The use of chemical sensors in metal production

In the processing of molten metals, one important parameter is the chemical composition of the molten metal. Interactions between a molten metal and the atmosphere can change the composition of the metal. In some cases, undesired elements, such as oxygen or hydrogen, can be incorporated into the molten metal. Chemical sensors have been developed to monitor the chemical composition during this processing stage.

Electrochemical sensors

Sensors based on solid electrolytes have several advantages in the processing of molten metals. The conductivities of solid electrolytes increase with increasing temperature, so the high operating temperature required during the processing of molten metals is well suited to solid electrolyte based sensors.

The output of solid electrolyte based sensors is determined by the thermodynamic properties of the molten metal and reference electrode, so the sensor does not require calibration. The supporting electronics are relatively simple, since the output of an electrochemical sensor is a DC voltage. In addition, solid electrolytes are generally stable compounds, which can withstand the harsh chemical environment in molten metals.

Electrochemical sensors are not limited to detecting the species that is mobile in the electrolyte. The equilibrium between an immobile species and mobile species establishes a concentration of the mobile species, which can generate a measurable voltage that is related to the concentration of the immobile species.

Furthermore, an additional phase (referred to as an auxiliary electrode) can be added to provide sensitivity to a species that is not present in the electrolyte. Thus, electrochemical sensors can be designed for detecting a wide variety of species through judicious selection of the electrolyte and electrode materials.

Oxygen sensor

Oxygen from reaction with the atmosphere is removed from molten steel by adding aluminium or silicon alloys, which react with the oxygen to form oxides. Determining the optimal amounts of these alloys to be added requires knowledge of the amount of oxygen in the steel, which is provided by an oxygen sensor.

The most successful and widely used sensor in molten metals is the steel making oxygen sensor. This sensor is based on a stabilised zirconia electrolyte. The reference electrode is a metal/metal-oxide mixture (most commonly Cr/Cr2O3), the equilibrium of which establishes a reference oxygen partial pressure. Although oxygen sensors have been used for many years, there are areas for improvement.

Current sensors are used for one measurement and then discarded. Replacing these disposable sensors with extended-life sensors would both improve the quality of data obtained (ie, continuous measurements could be made) and reduce the costs.

One approach for extending the lifetime of current oxygen sensors has been alternative fabrication techniques, which improve the seal between the reference electrode and the molten metal. Another approach has been in the design of a non-isothermal sensor, in which the reference electrode is outside the molten steel.

This reduced temperature lessens the requirements on the reference electrode seal, but introduces an additional voltage due to the temperature difference between the two electrodes. Another approach to extending the life of oxygen sensors is to use an applied voltage to electrochemically reverse the degradation of the reference.

Hydrogen in aluminium

During molten aluminium processing, the most important dissolved gas is hydrogen, which is produced when aluminium reacts with moisture to form aluminium oxide and hydrogen. The solubility of hydrogen in the liquid aluminium is much higher than that in solid aluminium, so dissolved hydrogen can lead to porosity during solidification.

Systems for measuring the hydrogen content in molten aluminium are currently commercially available. In the most common systems, an inert gas (usually nitrogen) flows through a probe and over the molten metal, such that chemical equilibrium between the hydrogen partial pressure in the gas and the concentration of hydrogen in the molten aluminium can be established.

In response to the need for lower cost hydrogen sensors, research has been performed on developing low-cost solid-state electrochemical sensors.

A direct electrochemical sensor for hydrogen should conduct hydrogen ions. Researchers have modified sodium ion conducting materials, so that they conduct protons to form proton-conducting electrolytes.

However, some of these electrolytes cannot withstand the operation temperature in molten aluminium. In addition, sodium is often present in aluminium alloys and may interfere with the sensor output.

A new class of proton-conducting oxides has been developed that offers potential materials for use in hydrogen sensors. The most widely studied of these proton-conducting oxides are based on strontium and barium cerate.

However, because of its superior stability in molten aluminium, calcium zirconate doped with indium has received the most attention for hydrogen sensors for use in molten metals. In addition to being used in hydrogen sensors for molten aluminium, indium-doped calcium zirconate has been used in hydrogen sensors for other metals, including copper, copper-zinc and silver.

The solid electrolyte sensor is similar to the commercial nitrogen carrier gas sensor in that rather than directly measuring the hydrogen concentration dissolved in the molten metal, the hydrogen partial pressure in a gas, which is equilibrated with the metal, is measured. However, in the solid electrolyte based system (with the hydrogen gas reference electrode), the solid electrolyte forms a sample chamber, which is directly in contact with the molten metal. In the nitrogen carrier gas system, the sample gas must be transported from the melt to the analyser.

Elimination of this need for transport of the sample gas in the solid electrolyte based sensor simplifies the system and eliminates potential errors associated with the sampling process. Chemical sensors can also be used to monitor and control alloying additions.

Magnesium sensors

The first magnesium sensors were developed using molten chloride electrolytes, which had been used to measure the chemical activity of magnesium in molten aluminium. Although sensors using molten electrolytes have been successfully developed, there are potential improvements by using solid electrolytes.

Solid electrolytes do not require a crucible to contain the electrolyte or frits to separate the electrodes from the electrolyte, as is required for a molten electrolyte.

However, the system is further simplified by using a solid electrolyte, which would not require impregnation. Thus, a solid electrolyte based sensor has fewer components and a simpler design, which can reduce the cost of fabrication.

Solid electrolyte sensors have been reported using either ß-alumina or MgF248 electrolyte. The ß-alumina based sensor uses the equilibrium between magnesium (dissolved in the molten aluminium alloy), magnesium oxide and sodium oxide to generate a voltage, which corresponds to the magnesium activity in the molten alloy.

The inclusion of sodium oxide in the electrode equilibrium results in the sensor output, under some conditions, being affected by sodium impurities in the melt.

Microstructure control

Another application for chemical sensors in processing molten metals is monitoring the concentration of alloying elements. This is particularly important in cases where a small amount of a reactive or volatile alloying element is added. In such cases, the alloying element may be preferentially lost, which can cause significant changes to a small initial concentration.

An important example of this is the eutectic modification of aluminium alloys. One method for controlling the eutectic microstructure formed during casting of aluminium is to add small amounts of sodium or strontium (a given foundry will generally use either strontium or sodium).

Both of these elements are reactive and can preferentially oxidise or vaporise during processing. In addition, excess amounts of alkali metals can be detrimental by causing edge cracking or hot shortness during rolling.

Since the concentration of these two elements is critical for controlling the cast microstructure and the concentrations may change during processing, strontium and sodium sensors have been developed.

Sodium sensors

The sodium sensor provides valuable information on the actual sodium content in the alloy, which can be used to compensate for sodium lost due to preferential vaporisation or oxidation.

Sodium sensors, sensors based on fluoride, or other non-oxide electrolytes, can potentially be more resistant to interference from water vapour compared to oxide electrolytes-based sensors.

Strontium sensors

Strontium sensors for use in molten aluminium have been reported using both oxide and fluoride (SrF2) electrolytes. In the case of the strontium ß-alumina based sensors, sodium is exchanged with strontium so that the electrolyte is a strontium-ion conductor. The SrF2-based sensor uses the equilibrium between strontium dissolved in the alloy and the SrF2 electrolyte to establish the measurable fluorine partial.

Although the simplest reference electrode would be pure strontium, its reactivity and high melting point (relative to aluminium and magnesium) may be problematic for sensor operation and stability. Thus, a magnesium/MgF2 mixture has been used as the reference electrode in the SrF2-based strontium sensor.

Aluminium sensors

Researchers have developed aluminium sensors based on solid electrolytes, because such sensors have the potential for a simpler design and longer life, both of which can improve the cost effectiveness of the sensor.

Sensors using zirconia-based electrolytes with an Al2O3 auxiliary electrode have been reported. One problem with zirconia-based aluminium sensors is the formation of a continuous Al2O3 layer on the surface of the electrolyte, which eliminates the three-phase contact (solid electrolyte, auxiliary electrode, melt) needed to maintain the reference potential. Aluminium sensors have also been developed using fluoride electrolytes (SrF2-LaF3,84 CaF2,85 and MgF286,87), which remain pure ionic conductors in more reducing conditions as compared to zirconia. All these fluoride-based sensors use AlF3 as the auxiliary electrode and all have been shown to respond to aluminium concentration. In general, the response time and reproducibility of the solid electrolyte-based sensors is inferior to those of the molten chloride-based electrolytes.

However, the potential for lower fabrication cost and longer life may make solid electrolyte-based sensors a more cost-effective alternative in the future.

Antimony sensors

Another alloying element used to control the microstructure of hot-dip galvanised coatings is antimony. Specifically, control of the antimony concentration is used to control the degree to which large grains (referred to as spangles) form on the galvanised coating. Spangle formation can affect both the appearance and properties, such as corrosion resistance and paintability, of the coating.

An antimony sensor using a ß-alumina electrolyte has been reported. The sensor uses a NaSbO3 auxiliary electrode to provide the sensitivity to antimony and has been shown to respond to antimony concentrations between 0.02 and 1.0 per cent.

Conclusions

Chemical sensors are valuable tools for improving the efficiency and quality control in the processing of molten metals. Oxygen sensors in molten steel are the most successful and widely used example, and their use will expand in the future as the sensors are further improved.

Other sensors, such as hydrogen sensors for molten aluminium and aluminium sensors for molten zinc, are commercially available, but are cost prohibitive for some applications. In addition, other sensors, such as a magnesium sensor for molten aluminium, have shown promise in laboratory testing, but need additional development for commercialisation. The use of chemical sensors in metallurgical processing will continue to expand as the performance, reliability and cost effectiveness of current sensors are improved and as new sensors are developed.