This is essentially a measure of the amount of CO2 in the water. CO2 makes carbonic acid (HCO3-) is the the anion.
This is NOT easy to measure accurately, simply because just breathing on the sample is enough to change it. Fluctuations in the CO2 level in the atmosphere will change it. Small changes in temperature will shift the equilibrium quite a bit. Addition of other species in solution will effect it quite a bit.
It makes the water more acidic, yes, but I am not sure I would use this to determine how corrosive water is. I will also point out that corrosion happens by different mechanisms depending on what you are looking at.