Zinc alloy is a metal mixture combining zinc with elements like aluminum, copper, or magnesium to enhance its strength and casting properties. Due to its versatility and low cost, it is widely used in everyday items, including jewelry, hardware, and automotive parts. While it does not typically form the dark sulfide layer associated with silver tarnish, zinc alloy undergoes surface degradation through corrosion or oxidation. This surface change is an inevitable chemical reaction when the metal is exposed to the environment.
How Zinc Alloy Reacts to the Environment
The chemical process affecting zinc alloy is oxidation. When zinc is exposed to air and moisture, it forms zinc oxide, which then reacts with carbon dioxide to create a layer of zinc carbonate. This zinc carbonate serves as a natural, self-limiting protective barrier on the surface, significantly slowing the rate of further corrosion into the underlying metal.
The visual result is not a dark tarnish, but a dull, whitish-gray, or powdery film. This corrosion product is sometimes called “white rust” and is noticeable if the item is frequently exposed to moisture that cannot quickly evaporate. Although the metal loses its original metallic luster, this patina is a sign of the alloy’s natural defense mechanism protecting its core structure.
Factors That Accelerate Corrosion
While the zinc carbonate layer is protective, various environmental factors can break down this barrier and accelerate corrosion. High humidity and prolonged contact with water are significant contributors, as moisture provides the medium for corrosive chemical reactions. Saltwater environments, including sweat, are particularly aggressive due to chloride ions, which actively break down the protective film.
Acidic substances also pose a threat, as zinc is highly reactive outside of a specific neutral to mildly alkaline pH range (5.5 to 12.5). This includes human sweat, perfumes, lotions, and common household cleaning chemicals. Furthermore, elevated temperatures, especially above 55°C, can cause the protective corrosion products to form with a coarse structure, leading to poor adhesion and exposing fresh metal to continued attack.
Protecting Zinc Alloy from Degradation
The most effective way to prevent degradation is by applying a physical barrier between the metal and the environment. Manufacturers often use electroplating with noble metals like copper, nickel, or chromium to provide superior wear and corrosion resistance, especially for decorative items. A clear protective lacquer or organic coating, such as a clear coat or powder coating, provides a strong physical seal against moisture and air for larger components.
For smaller items like jewelry, applying a thin layer of specialized jewelry sealant or clear nail polish creates a temporary, effective shield that should be reapplied periodically. Proper handling is equally important. Items should be removed before activities involving high sweat, swimming, or showering. Always apply perfumes, hairspray, and lotions before putting on zinc alloy items to prevent direct chemical contact.
Long-term storage practices significantly influence the alloy’s lifespan. Zinc alloy should be stored in a cool, dry place away from high humidity areas, such as bathrooms. Using airtight containers or sealed plastic bags minimizes exposure to oxygen and airborne pollutants. Adding silica gel packets or anti-tarnish strips helps absorb residual moisture and further protects the surface.
Cleaning and Restoring Corroded Surfaces
Once the white or gray corrosion layer has formed, a gentle, non-abrasive cleaning method is needed to restore the finish without damaging underlying plating. A simple solution of mild dish soap and warm water is the safest approach for routine cleaning and removing surface dirt and oils. The surface should be gently scrubbed with a soft cloth or soft-bristled toothbrush to dislodge the powdery residue.
For more stubborn white rust, a paste made of baking soda and water can be applied and gently rubbed in, as it is a mild abrasive that will not etch the surface. It is important to avoid using harsh cleaners, strong acids, or abrasive pads, which strip away protective coatings or accelerate corrosion. After cleaning, the item must be rinsed thoroughly to remove all cleaning agents and dried immediately with a soft, lint-free cloth to prevent new moisture spots. Following this process, reapplying a protective coating, such as a specialized metal wax or lacquer, is recommended to reseal the surface and protect against future degradation.