Re: Boiler treatment (candles)
Posted by:
Peter Heid (IP Logged)
Date: March 18, 2002 03:02PM
<HTML>Corrosion is the action resulting in the ionic equilibrium of metals and oxygen. According to the electro chemical theroy, when a metal goes into solution, something must be displaced by the dissolved metal. In the case of iron or steel, the iron dissolves into a solution of iron oxide and tiny hydrogen bubbles evolve. The rate at which it dissolves is dependant on fluid pressure and the amount of oxygen dissolved in it, which is based on the temperature. The dissolved O2 can react with the sub-micro coating of hydrogen that protects the iron or it can combine with dissolved ferrous hydroxide which forms insoluble hydrated ferric compounds (rust) that makes more room for more for the dissolved ferrous hydroxide to form.
There are many factors that influence corrosion and they are of two types, primary, built into the material and secondary, enviromental. Other than improper selection of materials, we have little control over primary factors.
Secondary factors can work independantly or in combination to increase the rate of corrosion. Oxygen concentration is one of the most important factors, the more oxygen you have, the more corrosion is possable. Water line corrosion is the result of higher O2 concentration at the waters surface and some light wave action. Corrosion can indeed occurr when there is no oxygen present, from conditions that cause the evolution of hydrogen.
The pH, the strength or degree of ionization is very important in the corrosion enviroment. All hydrogen ions in solution try to reach a state of electro chemical equilibrium through corrosion. The more available ions (the stronger the acid) the more it will react before reaching the balance point. Pure water is neutral with 1X10 -7 moles per litre of H2 ions with the same number of their neutralizing counterpart, the hydroxyl ion. A lower pH number actually means more H2 ions or stronger acid. A pH of 6 is actually 1X10 -6 H2 ions, or a ten fold increase on a logarithmic scale. An acid pH can dissolve the slight protective coating on the iron and it can increase the amount of H2 gas on the corroding surface.
Another important secondary factor influencing corrosion is the contact of other corrosive fluids such as those containing chloride ions. Clorinated water should be always avoided.
The rate of flow can influence the rate of corrosion when velocities are very high. The action of the fluids can disturb the self protective coatings and any suspended matter is abrasive at high speeds.
Carbon dioxide in solution will form carbonic acid, though not highly ionic, it will still attack most metals. Carbon dioxide is chemically bonded to the water so deairation won't help. Any water having dissolved calcium carbonate in it, is in balance dissolved CO2 and its subsequent use should be avoided.
The junctions of dissimilar metals cause galvanatic corrosion, much like a battery powered by corrosion. Dissimilar metals should be joined with a nonmetalic gasket.
Few conditions will prevent corrosion and the complete removal of water is one. This means the complete removal, not the apparent removal. Steel rusts in the desert. During storage, a dessicant may help to remove the last bit of water. The complete displacement of all other gasses by an inert gas such as nitrogen will almost completely stop corrosion. Another possability would be cathodic protection where the free ions are absorbed in a battery like situtation, preventing their availability for corrosive action. Pipe lines and ocean going vessels are protected in this way as well as devices sold to protect your auto. If the device were purchased to protect your vehicle from road corrosion, this may also prevent boiler and plumbing corrosion during peroids of storage if these parts are in the circuit. Does anyone have any experience with cathodic protection for autos ?
Peter Heid</HTML>