Corrosion fatigue behavior of magnesium alloys under oil environments

Materials Science and Engineering A 477 (2008) 129–136

Corrosion fatigue behavior of magnesium alloys under oil environments A. Eliezer a,∗ , O. Medlinsky b , J. Haddad a , G. Ben-Hamu c a

Sami Shamoon College of Engineering, Corrosion Research Center, Bialik/Basel Sts Beer-Sheva 84100, Israel b N.Z.M.-Israel Chevron Texaco Agency, Israel c Department of Materials Engineering, Ben-Gurion University of the Negev, P.O.B. 653, Beer-Sheva 84105, Israel Received 12 December 2006; received in revised form 1 May 2007; accepted 13 May 2007

Abstract The fatigue and mechanochemical behavior of die-cast AZ91D (Mg–9% Al–l% Zn) and AM50 (Mg–5% Al–0.4% Mn) alloys in air, transmission oil and natural mineral oil at 25 ◦ C were studied. New methods were developed in this research in order to achieve better understanding of the mechanochemical effects in Mg alloys during exposure to different oil environments. It is found that the mineral oil environment causes an increase in the lifetime of both alloys in comparison with the gear oil. AM50 alloy shows excellent fatigue lifetime in oil in comparison with other environments. AZ91 shows a significantly higher sensitivity to the environment in comparison with AM50 alloy. © 2007 Published by Elsevier B.V. Keywords: Magnesium alloys; Oil; Corrosion fatigue; Lifetime; Prediction

1. Introduction Magnesium is the lightest of all commonly used metals and is very attractive for applications in transportation, with desirable features including low density, good ductility, high specific strength, and excellent castability. In order to use magnesium alloys in the automotive industry, for example to produce wheels and gearbox housing [1], it is very important to investigate their fatigue behavior. Understanding this phenomenon will increase magnesium consumption; besides, the environment effect is one of main factors determining the fatigue life. Today the most widely used magnesium alloy in the diecasting industry is AZ91D (9% Al, 1% Zn), which has excellent mechanical properties. AM50 (5% Al, 0.3% Mn) has a high ductility making it advantageous when energy absorption is needed during the service [1]. The fatigue life of magnesium alloys in corrosive solutions such as, for example, NaCl, is always less than that in air [2–7]. The corrosion fatigue life of magnesium alloys depends significantly on the solution acidity. For example, in NaCl–KCrO4 solution, a decrease in pH to below 5 results in a considerable increase in the rate of stress-corrosion cracking of a strained Mg alloy (6.5% Al, 1% Zn, and 0.2% Mn, 92.3% Mg). In the pH

∗

Corresponding author. Tel.: +972 52 2701285; fax: +972 8 6460127. E-mail address: [email protected] (A. Eliezer).

0921-5093/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.msea.2007.05.068

interval from 5 to 12 the rate of stress-corrosion cracking remains stable, and at a pH exceeding 12 it begins to decrease rapidly [8]. Due to the anodic dissolution of magnesium and instability of pH in basic electrolytes, the borate 0.1N Na2 B4 O7 buffer solution (pH 9.3) was used in high cyclic fatigue tests [7,9]. In this solution, an unusual result was observed: the fatigue life of AM50 and AZ91D alloys was longer than in air [3]. At the lowest Al content in die-cast and wrought Mg alloys, the weakest environmental effect in static fatigue (creep) [7,9,10] and dynamic (cyclic) fatigue was observed [2,3,7,9]. It is well known that the stress-corrosion cracking rate in magnesium alloys essentially increases with the increase of aluminum content [11–13]. Such additions to aqueous solutions as I− , SO3 −2 , Cl− , and Br− , which can accelerate stress-corrosion crack growth, also accelerate corrosion fatigue crack growth in Mg alloy ZK60A [14]. Under static (stress-corrosion cracking) and cyclic (corrosion fatigue) loading, the subcritical crack growth rate in this alloy is slower in air, argon, and distilled water [14]. The degradation in fatigue strength for the high strength AZ80 or AZ91D alloys due to NaCl is more pronounced than that of lower-strength alloys AZ31 [2] and AM20–AM40 [3], apparently, due to a higher percentage of the second phase in the former [2,3]. It also agrees with the known fact of a higher sensitivity of AZ91D alloy to high cyclic fatigue in 3.5% NaCl solution at stresses below 130 MPa compared with AM50 alloy [9]. One of the most abundant environments in any industry, especially automotive, is oil. Oil is used with almost every

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Table 1 Die-casting parameters

Table 3 Chemical analysis, ppm

Alloy

Die temperature (◦ C)

Casting temperature (◦ C)

Injection pressure (atm)

AM50 AZ91D

150–200 150–200

690 670

300 300

moving part made of metal, so it is important to understand its influence on the metal. Today there are no data on the effect of oil-based solutions on the fatigue behavior of magnesium alloys. An important application in the automotive industry is the transmission housing. In order to succeed in this function, magnesium alloys should not be affected by corrosion attack from the transmission fluid combined with fatigue stress that decreases the lifetime of alloys. During last few years, studies of a correlation between the fatigue phenomenon and the corrosion problem that reduces the number of cycles to fracture were published. It is well known that different environments influence the lifetime, for instance, 3.5% NaCl solution reduces the lifetime of magnesium by 20–30% [15,16]. The present paper is devoted to the effect of industrial and pure mineral oils on fatigue and behavior of magnesium alloys AZ91D and AM50. 2. Experimental Die-cast specimens (with gauge diameter and length of 5.9 and 75 mm, respectively) of Mg alloys AZ91D and AM50 were produced on a cold-chamber machine with the locking force of 3450 kN (Magnesium Research Institute of Dead Sea Magnesium Works, Israel). In addition, we carried out fatigue tests using die-cast hour-glass shaped specimens (with minimum gauge diameter of 7.9 mm and gauge length of 67 mm). Diecast specimens were used with no mechanical treatment of the gauge. Die parameters and chemical composition are given in Tables 1 and 2. All the specimens were checked by radiography to ensure the absence of porosity in the specimens. The ultimate tensile strength (UTS), tensile yield strength (TYS), elongation to fracture of AZ91D are: 249, 182 MPa, 4%, respectively; for AM50: 230, 142 MPa, 11.2%. In order to understand the influence of the quantity and density of ␤-phase in the surface due to the casting process (i.e., die-cast), the skin (0.5 mm diameter) was removed by mechanical process (metalworking). The mechanical process was done in five steps, 100 ␮m in each step, in order to decrease the influence of the process on the residual stresses on the surface. The

Elements Fe Cu Cr Al Pb Ni Sn Ag Ca Mg Ba P Si Na K Mo B

<1 <1 <1 <1 <1 <1 <1 <1 40 4 <1 770 <1 <1 <1 <1 <1

diameter of the sample after the removal of the skin was 5.5 mm; these samples are identified as S.E. (SKIN EFFECT). Fatigue tests were performed on a rotating-beam type (R = −1) fatigue machine (Satec System, Inc., USA) equipped with a special cell at 25 ◦ C and at 30 Hz under stresses that varied from 0.65 to 0.9 of TYS. Cells for these tests were produced from glass [4,5]. For these tests, we used gear oil for industrial transmission applications (TEXACO GEARTEX EP-B SAE 85W90) that is supposed to simulate the environment of the gear housing, and industrial mineral oil (light white oil with the density of 0.84 g/ml) for reference and for better understanding of the results. The TEXACO GEARTEX EP-B SAE 85W90 is a commercial manual transmission oil with EP additives (extreme pressure additives based on chloride or phosphorus components). In order to determine the additives type (commercial secret) we ran a Lubrication Analysis (Table 3). The results show that the main element in the gear oil is phosphorus, with 770 ppm, leading us to the conclusion that this type of oil has phosphorus components as its EP additives. Microstructural studies were carried out using optical microscopy and scanning electron microscopy with an energy dispersive spectroscope (SEM-EDS). All polarization regimes for electrochemical tests were achieved using a corrosion measurement system consisting of a Versastat Potentiostat/Galva-nostat with the floating option. All of the electrochemical measurements were made relative to a saturated calomel electrode (SCE). Potentiodynamic polarization tests were conducted at the scanning rate of 5 mV/s in the range from −1850 to −1300 mV for both undeformed specimens and spec-

Table 2 Chemical analysis, wt% Alloy

Al

Mn

Zn

Si

Cu

Ni

Fe

Other

AM-50 AZ-91

4.4–5.4 8.3–9.7

0.26–0.60 0.15–0.50

Max 0.22 0.35–1.00

0.10 0.10

0.01 0.03

0.002 0.002

0.004 0.005

0.02 0.02

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imens after fatigue tests. During these tests, the electrode was activated by the application of cathodic current before anodic polarization. Specimens for mechanoelectrochemical investigations were studied with as-cast surfaces after polishing, rinsing with distilled water, and wiping with acetone. The compositions of the electrolytes used were: 0.1N Na2 B4 O7 saturated with Mg(OH)2 , 1% NaCl + 0.1N Na2 B4 O7 (1:1)—the ratio of 1 l of 0.1N Na2 B4 O7 mixed with 1 l of 1% NaCl. All solutions were prepared from analytical grade chemicals and distilled water. All polarization regimes for mechanoelectrochemical tests were achieved using a corrosion measurement system consisting of a Versastat Potentiostat/Galva-nostat with the floating option. Clamping an electrochemical cell to a specimen in a tensile testing machine made it possible to conduct electrochemical investigations under static or dynamic conditions of loading. The unit consisted of a Zwick Type 1445 tensile testing machine with a computer, an electrochemical cell, and a potentiostat with a computer. All of the electrochemical measurements were made relative to a saturated calomel electrode (SCE). Potentiodynamic polarisation tests were conducted at the scanning rate of 5 mV/s in the range from −1800 to 1600 mV for both undeformed specimens and specimens that had been strained plastically to specified levels of up to 10–15%. During these tests, the electrode was activated by the application of cathodic current before anodic polarization. This experimental technique is convenient to use with samples of both flat and cylindrical geometry. Corrosion rates were derived from the polarization data in the usual way (Tafel extrapolation) [17]. The elemental composition of the magnesium alloys at the surface and in depth concentration distributions (sputter depth-profiling) were estimated using surface sensitive AES combined with controlled Ar ion bombardment. A PHI 549 SAM/AES/XPS apparatus with standard pressure of 2 × 109 Torr in the chamber and cylindrical mirror analyzer were used for AES measurements. Quantitative analysis was based on atomic sensitivity factors and was carried out proceeding from general survey spectra. The sputtering procedure was done using a differential pumped small-spot ion gun (IQE 12/38) under an Ar pressure of 2.5 × 107 Torr and a power supply voltage of 6 kV with the sputtering yield of ∼20 nm/min at the maximum sputtering duration of 100 min. This corresponds, to the first approximation, to the thickness of the AES surface layer under study of about 600 nm. Specimens for Auger investigations, in air and immersion condition, were studied after polishing, rinsing with distilled water, and wiping with acetone and alcohol. The SSRTs (slow strain rate test) were carried out in all three environments. All measurements were taken from the same batch of specimens that was used for the fatigue tests. The strain rate for the SSRT measurements was 8 × 10−5 s−1 .

Fig. 1. S–N curve of AZ91D alloy in different environments.

eter for two reasons. The first is called the “size effect”—as the part becomes smaller the probability of a defect decreases. The second is related to the die-cast process; a smaller specimen needs less time to solidify so the grain is finer and the porosity and shrinkage decrease. The influence of the environment on the lifetime of the alloy is rather significant; mineral oil almost doubles the lifetime as compared to air, while gear oil increases the lifetime only by half. At high strength (169 MPa), numbers of cycles to fracture are approximately the same in all environments. At 121 MPa, AZ91D is more sensitive to the environment than AM50. In comparison with air, mineral oil causes a fourfold increase in the lifetime of AZ91D, while gear oil causes only a twofold increase. At 169 MPa, mineral oil reduces the lifetime of AZ91D compared to gear oil and air that have almost the same lifetime. In our studies [17–19], it was seen that in the non-stressed conditions, the corrosion rate of the alloy with higher Al content is lower than that for the alloy with less Al. This is largely due to the presence and distribution of the ␤-phase (Fig. 3(a)) that better protects the AZ91D alloy with a higher Al content and hence with an elevated quantity of ␤-phase. For AM50 with a low Al content, the fraction of ␤-phase is small and is only formed as discrete islands in the structure (Fig. 3(b)). However, hard secondary phase promotes strain hardening and thus, increases the chemical potential of atoms, i.e., they create the necessary conditions for mechanochemical dissolution [20]. Consequently, the greatest resistance to developing a mechanochemical effect will be obtained for the alloys with the lowest Al content. Indeed, deep pitting corrosion occurs and

3. Results and discussion S–N curves for AZ91D and AM50 alloys in different kinds of environment and for different diameters are given in Figs. 1 and 2, respectively. As expected, the specimens 6 mm in diameter show better stability than specimens 7.9 mm in diam-

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Fig. 2. S–N curve of AM50 alloy in different environments.

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Fig. 4. Mg alloys under the loading of 121 MPa.

Fig. 3. Typical microstructures of die-cast (a) AZ91D and (b) AM50 alloys observed by optical microscope.

the lifetime of the alloy decreases as the quantity of ␤-phase increases [21]. The microstructure of 6 mm diameter alloys is characterized by the presence of extended deformation bands and ␤-phase stringers, which lead to increased corrosion rates compared to the 7.9 mm diameter alloy, especially during loading with an external stress. Furthermore, the distribution of the ␤-phase is different due to the different diameter. In the die-cast condition 6 mm in diameter, the ␤ precipitates are finely and continuously distributed along the grain boundaries in the surface region of the casting, so the barrier effect of the ␤-phase is more effective in impeding corrosion [17,22–24]. However, the distribution of the ␤-phase in the die-cast condition 7.9 mm in diameter was different. The ␤ precipitates are not continuously distributed along the grain boundaries (rod shaped ␤-phase). The ␤-phase should act mainly as a galvanic cathode during the initial period of corrosion. A 6 mm diameter specimen of AM50 in mineral oil shows a superior lifetime in the full range of stress. At 121 MPa in air, this specimen shows a better lifetime than the 7.9 mm diameter sample in mineral oil; however, at 169 MPa the 6 mm diameter specimen breaks in air a few cycles before a thicker specimen in mineral oil.

AM50 alloy shows a lower sensitivity to the oil environment compared with AZ91D (Fig. 4). In comparison with air, the increase in the lifetime of AM50 is approximately twice in mineral oil and 70% in gear oil. In order to understand the effect of oil on different Al contents, fracture surface of the fatigue samples were studied by SEM (Fig. 5). The fracture surface of the two Mg alloys, AZ91D and AM50, was divided into two areas; one infancies by the mechanical stresses and the other by the corrosion reaction during the fatigue tests. In the alloys containing high Al content (AZ91D), the area that is influenced by the corrosion reaction was higher in comparison to the other area and to the same area in AM50. The higher corrosion area in AZ91D is due to the high content and density of the ␤-phase that act as a galvanic area and increased the dissolution rate and decreased the fatigue lifetime of AZ91D in comparison to AM50 magnesium alloy. The initiation stage of the fatigue fracture in the studied alloys is due to the Al content. Higher Al content in the alloys increased the dissolution rate of the Mg matrix (the ␤-phase act as a galvanic area)—forming corrosion pits. These pits were the initiation sites for the corrosion fatigue failures (Fig. 6(a)). During the corrosion fatigue tests, the pits are initiated and grow into corrosion fatigue cracks (Fig. 6(b) and (d)). For AM50, the corroded surface was more homogenous, fewer corrosion pits were observed (Fig. 6(c)), so fewer cracks were formed during corrosion fatigue tests (Fig. 6(d)). It may be mentioned that the oil energy adsorption interaction decreases the pitting formation initiation. After determining the fatigue life of each alloy in different environments and under different stresses, potentiodynamic polarization was measured from specimens after different fatigue life times to fracture (i.e., 0.25, 0.50, and 0.75). The potentiodynamic curves of AM50 after a fatigue test in gear oil performed in the fatigue machine for 0.25 of its lifetime at 169 MPa are given in Fig. 7. The curves indicate six different time intervals of immersion: 0, 30, 60, 120, 180, and 240 min. The corrosion behavior (from potentiodynamic polarization) of AZ91D after 0.25 of the fatigue life at 121 MPa, 0.25 of the fatigue life at 169 MPa, and as-cast are shown in Fig. 8. At the “0 time” (after 15 min of immersion), a sharp decrease in the

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Fig. 5. SEM fracture-graph of (a) AM50 and (b) AZ91D magnesium alloys after fatigue tests in 121 MPa.

Fig. 6. SEM micrographs of corroded surfaces without stress (a) AZ91D and (c) AM50, and corroded surface under stress (b) AZ91D and (d) AM50, in 0.1N Na2 B4 O7 solution saturated with Mg(OH)2 .

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Fig. 8. Corrosion rate of AZ91D alloy in the gear oil.

Fig. 7. The potentiodynamic curves of AM50 Mg alloy tested in the fatigue machine for 0.25 of its lifetime under 169 MPa in gear oil.

corrosion rate of AZ91D at 121 MPa was observed. A higher corrosion rate was observed in specimens tested in the fatigue machine for 0.25 of their lifetime as compared with as-cast specimens. Corrosion rate increases with decreasing applied fatigue stress that increases the number of cycles to failure. The residual stress in the specimen increases in every cycle; therefore, we observed an increase in the corrosion rate at a lower fatigue stress. Probably the stored energy, which remains in a specimen after the fatigue test, is freed by the electrochemical reaction until the alloy achieves equilibrium with the environment. In order to understand better the influence of ␤-phase and to find the correlation between the quantity and the high density

of ␤-phase in the surface of the die-cast samples due to the diecast process, 0.5 mm was removed from the surface of die-cast samples by a mechanical process (metalworking). The surface microstructure of AZ91D and AM50 in die-cast and “without skin” conditions are shown in Fig. 9. The “new surface” of the sample exhibits a low quantity of ␤-phase compared to the surface after the die-cast process. Due to the removal of the skin from the die-cast samples, the density of the ␤-phase was decreased. Furthermore, the mechanoelectrochemical behavior of the alloys without the skin was different compared to the die-cast alloys. The anodic dissolution of AM50 without skin (S.E.) (Fig. 10) was lower in comparison to the die-cast surface due to different quantity of ␤-phase. The anodic dissolution of die-cast (D.C.) [17] and no skin (S.E.) AM50 magnesium alloys during mecha-

Fig. 9. Typical surface microstructures of die-cast (a) AZ91D and (c) AM50, and (b) “no skin” AZ91D and (d) AM50 alloys observed by optical microscope.

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Fig. 10. Cross-section of anodic polarization curve for anodic potential (ϕ = −1.35 VSCE ) of AM50 as-cast and no skin Mg alloys in different media.

noelectrochemical tests is shown in Fig. 10. This test was done in different solutions —buffer and buffer containing 1% NaCl. The addition of Cl ion to the solution increased the anodic dissolution of the AM50 magnesium alloys in both conditions (die-cast and no skin). The corrosion fatigue behavior of AZ91D magnesium alloys in 169 MPa in three different solutions, 3.5% NaCl, gear oil, and gear oil with 3.5% NaCl, is presented in Fig. 11. The sample without the skin exhibits a higher fatigue lifetime in comparison to the die-cast surface due to different Al contents in the surface. In this study the relation between mechanoelectrochemical (static stress—Fig. 10) and corrosion fatigue (dynamic stress—Fig. 11) was observed. High ␤-phase in the surface increased the anodic dissolution and increased the fatigue lifetime (time to fracture). In order to understand the interaction between the oil and the magnesium surface, this work employed Auger measurement to analyze the element of the surface after immersion in different oils (gear and mineral). AZ91D magnesium alloy was immersed in mineral and gear oils for 4 h after polishing with 1 ␮m diamond. Depth profile was used in order to understand the adsorption mechanism of the oil and the depth of the adsorption layer. The depth profile of the carbon content during the sputtering is shown in Fig. 12. AZ91D without immersion was measured

Fig. 11. AZ91D Mg alloys under the loading of 169 MPa, in as-cast and no skin conditions.

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Fig. 12. Variation of carbon concentration of the AZ91D alloy vs. depth penetration (sputtering yield of ∼20 nm/min).

as a reference. The sample after immersion in gear oil exhibits a higher concentration of carbon in the surface and a thicker of adsorption layer. The reference sample and the sample after immersion in mineral oil exhibit the same behavior concentration and adsorption layer of carbon. The carbon content on the surface of the reference sample can be due to the diamond polishing process. The oil adsorption to the magnesium surface can be a result of physical adsorption, chemical adsorption, or chemical reaction (when the oil contains EP additives). In this work, the gear oil contained an EP additive, so it can be concluded that the interaction between the oil and the magnesium surface is a chemical reaction. After determining the fatigue life of each alloy in different environments and under different stresses, and we understood the interaction between the oil and the magnesium surface, SSRT tests were examined under the single direction stress test in notch samples of AZ91D magnesium alloy. Fatigue lifetime accumulates by two processes—crack initiation and crack propagation. In order to understand the effect of oil in the crack propagation rate, SSRT was used (Fig. 13). The results of SSRT show a common trend between the three environments, but in 3.5% NaCl there is a decrease at the stress of 166 MPa and 1.5% strain. The oil decreased the crack propagation rate and increased the lifetime of AZ91D magnesium alloy. The oil solution adsorbs to the magnesium surface; due to the EP addition in the oil, the magnesium corrodes. However, in

Fig. 13. SSRT of AZ91 in different environments.

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stress conditions the oil reduces the crack propagation rate due to adsorption of the oil to the crack tip. 4. Conclusions 1. It was found that pure mineral oil reveals very high protective properties during fatigue tests. In comparison with air, mineral oil causes a fourfold increase in the lifetime of AZ91D, while gear oil causes only a twofold increase. 2. AM50 alloy shows a lower sensitivity to the oil environment in comparison with AZ91D. Compared with air, the increase in the lifetime of AM50 is approximately twice in mineral oil and 70% in gear oil. AZ91D is more sensitive to the environment than AM50. 3. Finer grains and fewer casting defects (such as porosity, shrinkage) improve fatigue life of a sample 6 mm in diameter, compared to a sample 7.9 mm in diameter. To reduce corrosion and extend fatigue life, protective environment, thickness, and fatigue strength must be considered in designing components for various industries. 4. AZ91D and AM50 magnesium alloys at two different surface conditions (i.e., die-cast and without skin) exhibit different stress corrosion behavior. The alloys without the skin increased the fatigue lifetime and reduced anodic dissolutions. 5. The interaction between the oil and the magnesium surface was studied by Auger measurements. The interaction was a chemical reaction due to the EP addition in the gear oil. 6. Oil environment influenced the SSRT behavior of AZ91D magnesium alloy. The oil reduced the crack propagation rate and increased the lifetime. References [1] S. Shumann, F. Friedrich, The Use of Magnesium in Car—Today and in the Future, Volkswagen AG Wolfsburg, pp. 355–362. [2] M. Hilpert, L. Wagner, Proceedings of the International Congress: Magnesium 2000, Magnesium Alloys and their Applications, Munich, 2000, pp. 304–311.

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