Corrosion is an electrochemical process that requires that several conditions be present if it is to occur. There must be:
- A supply of oxygen
- An anode (where corrosion occurs)
- A cathode
- An electrolyte that permits the flow of ions and electric current
- A conductive path for the return flow of current
The anode and cathode are electrically distinct areas that exhibit properties similar to the poles of a battery. They may be different structures (i.e., two different pieces of metal), or they may be two different locations in the same structure. The anode is the location or structure that suffers metal loss during corrosion. An electrolyte can be almost any material that contains moisture. Air, water, soil and concrete are a few common examples.
Its purpose is to provide a path for the flow of metal ions between the anode and cathode. It alsoprovides a conduit for the electrical current that the ion flow generates. The greater its conductivity, the better its electrolytic capability. Salt water, for example, provides a much better path for ion and current flow than fresh water and therefore hastens the corrosion process. In a similar fashion, the metal itself provides a return path for the flow of current from the cathode back to the anode.
Corrosion chemists refer to the different environments where these conditions can occur as corrosion cells. Three types of corrosion cells are common in pumping applications:
- Dissimilar Metal Cells-Commonly referred to as galvanic corrosion, these cells occur when two different metals contact one another in a common electrolyte.
- Dissimilar Electrolyte Cells-This type of corrosion occurs when a single structure passes through an electrolyte of varying properties.
Differential Aeration Cells- These cells occur commonly in soils where a single structure passes through areas of differing oxygen concentration.
Corrosion, if left unchecked, can be an extremely efficient and swift process. For example, a current of a single ampere flowing from a steel pipeline into the soil can consume 20 pounds of pipe in a year.. Fortunately, it can be prevented if any one of the required conditions can be eliminated.
Probably the most common method of eliminating one of the required conditions is to coat the surface of a metal. A good coat of paint or epoxy can electrically isolate the surface of a metal structure from a potential electrolyte and therefore prevent it from corroding. Some of the epoxy coatings available today can increase the corrosion resistance of cast and ductile iron pumps in salt water applications.
In fact, the product of corrosion itself can, in some cases, provide a protective coating. The iron oxide (rust) formed as iron or steel corrodes is not a good conductor. But, unfortunately, it does not adhere well and tends to flake away, thus exposing the surface to additional degradation. Aluminum oxide, on the other hand, is an extremely hard and durable substance and often does a good job of protecting aluminum surfaces from further corrosion.
Another method of corrosion prevention that has been used for more than 100 years is passive cathodic protection. I mentioned earlier that it is the anode that suffers metal loss during corrosion. If we transform a structure that is normally an anode into a cathode and let some other structure become the anode, we can protect the original structure. Cathodic protection does this by adding a sacrificial metal that gives up electrons more easily than the metal to be protected and thus becomes the anode. This allows the original structure to undergo a major life change (to that of a cathode). The process is termed passive because current flow between the anode and cathode occurs naturally and is due to galvanic action. It is an example of a dissimilar metal corrosion cell and illustrates how we can use a normal corrosion process to selectively protect a structure that would otherwise undergo corrosion.
One of the most common forms of passive cathodic protection is the galvanized coating of zinc found on sheet metal and steel piping. Since zinc is the less noble (looser electrons) metal, it becomes the anode and is sacrificed to preserve the primary metal. In submersible pump installations we can often attach a sacrificial anode directly to the pump body and achieve the same results as coating its entire internal and external surfaces. In either case, the anode, depending on its rate of decomposition, must be renewed periodically.
Another common type of cathodic protection is impressed current cathodic protection. Instead of relying on a natural flow of current between the sacrificial anode and cathode, an external direct current is applied to both. The impressed current makes the anode, regardless of its nobility, more positive than the structure to be protected. An advantage of this method is that almost any conductive material may be used as an impressed current anode. In fact, materials with low consumption rates are the most desirable since they require less frequent replacement.
Another form of impressed current cathodic protection is known as capacitive discharge oxidation interference (CDOI). This system uses a capacitor bank to affect a bulk transfer of electrons through an electrolyte. It can increase the effectiveness of cathodic protection in instances where small amounts of electrolyte are present.
In pumping applications, dissimilar electrolyte and differential aeration corrosion are typically found in connecting piping that is buried or in contact with an aqueous solution. In both cases, a single structure (a length of pipe) contains both anodic and cathodic areas. In a differential aeration cell, differing concentrations of oxygen in the soil (the electrolyte) will identify the different areas. The section of pipe in soil with a good supply of oxygen (well aerated) will become the cathode and the poorly aerated section the anode.
In the case of a dissimilar electrolyte cell, variations in resistivity in the electrolyte can occur due to differences in chemical composition. The area of the pipe in contact with lower resistivity will become the anodic area. Corrosion protection for these systems is usually accomplished via coating, impressed current cathodic protection or a combination of the two.
Pumps & Systems, November 2009
