Normal Corrosion

• The most common materials used for heat exchanger tubes are stainless steel and copper alloys. The corrosion of these two metals includes rusting of the material in the case of the stainless steel and thinning of the walls by copper-based metals. Copper ions are toxic to microorganism. Copper undergoes a slow dissolution, which can transport the copper to other areas of the system. The thinning of the heat exchanger tubes in copper alloy based systems may result in premature failure of the heat exchangers.
  
  

Pitting

• Pitting is a highly localized form of corrosion as opposed to the general corrosion previously described. In pitting, a small localized region of the tubing wall develops an active area by the removal of the protective layer. Corrosion begins in this active region and an electrochemical reaction drives the rupture of the tubing wall. The corrosion proceeds in a rapid manner working through the tubing wall in a matter of a few hours. The corrosion leaves small tubes connecting the two sides of the tubing wall.
  
  

Solution Conditions Resulting in Pitting

• When the water in the heat exchangers becomes stagnant, the corrosion attack can become quite localized. The immediate environment of the corrosive attack has a different concentration of chlorides and a lower pH than the bulk of the solution. High chloride content at the surface of the metal tube promotes a greater degree of dissolution as does a low pH. The progression of corrosion at these sites works its way through the thickness of the exchanger tubing. The breach of integrity in the tubing walls weakens its strength and leads to tubing failure and possible rupture.
  
  

Ways to Inhibit Pitting

• A careful watch and control of the solution running through heat exchanger tubes will prolong and guard against failure. If the temperature of the solution is higher than 35 degrees Celsius, you need to lower the maximum allowed chloride level. Because pH influences pitting to such a great degree, the allowed chloride level needs to drop as the pH drops. Use additives to control a microbe growth in the solution running through the heat exchanger tubes. If a microbe layer develops on the inside of the tubes, the likelihood of developing pitting greatly increases. The environment beneath such a layer is considerably different and tends to favor high chloride content and low pH values.