CORROSION, the basics .... Part 2

Other factors affecting corrosion:
a) Type of metal: the "galvanic series"
The easier that a metal gives up electrons the more likely and the faster it will corrode. The metals at the top of the series will corrode more readily than those at the bottom.

Galvanic Series for Water System Metals
Magnesium ........the "Anode" (corroded region)(most active)
Zinc
Aluminum
Cadmium
Mild Steel
Cast iron
Lead
Tin
Brass
Copper
Stainless Steel.......the "Cathode" (least active)

If there are two dissimilar metals connected "electrically", meaning that an electrical current can pass between them, the metal at the top of the series will be the anode, and the metal at the bottom will be the cathode. The greater the separation of the metals on the list, the greater the "galvanic corrosion" will be.

Some important corrosion considerations:
a) pH of the water: as it increases the rate of corrosion decreases
b) temperature of the water and the pipe: as it increases the rate of corrosion increases
c) quantity of dissolved oxygen in the water: as the concentration increases so does the rate of corrosion
d) alkalinity: as it increases the rate of corrosion decreases
e) Total dissolved solids in the water: as the level of solids increases, so does the corrosion, as the water is a better conductor with these ions in it.
f) fluid velocity: as the velocity increases, there may be a greater release of oxygen, and an increase in corrosion. There may be a reduction if the D.O. is low, and the velocity is high enough to remove corrosion byproducts and bring in inhibitors such as phosphates.

Corrosion depends on chemical reactions of the metal with the water and on the flow of electric current (the electrons) in the pipe material from the anode to the cathode.

Corrosion may be viewed as a combination of two types of processes, (1) anodic processes (oxidation) which releases electrons (2) cathodic processes (reduction) which accepts electrons. Reactions at the cathode are normally slower than those at the anode, so they control the rate of corrosion.

Corrosion Control Methods
There are a few methods that are available to reduce and/or control corrosion. The method selected will be determined on the local site specific conditions.
Use of

a) metallic coatings such as galvanizing or aluminum to protect metals
b) non-metallic coatings such as glass coatings to protect metals
c) non-metallic materials or corrosion-resistant materials
d) non-metallic materials like PVC, asbestos-cement, reinforced concrete, fiberglass, and plastic
e) corrosion-resistant metals such as stainless steel, aluminum, copper, bronze, and brass.
f) protective coatings and materials such as coal tar epoxies and enamels (bituminous), paints, cement mortar, epoxy resins, vinyl resins and inorganic zinc silicate paints

In our section on Stabilization, we will discuss more in depth the chemical methods of control. But for now a broad overview is in order:
Operationally, chemicals and treatment process of the water include:
a) reduction/removal of free carbon dioxide to reduce the acidity and raise the pH of the water
b) pH adjustment, increase the pH of the water by adding an appropriate chemical for your system, such as sodium hydroxide (NaOH)
c) reduction/removal of oxygen to reduce the corrosivity of the water
d) cathodic protection which is expensive, and used only when necessary.

Cathodic protection is when an outside electric current is applied to the metal in order to reverse the electrochemical corrosion process. A sacrificial anode is added to the corrosion cell or battery and acts as a substitute for the original anode with the metal being protected now acting as the cathode. The system may be either energized by an external source of direct current or may actually be a metal higher in the electromotive force series than the metal being protected. Metallic ions from the substitute anode, rather than from the metal being protected, go into solution. The sacrificial anodes are usually made of aluminum, magnesium, or zinc There are non-expendable anodes made of graphite or carbon that have been used successfully . Direct current must flow from the alternative source to the metal to maintain metal to water potentials of 0.80-0.85 volts for the system to function properly.

e) addition of chemicals to create a thin film or coating. Cathodic inhibitors reduce the rate of corrosion by preventing dissolved oxygen from reacting the cathodic areas, thus inhibiting the rate-controlling cathode reactions. Anodic inhibitors are less effective and not commonly used. Inhibitors that are known to make certain metals more passive or resistant to corrosion are called passivators; nitrites and chromates passivate iron.
1) Calcium carbonate.
2) Sodiumhexametaphosphates (polyphosphate).

There are many corrosion inhibiting chemicals available, but one MUST insure that they are approved for potable water before using!

Community Impacts
Obviously involves pipe, valve, and fittings repair and replacement in plumbing fixtures, appliances, and the water distribution system. In some extreme instances, public health may be impacted, most likely by lead and copper leaching into the water.

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