What can be done to counter the effects of corrosion?
A custom-designed anode rack ready for deployment. This rack will be used in an impressed-current cathodic protection system.
A variety of solutions exist, and deciding upon a particular solution first requires a thorough understanding of the corrosion issues facing a particular facility or vessel.
This is normally accomplished by conducting a comprehensive on-site inspection. Such an inspection normally includes a detailed examination of the structure, any available construction blueprints or prior reports, the local environmental conditions, current and tidal data, and an analysis of the water to determine electrical conductivity. This last point is a factor of salinity, mineral and impurity content, and temperature.
Once completed, the inspection data is reduced to written form and is accompanied by digital photographs and/or digital video.
Based upon the inspection data, one of a number of corrosion control techniques may be warranted. In the case of galvanic corrosion, cathodic protection (CP) is often warranted. In its simplest terms, CP is an electrochemical means of corrosion control in which the oxidation reaction in a galvanic cell is concentrated at the anode, thereby suppressing corrosion of the cathode in the same cell.
Cathodic Protection
Cathodic protection is accomplished by two widely used methods:
- The first method involves coupling a given structure or vessel with a more active metal such as zinc or magnesium. This produces a galvanic cell in which the active metal works as an anode and provides a flux of electrons to the structure, which then becomes the cathode. The cathode is protected and the anode is progressively destroyed. Hence, the anode is called a sacrificial anode.
- The second method involves impressing a direct current between an inert anode and the structure to be protected. Since electrons flow to the structure, it is protected from becoming the source of electrons (anode). In impressed current systems, a low voltage DC current is “impressed” between the anode and the cathode.
Sacrificial Anode Systems vs. Impressed-Current Systems
Sacrificial anode systems are generally simpler than impressed current systems, but often require a multitude of sacrificial anodes placed at precise locations. They require only a material anodic to the protected steel in the environment of interest.
Impressed-current systems, while more complex, are also generally more effective since they can utilize greater voltage differences. The voltage differences between anode and cathode are limited in sacrificial anode systems to approximately 1 V or even less, depending on the anodic material and the specific environment.
The larger voltages available with impressed-currents allow remote anode locations, which produce more efficient current distribution patterns along the protected cathode. These larger voltages are also useful in low-conductivity environments, such as freshwater and concrete, in which sacrificial anodes would have insufficient throwing power.
Marine Coatings and Encapsulations
As corrosion in a marine environment is worst in the tidal zone or “splash zone” at the air-water interface, special attention must be paid to this area. Here, protective coatings or encapsulations are warranted in addition to any cathodic protection system.
As all cathodic protection systems require an anode, a cathode, an electric circuit between the anode and cathode, and an electrolyte, cathodic protection will not work on structures exposed to air environments. The air is a poor electrolyte, and it prevents current from flowing from the anode to the cathode. Additional protection is necessary.
Differences Among Available Coatings & Encapsulations
Coatings and encapsulations come in a variety of types, and often adhere to the age-old adage that “you get what you pay for.” Coating and encapsulation manufacturers are quick to advance the benefits of their products without any real comparative data to support their claims. Many companies engage in a sleight of hand by selling products as their own that they themselves do not manufacture and in fact have never subjected to any quantitative testing.
Many coatings and encapsulations have documented failures after only a few years, and many installers of such products lack a broad base of knowledge of the range of products that exist on the market. This latter limitation seriously compromises the validity of the advice that such an installer can provide to a prospective client.
Over the years, Substructure has researched, purchased, field-tested, or installed literally hundreds of different types of corrosion control products and techniques. Our advice is sound and is based upon knowledge, not guesswork.
Don't Wait to Install a Corrosion Control System
It must be remembered that effectively designed and installed corrosion control systems can prevent corrosion of a new structure or can stop corrosion on an existing structure, but they cannot replace metal lost by corrosion of an existing structure. Permitting a corrosion problem to continue unchecked can have costly results.