Molten aluminum purification process.

Can Boron Really Clean Up Aluminum? Exploring Chromium Removal

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Jordan Keane in Tech & Innovation December 2025 • 5 min read.

"Unlocking the Secrets of Aluminum Alloys: Investigating How Boron Additives Can Purify Molten Aluminum by Eliminating Chromium Contamination"


In the world of aluminum alloys, even tiny amounts of transition metals like chromium (Cr) can significantly reduce electrical conductivity, making the metal less efficient for many applications. To combat this, industries often use a process called boron treatment, where boron-bearing substances are added to remove these unwanted metals during production. This method can remove everything from trace amounts to as much as 1400 parts per million (ppm) of these metals.

Aluminum-boron (Al-B) master alloys, which contain compounds like AlB2 or AlB12, are typically used to bind transition metals into stable borides, effectively isolating them. Previous studies have shown that this technique can successfully remove zirconium (Zr) and vanadium (V) from molten aluminum. A phase diagram analysis of the Al-Cr system indicates that Al7Cr is the stable phase in Al-1wt%Cr at room temperature. Chromium is known to form several borides (CrB2, Cr2B, CrB, Cr3B4, Cr5B3, and CrB4) in the Cr-B binary system.

Research indicates that removing chromium can increase the electrical conductivity of smelter-grade aluminum by up to 22 times. However, it's still unclear whether chromium can be extracted from molten aluminum in the same way as zirconium and vanadium, forming stable diborides like CrB2. While there are conflicting reports on chromium removal, this article explores the possibility of using Al-B(AlB12) master alloy at 750°C to remove chromium from molten aluminum alloy.

The Experiment: Can Boron Really Grab Chromium?

Molten aluminum purification process.

Researchers prepared an Al-1wt%Cr alloy using an induction furnace and introduced aluminum boron master alloy ingots containing AlB12 into the molten mix. The goal was to observe if CrB2 would form as chromium reacted with boron from the AlB12. Samples were taken regularly for analysis using a scanning electron microscope (SEM) and inductively coupled plasma atomic emission spectroscopy (ICP-AES).

Initial SEM analysis of the Al-1wt%Cr-0.415wt%B alloy, taken 15 minutes after adding the Al-B master alloys, showed no immediate reaction products. Ideally, a reaction between Cr and AlB12/B would produce continuous or discontinuous layers of chromium borides at the interface. Energy dispersive X-ray (EDS) analysis, however, revealed that the grey particles present were primarily aluminum and chromium, likely Al7Cr. These particles appeared to nucleate on existing AlB12 or undissolved Al7Cr particles and aggregated during solidification.

  • Alloy Preparation: An Al-1wt%Cr alloy was created using an induction furnace.
  • Boron Addition: Aluminum boron master alloy ingots containing AlB12 were introduced.
  • Reaction Goal: To observe the formation of CrB2 as chromium reacted with boron.
  • Sampling: Samples were taken at regular intervals for detailed analysis.
  • Analysis Techniques: Scanning electron microscope (SEM) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were used.
ICP analysis measured the chromium concentration in the Al-1wt%Cr-0.415wt%B alloy over time. Surprisingly, the chromium concentration was initially lower than 1wt% but increased over time, likely due to the dissolution of Al7Cr intermetallic particles releasing chromium back into the molten alloy. Overall, the experiment found no evidence that chromium levels decreased with the AlB12 reaction time, suggesting this method may not effectively remove chromium from molten aluminum.

Key Takeaways: The Ongoing Quest for Aluminum Purification

The study revealed that using Al-B(AlB12) master alloy at 750°C might not be a feasible method to remove chromium from molten aluminum. The chromium concentration did not decrease as expected; instead, it increased due to the dissolution of Al7Cr intermetallic particles.

Previous research suggested that chromium in sludge might be due to small Al-Cr intermetallic compounds rather than borides, aligning with the findings of this study. The presence of these intermetallic compounds complicates the purification process, requiring alternative strategies.

Further research is needed to explore different methods for chromium removal in aluminum alloys. Understanding the behavior of Al7Cr intermetallic particles and developing techniques to prevent their dissolution could lead to more effective purification processes and improved aluminum alloy quality.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1017/s1431927618011844, Alternate LINK

Title: Exploring Possibility Of The Chromium (Cr) Removal From Molten Aluminum By Adding Boron Bearing Additive (Aluminum-Boron Master Alloy)

Subject: Instrumentation

Journal: Microscopy and Microanalysis

Publisher: Cambridge University Press (CUP)

Authors: Abdul Khaliq, Muhammad Akbar Rhamdhani, Rahila Batul

Published: 2018-08-01

Everything You Need To Know

1

Why is it important to remove transition metals such as chromium from aluminum alloys?

In aluminum alloys, even small amounts of transition metals like chromium can reduce electrical conductivity. To address this, industries use boron treatment, adding boron-bearing substances to remove unwanted metals during production. This process can eliminate trace amounts up to 1400 parts per million of these metals, improving the metal's efficiency for various applications. Using aluminum-boron master alloys is a prevalent method of boron treatment.

2

How do aluminum-boron master alloys work to remove unwanted metals from molten aluminum?

Aluminum-boron master alloys, containing compounds such as AlB2 or AlB12, are used to bind transition metals into stable borides, effectively isolating them. These alloys have been proven effective in removing zirconium and vanadium from molten aluminum. The intention is that the boron will react with the unwanted transition metal to form stable borides, thereby preventing the transition metal from negatively impacting the aluminum.

3

How was the experiment conducted to test the removal of chromium using Al-B(AlB12) master alloy?

Researchers prepared an Al-1wt%Cr alloy using an induction furnace and introduced aluminum boron master alloy ingots containing AlB12 into the molten mix. Samples were taken regularly for analysis using a scanning electron microscope (SEM) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The goal was to observe if CrB2 would form as chromium reacted with boron from the AlB12, however it was discovered that it may not be a feasible method to remove chromium from molten aluminum at 750°C.

4

What were the unexpected findings regarding chromium concentration during the AlB12 reaction time, and what could explain this?

The experiment involved mixing Al-1wt%Cr alloy with AlB12 at 750°C to observe if CrB2 would form, indicating chromium removal. However, the chromium concentration did not decrease; instead, it increased due to the dissolution of Al7Cr intermetallic particles, suggesting that this method may not effectively remove chromium from molten aluminum. To further investigate this unexpected behavior, future studies could explore different temperatures, boron concentrations, and alloy compositions.

5

What does the study suggest about using Al-B(AlB12) master alloy to remove chromium from molten aluminum, and what further research could be done?

The study indicates that using Al-B(AlB12) master alloy at 750°C may not be a feasible method to remove chromium from molten aluminum because the chromium concentration increased due to the dissolution of Al7Cr intermetallic particles. Further research could investigate alternative temperatures, boron concentrations, or different types of aluminum-boron master alloys to identify more effective methods for chromium removal. It's possible other reaction kinetics are present, or a catalyst may be required.

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