What measurement is used to express the rate of metal corrosion in cooling water treatment?

The measurement used to express the rate of metal corrosion in cooling water treatment is MPY, which stands for mils per year. Mils per year is a unit that quantifies the thickness of metal lost due to corrosion over the span of a year. This measurement is particularly useful in the context of cooling water treatment, as it provides a clear and quantifiable assessment of how quickly corrosion is occurring in the system. Understanding corrosion rates in terms of thickness loss helps engineers and technicians make informed decisions on maintenance schedules, material selection, and necessary chemical treatments to mitigate corrosion.

The other options, such as parts per million, gallons per day, and gallons per hour, pertain to different measurements that do not directly relate to the specific evaluation of corrosion rates. Parts per million is typically used to express concentrations of contaminants or chemicals in water, while gallons per day and gallons per hour are measurements of flow rates, not corrosion rates. Therefore, MPY is the appropriate and relevant choice for measuring the rate of metal corrosion in cooling water systems.