Definition - What does Surface Defects mean?
Surface defects are the boundaries, or planes, that separate a material into regions, each region having the same crystal structure but different orientations. Surface defects may cause corrosion and coating failure.
The presence of surface defects can dramatically change the corrosion resistance and mechanical properties of a material. A combination of corrosion and cyclic loading could destroy the materials, resulting in an unpredictably short service life and enormous release of harmful corrosion products.
Pitting can be initiated by a small surface defect, being a scratch or a local change in composition, or damage to protective coating. Polished surfaces display higher resistance to pitting.
Corrosionpedia explains Surface Defects
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 atom arrangement, or crystal 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. Movement of twin boundaries can also cause a metal to deform.
The effectiveness of the surface defects in interfering with the slip process can be judged from the surface energies. The high-energy grain boundaries are much more effective in blocking dislocations than either stacking faults or twin boundaries.