Types of Corrosion & Scale Build Up in Closed Loops
What Happens To Corrosion By-Products In A Closed Loop?
The corrosion inhibitor treatment level decreases in the loop as make-up water is added to replace leakage. Corrosion by-products tend to accumulate in closed loops since the water recirculates without blow down. Iron is the most common corrosion by-product in closed loops. Even though some iron is lost through leakage, the reduced treatment level rapidly exceeds the ability of the leak to control the amount of iron dissolved in the water. As the concentration of iron increases, the solubility is exceeded, and precipitation occurs. This results in iron deposits.
What is Microbiologically Influenced Corrosion?
Often referred to as MIC, Microbiologically Influenced Corrosion occurs when one or more species of bacteria colonize and begin feeding on the iron surfaces in a system. Colonies can attack all types of ferrous metals including iron, mild steel, galvanized and stainless steel. The metabolic process of Sulfate-Reducing Bacteria (SRB) uses the conversion of iron to iron oxide to create energy.
Galionella also converts iron to iron oxide. Clostridium excretes hydrogen ions which react with water to form strong organic acids. The common result in pits in the surface of the metal that are hidden under tubercles of iron oxide.
How Can I Clean Microbiologically Influenced Corrosion Up?
Removing the iron deposits involves a non-acid chemical cleaning, a thorough flushing of the system to remove the iron, and a biocide to sterilize the system. Here are the steps:
- If the water in the loop is already heavily fouled or dark in color, begin by flushing at a rate of 1-2 gallons per minute from a valve at a low point in the system. It is important to monitor the pressure on the circulating pump during this stage. If the flushing rate exceeds the make-up rate, the system volume will drop and the pump will cavitate. This can cause expensive damage to the pump bearings and impellor. By making sure the pressure is maintained in the system, you ensure that the pump will not run dry. Continue flushing until the water consistently runs clear. Depending on the volume in the loop, this may take as long as 5-7 days. Count on approximately 12 hours of flushing for every 1,000 gallon in the loop.
- Once the water is clear, add the chemical loop treatment as recommended by your Chardon Service Technician
- Circulate this cleaning solution for at least 48 hours. Circulate for 96 hours in heavily fouled systems.
- After the cleaning period, flush the loop using the guidelines in the first step. Continue flushing until the conductivity of the water in the loop equals that of the make-up water.
- Retreat with the recommended closed loop treatment.
How Can Scale From In A Closed Loop?
Closed loops rarely have scale problems, but mineral scale can form in systems with high hardness and alkalinity make-up sources. Mold surfaces are generally the hottest point in the system and high skin surface temperatures are prone to scale precipitation. Scale the thickness of a human hair can result in a 20% loss of heat exchange efficiency and that translates directly to an increased Cycle Time.
