Mixture Corrosion is an important principle of the dissolution of limestone by groundwater in karst areas.
The mixing of two different waters with different amounts of carbon dioxide (CO2), both originally saturated with calzite, results in a mixture which is undersaturated and capable of further calcite dissolution. The reason for this behaviour is the non-linear relationship between calcite solubility and carbon dioxide partial pressure.
The black curve shows the non-linear relationship between calcite solubility and carbon dioxide partial pressure. It shows the maximum CaCO3 water can hold with a certain amount of CO2. All waters with less CaCO3, represented by positions below the curve, are called undersaturated.
Waters W1 and W2, represented by two red dots, have a different amount of CO2, but both are saturated. Mixing the two waters is a linear operation, symbolized by the linear red line. So the mixing at a certain proportion, in this example 1:1, leads to point M, representing the mixture.
As you can see, this point - like every point on the red line - lies below the black curve, which means understuration!
There are no dimensions, as this scetch is intended to explain the basic principle of mixing corrosion.