In recirculating cooling water systems, plant owners want their assets, such as heat exchange equipment, pumps, and piping, to have lifetimes greater than 20 years. Many of these assets are mild steel, which is attacked by high levels of aggressive anions, i.e. chloride and sulfate, in the cooling water. Component failures typically occur due to localized corrosion rather than general corrosion. Further, plant owners need to increase their re-use of water. This re-use typically results in increased concentrations of ions in the water and further challenges for cooling water treatment programs. Control of (localized) corrosion in these challenging systems without the use of phosphorous-containing compounds and additionally without the use of zinc, tin, or molybdate is the subject of this RFP.
Limitations of Phosphorous-Based Programs
Today’s phosphate and zinc chemistries perform well under most circumstances. The concentrations of phosphate and zinc must be balanced carefully with calcium, polymeric dispersant, pH, and temperature. If all six factors are not always kept well-balanced in the system, either corrosion or fouling can occur. This is particularly problematic in the chemical and refining industries due to the presence of high temperature, low flow heat exchangers. Apart from challenging program control requirements such as tight pH operating windows, phosphate/zinc programs have some additional weaknesses, including:
- Low hardness, i.e. “soft”, water require much higher levels of phosphate and zinc to achieve good performance
- Little or no beneficial effect on copper or aluminum corrosion
Recently, US regulations have begun to restrict the industrial discharge of zinc as a primary pollutant to aquatic life species and phosphorus as an undesirable aquatic nutrient for cyanobacteria and algae in the environment. Molybdate suffers from being both very expensive and often regulated in discharges as well. The Chinese government has released a series of regulations to limit the discharge of phosphorous. The discharging limit, 0.5 ppm as elemental phosphorus, is too low for the current phosphorus containing cooling water treatment programs.
Desired Performance Criteria of New Non-P, Non-Metal Program
As in the late 1970’s when chromate was banned, the cooling water treatment paradigm is shifting again. The new goal is to find a corrosion inhibition approach for open recirculating cooling water systems that is free of regulated metals (Zn, Sn, and Mo) and phosphorous species, while maintaining most of the operational and economic benchmarks that the zinc phosphate approach provided. Some “typical” water system chemistries where program performance would need to be proven are shown below in Table 1. The two most challenging waters from a corrosion treatment perspective are Water 1 and Water 3.
Key program stability criteria and performance expectations are shown in Table 2. The cooling water environment is obviously oxygen saturated, which the program must be able to tolerate. Likewise, most cooling towers are now treated with upward of 0.5 ppm free residual chlorine (FRC) resulting from bleach or chlorine gas feed, so at least this level of oxidant must be tolerated on a continuous basis. Ability to withstand periodic excursions to a maximum of 2 ppm FRC is desired. Metal surface temperatures and flow velocity refer to typical shell and tube heat exchanger conditions with cooling water on the tube side. Within these specified conditions, the program should be able to perform for a minimum of 150 hours with minimal loss of potency, which could occur for example by halogen degradation of treatment chemistries. Corrosion performance expectations for AISI C1010 corrosion test specimen are shown in the last 5 rows of the table.