Module Concept:

Corrosion Control in a Water/Wastewater Plant

Objectives for the Module:

1. Corrosion can cause failure of pipes, tanks, etc.
2. Corrosion puts impurities in the water which must be removed.
3. Corrosion is a costly item for all water/wastewater plants, lines that
 conduct water, and tanks.
4. Corrosion can cause a reduction or loss of electrical conductivity which can cause the plant to operate inefficiently or fail.

Overview of the Module:

1. Electro-galvanic
2. Electrolysis
3. Bacterial
4. Chemical

Importance of this Topic for Water/Wastewater Operator:

Corrosion is costly, difficult to control, and creates hazards for water
plant operators and customers. It can also contaminate the water.

Explanation of Topic(Theory, Procedure, Control Methods, Etc.)

1. Electro-galvanic : Iron is not in a pure state in nature. Refining efforts make it relatively pure. When electrical current is passed through it, the outer electron is removed causing the atom to have a positive charge(ionized). Oxygen molecules in water are relatively electrically negative. Therefore, the iron molecules which are slightly positively charged are attracted to the oxygen molecules which are negatively charged with Vander Waals forces and follows the water molecule anywhere it might go. It then becomes, in the presence of free oxygen, FE₂O₃ which is what we call common rust.
2. Electrolysis : In electrolysis, acids and bases tend to cause current to flow like in batteries. In the generation of current, the atoms become ionized. Then the ionized iron is attracted to the oxygen molecules which in water is relatively electrically negative. At that point, it does the same as above.
3. Bacteria : Some bacteria can actually metabolize iron.
4. Chemical : Chlorine, oxygen, and other compounds can cause direct corrosion without water.

Control:

1. Painting and other surface protection (i.e. plating, etc.) - creates a barrier and keeps water from getting to the metal.
2. Cathodic protection - Use of a sacrificial electrode to supply electrons to the iron for the reduction of oxygen and slows process of corrosion.
3. Selection of materials - Corrosion resistant materials like plastics, stainless steels, and brass.

How do you visualize the corrosion process?

                        Fe -----> Current  ------> Fe++
                                   or
                                  Acid


                                H
                               /
                        Fe++--(O) -->O2--->Fe2O3
                               \          Fe3O4
                                H

Things necessary to occur:

H20                     Bacteria
Metal                   Current
O2 or oxidant           Dissimilar Metals

Examples:

Copper, Steel - Current causing rotting of steel

 Iron Water Stains - Especially from ground water sources.

Insights:

Why would a weldment deteriorate faster at the level of changing water in a tank?
Why do heat and other energies increase corrosion?
How long would 1 ampere take to ionize 20 pounds of iron?

Discussion:

Suggestions:

1. Weld strips over existing welds
2. Install bands around tank
3. Decrease corrosion of water - pH and corrosion inhibitor
4. Test with coupon - see where at
5. Use less corrosive materials (plastic, fiberglass, possibly liners - but there are problems with liners if moisture gets behind the liner-)

Visuals: (Pictures, Film, Etc.)

 Tank at Pennington Gap, VA
Pictures of tank and lines that have corroded

References:

Standard Methods
Chemistry (Molecules, Matter and Change) Third Edition, Peter Adkins, Lorretta Jones, 1997, W.H. Freeman and Company, New York.

Best Fit:

1. Hard Copy *
2. Pictures *
3. Film

Also refer to pH, acid, base, deposition or corrosion in accordance with pH and hardness, CO₃, concentration equilibrium, Baylis Curve, Langelier Index.

Credit:

Jay Blevins, Associate Professor
Mountain Empire Community College
Drawer 700
Big Stone Gap, VA  24219

Email: jblevins@me.cc.va.us