Filiform corrosion
Filiform corrosion (also filigree, worm-eaten or snail-track corrosion ) was first described by CF Sharman on coated steel surfaces and describes a thread-like corrosion phenomenon that occurs as a special form of anodic infiltration, especially under organic coatings of aluminum and low-alloy steels. Typically, the threads are 0.1 to 0.5 mm wide and grow at a constant rate of about 0.4 mm per day in different directions, but never cross each other. Filiform corrosion has been observed on a variety of metals including steel , zinc , aluminum , magnesium, and chrome-plated nickel . In the case of steel, this type of corrosion only occurs when the air humidity is relatively high (e.g. 65–95%). At 100% humidity, the threads can widen and form bubbles.
In general, the following conditions are important prerequisites for filiform corrosion to develop:
- high relative humidity, ideal conditions are 80 to 95% rel. Humidity and 40 ° C
- water-permeable layers
- Impurities in the metal, trapped particles, microcrystalline deformation layer
- Presence of salts, especially chlorides
- Defects in the coating, such as mechanical damage to the material, scratches, pores, holes, sharp edges with smaller paint layer thicknesses (e.g. in the manufacture of aluminum windows), edge alignment
All or more of these factors usually come together in coastal, marine or industrial atmospheres and also when de-icing salt is used on roads.
As the layer thickness of the organic coating decreases, the susceptibility to filiform corrosion increases. In addition, the rolling direction has an influence on rolled substrates. The corrosion takes place preferentially in the rolling direction.
The microcrystalline deformation layer, which is located under the naturally formed oxide layer of the aluminum, arises after a strong heat input into the aluminum. It is therefore to be regarded as a result of rolling, annealing or extrusion processes. These deformation layers contain metal oxides and intermetallic phases that can have a corrosive effect. The layer thicknesses of these deformation layers depend on the heat input and can vary widely. For many aluminum alloys, a pickling rate of> 1 g / m² is recommended for the pickling process, with the last pickling step taking place in an acidic environment (pickling). Too little stain removal does not sufficiently remove the deformation layer. Excessive pickling leads to the more easily soluble aluminum matrix being dissolved out and less soluble intermetallic phases remaining on the surface as a partially wipeable coating and can interfere with the build-up of the subsequent conversion layer.
Pure filiform corrosion results in an external attack on the surface, which leads to infiltration, lifting and flaking of the top layer. A deep attack can only be observed in combination with other types of corrosion.
Filiform corrosion can generally be divided into two components:
- a thread-like, laterally spreading corrosion phenomenon between substrate and coating
- an attack on the substrate material under the filiform corrosion threads, the depth of attack and appearance being determined by the alloy composition
literature
- Judith Pietschmann: Industrial powder coating: basics, processes, practical use . Springer Vieweg, Wiesbaden 2013, p. 397ff
Individual evidence
- ^ WF Bogaerts, KS Agema: Reference Cube Section of the Active Library on Corrosion (CD-ROM) . Ed .: Elsevier Science Publishers. Amsterdam 1992.
- ↑ CF SHARMAN: Filiform Underfilm Corrosion of Lacquered Steel Surfaces . In: Nature . tape 153 , no. 3890 , p. 621-622 , doi : 10.1038 / 153621a0 ( nature.com ).
- ↑ WH Slabaugh, W. Dejager, SE Hoover: 44 (566) . Ed .: J. PAINT TECHNOL. 1972, p. 76-83 .
- ↑ H. Knufinke: Current findings on avoiding filiform corrosion . In: GSB International (ed.): Info letter . No. 7 , 2009.
- ↑ Institute for Corrosion Protection Dresden GmbH: Investigations into corrosion in injuries to coatings on galvanized steel and aluminum. Institute for Corrosion Protection Dresden GmbH, accessed on April 28, 2016 .