What is Dezincification?
Dezincification is a process which selectively removes zinc from an alloy, leaving behind a porous, copper-rich structure that has little mechanical strength. The bronze alloys that we use on our boats for thru-hulls, seacocks, and propellers are made up of a mixture of different metals with the primary metal being copper. For example Naval Brass, a popular marine grade alloy with high strength and corrosion-resistance, contains 60 percent copper, .75 percent tin and 39.2 percent zinc. When properly protected against galvanic corrosion naval brass is a great option for use below the waterline in salt water.
Most boaters understand that we use zinc anodes in salt water to protect against galvanic corrosion by allowing the zinc to be ‘sacrificial’ and serve as the anode in a galvanic cell, thereby protecting the other underwater metals. We use zinc for this purpose because it is one of the least noble metals and will become anodic when in a galvanic cell with most other metals. Because zinc is so susceptible to galvanic corrosion it is extremely important that we protect our underwater bronze materials from this corrosion by using sacrificial anodes. But what happens to that bronze or naval brass material when we don’t protect it?
When the zinc is removed from the bronze structure (dezincification) we are left with a mostly copper alloy that is extremely brittle. As you can see in the picture above, the prop is left with a pinkish hue because it is mostly copper and all of the zinc has been corroded out of the alloy. An extreme case like this propeller must be replaced as it no longer has the structural strength necessary to perform it’s job and could lose a blade at any time.
A common question is “How can a zinc anode protect the zinc in bronze, if it’s the same metal? The reality is that our ‘zinc anodes’ are not made up of only zinc, but rather they are also an alloy comprising other materials which make them more anodic. For the same reason we often use aluminum anodes to protect underwater aluminum. Aluminum anodes are also the preferred material for protecting under water metals in brackish water, and many boaters successfully use aluminum anodes in saltwater as well. (Some prefer aluminum anodes in saltwater for environmental reasons because the zinc anodes contain trace amounts of cadmium.)
As an ABYC Corrosion Certified Marine Surveyor this is one of the checks I make on every vessel survey. If a prop has been coated with prop speed, bottom paint, or some other coating you will see me with a scraper removing some of the coating on the outer edges of all the blades to check for this pink hue which indicates there has been dezincification of the prop.
The image above is from the same vessel that the dezincified prop was on, and shows the destruction of bronze fittings on a vessel when galvanic protection has been ignored for too long a time. Significant cost to repair comes from simply failing to renew your anodes in a timely fashion.
Contact Andy Lowe Marine Surveying today to set up your vessel survey or corrosion inspection with a SAMS surveyor, ABYC Master Technician and Corrosion Expert.