Carbonate is essential ...
Published by Ramlan Ramli, Utility & Wastewater
Carbonate is essential natural corrosion inhibitor for carbon steel in contact with water. Balance between carbonate and calcium will enhance the effectiveness of this inhibition. I suggest from brief data that you have provided, the natural inhibition effect is not sufficient in your system thus will need chemical supplement to reduce corrosion potential.
The most straight forward way is by adjusting alkalinity and calcium hardness such that water quality to fall into non corrosive zone as per LSI calculation. However, when water temperature is exceeding 35 Celsius this approach will be very challenging since scale and deposition risk start to present.