2,808
edits
Changes
→13.5.2 Special Types of Corrosion
*Contact corrosion: <br />Corrosion of a metal object after coming into physical contact with another metallic body. This can occur also on metallic impurities in alloys, on chemically and physically heterogeneous surfaces and on heterogeneous solutions on homogeneous surfaces, as well as through contacting a metal object by non-metallic materials through formation of corrosion compounds.<br />
*Hole corrosion (pitting corrosion): <br />Local narrowly limited corrosion growing by dissolving material in small pin holes or craters to a depth that can lead to holes all the way through the material.<br />
*Inter-crystalline corrosion: <br />Corrosion along the grain boundaries with the danger for the material to lose all its mechanical strength by decomposition (for example: at weld seams in austenitic stainless steels).<br />
*Selective corrosion: <br />Preferred corrosion in specific microstructure areas (for example: loss of zinc in brasses with formation of copper enrichments).<br />
*Air access corrosion: <br />Through differences in the amount of exposure to air or oxygen surface areas of a metal are becoming cathodes at the more exposed spots and therefore corrode less than those protected (for example: gap corrosion in screw or press connections).<br />
*Tension stress corrosion: <br />Crack formation of stress corrosion sensitive materials which are under mechanical pull stresses while exposed to corrosive media. Especially affected are zinc containing copper alloys (brasses) under the influence of ammonia or nitrates.<br />
*Oxygen corrosion: <br />Cathodic reaction in aqueous solutions forms reduced molecular, in water dissolved oxygen. Corrosion occurs when the electrochemical potential of the metal is below that of oxygen.<br />
*Hydrogen corrosion: <br />Cathodic reduction of H to H<sub>2</sub> (in acidic solutions) under conditions where the potential of the metal is less precious.<br />
*Fretting (frictional) corrosion: <br />Enrichment of oxide particles of non-precious metal (especially tinned) surfaces during relative movements at small ampliyudes (< 100 μm). They occur through transfer of oscillation ot thermal displacement energy because of the difference in thermal expansion of the two contacting metals. This effect can be especially detrimental in connectors with tin plated surfaces, such as for example in automotive applications.<br />
*Fatigue corrosion: <br />Fatigue fracture during repeated mechanical stresses in corrosive environments. This effect is often observed on brittle electroplated surface coatings that are exposed to repeated cycling between mechanical stresses and corrosive chemicals. Air access corrosion:
Through differences in the amount of exposure to air or oxygen surface areas of a metal are becoming cathodes at the more exposed spots and therefore corrode less than those protected (for example: gap corrosion in screw or press connections).<br />
