What Does Surface Defects Mean?
Surface defects are the boundaries or planes that separate a material into regions, with each region having the same crystalline structure but a different orientation. Surface defects are usually formed by surface finishing methods like embossing or by degradation caused by weathering or environmental stress cracking. Defects may also be produced during the processing of and use of metals in service.
When defects are created early in the processing chain they usually tend to cause processing problems or initiate failure in later stages. Some of these defects are typical of complex metallurgical, chemical and physical reactions that metals undergo during these processing operations and are sometimes impossible to avoid.
Surface defects may cause corrosion and coating failures. The presence of surface defects can dramatically change a material’s corrosion resistance and mechanical properties. A combination of corrosion and cyclic loading can potentially destroy the materials, resulting in an unpredictably short service life and a release of harmful corrosion products.
Pitting can be initiated by a small surface defect such as a scratch or a local change in composition, or by damage to a protective coating. Various surface defects unavoidably occur in areas of production like during the steel strip production process due to its physical and chemical properties. Defects tend to ruin not only the appearance of steel strips, but also their important performance characteristics such as corrosion resistance, wear resistance and fatigue strength. Conversely, polished surfaces display a higher resistance to pitting.
Corrosionpedia Explains Surface Defects
Surface Defects on Crystalline Materials
Surface defects are boundaries that have two dimensions, and normally separate regions of the materials that have different crystal structures and/or crystallographic orientations. These imperfections include:
- External surfaces - The exterior dimensions of the material represent surfaces at which the lattice abruptly ends. The exterior surface may also be very rough, may contain tiny notches, and may be much more reactive than the bulk of the material.
- Grain boundaries - The orientation of the atomic arrangement, or crystalline structure, is different for each adjoining grain. The atoms are so close at some locations in the grain boundary that they cause a region of compression, and in other areas they are so far apart that they cause a region of tension.
- Stacking faults - These occur in face centered cubic (FCC) metals and represent an error in the stacking sequence of close-packed planes. Stacking faults interfere with the slip process.
- Twin boundaries - A twin boundary is a special type of grain boundary across which there is a specific mirror lattice symmetry. Twin boundaries interfere with the slip process and increase the strength of a metal. The movement of twin boundaries can also cause a metal to deform.
The effectiveness of the surface defect in interfering with the slip process can be judged from the surface energies. The high-energy grain boundaries are much more effective at blocking dislocations than either stacking faults or twin boundaries. Surface defects are usually observed at the boundary between two grains or between small crystals within a larger crystal. This is due to the slightly different directions that rows of atoms in two different grains may run in, leading to a mismatch across the grain boundary.
The external surface of a crystal is also technically a surface defect because atoms on the surface adjust their positions in order to accommodate the absence of neighboring atoms outside the surface.