It's very simple.  No study has ever found a calcium carbonate buildup in a passive layer.  But they have found ferrous carbonate. LSI appears to work some of the time because you get a high LSI when your alkalinity and pH are high - both criteria that are also applicable for ferrous carbonate formation.  However, if the alkalinity is insufficient, and the LSI is high because of high calcium, you will experience corrosion. I've had clients that have dosed calcium chloride to increase their LSI and encountered severe red water problems.  Our corrosion coupon studies found a slightly higher rate of corrosion than the control on sidestream racks.  But when they discontinued calcium chloride dosing, turned off their decarbonator, and adjusted their pH up to ~8, their red water issues immediately ceased.

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It's very simple.  No study has ever found a calcium carbonate buildup in a passive layer.  But they have found ferrous carbonate.

LSI appears to work some of the time because you get a high LSI when your alkalinity and pH are high - both criteria that are also applicable for ferrous carbonate formation.  However, if the alkalinity is insufficient, and the LSI is high because of high calcium, you will experience corrosion.

I've had clients that have dosed calcium chloride to increase their LSI and encountered severe red water problems.  Our corrosion coupon studies found a slightly higher rate of corrosion than the control on sidestream racks.  But when they discontinued calcium chloride dosing, turned off their decarbonator, and adjusted their pH up to ~8, their red water issues immediately ceased.