What Does Porosity Mean?
Porosity is the open spaces between grains or trapped in grains in a microstructure — the presence of tiny openings or spaces within a material. Porous materials can absorb fluids or moisture, which causes corrosion. Porosity is a fraction between 0 and 1, typically ranging from less than 0.01 for solid granite to more than 0.5 for peat and clay. It may also be represented in percent terms by multiplying the fraction by 100.
Porosity is also known as void fraction.
Corrosionpedia Explains Porosity
Porosity is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume. It is a phenomenon that occurs in materials, especially castings. It is weld metal contamination in the form of a trapped gas. Porosity causes a casting to leak and makes it unusable for holding pressure in applications like pumps, compressors, transmissions and plumbing fixtures.
Porosity is used in multiple fields, including:
- Earth sciences
- Soil mechanics
There are two types of porosity:
- Surface porosity - Occurs on the surface of the metal and can be detected with the naked eye
- Subsurface porosity - Occurs within the metal and can be detected only with specialized testing
There are many ways to test porosity, such as industrial CT scanning. Pores transmit water, so smaller pores and smaller void ratios are important in increasing barrier performance. In general, high total void ratios result in higher permeability.
Porosity also causes poor results in decorative coatings on structural castings, and many times porosity is the root cause of coating failures, which manifest themselves as surface pitting, spotting or corrosion. Any of these types of failures can ultimately make a cast part unacceptable for its intended purpose. Impregnation is a cost-effective, permanent solution to the problems that may be encountered as a result of such porosity in castings.