting Corrosion Environment and Development of Local Environment.Classical pitting corrosion caused by passive ?lm breakdown will only occur in the presence of aggressive anionic species,and chlo-ride ions are usually,although not always,the cause.The severity of pitting tends to vary with the logarithm of the bulk chloride concentration (Ref 1).The reason for the aggressiveness of chloride has been pondered for some time,and a number of notions have been put forth.Chlo-ride is an anion of a strong acid,and many metal cations exhibit considerable solubility in chlo-ride solutions (Ref 2).Chloride is a relatively small anion with a high diffusivity;it interferes with passivation,and it is ubiquitous as a con-taminant. The presence of oxidizing agents in a chlo-ride-containing environment is usually ex-tremely detrimental and will further enhance lo-calized corrosion.It should be noted that chromate is an oxidizing agent that typically in-hibits corrosion by reducing to form Cr(III)?lm.Most oxidizing agents enhance the likelihood of pitting corrosion by providing extra cathodic re-actants and increasing the local potential.Of course,dissolved oxygen is the most common oxidizing agent.One of the reactions by which oxygen reduction occurs is: O H O 4OH pH vs. SHE 22rev ++=???241230059e E →() ..(Eq 1) where SHE is standard hydrogen electrode.Removal of oxidizing agents,such as removal of dissolved oxygen by deaeration,is one pow-erful approach for reducing susceptibility to lo-calized corrosion.The in?uence of potential on pitting corrosion is described subsequently.Pitting is considered to be autocatalytic in na-ture;once a pit starts to grow,the local condi-tions are altered such that further pit growth is promoted.The anodic and cathodic electrochem-ical reactions that comprise corrosion separate spatially during pitting (Fig.2).The local pit en-vironment becomes depleted in cathodic reactant (e.g.,oxygen),which shifts most of the cathodic reaction (such as is given by Eq 1)to the boldly exposed surface outside of the pit cavity,where this reactant is more plentiful.The pit environ-ment becomes enriched in metal cations as a re-sult of the dissolution process in the pit (written for a generic metallic element,M): M r M n ??ne ? (Eq 2) The concentration of an anionic species such as chloride must also increase within the pit in order to balance the charge associated with the cation concentration and to maintain charge neu-trality.This enrichment of anions occurs by elec-tromigration from the bulk solution in response to the potential gradient that develops as a result of the ohmic potential drop along the current path between the inside of the pit and the cath-odic sites on the boldly exposed surface.The ?-nal aspect of the local pit environment that must be considered is the pH,which decreases,owing to cation hydrolysis: M H O M OH H O M OH H 22e e e 2222++ ++ ++Η++→()→()(Eq 3) The common cathodic reactions that must ac-company the dissolution occurring in the pit,such as the oxygen reduction reaction (Eq 1),result in a local increase in the pH at the cathodic sites.The acidity developed in the pit is not neu-tralized by the cathodic reaction because of the spacial separation of the anodic and cathodic re-actions.In summary,the local pit environment is depleted in the cathodic reactant,such as dis-solved oxygen;enriched in metal cation and an anionic species,such as chloride;and acidi?ed.This acidic chloride environment is aggressive to most metals and tends to prevent repassivation and promote continued propagation of the pit.
1) pit corrosion depth 点蚀深度 In combination with corrosion analysis, especially with the pit corrosion analysis, a model for calculating collecting line pit corrosion depth and a model for statistically predicting the leak off in the collecting line caused by pit corrosion are established on the basis of corrosion investigation. 以腐蚀分析特别是局部点蚀分析为基础,以腐蚀调查为依据,建立了集油管道点蚀深度的统计模型,以及集油管道点蚀泄漏的统计预测模型。 2) maximum pit depth 最大点蚀深度 Based on the corresponding pitting corrosion mechanics and Melchers pitting corrosion model, a simplified maximum pit depth model is proposed, which will be more concisely to predict the statistical characteristics of maximum macro-pit depth. 此外,依据青岛、厦门、榆林海域碳钢试验数据,文中还对海水环境因素,如:溶解氧浓度、平均温度、盐度、PH值等,以及钢材成分变化对新型最大点蚀深度模型各参数的影响进行了初步探讨,得出了相应的函数关系式及相关结论。 3) pitting 点蚀 The effects of two types of electrode pitting morphologies,ring type and hole type on resistance spot welding of aluminum alloy 5182 were investigated by the combination of finite element analysis and physical modeling methods. 采用有限元分析和物理模拟相结合的方法,研究了环形和孔形两种电极点蚀形貌对铝合金AA5182电阻点焊的影响。 2.Results show that the material is 316 stainless steel,and the failure was due to pitting induced by chloride which was enriched from the enviroment in the medium in local areas of it. 针对某炼油厂加氢裂化装置中的循环氢压缩机出口仪表引压管的氢泄漏事故,对该管的材料成分、金相组织、腐蚀状况、裂纹及断口等进行了分析.结果表明,该管的材质为316不锈钢;失效原因是由于在其管外壁的局部环境中氯化物的富集而发生点蚀,进而引发管子应力腐蚀开裂,最终造成管内氢气的泄漏 3.On the other hand,it is demonstrated that pitting sensitivity of Al-6Mg-Zr-Sc alloy in the SRB solution is decreased and its microbiologically influenced corrosion r. 结果表明,添加Sc元素后,合金的氧化膜更加致密,具有更好的保护作用,使电极开路电位正移100 mV左右,同时Sc元素的加入降低了材料在SRB菌液中的点蚀敏感性,使材料耐微生物腐蚀性能得到提高,但是添加Sc后的合金对SRB更加敏感。 4) pitting corrosion 点蚀 A computational method,FEM is adopted to calculate the collapse load of pressurized elbow with pitting corrosion,by taking the immense deformation effects and stress stiffening into consideration. 应用有限元分析软件ANSYS,对含点蚀缺陷的弯管在内压载荷作用下进行有限元分析,分析中考虑材料非线性和几何非线性,根据腐蚀区的载荷-应变图,对模型的
MATERIALS FORUM VOLUME 30 - 2006 Edited by R. Wuhrer and M. Cortie ? Institute of Materials Engineering Australasia Ltd CORROSION AND MATERIALS SELECTION FOR AMINE SERVICE S. Rennie Woodside Energy Ltd ABSTRACT Amines are used in refineries and gas plants around the world to remove both H2S and CO2 from feed gas. The corrosion in amine plants is not caused by the amine itself, but is caused by the hydrogen sulphide, CO2 and by amine degradation products. Amines are compounds formed by replacing hydrogen atoms of ammonia, NH3 by organic radicals. Problems with amine units can be corrosion or by cracking (sulphide stress cracking, hydrogen induced cracking associated with hydrogen blistering, stress-orientated hydrogen induced cracking, and alkaline stress corrosion cracking). Areas to have a close look at is where impingement erosion could occur, where there is elevated temperatures, where flashing off of CO2 could occur, and where heat stable salts could cause scaling. Carbon steel and stainless steel are suitable for most applications, with the choice of material being determined by the susceptibility to the damage modes in each location. Careful consideration should also be given to the repair of plant (controlling weldment hardness) 1. INTRODUCTION Amines are used in refineries and gas plants around the world to remove both H2S and CO2 from feed gas. The gas may be either feed gas in terms of a Gas plant, or it could be off-gases and olefinic and saturated liquefied Petroleum Gas (LPG) streams generated during the operation of Catalytic Crackers1. Sulphur recovery units in Refinery operations also use amines. CO2 can cause problems in gas processing plants and refineries alike. It may cause problems in hydrate formation, and affect specification of products such as ethylene in gas cracking units2 . The corrosion in amine plants is not caused by the amine itself, but is caused by the hydrogen sulphide, CO2, and by amine degradation products (Metals Handbook, Vol 13). The interest in amine units has been particularly strong by NACE and the like since the Unocal disaster in 1984. Amines are compounds formed by replacing hydrogen atoms of ammonia, NH3 by organic radicals. The chemistry of acid gas removal by amine solutions is relatively complex, but the simplified reactions (exothermic) are: H2S + R2NH ←→ R2NH2+ + HS- CO2 + R2NH ←→ R2NH2+ + R2NCOO- Types of Amines used include primary amines (NH2R) such as Methanolamine (MEA) and diglycolamine (DGA), and secondary (NHR2) and tertiary amines (NR3) such as diethanolamine (DEA), diisopropanolamine (DIPA) and methyldiethanolamine (MDEA). In the case of the Methanolamine (MEA), the methanol molecule loses an H bond and attaches to the amine molecule (which also loses an H bond. The formula is CH2OHNH2. Secondary amines such as diethanolamine (DEA) and diisopropanolamine (DIPA) are similar except that there are two alcohol molecules for the one amine molecule (i.e NHR2), where the R represents the alcohol radical. A tertiary amine has 3 alcohol radicals to the N atom (i.e NR3). MEA has been reported to be responsible for stress corrosion cracking (SCC) failure of vessels in the late 1980’s3. Conversion to MDEA eventually leads to low corrosion rates and elimination of SCC 2. PROBLEMS IN AMINE UNITS Problems with amine units can be split in to two categories: Corrosion Corrosion of amine units is usually not caused by the amine itself, but more the dissolved acid gases, primarily hydrogen sulphide and carbon dioxide. Heat stable salts resulting from the amine degradation can also cause corrosion problems. API RP945, Avoiding Environmental Cracking in Amine Units,4 indicate that, in low pressure systems, corrosion of amine units used primarily to remove carbon dioxide has been historically more severe than that of units used to remove either hydrogen sulphide, or carbon dioxide and hydrogen sulphide. This is supported by Schmeal et al5. In high pressure systems
A useful tool in determining corrosion resistance is the "Y" of corrosion shown in Figure 1. This chart divides the alloys into three classes: those resistant to oxidizing acids on the left, those resistant to reducing acids on the right, and those resistant to a mixture of the two in the center. Oxidizing acids are those acids that oxidize the metals they come in contact with, and are themselves, reduced in the process. Reducing simply dissolves the metal without a change in valence or a release of hydrogen in the process. Corrosion resistance increases as you move up the chart. This chart indicates relative corrosion resistance. By using the published tables of general corrosion rates, it is possible to determine the resistance of a given alloy to a given environment. The Corrosion Data Survey or the computer program, Corsur-both published by the National Association of Corrosion Engineers (NACE)-are excellent resources. Alloy selection can be simpli-fied, or at least narrowed down, using these tables. Corrosion tables are based on isocorrosion curves. An isocorrosion curve for type 316 stainless steel in sulfuric acid is presented in Figure 2 (page 8). This curve shows the variation in corrosion rate with temperature and concentration. Similar curves are available for most alloys in many media, and generally are available from reputable material producers. Figure 2:Isocorrosion curve for Type 316 in sulfuric acid at temperatures up to 350oF (175oC) The boiling point curve represents the boiling point of the sulfuric acid -- water https://www.doczj.com/doc/ec6413826.html,ls per year is 0.001 x mpy = inches per year. Figure 1:The “Y” of uniform corrosion.Increasing chromium content on the left means increasing corrosion resistance to oxidizing acids, such as nitric or citric.Increasing alloy content on the right indi-cates increasing resistance to the halide ions or reducing acids such as hydrochloric acid. When both the chromium and molyb-denum content increase, as in the center,resistance to both types of acids increases. 10
CORROSION COUPON MANUAL
CORROSION COUPON MANUAL CONFIDENTIAL CORROSION COUPONS Overview Corrosion coupons have long been used as a monitoring tool in cooling water systems. The information they provide has been used to evaluate current treatment programs as well as to compare current results with those seen in the past. By comparing the final coupon weight with the initial weight, it is possible to calculate a projected annual mils per year (mpy) corrosion rate. Historically, this has been the primary way in which coupons were used. There is, however, far more information that can be gathered from a corrosion coupon than corrosion rate. Some indication of scaling tendencies as well as the deposition resulting from corrosion can be observed upon visual examination of the coupon surface. Microbiologically Influenced Corrosion (MIC) can also be observed on these test strips. This manual of photographs has been assembled to assist in the evaluation of cooling water treatment performance. An attempt was made to secure a cross-section of coupon conditions for inclusion in this manual. Conditions such as dezincification, microbiologically influenced corrosion, cathodic protection, and fouling are presented. Included with each set of photographs is an explanation of the probable cause of the condition observed and appropriate recommendations. It is hoped that the field person, by comparing actual coupons to these photographs, will be able to give a reasonably accurate evaluation of the cause of the condition. Remember, however, that coupons are only one tool in evaluating program performance. Coupon corrosion performance must be assessed in relation to water chemistry, treatment residuals and control of operating parameters (pH, halogen, cycles, etc.). It is important to remember how to properly use corrosion coupons. They are most valuable when used to compare with one another and to evaluate such things as program changes or system upsets. By maintaining a constant size, metallurgy, location, water flow and replacement time, the only variable becomes the water condition.