Why Does Bronze Turn Red or Pink? The Dezincification of Bronze Fasteners

If you've ever worked with old bronze fasteners, you may have noticed a curious phenomenon: over time, some bronze components, especially those exposed to marine or corrosive environments, can take on a reddish or pinkish hue. This color change is caused by a process called dezincification, a type of corrosion that affects certain copper alloys, including brass and some types of bronze. But why does this happen, and what does it mean for the integrity of your fasteners?

What Is Dezincification?

Dezincification is a selective corrosion process in which zinc is leached out of an alloy, leaving behind a porous, weakened structure primarily composed of copper. This process occurs in environments where chlorides, acids, or oxygen-rich seawaters are present—conditions commonly found in marine settings.

While dezincification is most commonly associated with brass, certain types of bronze are also susceptible, particularly those that contain significant amounts of zinc, such as manganese bronze and naval brass. The characteristic pink or red color is due to the remaining copper-rich matrix, which no longer retains the original golden hue of the alloy.

Why Does Dezincification Occur in Bronze Fasteners?

Marine-grade fasteners are typically chosen for their superior corrosion resistance, but not all bronze alloys are equally resistant to dezincification. The following factors influence whether a bronze fastener will experience this type of corrosion:

1. Zinc Content

Bronze is an alloy primarily composed of copper, but some variants contain zinc as a major or minor alloying element. Alloys such as manganese bronzes (which can contain 20-40% zinc) are particularly vulnerable. When exposed to aggressive environments, the zinc atoms are more reactive and preferentially dissolve, leaving behind a weakened copper skeleton.

2. Environmental Exposure

Bronze fasteners in seawater, brackish water, or acidic conditions are more likely to experience dezincification. Chloride ions, which are abundant in marine environments, accelerate the process by attacking the zinc-rich areas of the metal.

3. Oxygen and Galvanic Corrosion

In conditions where oxygen levels vary—such as in crevices, joints, or areas where fasteners are partially submerged—galvanic corrosion can drive dezincification. If the fastener is coupled with a less noble metal, such as aluminum or steel, an electrochemical reaction may also accelerate zinc leaching.

4. Poor Alloying and Impurities

Low-quality or improperly alloyed bronze may have uneven zinc distribution, leading to localized dezincification. The manufacturing process also plays a role—casting flaws, improper heat treatments, or poor metallurgical bonding can all contribute to premature failure.

How to Identify Dezincification in Bronze Fasteners

Dezincification often starts as a color change, where the once bright, golden bronze takes on a reddish tint. However, the real danger is structural weakening. As zinc is removed, the metal becomes porous and brittle. In severe cases, affected fasteners may:

• Show a chalky white or pinkish-red appearance
• Develop cracks or crumbling areas
• Lose mechanical strength, leading to failure under stress

One common test for dezincification is the Ferric Chloride Test, where a small amount of ferric chloride solution is applied to the metal surface. If zinc has been leached out, the affected areas will turn red quickly, confirming the presence of dezincification.

Preventing Dezincification in Marine Fasteners

While dezincification is a serious concern, there are ways to prevent it and extend the lifespan of bronze fasteners:

1. Choose Silicon Bronze Over Manganese Bronze

If you need corrosion-resistant fasteners, silicon bronze (C65500 or C65100) is the superior choice. Unlike manganese bronze, silicon bronze contains little to no zinc, making it virtually immune to dezincification. This is why proper marine-grade fasteners are almost always made from silicon bronze rather than manganese or one of the other numerous bronze alloys.

2. Use Cathodic Protection

For applications where bronze fasteners must be used in marine environments, cathodic protection (such as sacrificial anodes) can help reduce corrosion. This method diverts the electrochemical attack away from the fasteners and on to the anode, preserving the fastener's integrity. This is usually accomplished with zinc anodes and on the vessel, but keep in mind that cathodic protection on wooden vessels in particular is a highly complex science/art and the level of protection needs to be well balanced. A wooden boat can experience delignification, a breakdown of the wood's fibers, if cathodic protection is not properly balanced.

3. Apply Protective Coatings

Sealants, epoxy coatings, and other protective barriers can help shield bronze fasteners from direct exposure to aggressive environments. However, coatings must be properly applied and maintained, as any breach can lead to accelerated corrosion in exposed areas.

4. Avoid Mixing Dissimilar Metals

To prevent galvanic corrosion, avoid pairing bronze fasteners with reactive metals like aluminum or carbon steel in a marine environment. If mixed metals must be used, insulating washers or dielectric coatings can help minimize direct contact and slow corrosion.

5. Regular Inspection and Replacement

Since dezincification weakens metal over time, periodic inspection of fasteners in high-risk environments is essential. Look for signs of red discoloration, pitting, or weakened areas, and replace any compromised fasteners before they fail.

If you’ve ever wondered why bronze turns red, the answer is most likely dezincification. This process selectively removes zinc from the alloy, weakening the structure and altering the color. While some types of bronze are vulnerable, proper material selection can prevent issues.

For marine and corrosive environments, silicon bronze fasteners are the best choice, as they resist dezincification and provide long-term reliability. By understanding the factors that contribute to dezincification and taking steps to prevent it, you can ensure the longevity and performance of your bronze fasteners in even the harshest conditions.

Fair Wind Fasteners: Your Source for Dezincification-Resistant Bronze Fasteners

At Fair Wind Fasteners, we specialize in high-quality, marine grade silicon bronze fasteners designed to withstand the toughest marine conditions. Unlike manganese bronze or other mystery bronzes, our fasteners are dezincification-resistant and built to last. Whether you're restoring a classic wooden boat or working on a high-performance marine project, you can trust Fair Wind Fasteners for the best in marine-grade hardware.