What Does Salinity Mean?
Salinity is a measure of dissolved salts in a body of water. It is calculated as the amount of salt (in grams) dissolved in 1,000 grams (1 kilogram) of seawater.
Salinity is an important factor in determining the chemistry of natural waters and of biological processes within it, and is a thermodynamic state variable that, along with temperature and pressure, governs physical characteristics like the density and heat capacity of the water.
Salinity in marine atmospheres accelerates metallic corrosion and can vary drastically.
Corrosionpedia Explains Salinity
Salinity may be expressed in a number of ways; parts per thousand and parts per million are the two most common measurements, and it is sometimes expressed as a percentage as well.
Sodium and chloride are the predominant ions in seawater, and the concentrations of magnesium, calcium, and sulfate ions are also substantial. Naturally occurring waters vary in salinity from almost pure water, devoid of salts, in snowmelt to the saturated solutions in salt lakes such as the Dead Sea.
Since salinity is a measure of water's conductivity by increase in electrolyte, and galvanic corrosion requires dissimilar metals in an electrolytic solution, salinity and corrosion rate are directly linked. The increase in salinity provides more free electrons to act as an escape path for corroding metals.
Corrosion is the shifting of electrons from one material to the other, therefore salt present in water increases the capability of water to transmit electrons via redox reactions.
There are two main methods of determining the salt content of water:
- Total dissolved salts (or solids) - Measured by evaporating a known volume of water to dryness, then weighing the solid residue remaining
- Electrical conductivity - Measured by passing an electric current between two electrodes in the water sample and measuring how readily current flows between the plates