Packaged or bare?

Bare Anode Ingots

Last week was the Rocky Mountain Section of NACE's short course. It was the biggest turn out yet with over 300 people attending! It's a great opportunity to meet customers (and competitors), and learn more about this rusty world. Every year is a Basic CP course that goes over the basics of electrochemistry, and what the corrosion cycle is. The nugget I garnered this year has to do with sacrifical anodes. A question I always have to ask when someone orders a mag (or zinc) anode is, do you want that packaged or bare?

When we talk about anodes, we talk about the bare weight of the ingot. "I want a 17 pound mag anode," means, I want an anode with 17 pounds of magnesium. The ingot is then silver soldered for a strong connection between the lead wire and the bare metal. Some anodes--bare anodes--are then finished.

The pictures on the left show bare ingot dimensions, where
the picture on the right shows a cross-section of a packaged
anode.

For a packaged anode, the ingot is then surrounded by a backfill and a cloth bag. This adds a significant amount of weight (at least doubling it; a 17# anode becomes 50#). The backfill is a standard mix of 75% gypsum, 20% bentonite, and 5% sodium sulfate. Each part plays an important part:

• The gypsum helps maintain a constant resistance between the anode and the structure being protected.
• The bentonite (clay) helps retain moisture (the electrolyte) against the ingot to help make sure the ingot is what corrodes, not the metal structure.
• The sodium sulfate helps keep the ingot from passivation, where the metal forms a film to protect itself.

The packaging of an anode obviously adds to freight costs, and the cost of the anode goes up by about 33%, but the benefits of ensuring an anode is protecting the neighboring communities explains why every specification I've worked with requires packaging.