Evaluation of the BEASY program using linear and piecewise linear approaches for the boundary conditions

Self Cite

The study of the galvanic corrosion of magnesium becomes increasingly important as the use of magnesium alloys increases rapidly in the auto and aerospace industries due to their advantages of light-weight, adequate mechanical properties and moderate cost. Corrosion, however, limits the application of magnesium alloys. [1][2][3][4][5][6][7] A number of different mechanisms are important for the corrosion. [8][9][10][11][12][13][14] But galvanic corrosion is probably the most important for magnesium because magnesium is the most active structural metal and consequently may suffer serious corrosion when joined to all other common metals of construction, such as aluminum or steel. [1,2,15] Fasteners and their galvanic corrosion is of major concern in automotive applications. [16][17][18] Skar [17] showed that 6000 series aluminum alloy fasteners caused negligible galvanic corrosion of magnesium in the salt spray test. However, steel fasteners are desired for many applications due to their inherently better mechanical properties. Poor compatibility with magnesium was shown by aluminium-coated steel fasteners [18] whereas steel fasteners with zinc or tin-zinc alloy coatings were compatible with magnesium in salt-water exposure. [16] To be able to design a structural component, incorporating galvanic corrosion, it is useful to be able to simulate the galvanic corrosion distribution qualitatively. The research presented in this paper has been undertaken as part of a program to explore that aim. The total corrosion in the area of galvanic corrosion can be considered to be made up of the following two components: (1) galvanic corrosion and (2) self corrosion. The galvanic corrosion is that part of the corrosion, which is directly caused by the coupling of the magnesium to a steel fastener. The self-corrosion is defined as the extra corrosion. Both the galvanic corrosion and the self-corrosion may take the form of more or less general corrosion, or the form of localized corrosion or pitting corrosion.Prior studies of galvanic corrosion of magnesium have been scarce. The earliest study, started in the 1950s by Teeple, [19] investigated the influence of location and climate. This study revealed that different locations produced different corrosion rates because of the different electrolyte properties of the condensed film on the metal surface. [19] Atmospheric galvanic corrosion could be detrimental for magnesium in one location whilst it was almost harmless in another location. This provided helpful information to select magnesium for a particular location. However, the study was time consuming. It is often not practical to wait years to have the test results for each particular service location. Limited studies have addressed the effect of electrolyte on the galvanic corrosion of magnesium. More effort has been focused on general corrosion, particularly on the influence of the ion species in solution, and the influence of cathodic impurity elements in the alloy. [20][21][22][23][24][25][26][27][28] The influence of the ele...

mentioning

confidence: 71%

Experimental Measurement and Computer Simulation of Galvanic Corrosion of Magnesium Coupled to Steel

Jia¹,

Song²,

Atrens³

2007

Self Cite

“…Recent research [5,20,21,22,26,29,30] has investigated the influence of geometric factors on the galvanic current distribution for AZ91D coupled to steel using a Boundary Element Method (BEM) model and experimental measurements. The experimental measurements used a multi-channel zero resistance ammeter and an idealized 1D galvanic corrosion assembly.…”

Section: Galvanic Corrosionmentioning

confidence: 99%

Recent Insights into the Mechanism of Magnesium Corrosion and Research Suggestions

Song¹,

Atrens²

2007

Self Cite

280

121

Magnesium (Mg) is the most reactive engineering material and corrosion protection continues to be an issue of great importance. [1][2][3][4] This note builds on prior reviews [1][2][3][4][5][6][7][8] and our research on Mg corrosion. [1,2, The corrosion performance of Mg alloys results from (1) the high intrinsic dissolution tendency of Mg, which is only weakly inhibited by corrosion product films and (2) the presence of impurities and second phases acting as local cathodes and thus causing local microgalvanic acceleration of corrosion.The corrosion mechanism includes all physical features and chemical reactions when metallic Mg is exposed to an environment. Our prior work [2,5,11,12] has indicated that the Mg corrosion mechanism has the following key points. In the general case, a partially protective film covers the surface of Mg and Mg dissolution occurs at breaks in this film. Hydrogen (H) evolution is associated with Mg dissolution in two separate ways. (a) An electrochemical reaction, Equation 1, balances the Mg dissolution reaction, Equation 2. (b) H is also produced directly by the reaction of Mg + with water, Equation 3.The overall reaction consumes H + and produces OH -, i.e. the pH increases, which favours the formation of Mg hydroxide film by the precipitation reaction. Mg has a negative corrosion potential, with a slightly more negative pitting potential, in solutions of practical importance like 3 % NaCl. The chance development of areas of localized corrosion leads to undermining and falling out of particles of Mg, even for the corrosion of pure Mg. New Insights: Mg +There continues to be some controversy as to the existence of Mg + as an intermediate in the reaction sequence between metallic Mg and Mg 2+ , the stable Mg ionic species in aqueous solutions. The evidence in support of the existence of Mg + is strong but circumstantial. The evidence in favor is based on the fact that the simplest possible reaction sequence, which includes Reactions 2 and 3, is consistent with all the experimental evidence, particularly the electrochemical impedance spectroscopy. [12] It is also supported by the observations of Song and co-workers [5,16,19] that the Mg corrosion mechanism involves "anodic H evolution" which is different to "cathodic H evolution".Negative Difference Effect (NDE): Mg has an exceedingly strange phenomenon, the negative difference effect (NDE). The NDE continues to receive considerable discussion. The NDE has been defined in our previous publications [1,2,5,11,12] in terms of the difference D:where I s is the spontaneous rate of the H evolution reaction (HER) on the Mg surface at the free corrosion potential, I H,m is the measured HER rate for an applied galvanostatic current I appl , which is equal to I appl I Mg;m À I H;m 5where I Mg,m is the measured rate of Mg corrosion. For most metals like Fe and Zn, in an acid solution, when the potential is increased, the anodic dissolution rate increases and the cathodic H evolution rate decreases. For these cases, D is greater than zero because I ...

“…Petty et al [18] interpreted their experiments as showing that the first step in the anodic oxidation of Mg was the unipositive ion Mg + , produced by means of Mg = Mg + + e (10) and that this Mg + ion could be further oxidized electrochemically to the stable species Mg ++ by:…”

Section: Existence Of the Unipositive Magnesium Ionmentioning

confidence: 99%

“…This interest is warranted by the fact that the NDE is of central importance to the corrosion of magnesium, [2][3][4][5] including galvanic corrosion [6][7][8][9][10] and stress corrosion cracking. [11][12][13][14][15] The NDE is also probably an issue for magnesium anodes used in cathodic protection, (see e.g.…”

mentioning

confidence: 99%

The Negative Difference Effect and Unipositive Mg⁺

Atrens

Dietzel²

2007

Self Cite

172

There continues to be significant interest in the negative difference effect (NDE) [1] for magnesium corrosion. This interest is warranted by the fact that the NDE is of central importance to the corrosion of magnesium, [2][3][4][5] including galvanic corrosion [6][7][8][9][10] and stress corrosion cracking. [11][12][13][14][15] The NDE is also probably an issue for magnesium anodes used in cathodic protection, (see e.g. [16,17] ). Furthermore, as discussed herein, there are aspects of the NDE that still need to be resolved even though it is more than fifty years since the paper by Petty et al. [18] in which strong evidence was presented for the existence of the unipositive magnesium ion, Mg + .The electrochemical corrosion of magnesium is in many respect a model system for research as it is relatively easy to measure all the quantities of importance, including (i) the magnesium metal mass loss, (ii) the amount of the stable ion Mg ++ in solution and (iii) the amount/volume of hydrogen produced, [19] (hydrogen is associated with the cathodic partial reaction). Furthermore, it is also worth stressing that magnesium corrosion is inherently complicated and includes [2,3] (i) a partially protective surface film in many aqueous solutions, (ii) the equilibrium ion is the Mg ++ ion, (iii) localised corrosion is often the dominant form of corrosion in many solutions, (iv) microgalvanic corrosion [20] between microstructural features is important during the corrosion of magnesium alloys (v) magnesium has a very negative corrosion potential and (vi) there is the possibility of a partially protective surface hydride. [21] There are currently [1,2,3] two mechanistic approaches to the magnesium NDE and these are designated herein as: (i) "the unipositive Mg + ion NDE mechanism" and (ii) "the magnesium self-corrosion mechanism". "The unipositive Mg + ion NDE mechanism" derives from the research work of Song et al. [22,23] which examined the then existing [24] four theories for the magnesium NDE, the prior experimental results, the results of a targeted experimental program and, as a synthesis, formulated stepwise a new mechanism based on what are still considered [2,3] to be the two critical aspects of magnesium corrosion: (i) the surface of magnesium, in most aqueous solutions, is covered by a partially protective surface film, and (ii) the postulate that the unipositive Mg + is involved as an early step in the electrochemical oxidation of magnesium and moreover that the unipositive Mg + has a sufficient lifetime to react chemically with other available species. It is clear that, during the anodic oxidation of magnesium metal, the transfer simultaneously of two electrons to form Mg ++ is much less probable than the transfer first of one electron to form Mg + . However, a key issue is the lifetime of the Mg + ion. Does the Mg + ion "immediately" attract a second electron to form the equilibrium species Mg ++ , or does the Mg + ion exist for a sufficiently long time period so that it can react with other species in a chemical r...