Hydrogen gas porosity
Encyclopedia
Hydrogen gas porosity is an aluminium
casting defect
under the form of a porosity or void in an aluminium casting caused by a high level of hydrogen
gas (H2) dissolved in the aluminium at liquid phase. Because the solubility
of hydrogen in solid aluminium is much smaller than in liquid aluminium, when the aluminium freezes, the dissolved hydrogen gas creates porosity in solid aluminium.
Aluminum smelters and aluminum foundries want to produce high quality aluminum and shape castings with minimum porosity. This can be obtained by reducing the amount of hydrogen in the liquid aluminium alloy.
The detrimental effects arising from the presence of an excess of dissolved hydrogen in aluminium are numerous. Hydrogen causes porosity in aluminum products leading to many casting defects, reduced mechanical properties like fatigue and lower corrosion resistance. Several methods are used to reduce the amount of dissolved hydrogen from the melt, such as furnace fluxing prior to the casting process or using in-line degasing equipment during the casting process.
. The Reduced Pressure Test (RPT) is often used on the foundry floor. The RPT is a semi-quantitative method with limited accuracy that provides an indication of the hydrogen level.
Since its introduction in 1989, this gas recirculation method has been used more and more by all major aluminum producers.
The amount of H2 in the gas loop of the instrument is determined by a thermal conductivity
sensor, which provides high reproducibility
and a broad measurement range.
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
casting defect
Casting defect
A casting defect is an irregularity in the metal casting process that is undesired. Some defects can be tolerated while others can be repaired otherwise they must be eliminated...
under the form of a porosity or void in an aluminium casting caused by a high level of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas (H2) dissolved in the aluminium at liquid phase. Because the solubility
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
of hydrogen in solid aluminium is much smaller than in liquid aluminium, when the aluminium freezes, the dissolved hydrogen gas creates porosity in solid aluminium.
Aluminum smelters and aluminum foundries want to produce high quality aluminum and shape castings with minimum porosity. This can be obtained by reducing the amount of hydrogen in the liquid aluminium alloy.
The hydrogen problem
Hydrogen forms whenever molten aluminium comes into contact with water vapor, and easily dissolves into the melt. The gas tends to come out of the solution and forms bubbles when the melt solidifies.The detrimental effects arising from the presence of an excess of dissolved hydrogen in aluminium are numerous. Hydrogen causes porosity in aluminum products leading to many casting defects, reduced mechanical properties like fatigue and lower corrosion resistance. Several methods are used to reduce the amount of dissolved hydrogen from the melt, such as furnace fluxing prior to the casting process or using in-line degasing equipment during the casting process.
Direct hydrogen measurement
An on-line method of measuring hydrogen in aluminum is then required to characterize and optimize the process, which helps ensure the quality of outgoing products and monitors the performance of these degassing processes. Traditional laboratory methods, such as hot extraction, are too expensive for routine quality assurance, and too slow for effective process controlProcess control
Process control is a statistics and engineering discipline that deals with architectures, mechanisms and algorithms for maintaining the output of a specific process within a desired range...
. The Reduced Pressure Test (RPT) is often used on the foundry floor. The RPT is a semi-quantitative method with limited accuracy that provides an indication of the hydrogen level.
Hydrogen analyzer
An hydrogen analyzer can be used for direct measurement of hydrogen in liquid aluminium. Direct monitoring of hydrogen is possible using an on-line quantitative measurement technology based on a closed-loop gas recirculation method though a porous ceramic probe.Since its introduction in 1989, this gas recirculation method has been used more and more by all major aluminum producers.
Operation principle
The closed loop recirculation is a proven method of directly monitoring hydrogen in molten aluminium. A small volume of carrier gas, usually nitrogen, is brought in contact with the melt by means of an immersed probe, and is continuously recirculated in the closed loop until its hydrogen content reaches equilibrium with the vapor pressure of H2 in the melt. The H2 concentration in the gas is measured and converted into a reading of the gas concentration in the metal. This method is fast, reproducible and accurate, and can be used “on-line” on the cast shop floor.The amount of H2 in the gas loop of the instrument is determined by a thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
sensor, which provides high reproducibility
Reproducibility
Reproducibility is the ability of an experiment or study to be accurately reproduced, or replicated, by someone else working independently...
and a broad measurement range.