The galvanic current increases with the potential difference, that is, the bigger potential difference between Mg and dissimilar metal, the easier for Mg subject to corrode. The intensity of corrosion depends on the relative Volta potential differences of the micro-constituents, as well as their amounts, compositions and distributions within the alloys by KPFM [ 10 ].
Novel Approaches to Improving High Temperature Corrosion Resistance
The variations of pH may change the electrode reaction and polarity of galvanic metals. For instance, in an Al-Mg galvanic coupling in a neutral or weakly alkaline NaCl solution, Al acts as cathode. But with the dissolution of Mg, the solution changes into alkaline and aluminum turns into anode, which is the alkalinized effect of corrosion products of Mg alloys. Mg alloys in acidic and neutral solutions, except for chromic acid, phosphoric acid and hydrofluoric acid that can form passive film, are susceptible to corrosion.
The distance between galvanic couples is also important for corrosion behavior of Mg alloys. The distance of the galvanic current of the corrosion behavior of Mg alloys is limited. Corrosion generally occurs near the edge of such a specimen [ 11 ]. The declining amplitude decreases with the increase in the distance similarly to an exponential decay.
Pitting corrosion is one of the most common corrosion types of Mg alloy. The generation of pits is related to chemical composition and specific microstructural features at the exposed Mg alloy surface. Chemical composition, grain size, heat treatment condition, morphology and distribution of the second phases all have an important influence on the corrosion of Mg alloys. In a subsequent immersion, the Zr-rich zones located in the center of grains become preferential corrosion sites.
Small pits appear in the zones and severe attack develops. Occasionally, pitting corrosion is competitive when a number of second phases exist in Mg alloys. And corrosion preferentially occurs around AlMnFe second phase due to the fact that the potential difference between the AlMnFe phase and matrix is the biggest [ 18 ]. For Mg-Al alloys, pitting corrosion occurs along the net structure of Mg 17 Al 12 selectively. At first, corrosion pits were formed in Mn-bearing particles, then developed into corrosion cracks and extended and terminated in the high Al region or GBs of the crystals.
Corrosion Types of Magnesium Alloys
Pitting corrosion can be affected by grain size. Refined grains make the corrosion morphology from pitting to homogeneous or uniform corrosion. In addition, there are a small number of Mg, Si elements and a small amount of deformation twins. The AlMn phase is major in the grain that forms a cubic shape. There are both fine and coarse grains in the microstructure. Simultaneously, recrystallized grains form in the coarse grains. This result is due to the hot extrusion processing.
The thick plate has a faster cooling rate for the outer layer and relatively slower cooling rate in the interiors, so localized recrystallization and grain abnormal growth phenomenon may occur. In addition, pH value has a great influence on corrosion morphology of Mg alloys [ 19 ]. Thus, pitting corrosion of Mg alloy AM60 occurs. In alkaline solutions, uniform corrosion of high alumina regions is observed with cellular corrosion morphology.
From the morphology of the pits, the site of incipient pitting corrosion is related to the AlMn phase. Here, the corrosion pits of Mg alloys are hemispherical. This is the so-called occluded cell. When an extruded Mg The pitting corrosion occurs on as-cast dual-phase Mg-Li-Ca alloy due to the presence of Mg 2 Ca particles, whereas uniform corrosion appears after an extrusion processing [ 20 ]. Schematic diagrams of the corrosion mechanisms of Mg-9Li-1Ca alloys: a pitting corrosion on the cast alloy and b tubular paths in the oxide film on the extruded alloy were jammed by corrosion products .
Pitting corrosion of Mg-Li alloys can be suppressed through grain refinement and formation of quasicrystalline or icosahedral phase I-phase.
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Filiform corrosion is also the most common type of corrosion for Mg alloys. Filiform corrosion of steel surfaces often occurs under coatings in high humidity environments.
The filiform corrosion of Mg alloys is as common as pitting corrosion, but different from filiform corrosion of steels. Filiform corrosion is common on bare Mg alloy surfaces exposed to air and NaCl solution.
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The pitting corrosion and filiform corrosion of Mg alloys usually occur simultaneously. Filiform corrosion originates from corrosion pits and extends forward along the active area. Corrosion of Mg alloys mainly is hydrogen evolution corrosion rather than oxygen absorption corrosion or oxygen concentration corrosion.
Filiform corrosion is caused by active corrosive cells moving through the metal surface. Filiform corrosion of Mg is driven by the difference in oxygen concentration between filament head and filament tail, and thus proposed a filiform corrosion model for Mg [ 22 ]. Earlier literature reported that filiform corrosion occurs under protective coatings and anodic oxide layers, and pure Mg without coatings does not undergo filiform corrosion [ 1 , 23 ].
Chloride-induced fliform corrosion of organic-coated Mg is studied by Williams et al. The rate of filiform corrosion advance is shown to be insensitive to the presence of oxygen, but highly dependent upon the relative humidity of the holding environment. The hydrolysis reaction acidifies the corrosion pits and becomes a filament tip. Then the tip continues to propagate forward. An earlier review [ 26 ] pointed out that IGC does not occur on Mg and its alloys.
It is regarded that the attack does not enter the grain along GBs, due to the fact that GBs are almost always cathodic relative to the grain interiors. Therefore, grains are anodic with regard to GBs, and the area next to GBs will be corroded. But this is not regarded as IGC, but granular corrosion, because corrosion tends to concentrate in areas adjacent to GBs. However, intergranular stress corrosion cracking ISCC has been observed in the process of stress corrosion of Mg alloys, which indicates that IGC of Mg alloys is truly possible. The depth of this corrosion profile is shallow.
Thus, pearl-like corrosion pits have been observed along the GBs in the extrusion direction. Because zirconium is mainly distributed in the center of grains, the crystal is relatively stable. Further study discloses that the attack mainly occurs on the periphery of the grains, and no attack is seen to penetrate into the alloy along GBs. EFC on wrought Al alloys has been extensively reported [ 28 , 29 , 30 ].
Two essential factors exist in developing EFC: elongated grains in microstructure and different potential between the matrix and intermetallic compound precipitates at GBs [ 31 ]. However, EFC of Mg alloys has sparsely been reported. At the initial stage of corrosion, oxide film formed rapidly on the surface of magnesium alloy.
There corrosion micro-cracks obviously emerge along GBs, which eventually results in denudation. After annealing, the corresponding EFC disappeared. The introduction of Al suppresses the exfoliation behavior of MgLi alloy. The excessive addition of Al leads to a decrease in corrosion resistance. Recently, the corrosion morphologies of an as-extruded Mg-1Li-1Ca alloy have been investigated after immersion in 3.
Novel Approaches to the Improvement of High Temperature Corrosion Resistance
Corrosion macro-morphology of extruded Mg-1Li-1Ca specimens immersed in 3. The cracks initiate along GBs or inside the grains, then coalescence, and arrest and propagation as well. The formation of voids or pores may be due to hydrogen blister and wedge effect resulted from the increase in volume of corrosion products. Optical image of cross-sectional morphologies of extruded Mg-1Li-1Ca alloy after an immersion in 3. Macro-galvanic corrosion may, thus, occur between the inner and outer layers.
Values of corrosion potential E corr and corrosion current density i corr were estimated through Tafel extrapolation fitting. For the interior zone, the Volta potential of extruded Mg-1Li-1Ca alloy fluctuates over a narrow range, while the skin layer in purple color corresponds to the most negative Volta potential. The Volta potential difference implies the possible presence of macro-galvanic corrosion between the interior zone and skin layer.
The corrosion current density, i corr , for the inner layer and the outer layer of the extruded Mg-1Li-1Ca alloy is 5. The skin layer with more refined grains shows better corrosion resistance, whereas the interior with coarse grains and Mg 2 Ca particles distribute along GBs and extrusion direction in a line. The inner zone exhibits a negative OCP in comparison with the skin layer.
The papers are divided into five topic areas: mechanisms; biofilms; prevention and control; case histories; and other. This book will be of interest to those already in the field and to the new generation of researchers and engineers. Inhibitors, organic coatings, inorganic coatings, passivity, and evaluation in special environments. New areas covered include high temperature corrosion, electrochemical and pre-normalisation research. The three titles included in the video are Corrosion Prevention by Design minutes , Corrosion Control by Preventive Coating minutes and Corrosion Prevention by Changing the Environment minutes.
A useful tool for the materials engineer providing a guide to materials selection and methods for life time estimation of real components in service. Includes microbial corrosion of copper, stainless steel, aluminium brass, carbon steels etc. JiscMail Tools. RSS 1. It also reviews research on the behavior of alloys in a wide range of service conditions such as furnace and boiler environments. The final part of the book discusses ways of modelling high-temperature corrosion processes to improve material performance and service life.
With its distinguished editors and team of contributors drawn from some of the leading centers of research in the field, Novel approaches to improving high-temperature corrosion resistance will be a standard reference for all those studying and dealing with high-temperature corrosion. Better World Marketplace. New ".
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