Metals and Metallic Coatings
Corrosion tests can be performed on metals, alloys and metallic coatings, such as hot-dip and electroplated surfaces. There are many different forms of corrosion which affects these materials, including uniform degradation, pitting, crevice corrosion, stress corrosion cracking, galvanic corrosion and intergranular corrosion.
Accelerated environmental tests including neutral salt spray and humidity testing can be used to measure the progression of uniform corrosion on metals and metallic coatings. With the testing of metallic coatings, the visual appearance of the corrosion products provides an indication of when the corrosion of the substrate metal has commenced.
Both D.C. electrochemical and impedance spectroscopy are also applicable to studying the corrosion of metals. Linear polarization resistance (LPR) is a simple means for acquiring a numerical value relating to the corrosion resistance of a sample. Further information can be obtained with a potentiodynamic polarization test, which will provide information on phenomena such as passivity or pitting. The appearance of impedance spectra can also elucidate the electrochemical corrosion processes occurring as metals corrode.
Localised corrosion measurement techniques are able to identify specific corrosion events on the surface of a sample. For example the scanning vibrating electrode technique (SVET) is able to map the presence of pits or defects in metallic coatings. The scanning droplet cell is also able to perform a range of electrochemical tests but at specific points of interest, for example welds.
Microscopy can also be used to support the corrosion measurement techniques listed above. An example of this has been the use of the infinite focus microscope (IFM) to measure the volume of corrosion pits and therefore determine the amount of material lost.
- The Electrochemical Encyclopedia has further information regarding the different types of corrosion which occur
- Illustrated examples of corrosion degradation can be seen on the website of the National Association of Corrosion Engineers
For more information please contact Dr Nick Farmilo