Effect of air-formed film on corrosion behavior of magnesium-lithium alloys

It is recently suggested that air-formed film plays an important role in controlling corrosion resistance of Mg-Li alloys. However, the structure of the air-formed film and its effect on corrosion resistance of Mg-Li alloys has not been fully understood. Firstly, the air-formed films formed on α and β phases in a dual-phase LZ91 Mg-Li alloy after exposure to laboratory air for up to 48 h have been examined by SEM under the assistance of ultramicrotomy. Then, the effect of the air-formed film on surface potential and, consequently, corrosion/oxidation behavior of the alloy has been investigated. Finally, in order to exclude the influence from α phase, the structure of the air-formed film on β phase and its effect on corrosion/oxidation behavior of Mg-Li alloys have been studied based on a single-phase LA141 Mg-Li alloy. The results show that the air-formed film is thin and negligible on α phase but thick on β phase after prolonged exposure to laboratory air. The thick air-formed film on β phase has a multilayer structure with an inner layer consisting of Mg O/Mg(OH)_(2) and outer layer consisting of Li_(2)CO_(3), which greatly elevates the surface potential of β phase in air. Both LZ91 and LA141 Mg-Li alloys firstly undergo uniform corrosion and then filiform corrosion when immersed in Na Cl solution and the pre-existed air-formed film on β-Li phase can retard the occurrence of filiform corrosion in the alloys.

Second phase refining induced optimization of Fe alloying in Zn:Significantly enhanced strengthening effect and corrosion uniformity

Many non-toxic alloying elements,such as Fe,Ca,and Sr,have negligible solid solubilities in Zn matrix,leading to formation of coarse second phase particles.They exhibit low strengthening effects but highly detrimental to ductility.So refining second phase is a common pursuit for Zn alloys.The present paper takes Zn-0.3Fe alloy suffered from coarse FeZn_(13) second phase particles as a touchstone to testify microstructure refining effect through solidification with an accelerated speed and multi-pass rolling.FeZn_(13) particles are refined from 24 to 2μm,and Zn grains are refined to 5μm.As a result,the strengthening effect of Fe is enhanced significantly,with yield strength and the ultimate tensile strength of the alloy increased from 132 to 218 MPa and from 159 to 264 MPa,respectively.Furthermore,corrosion non-uniformity and penetration are much alleviated.These results show that microstructure refinement,especially on coarse intermetallic second phases,has a great potential to improve mechanical and degradation properties of biodegradable Zn alloys.

Corrosion behavior of Mg-3Gd-1Zn-0.4Zr alloy with and without stacking faults

To develop biodegradable magnesium alloy with desirable corrosion properties,a low Gd-containing Mg-3Gd-1Zn-0.4Zr(wt%,GZ31K)alloy was prepared.The as-cast ingot was solution treated and then hot extruded.Microstructures were characterized by scanning electron microscopy(SEM).Corrosion behavior of the alloy under each condition was studied by hydrogen evolution and quasi in-situ corrosion methods.It has been found that the as-cast alloy is composed ofα-Mg,stacking faults(SFs)at the outer edge of the matrix grains,and eutectic phase along the grain boundaries.After solution treatment,the SFs disappear and precipitates rich in Zn and Zr elements form in the grain interior and boundaries.The microstructure is significantly refined after extrusion.Hydrogen evolution tests show that the as-cast alloy exhibits the best corrosion resistance,and the solution-treated alloy has the worst corrosion resistance.Corrosion rate of the alloy under each condition decreases first and then increases with prolonging immersion time.Corrosion experiments demonstrate thatα-Mg was corroded preferentially,the eutectic phase and precipitates exhibit better corrosion resistance.The as-extruded alloy demonstrates uniform corrosion due to fine and homogeneous microstructure.

Progressive Collapse Analysis of an FPSO Vessel Hull Girder Under Vertical Bending Considering Different Corrosion Models

Nowadays,with the increasing operational life of ships,the aging effects on their structural behavior need to be investigated precisely.With the corrosive marine environment taken into consideration,one of the important effects of aging that must be studied is thickness degradation.In this paper,with the use of previously proposed equivalent thickness formulations for corroded plates,the progressive collapse analysis software HULLST is enhanced,and then,the effects of different corrosion models of uniform,random,pitting,and tanker pattern types on the ultimate and residual strengths of a floating production,storage,and offloading vessel hull girder are evaluated for the ages of 0 to 25 years.Results reveal that the uniform corrosion and random corrosion models have close outcomes.The value of relative reduction in the ultimate strength of ship hull girder(compared with the intact condition)ranges roughly from 6%for the age of 5 years to 17%for the age of 25 years in the hogging mode.The relative reduction in the ultimate strength ranges from 4%to 16%in the sagging mode.Pitting corrosion and tanker pattern(random)corrosion models lead to higher relative reductions in ultimate strength.The pitting corrosion model leads to a 16%–32%relative reduction in the ultimate strength for the ages of 5–25 years of the ship in either hogging or sagging.The tanker pattern(random)corrosion model leads to a 6%–37%relative reduction in the ultimate strength in the hogging mode and 3%–31%in the sagging mode at ship ages of 5 to 25 years.

Opportunities and challenges for the biodegradable magnesium alloys as next-generation biomaterials

In recent years,biodegradable magnesium alloys emerge as a new class of biomaterials for tissue engineering and medical devices.Deploying biodegradable magnesium-based materials not only avoids a second surgical intervention for implant removal but also circumvents the long-term foreign body effect of permanent implants.However,these materials are often subjected to an uncontrolled and fast degradation,acute toxic responses and rapid structural failure presumably due to a localized,too rapid corrosion process.The patented Mg-Nd-Zn-based alloys(JiaoDa BioMg[JDBM])have been developed in Shanghai Jiao Tong University in recent years.The alloy series exhibit lower biodegradation rate and homogeneous nanophasic degradation patterns as compared with other biodegradable Mg alloys.The in vitro cytotoxicity tests using various types of cells indicate excellent biocompatibility of JDBM.Finally,bone implants using JDBM-1 alloy and cardiovascular stents using JDBM-2 alloy have been successfully fabricated and in vivo long-term assessment via implantation in animal model have been performed.The results confirmed the reduced degradation rate in vivo,excellent tissue compatibility and long-term structural and mechanical durability.Thus,this novel Mg-alloy series with highly uniform nanophasic biodegradation represent a major breakthrough in the field and a promising candidate for manufacturing the next generation biodegradable implants.

One-step PDA coating strategy on pure Zn for blood-contacting engineering

The mechanical performance degradation caused by pitting corrosion and the insufficient biocompatibility induced with ion release inhibit the application of Zn as the new biodegradable metallic materials for blood-contacting intervention therapy.This work successfully coats a uniform polydopamine(PDA)layer on pure Zn via a one-step deposition process without introducing heavy metal elements or detrimental additives.Enhanced corrosion resistance and uniform corrosion have been performed on PDA coated Zn with corrosion products and pits reduced significantly than pure Zn during in vitro degradation.Thrombosis could be prohibited by anticoagulation performance with prothrombin time and activated partial thromboplastin time prolonged after co-incubation,and an almost 40%decrease of platelets adhesion,indicating the improved blood compatibility of coated Zn.Additionally,the poly(lactic-co-glycolic acid)(PLGA)is filmed on PDA coated Zn to realize a smooth double-coated structure with superior mechanical performance under stress loading to stably serve the multifunctional application,such as drug loading.Our strategy not only represents a facile and biological friendly method for Zn surface modification but also puts forward new ideas on preparing PDA-based coatings on active biodegradable alloys,which are beneficial for better blood-contacting engineering.