For metal protection, conventional chemistries utilize the formation of an inorganic crystalline, hydrophilic corrosion product oxide (typically magnetite) formed on the virgin metal. This oxide's protection characteristics and sustainability are directly affected by operational physical and chemical variations, thus making complete metal protection an elusive goal.
Conventional and published international guidelines dictate chemical treatment of mixed metallurgy installations under a reducing environment, AVT(R).
A reducing environment is required to prevent oxidation of admiralty and/or copper alloys and the formation of copper hydroxides. The “protective copper patina” freely interacts with ammonia and/or the presence of amines to form weak complexes (e.g. ammonium copper cuprate complex) that are readily washed away promoting virgin base metal corrosion.
Copper(I) oxides formed under reducing conditions are less volatile than copper(II) oxides. Protection of copper is critically important to prevent transport of copper into the super heater and turbine. At pressures of > 2,300 psig copper readily transports into the steam and thus offers significant negative consequence to turbine integrity.
A trade off between protection of different metals has now been created. The lower operational pH required to protect copper alloys does not favor protection and oxide stability in the ferrous sections. The pH required for passivation of copper that may exist in the condensate (admiralty condenser), the low-pressure admiralty feed water heaters typically 70% copper 30% nickel, and the high-pressure feed water heaters typically 30 % copper and 70% nickel, now exponentially increases ones’ utilities exposure to the risks of FAC.
With AVT(O) and OT treatment regimes, FAC is almost eliminated in single-phase areas, however, two-phases areas are still an untreatable problem. With the excess oxygen, hematite oxide forms which is more stable and far less stable than magnetite. In two-phase areas, the oxygen naturally deaerates out of the liquid film into the vapor, leaving the remaining liquid film in a reducing environment. This causes two-phase flow-assisted corrosion.
The ultimate result is increased corrosion-related failures and problems. With increased corrosion, a list of problems occur including under-deposit corrosion, stress-corrosion cracking, hydrogen damage, increased boiler blowdown, increased fuel usage, wasted water, increased boiling cleaning frequency, and reduced safety & reliability.
Oxygen scavengers used in conventional regimes are often toxic, and extremely hazardous to operators and the environment.
Why sacrifice safety in pursuit of "protection"?
The deposition of corrosion products lead to under-deposit corrosion. Hydrogen cracking often occurs from low pH excursions. When load changes occur (physical flow is changed), stress-corrosion cracking, corrosion fatigue, and FAC often result.
There are published EPRI standards that outline procedures for plant protection during layup, such as nitrogen blanketing, dehumidified air, etc. These procedures are cumbersome, expensive, and difficult to follow or shutdown is given on such short notice that it's impossibe to correctly lay-up a unit in time.
High oxide transport and deposition leads to increased blowdown and inefficiency of heat transfer. Moreover, turbine efficiency is decreased over time. The end result is the costly procedure of chemical cleaning along with the associated hazardous disposals.
Protection is based on the formation of a corrosion product oxide, magnetite. Whilst magnetite improves passivation of metal, it suffers from many solubility and stability limitations that lead to increased oxide transport and flow-assisted corrosion failures.
Conventional chemistry utilizing ammonia alone cannot succesfully buffer the pH of the liquid film/early condensate. This limitation leads to liquid drop impingment and Flow-Assisted Corrosion.