clorine ion concentration

You are leaving out too much information, such as what is the nature of the fluid, and what else is in it besides the presence of chlorine ions? What is the PH of the liquid? How long a period before some significant corrosion is acceptable? Are you talking about salt water? And I don't understand what you mean by "5c". Is that the temperature (5°C)?

Since it is a cold drawn low carbon steel, it won't offer too much in the way of corrosion resistance.

You could install a Honeywell SmartCET corrosion transmitter and monitor the general and the pitting corrosion rates in real time based on the actual process conditions. You cannot control what you do not measure.

http://hpsweb.honeywell.com/Cultures/en-US/Products/Instrumentation/corrosion/default.htm