Ceramic filters, such as those used in the chemical industry, must have a precisely defined level of permeability. This depends on the size and quantity of pores in the material. The desired pore volume is formed during sintering and firing. The same applies to the fireclay bricks used to line fireplaces and pizza ovens. The optimum porosity here is about 20 percent of the volume.
Hydrogen in the engine block?
Pores in concrete, tiles or natural stone, especially outdoors, on the other hand, are less desirable. The small openings allow water to penetrate, which then freezes during frost and can thus burst the building material. Cracks can also form in die-cast aluminum engine blocks if the material is too porous.
In this case, it is due to hydrogen: The gas dissolves very easily in liquid aluminum and forms cavities in the material as it cools. This effect can be prevented or minimized during casting by using vacuum.
Precisely determining gas lockings
For quality assurance, it is important to accurately determine the porosity of the material. In aluminum die casting, a sample of the molten metal is placed in a small vacuum chamber where it cools. In a vacuum, gas lockings inflate to about ten times their original size and are often already clearly visible in a cross-sectional image. Density and porosity can be accurately determined by weighing in air and water.
With ceramic materials and building materials, the sample blocks are also subjected to a vacuum that sucks the air out of the open pores. Water is then allowed to penetrate the pores. The difference in weight indicates the degree of porosity. Small vacuum chambers are sufficient for the testing devices. BUSCH offers a wide range of small-volume vacuum pumps, each optimally suited to different requirements.
In some metals, hydrogen dissolves like sugar in tea. The degree of solubility depends on the temperature. For example, 100 grams of liquid aluminum at casting temperature absorbs one cubic centimeter of hydrogen. In the case of cold metal, it is only 0.05 cubic centimeters, or one-twentieth.
When hydrogen gets into a molten metal, it usually comes from the moisture in the air. The melting heat causes the water molecules in the air to split into hydrogen and oxygen. The released hydrogen atoms are absorbed by the liquid metal. After it has lost most of its hydrogen solubility during cooling, the dissolved hydrogen turns back into gas.
With aluminum, 95 percent of the solute undergoes this transformation, causing gas lockings. When the material is stressed, the hydrogen atoms in the material can begin to migrate. When they meet, they combine to form molecules (2 H à H2). In the process, their volume grows by leaps and bounds by a factor of a thousand! Stresses develop in the metal and so-called hydrogen cracks can form. In order to keep humidity away from casting processes, they are often carried out under inert gas atmosphere or vacuum.