Module 3.5 — Why some plants still have copper in the feedwater train, how AVT(R) protects it, and the corrosion and transport mechanisms specific to copper alloys.
Click either stage to see its role in this cycle. Copper doesn't have to leak in from outside — it can corrode from the plant's own copper alloy tubing and redeposit somewhere hotter.
Modules 3.1 and 3.4 both mentioned copper alloy tubing — in condensers and feedwater heaters — as a historically common design choice for heat transfer performance. Many older units still have some copper alloy equipment in service, since replacing tube bundles is a major capital project usually only undertaken during a significant overhaul or when leak rates make replacement unavoidable. Newer construction and major upgrades increasingly move toward titanium, stainless steel, or other non-copper materials specifically to sidestep the chemistry constraints copper alloys impose, but a large installed base of older equipment means copper alloy compatibility remains a real, practical concern across the industry.
Copper alloys corrode significantly faster under oxidizing conditions than under reducing (oxygen-free) conditions — the opposite sensitivity from the "small amount of oxygen helps protect ferrous metal" logic behind AVT(O), covered in Module 2.4. This is precisely why any feedwater train containing copper alloy equipment anywhere — condenser, LP or HP heaters — typically requires AVT(R), maintaining low dissolved oxygen throughout via a reliable reducing agent, rather than AVT(O)'s deliberate trace oxygen approach. Running AVT(O) chemistry on a system with copper alloys present would actively accelerate copper corrosion.
Even under well-controlled AVT(R) conditions, copper alloys corrode at some low background rate, releasing copper in dissolved or fine particulate form into the feedwater stream. This copper travels forward through the cycle and can redeposit on hotter surfaces downstream — notably boiler tubes and, significant for Track 6, turbine blades. Copper deposition on boiler tubes acts similarly to other deposits discussed in this course: it can insulate heat transfer surfaces and, in some cases, contribute to localized corrosion cells distinct from the mechanisms in Modules 2.1–2.3. Copper deposition on turbine blades affects aerodynamic performance and is covered specifically in Track 6.
Where copper alloys are present in the system, feedwater copper concentration becomes a monitored parameter in its own right, typically targeted below roughly 2 ppb, tighter than the iron targets often discussed alongside it. Elevated copper readings can indicate accelerating corrosion of copper alloy components — potentially from an oxygen excursion (tying back to the DA and scavenger discipline from Modules 2.6 and 3.3), from erosion-corrosion at high-velocity points in the system, or simply from aging equipment nearing end of service life.
Because copper alloy compatibility constrains chemistry program choice (mandating AVT(R) over the simpler AVT(O)) and introduces its own transport/redeposition risks, many plants undertaking major condenser or feedwater heater retubing projects choose non-copper materials specifically to remove this constraint going forward, even at higher upfront material cost. This is a slow, capital-driven shift — copper alloy equipment remains common enough that understanding its chemistry implications is still directly relevant across much of the currently operating fleet.