Inorganic phosphates have a long history of being effective corrosion inhibitors however, with the trend to lower phosphate levels in cooling water, a new set of operating guidelines are required to minimize the risk of scaling/deposition. This paper will explore the chemistry of this type of program and outline application criteria that should be used to govern the application, in order to balance corrosion and fouling issues. Cases studies will be used to highlight the primary conclusions.
The use of inorganic phosphates as corrosion and scale inhibitors date back over 50 years. In the area of scale control, polyphosphates (sometimes referred to as, condensed phosphates), have long ago been superseded by more effective materials. However, the application of ortho and polyphosphate for corrosion control remains a common practice, in spite of the development of newer technologies. While phosphate programs provide good corrosion protection, they require close control of operating guidelines to avoid excessive deposition and scaling.
BACKGROUND
Over the past two decades, cooling water treatments have developed along several distinct lines (see figure 1 for a general family tree) and while there is overlap, they belong to one of four distinct families:
1) Zinc-Phosphate - this is the traditional "metal" program that uses zinc, typically combined with orthophosphate for corrosion inhibition.
2) High Phosphate - sometimes referred to as "stabilized" phosphate programs, these use a combination of ortho and poly phosphate for corrosion control. Cooling water pH is kept in the neutral range.
3) Low Phosphate - the alternative name is "extended" phosphate programs and is commonly run under neutral to somewhat alkaline conditions and with lower PO4 concentrations.
4) Phosphonate - this is the commonly called "organic" treatment that has become the industry standard.
This paper will be discussing the general technology behind low (or extended) phosphate programs and why these programs can fail under conditions that are commonly encountered today.
