The effects of corrosion are heavily amplified inside a 500+ degF reactor vessel with large amounts of radiation. The other issue, is impurities can get activated in the core and become radioactive, increasing station dose. Also you can get CRUD buildup in the fuel which inhibits heat transfer out of the fuel rods. So you want that water very pure.
To prevent corrosion, you use very pure water and even put some specific additives that help protect the vessel and internals.
Before lake water can go into my plant, we run it through multiple sand filters, reverse osmosis, and ion exchangers. This makes the water nearly reactor grade pure.
When that water is transferred to the condensate system, it also gets deoxygenated, goes through iron filters, another set of heavy duty ion exchangers, then has zinc (for metal protection) and hydrogen (to further lower oxygen) added. We also occasionally add small amounts of noble metals like platinum to help coat the inside of surfaces susceptible to corrosion and protect them.
The things we worry about: chlorine (eats reactor internals rapidly), iron (corrosion product, can plate out on stuff), overall conductivity (how electrically responsive the water is, this can increase corrosion rates), Total Organic Content or TOC (hard to filter, becomes radioactive, clogs up filters and coats stuff), and Oxygen (keep it as low as possible to prevent rusting). We also measure sulfates, because when our ion exchangers get depleted, sulfer starts to leach out of them. Sulfur in high amounts also causes issues.