Corrosion Resistance Of Magnesium Alloys
2003 ASM International. All Rights Reserved.ASM Handbook, Volume 13A Corrosion: Fundamentals, Testing, and Protection (#06494G)www.asminternational.orgCorrosion Resistanceof Magnesium AlloysRevised by Barbara A. Shaw, Pennsylvania State UniversityMAGNESIUM AND MAGNESIUM ALLOYS are often thought of as rapidly corrodingmetals because of their active positions in boththe electromotive force (EMF) series (Table 1)and the galvanic series for seawater (see Fig. 1in the article “Evaluating Galvanic Corrosion”in this Volume). However, depending on the environment and certain design considerations, thecorrosion of magnesium can be well within acceptable design limits. Knowledge of environmental factors that influence degradation, typesof corrosion to which magnesium alloys are mostsusceptible, protection schemes, and design considerations can significantly minimize corrosionand increase use of this family of lightweightstructural metals.When unalloyed magnesium is exposed to theair at room temperature, a gray oxide forms onits surface. Moisture converts this oxide to magnesium hydroxide, which is stable in the basicrange of pH values, but is not in the neutral oracid ranges as shown in the Pourbaix diagram(Fig. 1). The immunity region of the diagram iswell below the region of water stability; as a result, in neutral and low pH environments magnesium dissolution is accompanied by hydrogenevolution. In basic environments, passivation ispossible as a result of the formation of aMg(OH)2 layer on the metal surface. Since thefilms that form on unalloyed magnesium areslightly soluble in water, they do not providelong-term protection (Ref 2). When chloride,bromide, sulfate, and chlorate are present, thesurface films break down. Likewise, as the CO2in air acidifies water, the films are not stable.Corrosion potentials for magnesium electrodesin a variety of aqueous solutions are presentedin Table 2, while corrosion rate data in water andseveral other media are presented in Fig. 2 andTable 3.Unalloyed magnesium is not extensively usedfor structural purposes. Consequently, the corrosion resistance of magnesium alloys is of primary concern. Two major magnesium alloy systems are available to the designer.The first includes alloys containing 2 to 10%Al, combined with minor additions of zinc and0.82H2O O2 4H 0.4H2 2H Potential (E ), V–0.4 2e 4–2.8B4ImmunityCorrosion rate, mils/yr01086 4e –6812Potential, Veⳮ r Lieⳮ r Keⳮ r Naeⳮ r Mgeⳮ r Aleⳮ r Zneⳮ r Feeⳮ r Cdeⳮ r Nieⳮ r Sneⳮ r Cueⳮ r 4ⳮ0.40ⳮ0.24ⳮ0.140.340.80ElectrolytepHFig. 1ReactionLiⳭ ⳭKⳭ ⳭNaⳭ ⳭMg2Ⳮ ⳭAl3Ⳮ ⳭZn2Ⳮ ⳭFe2Ⳮ ⳭCd2Ⳮ ⳭNi2Ⳮ ⳭSn2Ⳮ ⳭCu2Ⳮ ⳭAgⳭ Ⳮ0.40.210Li, LiⳭK, KⳭNa, NaⳭMg, Mg2ⳭAl, Al3ⳭZn, Zn2ⳭFe, Fe2ⳭCd, Cd2ⳭNi, Ni2ⳭSn, Sn2ⳭCu, Cu2ⳭAg, AgⳭTable 2 Rest potential of magnesiumelectrodes under various aqueous solutionsFig. 24Electrode1.00.80.6Potential-pH (Pourbaix) diagram for the systemof magnesium and water at 25 C (77 F), showing the theoretical domains of corrosion, immunity, andpassivation. Source: Ref 12Standard reduction potentialsA10Table 1214 16–2manganese. These alloys are widely available atmoderate cost, and their room-temperature mechanical properties are maintained to 95 to 120 C (200 to 250 F). Beyond this, elevated temperatures adversely affect mechanical propertiesand the corrosion properties deteriorate rapidlywith increasing temperature.The second group consists of magnesium alloyed with various elements (rare earths, zinc,thorium, and silver) except aluminum, all containing a small but effective zirconium contentthat imparts a fine grain structure and thus improved mechanical properties. These alloys generally possess much better elevated-temperatureproperties, but the more costly elemental additions combined with the specialized manufacturing technology required result in significantly246 8 1020Days on test40 60 80Corrosion rates as a function of time for commercially pure magnesium. Curve A, distilledwater vented to air through a caustic trap; curve B, distilledwater vented to atmospheric CO2. Source: Ref 2N NaClN Na2SO4N Na2CrO4N HClN HNO3N NaOHN NH3Ca(OH)2 saturatedBa(OH)2 saturatedN, normal. Source: Ref 3ER (vs .43ⳮ0.95ⳮ0.88
2003 ASM International. All Rights Reserved.ASM Handbook, Volume 13A Corrosion: Fundamentals, Testing, and Protection (#06494G)www.asminternational.orgCorrosion Resistance of Magnesium Alloys / 693higher costs. Table 4 lists some of the compositions commonly available in both systems.Note the aluminum group alloy designations begin with “A.”Metallurgical FactorsChemical Composition. As the galvanic series in seawater reveals, magnesium is anodic toall other structural metals and, as a result, galvanic interactions between magnesium and othermetals are a serious concern. The influence ofcathodic iron impurities on the corrosion of commercially pure magnesium is presented in Fig.3. Above the tolerance level of 170 ppm for ironin magnesium, the corrosion rate increases dramatically. Figure 4 shows the effects of iron and13 other elements on the saltwater corrosion performance of magnesium in binary alloys with100Corrosion rate, mg/cm2/day8060increasing levels of the individual elements. Sixof the elements included in Fig. 4 (aluminum,manganese, sodium, silicon, tin, and lead), aswell as thorium, zirconium, beryllium, cerium,praseodymium, and yttrium, have little if anydeleterious effect on the basic saltwater corrosion performance of pure magnesium when present at levels exceeding their solid solubility orup to a maximum of 5% (Ref 6). Four elementsin Fig. 4 (cadmium, zinc, calcium, and silver)have mild-to-moderate accelerating effects oncorrosion rates, whereas four others (iron, nickel,copper, and cobalt) have extremely deleteriouseffects because of their low solid-solubility limits and their ability to serve as active cathodicsites for the reduction of water at the sacrifice ofelemental magnesium. Although cobalt is seldom encountered at detrimental levels and cannot be introduced even through the long immersion of cobalt steels in magnesium melts, iron,nickel, and copper are common contaminantsthat can be readily introduced through poor molten-metal-handling practices. These elementsmust be held to levels under their individual solubility limits, or their activity must be moderatedthrough the use of alloying elements such asmanganese or zinc, to obtain good corrosion resistance. These limits are stated in Table 5 fordie-cast products.Figure 5 illustrates the effect of increasingiron, nickel, and copper contamination on thestandard ASTM B 117 salt-spray performance ofTable 3die-cast AZ91 test specimens as compared to therange of performance observed for cold-rolledsteel and die-cast aluminum alloy 380 samples.Such results have led to the definition of the critical contaminant limits for two magnesium-aluminum alloys in both low- and high-pressurecast form and the introduction of improved highpurity versions of the alloys. Table 6 lists someof the critical contaminant limits defined to date.The iron tolerance for the magnesium-aluminumalloys depends on the manganese present, a factsuggested many years ago but only recentlyproved. For AZ91 with a manganese content of0.15%, this means that the iron tolerance wouldbe 0.0048% (0.032 ⳯ 0.15%) (Ref 11).It should also be noted that the nickel tolerance depends strongly on the cast form, whichinfluences grain size, with the low-pressure castalloys showing just a 10 ppm tolerance for nickelin the as-cast (F) temper. Therefore, alloys intended for low-pressure cast applications shouldbe of the lowest possible nickel level (Ref 8).The low tolerance limits for the contaminants inAM60 alloy when compared to AZ91 alloy canbe related to the absence of zinc. Zinc is thoughtto improve the tolerance of magnesium-aluminum alloys for all three contaminants, but it islimited to 1 to 3% Zn because of its detrimentaleffects on microshrinkage porosity and its accelerating effect on corrosion above 3%.For magnesium-rare earth, -thorium, and -zincalloys containing zirconium, the normal saltwa-Corrosion rate of commercially pure magnesium in various media40Corrosion rateMedium20000.010.020.03Iron, %Humid airHumid air with condensationDistilled waterDistilled water exposed to acid gasesHot deionized water (100 C) (14 days stagnant immersion)Hot deionized water inhibited with 0.25 NaFSeawater3M MgCl2 solution3M NaCl (99.99% high-purity Mg with 10 ppm Fe)mm/yrmils/yr1.0 ⳯ 10ⳮ51.5 ⳯ 10ⳮ21.5 ⳯ 10ⳮ20.03–0.3165.5 ⳯ 10ⳮ20.253000.30.00040.60.61.2–126402.21012 ⳯ 10312Grades 9980, 9990, 9991, 9995, 9998 except for NaCl solution. Source: Compiled from Chapters 21–32 in Ref 4Fig. 3Effect of iron content on the corrosion rate ofcommercially pure magnesium subjected to alternate immersion in 3% NaCl. Source: Ref 4Table 4Typical magnesium alloy systems and nominal compositionsAlloy No.Fig. 4Effect of alloying and contaminant metals on thecorrosion rate of magnesium as determined byalternate immersion in 3% NaCl solution. Source: Ref 5Element(a), %ASTMUNSAlZnMnAgZrThReProduct 0.5.33.1.2.5.211.52.5.CWWCC, WCCWC, WCCCCCC, WC, W(a) For details, see alloying specifications. (b) C, castings; W, wrought products
2003 ASM International. All Rights Reserved.ASM Handbook, Volume 13A Corrosion: Fundamentals, Testing, and Protection (#06494G)www.asminternational.org694 / Methods of Corrosion ProtectionTable 5 Contaminant tolerances andmanganese limits for magnesium diecastingsTable 6Critical contaminant limit, %Alloy/formCritical contaminant limit(max), n contaminant tolerance limits in high- and low-pressure cast formsCuNiFeMn limit, �0.50(b)Per ASTM B 94. (a) In alloys AS41B, AM50A, AM60B, and AZ91D,if either the minimum manganese limit or the maximum iron limit is notmet, then the iron/manganese ratio shall not exceed 0.010, 0.015, 0.021,and 0.032, respectively. (b) Not specified, but included in the limits for“other metals”ter corrosion resistance is only moderately reduced when compared to high-purity magnesiumand magnesium-aluminum alloys—0.5 to 0.76mm/yr (20 to 30 mils/yr) as opposed to less than0.25 mm/yr (10 mils/yr) in 5% salt spray—butcontaminants again must be controlled. The zirconium alloying element is effective in this casebecause it serves as a strong grain refiner formagnesium alloys, and it precipitates the ironcontaminant from the alloys before casting.However, if alloys containing more than 0.5 to0.7% Ag or more than 2.7 to 3% Zn are used, asacrifice in corrosion resistance should be expected (Fig. 4). Nevertheless, when properly finished these alloys provide excellent service inharsh environments.Heat Treating, Grain Size, and Cold-WorkEffects. Heating influences the salt-spray corrosion rate of die-cast commercial magnesium-aluminum alloys as shown in Fig. 6, which showsthat alloys with higher residual-element (iron,nickel, and copper) concentrations were morenegatively impacted by temperature. Using controlled-purity AZ91 alloy cast in both high- andlow-pressure forms, the contaminant-tolerancelimits have been defined as summarized in Table7 for the as-cast (F), the solution treated (T4,held 16 h at 410 C, or 775 F, and quenched),and the solution treated and aged (T6, held 16 hat 410 C, or 775 F, quenched, and aged 4 h at215 C, or 420 F).Table 7 Contaminant tolerance limitsversus temper and cast form for AZ91 alloyHigh-pressure die cast, 5–10 lm average grain size; lowpressure cast, 100–200 lm average grain sizeUnalloyed magnesiumAZ91/high pressureAZ91/low pressureAM60/high pressureAM60/low pressureAZ63/low pressureK1A/low pressureGrain size, 032 Mn(a)0.032 Mn(a)0.021 Mn(b)0.021 Mn(b)0.003(c) .0400.0400.0100.010 0.45.58879510(a) Iron tolerance equals manganese content of alloy times 0.032. (b) Iron tolerance equals manganese content of alloy times 0.021. (c) Magnesiumcontent of AZ63 reported as 0.2%Table 8 compares the average 5% salt-spraycorrosion performance of sand-cast samples produced in a standard AZ91C and a high-purityAZ91E composition. The alloys were cast withand without standard grain-refining practicesused to evaluate physical and compositional effects. The cast samples were then tested in theF, T4, T6, and T5 (aged 4 h at 215 C, or 420 F) tempers. In the case of the high-iron-containing AZ91C, none of the variations tested significantly affected the poor corrosion performanceresulting from an iron level two to three timesthe alloy tolerance. In the case of the high-purityalloy, however, the T5 and T6 tempers consistently gave salt-spray corrosion rates less than0.25 mm/yr (10 mils/yr), whereas the as-cast andFig. 5Table 8solution-treated samples exhibited an inverse response to grain size and/or the grain-refiningagents. Welds on Mg-Al-Zn alloys should beaged or should be solution treated and aged toobtain good corrosion resistance in harsh environments and to reduce the risk of failure due tostress-corrosion cracking (SCC).Cold working of magnesium alloys, such asstretching or bending, has no appreciable effecton corrosion rate. Shot- or grit-blasted surfacesoften exhibit poor corrosion performance—notfrom induced cold work but from embedded contaminants. Acid pickling to a depth of 0.01 to0.05 mm (0.0004 to 0.002 in.) can be used toremove reactive contaminants, but unless theprocess is carefully controlled, reprecipitation ofEffect of nickel and copper contamination on the salt-spray corrosion performance of die-cast AZ91 alloy.Source: Ref 7Typical corrosion rates versus temper and grain size for two magnesium alloysASTM B 117 salt-spray testTemper corrosion rateCritical contaminant Low pressureFT4AlloyT60.032 Mn 0.032 Mn0.035 Mn 0.046 Mn0.00500.00100.0010.0010.0400.040 0.0100.040(a) Tolerance limits expressed in wt% except for iron, which is expressedas the fraction of the manganese content (for example, the iron toleranceof 0.2% Mn alloy ⳱ 0.0064% Fe in F temper)AZ91C (untreated)AZ91C (degassed and grain refined)AZ91E (untreated)AZ91E (degassed and grain refined)AZ91E (untreated)AZ91E (degassed and grain 0.1.5554FT4(a) Iron is expressed as a fraction of analyzed manganese content. Source: Ref 8–10T6T5
2003 ASM International. All Rights Reserved.ASM Handbook, Volume 13A Corrosion: Fundamentals, Testing, and Protection (#06494G)www.asminternational.orgCorrosion Resistance of Magnesium Alloys / 695the contaminant is possible, particularly withsteel shot residues. Therefore, fluoride anodizingis often used when complete removal of the contaminant is essential (Ref 9).Causes of CorrosionFailures in Magnesium AlloysCauses of corrosion failures typically includeheavy-metal contamination, blast residues, fluxinclusions, and galvanic attack.Heavy-metal contamination often results ingeneral pitting attack that is unassociated withfasteners or dissimilar-metal attachments. Therate of attack on unpainted surfaces will be essentially unaltered by surface condition, that is,freshly sanded, machined, or acid pickled. Figure 7 illustrates the effect of heavy-metal contamination on the ASTM salt-spray corrosionperformance of low-pressure cast AZ91.Blast residues can cause general pitting attack in saline environments. Attack is normallylimited to unmachined surfaces of sand .0400.5060AZ91Dlow residuals050100150200250300350Corrosion rate, mils/yrCorrosion rate, mm/yrAM60B200400Sanded or acid-etched (2% H2SO4 for 15 to 30s) samples will show vastly improved performance in saltwater immersion or salt-spray testsbecause of removal of the contaminant. Scanning electron microscopy and energy-dispersivex-ray analysis samples cleaned in chromic acid(H2CrO4) can be used to confirm and identify thepresence of the contaminant, which is usuallyiron (from steel shot blasting) or silica (fromsand blasting).Flux inclusions result in localized attack thatis clustered or distributed randomly on machinedsurfaces of castings. Freshly machined surfacesexposed to 70 to 90% relative humidity will develop active corrosion sites overnight. Scanningelectron microscopy/energy-dispersive x-rayanalysis of a freshly machined surface (free offingerprints or other sources of contamination)will reveal pockets of magnesium and potassiumchloride, as well as possible traces of calcium,barium, and sulfur. In zirconium-bearing alloys,elemental zirconium and zirconium-iron compounds may also be associated with the deposits.Chromic-acid pickling followed by chemicaltreatment and surface sealing can alleviate theproblem of inclusions in finished castings. Withthe use of sulfur hexafluoride (SF6) rapidly replacing fluxes for the protection of melts duringcasting, this problem should be eliminated in thefuture.Galvanic attack is usually observed as heavylocalized attack on the magnesium, normallywithin 3.2 to 4.8 mm (1 8 to 3 16 in.) of fastenersor an interface with other parts of dissimilarmetal. Proper design and assembly methods, especially in the area of joints, can minimize galvanic attack.Heating temperature, CFig. 6Effect of heating temperature on corrosion rate of die-cast AZ91D and AM60B in salt-spray test for 10 daysusing ASTM B 117 method. Data are for test specimens that were heated from 0.5 to 36 h. Source: Ref 12ACKNOWLEDGMENTThis article was adapted from the article byAllan Froats, Terje Kr. Aune, David Hawke, William Unsworth, and James Hillis, “Corrosion ofMagnesium and Magnesium Alloys,” which appeared in Volume 13, Corrosion, Metals Handbook, 9th ed., 1987; as well as the article by David L. Hawke, James E. Hillis, MihiribanPekguleryuz, and Isao Nakatsugawa, “CorrosionBehavior,” in ASM Specialty Handbook: Magnesium and Magnesium Alloys, ASM International, 1999.REFERENCESFig. 7Effect of heavy-metal contamination on the salt-spray performance of sand-cast AZ91 samples in the T6 temper,as determined by ASTM B 117 method. The samples, containing less than 10 ppm Ni and less than 100 ppmCu, were simultaneously exposed for 240 h. The sample at left contained 160 ppm Fe and had a corrosion rate of 15mm/yr (591 mils/yr). The sample at right contained 19 ppm Fe, and the corrosion rate was 0.15 mm/yr (5.9 mils/yr).1. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers, 1974, p1392. H.P. Godard, W.P. Jepson, M.R. Bothwell,and R.L. Lane, Ed., The Corrosion of LightMetals, John Wiley & Sons, 1967, p 2833. R.B. Mears and C.D. Brown, Corrosion, Vol1, 1945, p 1134. H.P. Godard, W.P. Jepson, M.R. Bothwell,and R.L. Lane, Ed., The Corrosion of LightMetals, John Wiley & Sons, 1967, Chap. 215. J.D. Hanawalt, C.E. Nelson, and J.A. Peloubet, Corrosion Studies of Magnesium
2003 ASM International. All Rights Reserved.ASM Handbook, Volume 13A Corrosion: Fundamentals, Testing, and Protection (#06494G)www.asminternational.org696
Corrosion Passivation 2H 2 O O 2 4H 4 e– H 2 2H 2 e– Fig. 1 C (77 F), show-ing the theoretical domains of corrosion, immunity, and passivation. Source: Ref 1 8 10 6 4 2 1.0 0.8 0.6 0.4 0.2 2 4 6 8 10 Days on test Corrosion rate, mils/yr 1 20 40 60 80 A B Fig. 2 Corrosion
Heat-Resistant Alloy Castings 8, 9 Aluminum Alloys 9, 10 Aluminum Casting Alloys 10, 11 Copper Alloys 12, 13 Copper Casting Alloys 13, 14 Magnesium Alloys/ Casting Alloys 14 Magnesium Alloys/Wrought Alloys 15 Nickel Alloys 15 Super Alloys 16-18 Tin Alloys 18 Zinc Alloys 19 Precious Metal Alloys 19 Ni-Cr-Mo alloys 19
Description: Synthetic magnesium may be obtained from magnesium carbonate, magnesium chloride, magnesium hydroxide, magnesium oxide, and magnesium sulfate. Nonsynthetic magnesium may be obtained from magnesium limestone and magnesium mica. NOP Rule: 205.237(a), 205.237(b)(2) & 205.603(d)(2) 6.2
Magnesium alloys: Magnesium with mixture of other metals is called as magnesium alloys. The major alloying elements are aluminium, zinc, manganese, silicon, copper, rare earths and zirconium. Magnesium alloys have a hexagonal lattice structure and is the lightest structural metal Cast
Metallography of Magnesium and its Alloys Pulised ueler a diision o Illinois ool ors olume Issue Magnesium and its alloys, regardless of the processing procedures employed, are among the most difficult metallic specimens to . Microstructures of AM60 (top) and AZ91D (bottom) alloys after etching with the glycol
1.2 magnesium alloys 2 1.3 mg-al-based alloys 3 1.4 high-pressure die-casting 6 1.5 deformation mechanisms of magnesium alloys 9 1.6 mechanical properties of mg-al alloys 11 1.7 def
Allyl Chloride (3-Chloropropene) CH2 CHCH2CI Almond Oil (artificial) . Magnesium Carbonate MgCO3 Magnesium Chloride MgCl2-6H2O Magnesium Hydroxide Mg(OH)2 Magnesium Nitrate Mg(NO3)2-6H2O Magnesium Oxide MgO Magnesium Sulfate MgSO4 Maleic Acid (CHCOOH)2 Maleic Anhydride C4H2O2
17. Magnesium has three naturally occurring isotopes. 78.70% of magnesium atoms exist as magnesium-24 (23.9850 amu), 10.03% exist as magnesium-25 (24.9858 amu) and 11.17% exist as magnesium-26 (25.9826 amu). Calculate the average atomic mass of magnesium 18. Which subatomic particle plays t
Magnesium Alloys - Design, Processing and Properties 266 Rare Earth addition and aging heat treatment on the microstructure, mechanical properties and creep resistance of AZ91-RE magnesium alloys will be investigated. The effects of Si additions and Al content on the microstructu
