CORROSION CHARACTERISTICS OF MG ALLOYS Guang-Ling Song Chemical Sciences and Materials System Laboratory, GM Global Research and Development Mail Code: 480-106-224 30500 Mound Road, Warren, MI48090, USA 586-986-1339,
[email protected]
ABSTRACT Magnesium alloys are great light structural and functional engineering materials for many applications. However, they have low corrosion resistance and exhibit unusual electrochemical corrosion behaviour in aqueous environments. This paper briefly summarizes their corrosion characteristics, such as hydrogen evolution, surface alkalization, galvanic corrosion, preferential corrosion of the matrix phase, secondary phase effect, and impurity influence. Keywords: Mg alloy, Corrosion
INTRODUCTION Mg and its alloys have low density, high strength, great damping capability, excellent fluidity for casting, good electric shielding effect, non-magnetic, satisfactory heat-conductivity, low heat capacity, negative electrochemical potential, acceptable recyclability and non-toxic. Therefore, Mg alloys are very attractive to industries where the strength/weight ratio is a critical issue, and have already found many applications in these industries. However, the application of Mg alloys in the automotive, aerospace and electronic industries is currently limited because of their poor corrosion resistance of the existing Mg alloys[1, 2, 3, 4, 5]. A large number of studies have been carried out to address the corrosion issue in order to improve the corrosion performance of Mg alloys. Many published results have clearly suggested that the corrosion of Mg and its alloys is quite special in terms of its electrochemical behaviors. This paper will systematically summarize the electrochemical corrosion
characteristics and behavior of Mg and its alloys. It is expected that such a summary will help researchers better understand the corrosion performance of Mg alloys. CORROSION CHARACTERISTICS
Thermodynamically Unstable Nature The standard Gibbs free energy change ( G0) for Mg oxidation is quite negative[6, 7, 8]. Therefore, Mg exposed to environments containing oxygen or water always tends to rapidly oxidize[9,10]. In pure water, many reactions can occur on Mg[11].The theoretical stability of Mg in water can be summarized in an E-pH diagram (refer to Figure 1)[12 ], where there is a large corrosion region, a very negative potential immunity region and a high alkaline (pH>10.5) possible passive range. In theoretically, metallic Mg is not stable in a natural aqueous environment with a neutral pH value at an open-circuit potential.
passivity region
Corrosion region
Immunity region
Figure 1. E-pH diagram with possible stable substances in a Mg-H2O electrochemical system (data source [3,24]) In practice, the stability of Mg and its alloys cannot determined by their thermodynamics only. The kinetics of the reactions on Mg or its alloy surface also determines its corrosion performance.
Non-protective Surface Film A surface film can to some degree slow down a metal corrosion process. From a thermodynamic point of view, the surface film should be composed of Mg(OH)2 and MgO. In dry atmospheric conditions MgO is the main surface film composition. Mg(OH)2 is more stable than MgO in an aqueous solution. After Mg is immersed in an aqueous solution, the surface film is mainly Mg(OH)2 [13]. If water vapor is present in air, a more stable hydrated oxide (containing hydroxyl or hydroxide species) will be formed on Mg[14,15]. As most atmospheric environments contain some moisture, the surface films of Mg formed in natural atmospheric environments often contain both MgO and Mg(OH)2[16, 17]. When the Mg substrate contains either trace or alloying elements, their oxides or hydroxides will more or less become constituents of the surface film. Mg typically has a much stronger affinity for oxygen and hydroxide than its alloying elements, such as Al, Zn, Mn, Zr, etc. It is generally believed that a surface film on a Mg-Al substrate has a Mg and Al oxides inner layer and Mg hydroxide outer layer (see Figure 2)[18]. It has been confirmed[19] that the surface films on some Mg-Al intermetallics contain significant amounts of Mg and Al oxides and hydroxides. However the ratios of Al/Mg in the films are lower than those in the substrate intermetallics which may be a result of the stronger affinity of Mg to O and hydroxide as compared to aluminum’s affinity. The presence of Al2O3 can certainly reduce the overall solubility of the surface film. More importantly, this inner species in the film can increase the activation energy for Mg transportation through the film and hence significantly improve the corrosion resistance of the film.
Figure 2. Schematic diagram of possible Mg surface film microstructure[18] Nevertheless, due to the presence of large amount of Mg oxides and hydroxides in the surface film and the film porosity, the surface film cannot be as corrosion protective as the film on Al or its alloys. Negative Difference Effect (NDE) For most metals, like iron, steels and zinc etc, an anodic increase in polarization potential can cause an increased anodic dissolution rate and simultaneously a decreased cathodic hydrogen evolution rate. However, for Mg and its alloys the hydrogen evolution rate increases
when the polarization potential or current density becomes more positive in the anodic region[20,,21,22,23]. This is a well known negative difference effect (NDE). Figure 3 shows the hydrogen evolution rate of a Mg alloy increases as anodic polarization current density increases in a corrosive solution. In fact, the increasing hydrogen evolution rate with increasing anodic current density actually starts at a potential in the cathodic region, ie. the NDE can even occur under a cathodic polarization condition.
Figure 3. Hydrogen evolution and Mg dissolution rates of deicast AZ91 in 1N NaCl (pH11). Because of the NDE, the corrosion damage of Mg and its alloys is always greater than that converted from a measure current density. In many cases, the corrosion rate estimation for a Mg alloy based on its polarization curve according to Tafel extrapolation is misleading. “Anodic Hydrogen Evolution” (AHE) The NDE phenomenon can be characterized by hydrogen evolution under an anodic polarization condition, ie. a hydrogen evolution process involved in the anodic dissolution of Mg or its alloy[ 24 , 25 , 26 , 27 , 28 ]. This “strange” hydrogen evolution under anodic polarization is defined[2,3] as “Anodic Hydrogen Evolution” (AHE) to distinguish it from the normal cathodic hydrogen evolution (CHE) resulting from cathodic reaction under cathodic polarization. This AHE behavior is schematically illustrated in Figure 4.
Figure 4. Schematic diagram for dissolution and hydrogen evolution from Mg or its alloy. The hydrogen evolution rate, HER (see curve Ha2 in Figure 4) first decreases and then increases
as the polarization potential or current density changes from a negative to a positive value. In experiment, it was found [2,3,24] that when a Mg alloy was anodically polarized to a value more positive than the corrosion potential, intense hydrogen evolution occurring in corroding areas (see Figure 5). It suggests that AHE is closely associated with the anodic dissolution of Mg in that location.
Figure 5. “Anodic hydrogen evolution” (AHE) from MEZ Mg alloy surface in 5wt% NaCl solution[2,3,26]. The “anodic” hydrogen evolution from corroding areas can be explained by the Mg corrosion model which involved in Mg+ in a surface film broken area [2,3,5,28]. Active Anodic Dissolution Mg and its alloys in many corrosive electrolytes exhibit a dramatically increasing anodic current density with increasing anodic polarization potential. Figure 6 clearly shows that the anodic current densities of Mg alloys in corrosive NaCl solutions increase dramatically as polarization potentials become more positive.
(a)
(b)
Figure 6. Polarization curves of Mg alloys in corrosive solutions: (a) Diecast AZ91 and sand cast MEZ alloys in 5wt% NaCl at pH11[29]; and (b) Mg-Al single phase in Mg(OH)2 saturated 5wt% NaCl [49] If these polarization curves are examined carefully, a sudden change in current density on a polarization curve can be identified (Figure 6). Such a sudden change in current density
occurring on an anodic polarization curve can easily be recognized as an onset of dramatic increase in anodic current density and denoted as Ept. Experimentally, Ept also always corresponds to a sudden increase in hydrogen evolution from a Mg alloy surface and initiation of localized corrosion damage. It is in some sense similar to “pitting” damage in morphology. In many corrosive environments, Ept is more negative than the corrosion potential Ecorr. Therefore, the experimentally measured anodic polarization curves of Mg and its alloys in corrosive solutions are actually intense “pitting” processes. Thus, the anodic current density increases dramatically with increasing polarization potential. Mg and its alloys display low anodic polarization resistance or a very active anodic dissolution process. No passivity can be measured for Mg and its alloys. Overall Corrosion Rate The overall corrosion reaction of Mg and its alloys can be written as: Mg+2H+ Mg2++ H2
(in acidic solution)
(1)
Mg+2H2O Mg2++2OH-+H2
(in neutral or basic media)
(2)
Many complicated detail reaction steps are involved in these two final corrosion reactions. For example, the anodic dissolution reaction is actually [3,25,30]: Mg+1/(1+y)H+
Mg2++ [1/(2+2y)]H2 + [(1+2y)/(1+y)]e
(3)
or Mg+[1/(1+y)]H2O Mg2++[1/(1+y)]OH-+[1/(2+2y)]H2+[(1+2y)/(1+y)]e
(4)
where y is the ratio of the amount of Mg+ involved in reaction: Mg+ + H+ =Mg2++ ½ H2
(5)
To that in reaction: Mg+ =Mg2++ e-
(6)
Based on the above anodic dissolution reactions proposed by Song [30], the experimental andoic valence (n) and dissolution efficiency ( ) should be: n = (1+2y)/(1+y) = [(1+2y)/(2+2y)]x100%
(7) (8)
They are less than 2 and far below 100%, which match the experimental results very well. It should be stressed that reaction (1) or (2) holds only when the entire electrode surface is considered. It does not mean that the anodic and cathodic reaction rates must be equal in a local surface area. Specifically, the cathodic hydrogen evolution reaction is more likely to occur
on particular impurity particles, secondary phases, and matrix phase areas with higher levels of specific solid solute alloying elements, while the anodic dissolution preferentially occurs at particular sites such as the regions adjacent to the impurity particles in the matrix phase and the matrix areas having lower concentrations of solid solute alloying elements. Total Hydrogen Evolution Hydrogen evolution phenomenon is one of the most important corrosion features of Mg and its alloys. The overall corrosion reaction (1) or (2) suggests that hydrogen evolution always accompanies the dissolution of Mg. If there is 1 mL of hydrogen evolved, there must be 1 mg of Mg dissolved[2,28]. Measuring the volume of evolved hydrogen is equivalent to measuring the weight-loss of Mg in corrosion, and the measured hydrogen evolution rate is equal to the weightloss rate if they both have been converted into the same unit (eg. mole per second) (see Figure 7). measured weight-loss rate (mg/cm2/day)
100000 y = 1.031x R2 = 1
10000 1000 100 10 1 0 0 0
1 100 10000 1000000 w eight-loss rate calculated from hydrogen evolution rate (mg/cm2/day)
Figure 7. Correlation between hydrogen evolution rate and corrosion rate [31] Therefore, a simple hydrogen evolution measurement technique can be employed[28] to estimate the corrosion rate of Mg[2, 3, 31] and it has now been widely used on Mg alloys[32,33,34, 35, 36, 37,38,39,40, 41 , 42] . It has several advantages over the traditional weight-loss measurement in estimating corrosion damage or corrosion rate of a Mg alloy: (1) smaller theoretical and experimental errors are introduced into the final estimated corrosion rate; (2) easy to set up and operate; (3) suitable for monitoring the corrosion of Mg and its alloys; and (4) no need for removal of corrosion products Surface Alkalization According to reaction (1) or (2), the dissolusion of Mg always corresponding to generation of hydroxyls or consumption of protons, ie. an increase in pH value of the solution. The pH increasing typically stops at ~10.5 even though the corrosion continues to proceed at this pH value. This is because the deposition of Mg(OH)2 dominates at this or higher pH levels and thus
the additional hydroxyls if generated in corrosion are consumed by dissolved Mg2+ via forming Mg(OH)2 deposition. Therefore, the surface of a corroding Mg alloy always experiences a local alkalization process. The degree of alkalization depends upon the ratio of the surface area of the Mg alloy to the volume of the solution it is exposed to. Under atmospheric corrosion condition, only a small amount of aqueous drops or a thin aqueous film can stay on the surface of the Mg specimens. The surface can be easily alkalized, and hence atmospheric corrosion is slow compared with immersion corrosion. Because of the surface alkalization effect, organic coatings which are unfavorable to hydroxides cannot offer effective corrosion protection on Mg alloys. Susceptibility to Galvanic Corrosion The standard equilibrium potential of Mg/Mg2+ is as negative as –2.4V[11,24], which is more negative than any other engineering metals (including coatings). As the surface films formed on Mg and its alloys are generally non-protective and their anodic polarization resistance is low, in a corrosive environment the corrosion potentials of Mg and its alloys are as negative as -1.7~1.6V/NHE. Therefore, in most aqueous solutions the corrosion potential of Mg or its alloy is the most negative among the common engineering metals. The negative potential means that Mg or its alloy acts as an anode if in contact with another engineering metal and suffers from galvanic corrosion attack. In theory, the galvanic corrosion rate (ig) is determined by[43,44] the open-circuit (corrosion) potentials of the cathode (Ec) and anode (Ea), the cathode and anode polarization resistance (Rc and Ra), and the solution RS between the anode and cathode, respectively. Simply based on their corrosion potentials, the engineering metals can be ranked into a galvanic series from active (negative) to passive (noble) when exposed to seawater[45]. Mg and its alloys are positioned at the most negative or active end in this series. When a Mg alloy is in contact with another metal, the driving force for the macro-galvanic corrosion is always large according to the difference between their corrosion potential difference (Ec-Ea). The galvanic corrosion is also dependent upon the anodic and cathodic polarization resistance (Ra and Rc). For a given Mg alloy in a given solution, the corrosion potential and the cathodic polarization resistance of a coupled cathode metal will have a decisive influence on the severity of the macro-galvanic corrosion of this Mg alloy. It is well known that Fe, Co, Ni, Cu, W, Ag and Au etc. are much nobler than Mg alloys in an aqueous solution and that they have relatively low hydrogen evolution over-potentials (lower than 500mV), i.e. low cathodic polarization resistance. They can form galvanic corrosion couples with Mg alloys resulting in serious galvanic corrosion damage to Mg alloys. In practice popular metals such as; aluminum, steel, galvanized steel and sometimes copper are quite often in contact with Mg alloys. For these metals, steel always has the most detrimental effect upon the corrosion of a Mg alloy and Al the least[43] (See Figure 8). Therefore, in industrial application, apart from the avoidance of direct contact with steel, some Al alloys are also used as isolators to separate Mg alloys parts from other metals to reduce the galvanic corrosion damage.
300 0
current density of the magnesium anode (uA/cm2)
250 0
Steel|Mg
200 0
150 0
Zn|Mg
100 0
5 00
A l|Mg 0 0
20 0
40 0 6 00 c ath o d e /an o d e ra tio ( %)
80 0
100 0
Figure 8. Galvanic current densities from AZ91D in contact with a steel, Zn and Al alloy exposed to 5wt% NaCl[43] The distribution of galvanic current density is dependent on the geometric configuration of the solution path for the galvanic current between the anode and cathode. For a simple onedimensional galvanic couple, an analytical prediction of galvanic current density distribution is possible[44], and this has been verified experimentally by directly measuring the distributions of galvanic current densities using a specially designed “sandwich”-like galvanic corrosion probe[43, 46 ]. When the geometry of the galvanic couple becomes complicated, computer modeling technique is an option[47,42]. It should be noted that due to the negative difference effect, alkalization effect, “poisoning” effect, and “short-circuit” effect[43], current computer modeling cannot does not comprehensively simulate a practical galvanic corrosion damage. It is not surprising that significant deviations are observed between computer-modeled data and experimentally measured galvanic corrosion results (Figure 9).
Figure 9. Computer modeled and experimental galvanic corrosion damage along a steel bolt in a Mg plate in 5wt.% NaCl solution.
Micro-galvanic Cell The non-uniformity of Mg and its alloys in terms of their composition, microstructure and even crystalline orientation, can result in non-uniform corrosion damage within Mg or a Mg alloy specimen because of different anodic and cathodic activities in different areas. The nonuniformity generates galvanic couples in Mg and its alloys. It is impossible for anodic and cathodic reactions to occur uniformly throughout entire surface of a Mg or Mg alloy specimen. These types of micro-galvanic cells actually dominate the corrosion behavior of Mg and its alloys. The Mg matrix always acts as a micro-anode and is preferentially corroded. There are many constituents, including the matrix phase itself with various solid solute concentrations which can act as micro-cathodes and therefore couple with the matrix to form micro-galvanic cells resulting in corrosion damage. Generally speaking, the following factors should be considered in analyzing the micro-galvanic effect: 1) the different crystal orientations of the matrix phase; 2) the different alloying element concentrations in the matrix phase; 3) the type and concentration of secondary phases along grain boundaries; and 4) the type and concentration of impurity particles in the matrix phase. Preferential Corrosion of Matrix Phase In a Mg alloy, the matrix phase is a major constituent and its corrosion performance is crucial to the alloy. In fact, the matrix phase is always preferentially corroded and is the main factor responsible for the alloy corrosion resistance. The corrosion of a Mg alloy in nature is a problem of the matrix phase. Figure 10 shows different corrosion rates (expressed by hydrogen evolution rates) on different crystallographic surfaces of AZ31B. The basal crystallographic plane is much more corrosion resistant than the prismatic planes[32].
CS
RS
Figure 10. Corrosion rates of the rolling surfaces and cross-section surface and their grain orientations for AZ31B sheet [32]. It has also been found on pure Mg, the corrosion depths of the three low index planes can be
ranked in the following order[48]:
The results suggest that even for a very uniform single phase, the difference in grain orientation can also result in micro-galvanic cells in Mg or its alloys. The matrix is a solid solution and as such, solutes normally have a significant influence on the corrosion behavior of the phase. For example, an Al containing Mg matrix phase can become more passive as the Al content increases and consequently, the corrosion rate of the Mg-Al matrix phase decreases as the Al content increases [49,50,51]. In a cast Mg-Al alloy, due to solidification induced segregation, the aluminum content in solid solution can vary from 1.5wt % at the grain center to about 12wt % along the grain boundary in vicinity of the phase [52]. Therefore, corrosion occurs mainly in the interior of the grain. In many cases, the corrosion stops at the grain boundary where the aluminum content is much higher than the grain centre. In non-Al containing alloys, the grain centre is rich in Zr and the role of Zr is as important as Al in the Al containing alloys. Many central areas of grains remain uncorroded while the grain boundaries have been severely corroded. In summary, Mg alloy matrix phase itself due to the non-uniform solid solute distribution is corroded nonuniformly. This non-uniformity within the matrix phase can also result in micro-galvanic cells. Dual Role of Secondary Phase Almost all the Mg intermetallic phases are more noble than the Mg matrix itself and many of them are secondary phases [2,25,24,53,54,55,56]. These phases are cathodic to the matrix phase and can act as micro-galvanic cathodes to accelerate the corrosion of the matrix. For example, the phase of a Mg-Al alloy has a corrosion potential about 400mV more positive than the matrix phase. It has a cathodic current density much larger than the matrix phase. This means that the phase is a very active cathode to the matrix phase and cathodic hydrogen evolution mainly occurs on this secondary phase (Figure 11 (a)).
(a)
(b)
Figure 11. (a) Hydrogen evolution and (b) corrosion damage of AZ91D in 5% NaCl
However, the secondary phase can also play another important role in a Mg alloy as a barrier against corrosion. The secondary phase in Mg alloys is much more stable than the Mg matrix. For example, in Al containing AZ alloys, the -phase is very corrosion resistant and normally not corroded if exposed to a NaCl solution (Figure 11(b)). The presence of high corrosion resistant secondary phase in some case can stop the development of corrosion in the alloy. Sometimes, even in the same alloy sample, some areas with a large amount of the secondary phase display much higher corrosion resistance than other areas with only a small amount of the phase [53]. It has also been reported[2,3,50] that the corrosion rate of a Mg-Al alloy decreases as the amount of the secondary phase increases. In summary, the secondary phase plays a dual-role in the corrosion of a Mg alloy[2,3,24,25]. Finely and continuously distributed secondary phase can effectively stop the development of corrosion, whereas the presence of a small amount of discontinuous secondary phase particles will accelerate the corrosion. This dual-role of the secondary phase can explain many corrosion phenomena of Mg alloys. Impurity Tolerance Fe, Cu and Ni, are impurities in a Mg alloy. A very small amount of Fe, Ni, Co, or Cu addition can dramatically increase the corrosion rate of a Mg alloy [57, 58, 59]. Among these impurities, Fe has been widely investigated as it is most likely to be incorporated into a Mg alloy via a production process. Purification significantly improve the corrosion resistance of Mg and Mg alloys[ 60 , 61 , 62 , 63 , 64 , 65 ,, 66 , 67 ]. Most commercial Mg alloys have strictly controlled impurity levels[68]. Each impurity has a tolerance limit, above which the corrosion rate increases dramatically[69]. If the impurity concentration is lower than its tolerance limit, the corrosion rate is low and the impurity level has an insignificant influence on the corrosion. It is calculated[70] that the Fe tolerance limit in Mg corresponds well to the solubility of Fe in Mg. There is a rough correspondence between critical concentrations and the solubility of some elements in Mg alloys [71] . Only after the element levels are greater than their solubility in the Mg matrix (which is a solid solution) and they start to form separate new phases in Mg alloys, is the corrosion of the alloys dramatically accelerated. Other alloying elements present in Mg can alter the impurity tolerance limit[ 72 ]. For example, when a few per cent of Al is added to Mg, the tolerance limit of iron decreases from 170 ppm to a few ppm. With a higher addition of Al in Mg, Fe and Al can combine to form Fe-Al phase (ie. FeAl3) particles which precipitate and act as galvanic cathodes in Mg-Al alloys[73,74,75,76]. This is why the tolerance limit for a Mg alloy becomes too low to be determined when the Al concentration is increased to 10 wt% [77]. Therefore, it is also understandable that different Mg alloys have different impurity tolerance limits[78]. Mn and Zr do not have a significant beneficial effect on corrosion resistance by themselves. However, they can also effectively reduce the impurity detrimental effect in Mg alloys. The iron tolerance limit of a Mg-Al alloy depends on the Mn concentration[79]. A small addition (0.2%) of Mn can lead to increased corrosion resistance of the Mg alloy[80,81] and reduce the detrimental effect of impurities when their tolerance limits are exceeded[82]. It can also increase the iron
tolerance limit to 20 ppm for Mg-Al alloys[83]. Similarly, addition of Zr can lead to a higher purity and hence a more corrosion resistant Mg alloy. The iron tolerance limit is dependent upon the Mn content in Mg or a Mg alloy, and the Fe/Mn ratio has been found to be a critical factor[84,85] determining the tolerance limit. A nearly direct proportionality has been observed between the Fe/Mn ratio and the corrosion rate[86]. Two mechanisms have been offered to explain why Mn reduces the corrosion rate. Firstly, Mn combines with iron in a molten Mg alloy during melting and forms intermetallic compounds which settle to the melt bottom thereby lowering the iron content of the alloy. Secondly, Mn encapsulates the iron particles that remain in the metal during solidification, thereby making them less active as local cathodes. The beneficial effect of Zr is also associated with the reaction of Zr with impurities to form intermetallic particles in a molten Mg alloy which quickly settle out due to their high density and therefore result in a purer Mg alloy. Non-uniform Damage Due to the micro-galvanic effect or electrochemical non-uniformity in Mg and its alloys, the corrosion damage of Mg or its alloys cannot be uniform. The non-uniformity can be an uneven distribution of micro-anodes and micro-cathodes in Mg or an alloy. Severe localized corrosion can result in particle undermining which is quite often observed for corroding Mg and its alloys. The localized corrosion of Mg and its alloy in most environments is initiated from some surface weak points and typically in the form of tiny irregular localized pits which then spread laterally over the surface. In contrast to typical pitting corrosion, the localized corrosion of Mg or a Mg alloy does not penetrate deeply. This is a result of the alkalization effect at the tips of the corroding pits which prevents the solution at the tips from acidifying or forming auto-catalytic cells. This self-limiting corrosion development tends to result in relatively wide spread corrosion damage although the degree of corrosion in different areas can vary markedly. Mg and its alloys are not sensitive to crevice corrosion attack, an important type of nonuniform corrosion damage. According to corrosion reaction (1) or (2), hydrogen evolution, not oxygen, is mainly responsible for the corrosion. Hence, even if there is a difference in oxygen concentration inside and outside a crevice, it cannot build up a significant galvanic effect to trigger the crevice corrosion mechanism. In practice, more severe corrosion may sometimes be observed in a crevice of a Mg alloy under atmospheric corrosion conditions. However, this is mainly due to the accumulation of moisture in the crevice, and thus the Mg alloy is exposed to the corrosive solution much longer within the crevice compared to the outside of the crevice. In this case, the corrosion in the crevices actually does not follow the traditional crevice corrosion mechanism that involves oxygen depletion and acidification inside the crevices. Corrosivity Different Mg alloys have different corrosion rates even in the same environment because of their different compositions and microstructures. Even for Mg alloys with similar levels of alloying elements, their corrosion rates are largely scattered, because their impurity levels and distributions of chemical composition and phase can be considerably different. A typical example to illustrate this point is the influence of heat-treatment on corrosion performance of
some Mg alloys. It is well known that without changing the purity level and chemical composition, T4, T5 or T6 heat-treatment alone can significantly modify the corrosion resistance of some Mg alloys[2,3,49, 55]. Fig.12 shows the effect of heat-treatment on corrosion rate of an AZ alloy. The dual role of the -phase is an important explanation for the variation of corrosion rate with heattreatment condition. Corrosion rate
Ascast solution treated
T5
T4 Heat-tratment
T6
Figure 12 Effect of heat-treatment on corrosion rate of an AZ Alloy High purity Mg or a Mg alloy generally has relatively good corrosion performance. Mg and its alloys are more corrosion resistant in a high alkaline and low chloride concentration solution. Many chemicals in solution can significantly affect the corrosion resistance of Mg alloys. The atmospheric corrosion of Mg and its alloys is relatively uniform while under immersed conditions the damage tends to be localized. This is a result of the more significantly microgalvanic effect within a Mg alloy under an immersion condition than under an atmospheric condition. For this reason, Mg and its alloys suffer from more severe localized corrosion damage during salt immersion than salt spray (assuming the same concentration of NaCl solution). Figure 13 shows the relatively lower corrosion rates of Mg alloys under salt spray than salt immersion.
Figure 13. Corrosion rate of Mg alloys under salt (5wt.% NaCl) immersion test (SIT) condition for 6 hours and salt spray test (SST) condition for 21 hours [87].
CONCLUSIONS 1. 2. 3. 4. 5.
6. 7. 8. 9. 10. 11. 12. 13. 14.
Mg is not stable in most natural environments. Corrosion of Mg is a spontaneous process. In most practical environments, there is a surface film on Mg alloys. Unfortunately, the surface film is not protective for Mg and its alloys. “Negative difference effect” phenomenon is normally involved in the anodic process of Mg or its alloy. It can be characterized by “Anodic hydrogen evolution” from corroding area An Mg+ involved anodic dissolution process in surface film free areas is responsible for the “Negative difference effect” and “anodic hydrogen evolution”. The anodic dissolution of Mg or its alloy is an active process. There is no passivity. The process has an apparent valence of dissolved Mg lower than 2, and a low anodic dissolution efficiency. The corrosion of Mg alloys is always accompanied by hydrogen evolution. The corrosion rate can always be estimated by the hydrogen evolution rate. Surface alkalization is an inevitable process due to the corrosion of Mg or its alloy. Mg and its alloys have very negative corrosion potentials and active anodic dissolution processes. They are susceptible to galvanic corrosion. Micro-galvanic cells within a Mg alloy can be formed due to different grain orientations, composition variations, presence of the secondary phases and impurities. The matrix phase of a Mg alloy is always preferentially corroded. It corrosion damage is not uniform. The dual role of the secondary phase determines the corrosion performance of a Mg alloy. The corrosion resistance of Mg and its alloys is very sensitive to Fe, Cu and Ni impurities. The corrosion damage of Mg or its alloys is mainly non-uniform or localized due to various galvanic effects in the materials. Mg and its alloys have very different corrosion performance than conventional metals because of their special electrochemistry. REFERENCES
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