Evaluation of Corrosion Behavior on Crept AlSi10MnMg (AA365) Alloy Produced by High-Pressure Die-Casting (HPDC)

This paper provides a detailed analysis of the corrosion behavior of crept AlSi10MnMg (AA365) alloy, a material widely used in automotive components exposed to high temperatures and corrosive environments. The study investigates the correlation between corrosion resistance and the microstructure, specifically focusing on intermetallic compounds and micro-voids formed under different creep temperatures.

1. Overview:

  • Title: Evaluation of Corrosion Behavior on Crept AlSi10MnMg (AA365) Alloy Produced by High-Pressure Die-Casting (HPDC)
  • Authors: Seonghwan Park, Cheolmin Ahn, and Eunkyung Lee
  • Publication Year: 2021
  • Publishing Journal: Applied Sciences, MDPI
  • Keywords: aluminum alloy; corrosion; microstructure; temperature; intermetallic compounds
Figure 2. EPMA micrograph and chemical element distribution maps of crept AA365 alloy at 373 K: Al, Mg, Fe, Si, Mn
Figure 2. EPMA micrograph and chemical element distribution maps of crept AA365 alloy at 373 K: Al, Mg, Fe, Si, Mn

2. Research Background:

  • Social/Academic Context of the Research Topic:
    • The increasing demand for lightweight materials in industries like automotive, marine, and aerospace is driven by environmental regulations and the need for improved fuel efficiency.
    • Aluminum alloys are gaining prominence due to their advantageous properties, including light weight, excellent corrosion resistance, and high strength-to-weight ratio [1].
    • AlSi10MnMg (Silafont-36, AA365) alloy is a leading aluminum-silicon alloy known for its good castability, wear resistance, corrosion resistance, and high elongation [2].
    • High-Pressure Die-Casting (HPDC) is the primary production method for AA365 alloy, enabling high productivity and the creation of complex components [3,4].
    • HPDC AA365 alloy is extensively used in automotive powertrain systems, such as cylinder heads and engine blocks, which operate in high-temperature and corrosive environments [2].
  • Limitations of Existing Research:
    • While previous studies have explored various characteristics of AlSi10MnMg alloys, including microstructure, casting defects, heat treatment, mechanical properties, and corrosion resistance [11-14], there is a gap in understanding the corrosion behavior under creep conditions.
    • Existing research has shown that the properties of AlSi10MnMg alloys vary with cooling rates after T4 and T6 heat treatments [9], and superior creep resistance has been observed under high temperature and stress [2]. However, the corrosion properties of AlSi10MnMg alloy affected by applied stress at high temperature remain under-investigated.
  • Necessity of the Research:
    • To address the limited understanding of corrosion behavior in crept AlSi10MnMg alloy, this research aims to investigate corrosion from a microstructural perspective.
    • The study seeks to contribute to enhancing the reliability and long-term structural performance of AA365 alloy in automotive applications by examining the correlation between corrosion resistance and microstructural features like intermetallic compounds and micro-voids under creep conditions.
    • Understanding this relationship is crucial for predicting and mitigating corrosion in components subjected to long-term temperature and stress.

3. Research Purpose and Research Questions:

  • Research Purpose:
    • The primary purpose is to investigate the corrosion behavior of crept AlSi10MnMg alloy (AA365) from a microstructural viewpoint.
    • The research aims to establish the correlation between corrosion resistance and microstructures, specifically intermetallic compounds and micro-voids, in crept AA365 alloys under varying temperatures and stresses.
    • Ultimately, the study seeks to contribute to improving the reliability and long-term structural performance of this alloy in automotive industry applications.
  • Key Research Questions:
    • How do different creep temperatures (373 K, 473 K, and 573 K) influence the microstructure of HPDC AA365 alloy, particularly the formation and distribution of intermetallic compounds and micro-voids?
    • What is the relationship between the microstructure of crept AA365 alloy and its corrosion resistance in a corrosive environment?
    • Does the density and type of intermetallic compounds, or the presence of micro-voids, have a more significant impact on the corrosion behavior of crept AA365 alloy?
  • Research Hypotheses:
    • The microstructure of crept AA365 alloy, specifically the density and characteristics of intermetallic compounds and micro-voids, will be significantly altered by the creep temperature.
    • Variations in microstructure, induced by different creep temperatures, will directly influence the corrosion resistance of the AA365 alloy.
    • Intermetallic compounds, rather than micro-voids, are hypothesized to be the primary factor influencing the corrosion behavior of crept AA365 alloy due to the potential for micro-galvanic corrosion.

4. Research Methodology:

  • Research Design:
    • The study employed an experimental research design to evaluate the corrosion behavior of crept HPDC AA365 alloy.
    • Creep tests were conducted at different temperatures to induce microstructural changes, followed by electrochemical corrosion tests and detailed microstructural analysis.
  • Data Collection Method:
    • Creep Tests: Cylindrical creep specimens of HPDC AA365 alloy were subjected to creep tests at three different temperatures: 373 K (190 MPa), 473 K (120 MPa), and 573 K (80 MPa). These conditions were selected based on preliminary experiments to ensure the formation and re-dissolution of intermetallic compounds.
    • Electrochemical Corrosion Tests: Cyclic potentiodynamic polarization (CPDP) and open circuit potential (OCP) measurements were performed using a computer-controlled GAMRY potentiostat and a three-electrode electrochemical cell system in a 3.5 wt% sodium chloride (NaCl) solution at room temperature.
    • Microstructural Analysis:
      • Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS): FE-SEM (MIRA3, Tescan) with EDS was used to investigate the microstructure of crept and corroded samples and to identify the elemental composition of intermetallic phases.
      • Electron Probe X-ray Microanalyzer (EPMA): EPMA (JXA-8230, JEOL) was utilized to analyze the chemical elements of intermetallic phases distributed in the Al matrix.
  • Analysis Method:
    • Electrochemical Data Analysis: Gamry Echem Analyst software was used to analyze CPDP curves. Tafel extrapolation method was employed to determine corrosion potential (Ecorr) and corrosion current density (Icorr).
    • Microstructural Analysis: SEM and EPMA images were analyzed to characterize the morphology, distribution, and composition of intermetallic compounds and micro-voids. EDS analysis was used to identify the types of intermetallic phases.
    • Correlation Analysis: The study correlated the microstructural features (density and type of intermetallic compounds, presence of micro-voids) with the electrochemical corrosion parameters (Ecorr and Icorr) to understand the relationship between microstructure and corrosion behavior.
  • Research Subjects and Scope:
    • The research focused on HPDC AA365 (Silafont 36) aluminum alloy supplied by Rio Tinto (USA).
    • Three crept samples were investigated: AA365 alloy crept at 373 K, 473 K, and 573 K under specific applied stresses.
    • Corrosion tests were conducted in a 3.5 wt% NaCl solution to simulate a corrosive environment.
    • Microstructural analysis was performed on both crept and corroded samples to assess the changes induced by creep and corrosion processes.

5. Main Research Results:

  • Key Research Results:
    • Microstructure: Crept AA365 alloy at 473 K exhibited a significantly higher density of intermetallic phases compared to alloys crept at 373 K and 573 K (Figure 1b). The alloy crept at 573 K showed the lowest density of intermetallic precipitates (Figure 1c). EDS and EPMA analysis identified the brittle intermetallic compounds as π-Al12FeMg1.5Si5 and α-Al16(Mn,Fe)Si3 (Figure 1d, Figure 2).
    • Corrosion Potential (Ecorr): The corrosion potentials were -687.0 mVSCE, -684.0 mVSCE, and -673.0 mVSCE for crept AA365 alloys at 373 K, 473 K, and 573 K, respectively (Table 2). The corrosion potential was highest (least negative) for the 573 K sample.
    • Corrosion Current Density (Icorr): The corrosion current density was highest for the crept AA365 alloy at 473 K, with a value of 13.3 × 10-6 Acm-2, compared to 8.02 × 10-6 Acm-2 at 373 K and 0.51 × 10-6 Acm-2 at 573 K (Table 2).
    • Corrosion Morphology: Localized corrosion was observed on the Al matrix in the vicinity of intermetallic compounds, indicating micro-galvanic corrosion (Figure 5). The alloy crept at 473 K showed the most severe corrosion.
  • Statistical/Qualitative Analysis Results:
    • Corrosion Potentials (Ecorr):
      • 373 K: -687.0 mVSCE
      • 473 K: -684.0 mVSCE
      • 573 K: -673.0 mVSCE
    • Corrosion Current Densities (Icorr):
      • 373 K: 8.02 × 10-6 Acm-2
      • 473 K: 13.3 × 10-6 Acm-2
      • 573 K: 0.51 × 10-6 Acm-2
  • Data Interpretation:
    • The higher density of intermetallic compounds in the AA365 alloy crept at 473 K is attributed to the precipitation of non-equilibrium solute atoms from the supersaturated α-Al matrix during creep.
    • The lower density of intermetallic compounds at 573 K is due to the re-dissolution of these phases back into the Al matrix at higher temperatures.
    • The corrosion potential results indicate that the corrosion process is slowest for the 573 K sample and fastest for the 473 K sample.
    • The significantly higher corrosion current density for the 473 K sample suggests that a large amount of intermetallic compounds promotes corrosion.
    • The localized corrosion around intermetallic compounds confirms that micro-galvanic corrosion between the intermetallic phases (cathodic) and the Al matrix (anodic) is the primary corrosion mechanism.
    • The study infers that the amount of intermetallic compounds is a more critical factor in corrosion than micro-voids in crept AA365 alloy.
  • Figure Name List:
    • Figure 1. SEM and EDS analysis of microstructure of crept HPDC AA365 alloys: (a) crept AA365 alloy at 373 K; (b) crept AA365 alloy at 473 K; (c) crept AA365 alloy at 573 K; (d) A higher magnification micrograph and EDS analysis of crept AA365 alloy at 473 K.
    • Figure 2. EPMA micrograph and chemical element distribution maps of crept AA365 alloy at 373 K: Al, Mg, Fe, Si, Mn.
    • Figure 3. Variation of the open circuit potential (OCP) of crept AA365 alloys in 3.5 wt.% NaCl during a hour of immersion: (a) crept AA365 alloy at 373 K; (b) crept AA365 alloy at 473 K; (c) crept AA365 alloy at 573 K.
    • Figure 4. Cyclic potentiodynamic polarization (CPDP) curves of crept HPDC AA365 alloys in 3.5 wt.% NaCl.
    • Figure 5. SEM and EDS analysis of corroded surface morphology of crept HPDC AA365 alloys of the attack after electrochemical testing in 3.5 wt.% NaCl: (a) crept HPDC AA365 alloy at 373 K; (b) crept HPDC AA365 alloy at 473 K; (c) crept HPDC AA365 alloy at 573 K; (d) BSE image of crept HPDC AA365 alloy at 473K along with the EDS elemental maps of Al, Si, Fe, Mg, and O.
Figure 1. SEM and EDS analysis of microstructure of crept HPDC AA365 alloys: (a) crept AA365 alloy at 373 K; (b) crept AA365 alloy at 473 K; (c) crept AA365 alloy at 573 K; (d) A higher magnification micrograph and EDS analysis of crept AA365 alloy at 473 K.
Figure 1. SEM and EDS analysis of microstructure of crept HPDC AA365 alloys: (a) crept AA365 alloy at 373 K; (b) crept AA365 alloy at 473 K; (c) crept AA365 alloy at 573 K; (d) A higher magnification micrograph and EDS analysis of crept AA365 alloy at 473 K.

6. Conclusion and Discussion:

  • Summary of Main Results:
    • Creep temperature significantly influences the microstructure and corrosion behavior of HPDC AA365 alloy.
    • Creep at 473 K resulted in the highest density of intermetallic compounds, leading to the highest corrosion current density (13.3 × 10-6 Acm-2) and thus the fastest corrosion rate.
    • Creep at 573 K, due to the re-dissolution of intermetallic compounds, resulted in the lowest corrosion current density (0.51 × 10-6 Acm-2) and the slowest corrosion rate.
    • Micro-galvanic corrosion, driven by the potential difference between intermetallic compounds and the Al matrix, is identified as the primary corrosion mechanism.
    • The study concludes that the density of intermetallic compounds is a more dominant factor in controlling the corrosion behavior of crept AA365 alloy than micro-voids.
  • Academic Significance of the Research:
    • This research provides valuable insights into the complex interplay between creep-induced microstructural evolution and corrosion behavior in HPDC AA365 alloy.
    • It enhances the fundamental understanding of corrosion mechanisms in aluminum alloys containing intermetallic compounds, particularly in the context of creep conditions.
    • The findings contribute to the broader field of materials science and engineering by highlighting the importance of microstructural control in achieving desired corrosion resistance in structural alloys.
  • Practical Implications:
    • The study's results are crucial for the reliable application of HPDC AA365 alloy in automotive components operating at elevated temperatures and in corrosive environments.
    • Understanding the detrimental effect of intermetallic compounds on corrosion resistance under creep conditions can guide the development of strategies to mitigate corrosion.
    • Potential strategies include optimizing alloy composition, controlling solidification and cooling rates during HPDC, or employing post-casting heat treatments to manage the formation and distribution of intermetallic phases and thereby improve corrosion performance.
  • Limitations of the Research:
    • The study focused on a specific set of creep conditions and a single alloy composition.
    • Further research is needed to generalize these findings to a broader range of creep conditions, stress levels, and variations in AA365 alloy composition or other similar aluminum alloys.
    • The long-term corrosion behavior under creep conditions requires further investigation to fully assess the durability of the material in real-world applications.

7. Future Follow-up Research:

  • Directions for Follow-up Research:
    • Long-term corrosion studies under various creep conditions are recommended to comprehensively evaluate the material's durability.
    • Investigating the influence of different types and distributions of intermetallic compounds on corrosion behavior would provide a more detailed understanding.
    • Exploring methods to modify the microstructure, such as tailored heat treatments or minor alloy modifications, to control intermetallic phase formation and improve corrosion resistance is a promising avenue.
  • Areas Requiring Further Exploration:
    • The precise role of different types of intermetallic compounds (β-Al5SiFe, α-Al15(Mn,Fe)3Si2, Mg2Si, π-Al8Mg3FeSi6) in the micro-galvanic corrosion process needs further clarification.
    • While the study suggests intermetallic compounds are more significant, the potential synergistic effect of micro-voids and intermetallic compounds on corrosion initiation and propagation warrants further exploration.
    • Investigating the corrosion behavior in more complex and realistic corrosive environments, beyond 3.5 wt% NaCl solution, would enhance the practical relevance of the research.

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9. Copyright:

  • This material is Seonghwan Park, Cheolmin Ahn and Eunkyung Lee's paper: Based on "Evaluation of Corrosion Behavior on Crept AlSi10MnMg (AA365) Alloy Produced by High-Pressure Die-Casting (HPDC)".
  • Paper Source: https://doi.org/10.3390/app11136227

This material was summarized based on the above paper, and unauthorized use for commercial purposes is prohibited.
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