•Cross-sections in SEM show that Mg2Ca dissolves before α-Mg within Mg–Ca alloys.•Mg6Ca2Zn3 locally, temporarily blocks corrosion; is undermined by corrosion.•More ternary phases by higher alloying ...contents of Zn and Ca increase corrosion rate.•At Zn/Ca=1.84, Zn-enrichment of α-Mg decreases potential differences between phases.•Larger, more dendritic grains for higher alloying contents in Mg–Ca–Zn alloys.
This investigation aims at providing deeper understanding of the interactive influence of the alloying elements calcium and zinc on biodegradable magnesium. Through the low-concentration variation of both alloying elements, the formation and distribution of microstructural phases and the in-vitro degradation behavior were investigated to gain mechanistic insights. Mg2Ca dissolves preferentially to α-Mg within Mg–Ca alloys. Mg6Ca2Zn3 temporarily blocks local corrosion but is ultimately undermined due to its discontinuity. More ternary phases obtained by higher alloying contents of Zn and Ca increase the corrosion rate. At a Zn/Ca (atomic) of 1.84, a Zn-enrichment of α-Mg decreases potential differences between the phases.
•Corrosion was determined through immersion and electrochemical testing.•Computational image analysis was adapted and automated for AZ91D micrographs.•Parabolic corrosion-time dependence was found ...for thixomolded AZ91D.•TRC promises satisfying corrosion properties.
The influence of different manufacturing processes on the microstructure and corrosion of the AZ91D magnesium alloy was investigated. The twin-roll casting process was compared with an established high-pressure die casting process and the Thixomolding® process. The best corrosion resistance was found in thixomolded specimens due to a continuous and small meshed β-phase network, a homogeneous and small grain size, a minimal extent of Al and Zn segregations and the highest amount of β-phase. Parabolic corrosion-time dependence was found for thixomolded AZ91D and attributed to microstructural features. A computational image analysis was adapted for AZ91D micrographs and utilised for microstructural quantification.
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•Al graded Mg-Al thin films were synthesized via combinatorial magnetron sputtering.•Al graded Mg-Al thin films were analyzed via in-situ electrochemistry.•The Mg-Al solid solution ...corrosion resistance increases with the Al concentration.•Exceeding cAl ∼ 4 wt% causes a distinct anodic current density decrease.•Exceeding cAl ∼ 4 wt% causes a predominant cathodic partial reaction on the surface.
The effect of varying Al concentrations on the electrochemical corrosion resistance of binary Mg-Al solid solutions thin films under alkaline immersion conditions was investigated via a combination of in-situ flow-cell, scanning vibrating electrode technique and microscopy analysis. These spatially resolving characterization techniques are employed along the Al concentration gradient of the combinatorically grown thin films enabling efficient screening of the Al concentration dependent electrochemical corrosion behaviour. The analysis revealed an increasing corrosion resistance with increasing Al concentration, as a consequence of Al induced hydroxide reinforcement. Specifically, the addition of >4 wt.% Al decreases the corrosion current density in the range of 70–90 % compared to pure Mg.
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•In the T4 condition, the Al wire was resistant to IGC.•Pitting was the dominant corrosion mechanism in T4.•Pitting was found to be driven by cathodic AlFeSi(Mn,Cu) ...particles.•Artificial aging of the T4 samples reintroduced IGC.•The IGC was attributed to Cu-rich particles at the grain boundary.
The influence of individual manufacturing steps during industrial wire drawing processes on the mechanical and corrosion properties of the 6056 aluminum alloy was investigated. These steps demonstrated an essential influence on the microstructure, and thus, the susceptibility to intergranular corrosion (IGC). No clear correlation between IGC susceptibility and hardness was observed. Although the highest resistance against intergranular attack was determined for those alloys in the solution annealed condition, pitting corrosion was identified to occur. Subsequent artificial aging of the solution annealed and quenched wires reintroduced IGC susceptibility; this phenomenon was attributed to the occurrence of galvanic coupling between the noble Cu-phases, located on the grain boundary, and the anodic grain boundary area.
The demand to reduce weight in the automobile industry necessitates an enhanced understanding of the stress corrosion cracking (SCC) mechanism of 7xxx series aluminum wrought alloys. Thus, the SCC ...susceptibility of 7075 aluminum wrought alloy wires, with respect to the heat treatment condition, solution annealed, peak-aged and overaged, was investigated via slow strain rate testing in 3.5 wt % sodium chloride solution. The SCC crack, initiated in a notched tensile specimen of the peak-aged condition, was investigated in detail by means of synchrotron-based high resolution holotomography and X-ray fluorescence as well as STEM in order to understand the correlation of microstructural features and cracking behavior. The local crack initiation and macroscopic crack propagation was found to be dependent on the stress fields, which were simulated by the finite element method. In the peak-aged condition, the SCC crack propagated microscopically along the grain boundaries leading to a brittle intergranular fracture. The SCC mechanism in the peak-aged condition was related to a combination of hydrogen embrittlement and an anodic-dissolution-induced notch effect that was attributed to a continuous seam of grain boundary (GB) precipitates.
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•The intergranular cracks in T4 are presumably due to hydrogen embrittlement.•Intergranular corrosion in T6 is due to a continuous seam of GB precipitates.•The SCC crack in T6 followed macroscopically the highest stress fields and microscopically the grain boundaries.•In T6, corrosion products at the crack tip and a brittle fracture surface were found.•No continuous corrosion path but large H trapping sides in T73 present.
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•The ORR/HER contribution to the total cathodic partial reaction has been determined via in situ respirometric measurements during the electrochemical corrosion of Mg-Al-Ca alloys in ...various electrolytes.•The relative ORR contribution is as high as ∼34% under alkaline conditions.•The absolute ORR current decreases under alkaline conditions.•Two Al-enriched interfaces have been observed in the formed oxide/hydroxide for the first time, and discussed in terms of their stability.
The electrochemical corrosion rate of magnesium depends on the stability of the surface layer formed. Based on the Mg substrate, the oxide structure comprises a dense MgO/Mg(OH)2 mixture underneath a porous plate-like Mg(OH)2 layer. While the kinetics of the anodic partial reaction have been mainly attributed to MgO, recent studies have showed an effect of the Mg(OH)2 layer thickness on the cathodic partial reaction. A thinner Mg(OH)2 layer has been associated with higher kinetics of the oxygen reduction reaction. In the present study, the mechanism has been further investigated via in situ respirometric measurements using Mg-Al-Ca solid solution in electrolytes with different pH values (pH = 8.0–13.0). The results indicate an additional effect based on the structure of the surface layer for Mg-Al-Ca alloys in the passive state. Furthermore, two different Al-enriched interfaces were observed and discussed in terms of their thermodynamic stability under alkaline immersion conditions.
The efficiency of material removal by electrochemical machining (ECM) and rim zone modifications is highly dependent on material composition, the chemical surface condition at the break through ...potential, the electrolyte, the machining parameters and the resulting current densities and local current density distribution at the surfaces. The ECM process is mechanistically determined by transpassive anodic metal dissolution and layer formation at high voltages and specific electrolytic compositions. The mechanisms of transpassive anodic metal dissolution and oxide formation are not fully understood yet for steels such as 42CrMo4. Therefore, martensitic 42CrMo4 was subjected to ECM in sodium nitrate solution with two different current densities and compared to the native oxide of ground 42CrMo4. The material removal rate as well as anodic dissolution and transpassive oxide formation were investigated by mass spectroscopic analysis (ICP-MS) and (angle-resolved) X-ray photoelectron spectroscopy ((AR)XPS) after ECM. The results revealed the formation of a Fe
O
mixed oxide and a change of the oxidation state for iron, chromium and molybdenum, e.g., 25% Fe (II) was present in the oxide at 20.6 A/cm
and was substituted by Fe (III) at 34.0 A/cm
to an amount of 10% Fe (II). Furthermore, ECM processing of 42CrMo4 in sodium nitrate solution was strongly determined by a stationary process with two parallel running steps: 1. Transpassive Fe
O
mixed oxide formation/repassivation; as well as 2. dissolution of the transpassive oxide at the metal surface.
The electrochemical machining (ECM) of 42CrMo4 steel in sodium nitrate solution is mechanistically characterized by transpassive material dissolution and the formation of a Fe
O
mixed oxide at the ...surface. It is assumed that the efficiency of material removal during ECM depends on the structure and composition of this oxide layer as well as on the microstructure of the material. Therefore, 42CrMo4 in different microstructures (ferritic-pearlitic and martensitic) was subjected to two ECM processes with current densities of about 20 A/cm
and 34 A/cm
, respectively. The composition of the process electrolyte was analyzed via mass spectrometry with inductively coupled plasma in order to obtain information on the efficiency of material removal and the reaction mechanisms. This was followed by an X-ray photoelectron spectroscopy analysis to detect the chemical composition and the binding states of chemical elements in the oxide formed during ECM. In summary, it has been demonstrated that the efficiency of material removal in both ECM processes is about 5-10% higher for martensitic 42CrMo4 than for ferritic-pearlitic 42CrMo4. This is on one hand attributed to the presence of the cementite phase at ferritic-pearlitic 42CrMo4, which promotes oxygen evolution and therefore has a negative effect on the material removal efficiency. On the other hand, it is assumed that an increasing proportion of Fe
O
in the mixed oxide leads to an increase in the process efficiency.
Microstructure optimization of Al-Zn-Mg-Cu-Zr aluminum alloys, particularly through recrystallization inhibition, for improved strength and corrosion resistance properties has been an important ...consideration in alloy development for aerospace applications. Addition of rare earth elements, sometimes combined with Cr, has been found to be beneficial in this regard. In this study, the role of a single addition of 0.1 wt.% Cr on microstructure evolution of an Al-Zn-Mg-Cu-Zr (7449) alloy during processing was systematically investigated by optical light microscopy, scanning electron microscopy, electron backscatter diffraction and scanning transmission electron microscopy. Susceptibility to localized corrosion after aging to T4, T6 and T76 conditions was evaluated by potentiodynamic polarization (PDP) measurements and an intergranular corrosion (IGC) test. A decrease in recrystallized fraction with 0.1 wt.% Cr addition was observed, which is attributed to the formation of Cu- and Zn-containing E (Al
Mg
Cr
) dispersoids and the larger as-cast grain size. Moreover, the depletion of alloying elements from solid solution due to the formation of the Cu- and Zn-containing E (Al
Mg
Cr
) dispersoids and
Mg(Zn,Cu,Al)
phase at its interface affects grain-boundary precipitation. The observed decrease in localized corrosion susceptibility with minor Cr addition is correlated with the microstructure and equally discussed.
In this paper, the high strength and lightweight Al-Cu-Li alloy (AA2099) is considered in as-built and preheated conditions (440 °C, 460 °C, 480 °C, 500 °C, and 520 °C). The purpose of this study is ...to investigate the influence of laser powder bed fusion (LPBF) in situ preheating on precipitation microstructure, mechanical and corrosive properties of LPBF-printed AA2099 alloy compared to the conventionally processed and heat-treated (T83) alloy. It is shown that precipitations evolve with increasing preheating temperatures from predominantly globular Cu-rich phases at lower temperatures (as-built, 440 °C) to more plate and rod-like precipitates (460 °C, 480 °C, 500 °C and 520 °C). Attendant increase with increasing preheating temperatures are the amount of low melting Cu-rich phases and precipitation-free zones (PFZ). Hardness of preheated LPBF samples peaks at 480 °C (93.6 HV0.1), and declines afterwards, although inferior to the T83 alloy (168.6 HV0.1). Preheated sample (500 °C) shows superior elongation (14.1%) compared to the T83 (11.3%) but falls short in tensile and yield strength properties. Potentiodynamic polarization results also show that increasing preheating temperature increases the corrosion current density (Icorr) and corrosion rate. Indicated by the lower oxide resistance (R
), the Cu-rich phases compromise the integrity of the oxide layer.
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