Gas bubble

from Wikipedia, the free encyclopedia
Gas bubbles in cast steel

Gas bubbles are particularly important in three areas:

As air bubbles , they have found innumerable applications in technology. Some examples are

Decompression for dissolved gases

Liquids usually also contain dissolved gases , the solubility of which strongly depends on the ambient pressure. In relief it is free and can dramatically effect phenomena - from sparkling wine and mineral water to the bends and heavy gas explosions or hot ash flow in some volcanic species.

From sparkling wine to diving disease

If you want a sparkling wine or mineral water open bottle, pearls previously dissolved gas under high pressure (in this case carbon dioxide ) up and escapes from the bottle. The faster the bottle is opened (or the harder the champagne cork pops), the faster the gas escapes and the more liquid it can entrain.

Decompression has a similar effect when a recreational diver ascends too quickly . The dissolved in the blood of nitrogen forms in relief at first small, then more and more gas bubbles, which eventually to embolism can lead to death and - see decompression sickness . The only antidote (apart from the diving bell ) is a slow ascent, interrupted by pauses, which gives time to degas .

Unlike pressure , the solubility of gases decreases slightly as the water temperature increases. In normal tap water, air is dissolved in the percentage range. Therefore, if you leave a glass of cool water to stand, small bubbles gradually collect on the inside of the glass as it is heated .

Gas explosions in volcanism

The decompression has a very drastic effect in some geological processes, e.g. B. in volcanism . The more gases the rock melt (the magma ) rising from the depths contains, the faster they are released when the pressure is released, which accelerates the ascent of the magma and can lead to gas explosions even in the volcanic vent . Often, volcanic eruptions with a large ash content create a gas-particle mixture that rushes down the slopes of the volcanic cone as a hot pyroclastic flow and can cost many lives.

Gas bubbles in the metal casting

This article or the following section is not adequately provided with supporting documents ( e.g. individual evidence ). Information without sufficient evidence could be removed soon. Please help Wikipedia by researching the information and including good evidence.

Gas bubbles also form in the metal during casting , but partly for other reasons. They are smooth-walled, spherical cavities, the diameter of which can exceed 2 to 3 mm.

Types of vesicles

The gas bubbles that occur in metal casting can be characterized as follows:

  • shiny bubbles,
  • oxidized bubbles,
  • blue colored bubbles,
  • closed air bubbles,
  • Pores and
  • pin holes.

Each of these phenomena represents an interruption in the metallic matrix and therefore reduces the strength of the cast material.

Pores are the cavities that are smaller than 2 mm and appear like nests.

Pinholes are elongated cavities that are perpendicular to the surface. They occur in certain areas of castings that have been produced using the wet casting process.

Closed air bubbles are rounded cavities that occur occasionally in the upper part of cast parts, usually just below the surface. Your walls can be described as shiny metallic.

Other cavities that can arise as a result of shrinkage of the casting when it solidifies are called blowholes . They can in part be reduced by using foundry methods.

Causes of Blistering

  • The metal was unable to completely fill the mold because a gas counter pressure equalized the ferrostatic pressure.
  • Formation of insoluble reaction gases in the metal melt or already existing dissolved gases.
  • The molding material has insufficient gas permeability.
  • Insufficient riser ( riser ) height.
  • Watering too slow or too cold.

literature