Spongiosis (corrosion)

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Spongiosis V = 100: 1
Spongiosis V = 500: 1

Spongiosis is a special form of selective corrosion .

Cast iron can be destroyed by local corrosion at room temperature. This form is known as spongiosis or spongy iron (lat. Spongus = sponge) and occurs in underground cast iron pipes or in bridge pipes suspended in hollow boxes. This is a long-term damage that is already supported by low levels of acids or chlorides in the ground, such as occurs in damp areas, near the sea or near chemical plants.

Spongiosis is a selective destruction of cast iron, that is, the iron-rich phases ( ferrite , pearlite ) corrode. Ferrite and pearlite form the anode, as they are less noble than the graphite cathode , and dissolve. The corrosion product is iron hydroxide oxide FeO (OH), also referred to in this context as sponge iron, which fills the interstices in the graphite network as a porous mass. The workpiece retains its shape, but loses its strength and is no longer mechanically resilient. In the advanced stage of cancellous bone, the cast iron becomes so soft that it can be cut with a knife. If the rust layer on the surface is removed, the sponge iron can be recognized by dark spots that stand out well from the metallic surface.

The structure of the spongiosis-affected cast iron consists of a dark gray base material, mainly made of iron hydroxide oxide FeO (OH), with embedded phosphide eutectic and black-brown graphite. Depending on the literature, spongiosis is possible with cast iron with lamellar graphite (GJL) or with cast iron with spheroidal graphite (GJS). In the case of cast iron with flake graphite, the corrosion attack along the graphite flakes can spread faster in the workpiece. This is not the case with spheroidal graphite cast iron due to the spherical shape of the graphite. The cast iron pipes can be coated (e.g. tar layer or paint) to protect against corrosion. A firmly adhering layer of rust, which is created by a lot of moisture, provides natural protection.

literature

  • Heinz W. Richter (Ed.): Repair of pipelines. Volume 2, rehabilitation of sewers and sewers, Vulkan-Verlag GmbH, Essen 2006, ISBN 978-3-8027-2731-3 .

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