Brass is an alloy primarily composed of copper and zinc. While it does not experience “rust” in the technical sense, it does corrode rapidly when exposed to salt water. Rust is a term specifically reserved for the oxidation of iron, which results in the formation of flaky, reddish-brown hydrated iron oxide. Since brass contains no iron, it is immune to true rust, but it is highly susceptible to other forms of electrochemical deterioration, especially in a marine environment.
Understanding Rust and Brass Composition
Rust is the common name for the corrosion of ferrous metals, meaning those containing iron, which reacts with oxygen and water to form iron oxide. This process creates a porous and structurally weak layer that flakes away, continuously exposing new metal to the corrosive environment. Brass is typically made of 60 to 90 percent copper and 10 to 40 percent zinc, with the specific ratio influencing its properties.
The high copper content in brass gives it a natural resistance to general oxidation compared to iron. When exposed to air and moisture, the copper component often forms a thin, protective layer of greenish-blue patina. This protective layer is a stable form of corrosion that shields the underlying metal from further decay. However, the presence of zinc fundamentally changes its behavior when introduced to a highly conductive solution like salt water.
Specific Corrosion Mechanisms in Salt Water
The primary corrosion danger for brass in salt water is a process called dezincification, a type of selective leaching. In this mechanism, the zinc component of the alloy is preferentially dissolved and removed, leaving behind a porous, weak, copper-rich residue. Zinc is significantly more reactive than copper, meaning it acts as the sacrificial metal within the alloy when exposed to the chloride ions present in sea water.
This loss of zinc transforms the brass component into a spongy material that retains the original shape and dimensions but is completely devoid of mechanical strength. Visually, dezincification may appear as dull, reddish or pinkish areas on the surface, which is the remaining copper layer. In severe cases, the entire cross-section of the metal can be affected, leading to sudden structural failure of fittings or valves.
Salt water acts as a strong electrolyte, which significantly accelerates general corrosion and increases the risk of galvanic corrosion when brass is coupled with other metals. Galvanic corrosion occurs when two dissimilar metals are electrically connected and immersed in a conductive liquid. Because brass contains a less noble metal (zinc), it can become the anode in a galvanic cell, corroding faster to protect the more noble metal it is connected to, such as stainless steel. This electrochemical reaction further compounds the risk of dezincification, as the saltwater environment encourages the rapid dissolution of the more reactive zinc.
Protecting Brass in Marine Environments
Protecting brass in a marine environment requires careful material selection and proactive maintenance. The most effective strategy involves choosing dezincification-resistant (DZR) alloys, sometimes referred to as naval brass or Admiralty brass. These specialized alloys often contain small amounts of inhibitor elements like arsenic, antimony, or phosphorus, which significantly slow the selective removal of zinc. Alloys with a naturally lower zinc content, typically less than 15%, are also inherently more resistant to this form of attack.
For brass components coupled with other metals underwater, sacrificial anodes made of zinc or aluminum can be installed nearby. These anodes are intentionally made to be the least noble metal in the system, forcing them to corrode first and protect the brass fittings from galvanic attack. Applying specialized marine-grade coatings creates a physical barrier that prevents direct contact between the brass surface and the corrosive salt water. Examples include:
- Clear lacquers.
- Epoxy paints.
- Polymer coatings.
Regular maintenance, including rinsing submerged components with fresh water, helps to remove salt deposits that accelerate the corrosion process. Using inhibited alloys and employing external protection methods significantly extends the serviceable life of brass in salt water applications.