The Corrosion Cycle

In order for corrosion to exist, there has to be four things present:

• A cathode
• An anode
• An electrolyte
• A mechanical path between the cathode and anode

After two years in the industry, I just realized that the test station I make in my head is kind of incomplete because of the fourth ingredient: in my head, I forget the mechanical pathway needed for the sacrificial anodes to connect to and protect the steel substructure.

Oops.

Hopefully the people in the field are better aware that the anode lead wire must somehow connect to one of the test leads run up from the pipe/tank/what-have-you. This could be done by:

1. Putting the two wires on the same terminal
2. Connecting the two terminals via bond strap
3. Connecting the two terminals via shunt

These are stated in the order of least preferable to best practice. Putting the two wires on the same terminal closes the mechanical connection so that the electricity is in a closed circuit, and the cycle of corrosion is completed so we can draw the electrons off of the anode instead of the steel. But it takes time and effort to remove them for testing.

Ideally, between the "test lead" terminal and the "anode" terminal you have a shunt, which makes for easy testing of the voltage drop.

What's a rectifier?

When I first started working in cathodic protection, I giggled all the time at the word rectifier. To an outsider (a mid-20s sometimes questionably mature outsider), rectifier sounds more like anatomy than an essential tool to protecting long-distance pipelines. So, what is a rectifier?

For impressed current systems, we don't rely on the natural chemistry between an anode and the steel structure. It's simply not cost-effective. For impressed current, there is generally a hole dug 300-500 feet deep with cast iron or graphite anodes lowered in specific increments with the super thick HMW-PE wire to ensure the cables are protected and will last the typical 20 years expected.

Through those anodes, electricity is applied to make them activate to get that magical -0.850mv reading. The application of electricity is accomplished through a rectifier. The rectifier will take alternating current (AC), which occasionally reverses direction, and convert it into direct current (DC), which only flows one direction. This "rectifies" the current, so it is always pushing through the anodes to the soil.

Rectifiers come with a variety of capabilities. Some of them are air-cooled, some are submersed in oil, others still are qualified as explosion proof. The volt-amp rating can vary from 20-10 to 100-80. Some units feature a convenience plug. All rectifiers need to have a log book to monitor for optimal operation. The rectifier will need a copper buss bar to distribute the electricity through shunts to the impressed current anodes, too.

The smaller units can run about $600, and Farwest Corrosion usually has some in stock. Larger units can have a 7-9 week lead time, and the cost can get pretty high. The highest I've seen is $5,500.

Standard v High Potential

Magnesium sacrificial anodes come in two different flavors, standard potential and high potential (sometimes abbreviated HP). Standard potential magnesium anodes have a less pure magnesium composition and thus has a reduced voltage output.

For impressed current systems, users depend on electricity applied to the anodes to provide coverage. For the sacrificial system, the protection is entirely based on the anode's ability to pump out voltage--the more negative the better for the steel structure.

A standard potential magnesium anode will rate approximately -1.52 volts in a closed circuit test, often conducted by a 3rd party to verify a company's anodes conform to industry standards. The high potential magnesium anode must rate more negative than -1.7 volts, as well as maintain a specific efficiency (50%, or 500 amp hours).

An anode's efficiency? What's that?

Well, in the cathodic protection cycle, there is the anode, cathode, electrolyte, and direct path. What we try to do is make the steel substructure the cathode (protected structure) and the magnesium the anode (what gets eaten). While the magnesium does a good job of protecting the steel, it also succumbs to self-corrosion. According to the definition for a high potential anode, then, at least 10# of a 20# magnesium anode must be available for protection of the pipeline.

In looking at pricing for a system, one could consider a standard potential anode, but even more of the 20# anode would be consumed, reducing the life of the anode. For the benefit of the structure--and the safety of nearby consumers--the high potential anode must be the prime candidate.

For additional information on Farwest Corrosion's fine product, additional information can be found in their white paper.

Cadwelding--come and get it!

 Another shot from the Farwest Corrosion--Denver warehouse, here you can see our Erico Cadwelding supplies (and a few pallets of anodes).

The yellow boxes you see are the basic weld metals; the plus versions of the shots are above them in the brown boxes. 100 CA15s are packaged a lot more concisely than the CA15Pluses.

To the left are boxes and boxes of specific molds,for almost any combination of steel/DIP/wire size. We have several of the vertical mold versions, as well as most of the horizontal ones.