A Mg alloy with no hydrogen evolution during dissolution

Hydrogen evolution is normally associated with the corrosion or dissolution of Mg alloys in aqueous solutions.This work studied the corrosion behavior of sputtered pure Mg,Mg82Zn18(at.%),Mg64Zn36(at.%),and pure Zn in 3.5%Na Cl solution.Mg64Zn36had(ⅰ)an amorphous microstructure with some nano-scale grains,(ⅱ)a corrosion rate substantially lower than that of pure Mg,and(ⅲ)no hydrogen evolution during corrosion or anodic dissolution,because the positive corrosion potential retarded the cathodic hydrogen evolution.This is a new route to prevent hydrogen evolution during Mg corrosion,which has never previously been realized.

Designing strategy for corrosion-resistant Mg alloys based on film-free and film-covered models

Mathematical models were proposed to clarify the effect of alloying on corrosion of magnesium alloys based on film-free and film-covered status. The models are applicable to explain the “barrier effect” by cathodes and the “analogous Hall-Petch relationship” between corrosion rates and grain size. The slope of corrosion rates versus alloying content is determined by the dissolution ability of film-free substrate and the hindering effects by corrosion product film. Designing strategy for corrosion-resistant Mg alloys is established.

Advances in Mg corrosion and research suggestions

Recent research is summarised with an emphasis on the use of Mg alloys for biodegradable medical applications.Mg melt purification using Zr has been shown to provide the opportunity to produce ultra-high-purity Mg alloys,which could lead to stainless Mg.Nor's solution may be a good starting model for the study of Mg for biodegradable medical implant applications.A systematic laboratory investigation is needed to elucidate the details of how the corrosion behaviour is controlled by the various constituents of the body fluids.In the evaluation of the Mg corrosion mechanism there is a critical lack of understanding of(i)the amount of hydrogen dissolved in the Mg metal during corrosion,and during anodic polarisation,and(ii)the size film-free area where corrosion occurs,and how to measure this area.In the evaluation of the apparent valence of Mg using an applied anodic current density,for reliable values,it is important to apply a sufficiently large applied current density.The available data are consistent with the slightly modified uni-positive Mg^(+)ion mechanism,which maintains that(i)the surface of Mg is covered by a partially protective film,and the film-free area increases as the potential becomes more positive(i.e.a catalytic activation process),(ii)corrosion occurs preferentially at breaks in the partial protective film,(iii)corrosion at the breaks in the partially protective film involves the uni-positive Mg ion,(iv)undermining of particles occurs when Mg is severely dissolved,and(v)there may be some self-corrosion not covered by these four processes,which may be associated with crevice-like features on a severely corroded surface or hydride dissolution at relatively negative potentials.Self-corrosion might also be possible under condition of essentially uniform corrosion.Mg^(+)has not been experimentally observed.Its existence is postulated as an extremely-short lifetime intermediate in the reaction sequence between metallic Mg and the equilibrium ion Mg^(++).There has been no direct experimental examination of this sequence,and a key challenge remains to devise an experimental approach to study the details of this reaction sequence and the intermediate steps.The apparent valence of Mg continues to be a critical question.If defendable values of effective valence for Mg less than 1.0 were measured,this would indicate that some phenomena contribute to these low values that are not currently accounted for in the uni-positive Mg^(+)corrosion mechanism.The most likely candidate would be self-corrosion.

Corrosivity of haze constituents to pure Mg

The corrosion behavior of pure Magnesium(Mg)in a Mg(OH)2-saturated solution containing different individual constituents of PM2.5 in haze were studied by hydrogen evolution,weight loss and electrochemical experiments.The results indicated that the corrosivity of these constituents to pure Mg decreased in the following order:(NH4)2SO4>Haze-contaminated-solution>NH4NO3>NH4Cl>NaCl≈KCl≈Na2SO4≈MgCl2≈CaSO4>Mg(OH)2(basic solution)>Ca(NO3)2.Possible mechanisms behind the different corrosion behaviors of Mg in response to these constituents were also briefly discussed in this paper.

Stress corrosion cracking of high-strength AZ31 processed by high-ratio differential speed rolling

Stress corrosion cracking(SCC)in distilled water was studied for AZ31,processed by differential-speed-rolling to different strengths,using Linear Increasing Stress Tests(LISTs).The stress corrosion crack velocity was 5.0±2.5×10^(−9) m s^(−1),independent of applied stress rate and independent of material strength.SCC susceptibility was greater at lower applied stress rates manifest most importantly as a lower threshold stress for stress corrosion crack initiation.SCC susceptibility could be characterised by the ratio of threshold stress to yield stress,which was dependent on processing details and was as low as 0.3.

A Green Conversion Coating on a Magnesium Alloy for Corrosion Protection

A novel coating on the Mg1Mn alloy was produced by anodic polarization combined with hydrothermal treatment(AP+H)in 0.1 M Na2CO3 solution.The microstructure and protection of the coating were evaluated.The coating consisted of MgCO3,Mg(OH)2 and MgO,and provided satisfactory protection in 3.5 wt%NaCl with a corrosion rate of 0.07 mm y−1 in 72 h.However,after that period,the corrosion rate of the specimen increased due to the damage of the coating.The failure of the coating was strongly related to the second phase particles(e.g.Zr particles)or impurities in the matrix.The AP+H coating is supposed to be used as a primer coating for Mg applications in kitchen ware,biomedical areas or industry.

Review of the atmospheric corrosion of magnesium alloys

Mg atmospheric corrosion is induced by a thin surface aqueous layer. Controlling factors are microgalvanic acceleration between different phases, protection by a continuous second phase distribution, protection by corrosion products, and degradation of protective layers by aggressive species such as chloride ions. The Mg atmospheric corrosion rate increases with relative humidity (RH) and concentrations of aggressive species. Temperature increases the corrosion rate unless a protective film causes a decrease.O2, SO2 and NO2 accelerate the atmospheric corrosion rate, whereas the corrosion rate is decreased by CO2. The traditional gravimetric method can evaluate effectively the corrosion behavior of Mg alloys.

A state-of-the-art review on passivation and biofouling of Ti and its alloys in marine environments

High strength-to-weight ratio, commendable biocompatibility and excellent corrosion resistance make Ti alloys widely applicable in aerospace, medical and marine industries. However, these alloys suffer from serious biofouling, and may become vulnerable to corrosion attack under some extreme marine conditions. The passivating and biofouling performance of Ti alloys can be attributed to their compact, stable and protective films. This paper comprehensively reviews the passivating and biofouling behavior, as well as their mechanisms, for typical Ti alloys in various marine environments. This review aims to help extend applications of Ti alloys in extremely harsh marine conditions.