ABSTRACT
Traditionally, phosphorous based corrosion inhibitors are applied to prevent corrosion occurring in heat exchangers of cooling tower operations. It is well known that a protective coating rich in phosphate material protects a surface from corrosion by forming a uniform layer with thickness in micrometer range. Although, efficient in corrosion prevention, there is detrimental environmental impact due to discharge of phosphorous downstream of application. Significant research is sustained to design and identify alternative chemistries to address the phosphorous discharge concerns. A novel corrosion inhibitor was developed and successfully applied in the field and an extended understanding of the inhibition mechanism was investigated.
In this paper, a combination of electrochemical and surface studies was used to shed light upon the corrosion inhibition mechanism through which the novel corrosion inhibitor protects the surface. X- ray Photoelectron Spectroscopy (XPS) was used to evaluate the protective coating layer at the surface. This study focuses on understanding the impact of reaction time and flow regime on protective coating formation and composition. The XPS results were correlated with the corrosion rates obtained via the electrochemical data.
INTRODUCTION
The three most common challenges in operating a cooling water system are corrosion, scale buildup on heat exchanger surfaces and microbiological growth.1,2 Controlling each of these factors is equally important to minimize operating costs and maximizing plant performance and profitability. A common approach to prevent corrosion and scale formation is by applying chemicals with targeted functional groups. Nowadays, aside from good inhibiting performance, government regulations and economic considerations largely dictate the choice of the chemical additives.
Corrosion is an electrochemical process that involves an oxidation-reduction type of reaction.(3) A corrosion inhibitor may act by either disrupting the electrochemical reactions occurring at the cathode or anode, or by forming a passive layer on the surface limiting its contact with the process water. Typically used corrosion inhibitors in cooling water systems include chromate, zinc, orthophosphate/polyphosphate, phosphonate, molybdate, and silicate species.4,5 Each of these inhibitors has its own limitations. For example, stringent environmental regulations have severely restricted the use of chromate. High amount of chlorides and sulfates negatively impact the corrosion inhibition performance of molybdate and nitrite. Consequently, the choice of corrosion inhibitors and their corresponding dosages vary with the quality of the make-up water and the system's operating conditions such as pH and temperature.
