A combination of laboratory experimental studies and the results of real-world plant experience is being used to optimize the corporate cooling water program. All aspects of the program are being considered, including passivation treatments for individual bundles, system pre-film solutions, and longterm cooling water chemistries. The aim of the optimization plan is to select the combination of passivation, pre-film and cooling water solutions that provides the most cost-effective protection of the system within defined performance targets that the vendor must meet. Electrochemical techniques are being used to investigate the passivation, system pre-film, and cooling water solutions and how they interact with each other. Developing an in-house experimental capability helps in making informed decisions between different vendors products. Here, the results of electrochemical tests to investigate the changes in corrosion behaviour of carbon steel on transitioning from a nitrite-based passivation solution to a phosphate-based cooling water program are described. The change in protectiveness of the film can be observed during the transition, as can the possible susceptibility to localized corrosion.
NOVA Chemicals is a major manufacturer of ethylene, polyethylene, styrene and polystyrene with manufacturing plants in Canada, the U.S. and South America. The company owns and operates the largest ethylene and polyethylene complex in the world near Joffre in Alberta, Canada. External vendors provide cooling water services at each of the plants, with multiple vendors sometimes employed at a given site. Recently, the corporation has developed a company-wide cooling water program under the guidance of a corporate cooling-water specialist in an attempt to rationalize the procurement and operation of cooling water services. This program defines various key performance indicators (KPI) which the vendor must meet, including specified corrosion rates for both carbon steel and Admiralty brass condensers. The work described here is part of an effort to understand changes to the surface of carbon steel upon transitioning from a nitrite-based passivation treatment to a phosphate-based pre-film or cooling water program. The interest in this system developed from a realization that the nature of the protective surface films was different (being an oxide in the nitrite solution and a Fe-phosphate film in the pre-film or cooling water), and a desire to understand how this transition occurred in practice. Nitrite and phosphate are both anodic inhibitors and are extensively used in closed re-circulating systems, requiring a well-controlled solution pH to be effective. For nitrite, the proposed pH is above 7.5 and for phosphate-dominated inhibitors (pH > 8)1. Research using various electrochemical techniques has been done previously to study the effect of phosphate-dominated1 and nitrite-dominated solutions2 on carbon steel. Electrochemical techniques, including cyclic voltammetry, measurement of the open-circuit (corrosion) potential, and electrochemical impedance spectroscopy (EIS), have been used to follow the progression of this transition on a laboratory scale.
Three different solutions were used in this study, each based on a proprietary formulation. Solution A is a nitrite-based passivation or pre-film solution, containing approximately 1000 ppm NO2 -, dispersants, bio-detergent and biocide, and azole-based yellow metal inhibitors.
