What Causes Corrosion in Metal Inserts within Plastic Products?

 Environmental Factors

Exposure to certain environments is the most common cause of corrosion.

Moisture/Humidity: The presence of water triggers the oxidation of iron (rust). In humid conditions, moisture condenses on the metal surface, initiating corrosion.

Chlorides: Exposure to salt (e.g., from coastal air, sweat, or deicing salts) drastically accelerates corrosion, causing severe "salt-induced rusting."

Acids/Alkalis: Contact with cleaning agents, sweat, or industrial chemicals can break down the metal's protective surface layer, increasing corrosion risk.

Insert-Specific Factors

The metal's inherent properties are critical.

Material: Carbon steel rusts easily unless protected. Some stainless steel grades (e.g., low-quality types) may still corrode in harsh environments.

Surface Coating: A lack of coating, or a poor-quality one (e.g., thin, porous plating like zinc or nickel), provides inadequate defense. Damage during handling also creates weak points for rust to start.

Design & Manufacturing Factors

The integration and assembly process plays a key role.

Residual Stress from Molding: The high heat of insert molding and subsequent cooling can create internal stress in the metal, making it more prone to "stress corrosion."

Over-Tightening during Assembly: Excessive force can strip protective coatings and create micro-cracks, offering a path for corrosion.

Galvanic Corrosion: If the insert contacts a different metal (e.g., an aluminum part) in a moist environment, an electrochemical reaction can rapidly corrode the less noble metal.

IV. Other Influences

Additional factors can contribute to the problem.

Plastic Properties: Some plastics (like PVC) can release acidic substances over time, which may slowly attack the metal.

Storage/Transport: Damp warehouses or inadequate packaging can expose products to moisture long before they reach the end user.

Summary of Mitigation Strategies

Material Selection: Choose appropriate corrosion-resistant alloys, such as 304 or 316 stainless steel, for the expected environment.

Surface Treatment: If using carbon steel, apply high-quality coatings like Dacromet or zinc phosphating, which are superior to standard electroplating.

Process Control: Optimize molding and assembly processes to minimize residual stress and avoid damaging the inserts.

Environmental Management: Control storage humidity and use protective packaging to limit exposure to moisture.