In short, the saturation index just tells you when the water is "saturated" with calcium carbonate. It depends on pH, Calcium Hardness, Total Alkalinity adjusted for CYA (to get carbonate alkalinity), TDS (a proxy for ionic strength) and temperature.

It is most definitely certain that if your water is not saturated (enough) with calcium carbonate then it will be "corrosive" to plaster/gunite/grout surfaces since these (mostly) contain calcium carbonate which will dissolve into the water since the water is not saturated.

It is also most definitely certain that if your water is over saturated (enough) with calcium carbonate then the excess will precipitate out as scale and will make the water cloudy until such precipitation is complete.

The "enough" in the above is somewhere in the neighborhood of plus or minus 0.7 to 1.0 and higher in the index though the exact point is debatable.

What is not at all certain is what water that is not saturated with calcium carbonate does to surfaces that do not contain calcium carbonate, such as vinyl or metal. In this context, the pH is by far more important as low pH (i.e. acidity) is most certainly corrosive to both vinyl and metal in light of having lots of oxidizers in the water (both oxygen and especially chlorine).

That's about as far as I can go without venturing into the controversial and debatable topics such as whether saturated calcium carbonate can form a protective coating on metal to prevent corrosion, etc.

Richard