In humid environments, oxidation and corrosion of switch series contacts primarily stem from the combined effects of moisture, oxygen, and impurities in the air, leading to electrochemical corrosion of the metal on the contact surface. Therefore, surface protection must focus on three key areas: isolating the corrosive media, enhancing surface corrosion resistance, and blocking electrochemical reactions. Through targeted surface treatment processes, a stable protective barrier is constructed for the contact.
The most common surface protection method for switch contacts is metal plating, which creates a physical barrier by coating the contact substrate with a highly corrosion-resistant metal. Common plating materials include gold, nickel, and tin, each suitable for different applications. Gold plating offers excellent chemical stability, effectively preventing moisture and oxygen from reaching the substrate while maintaining good conductivity, making it suitable for switches requiring high contact reliability. Nickel plating is more cost-effective, forming a dense oxide film that provides long-term resistance to humid environments and exhibits a certain degree of wear resistance, making it suitable for switches used in low-humidity environments such as household or industrial environments. Tin plating not only prevents corrosion but also improves the solderability of the contact. Tin coatings are less susceptible to rust formation in humid environments, preventing rust accumulation and poor contact. However, care should be taken to avoid excessive plating thickness, which could affect the elastic deformation capacity of the contact. These plating layers require a standardized electroplating process to form a uniform, pinhole-free film. If the plating has defects, moisture can still penetrate the substrate and cause corrosion, making process control crucial for effective protection.
Chemical conversion coatings are also an important protective method for switch contact plates. Through a chemical reaction, they form a stable inorganic compound film on the contact plate surface. This film bonds tightly to the substrate and effectively blocks moisture from contacting the metal. For example, chromate passivation forms a dense, chemically stable chromium oxide film on the contact plate surface. This film is resistant even to high humidity. Phosphating treatment creates a phosphate film. While its own corrosion resistance is somewhat weak, it serves as a base coat for subsequent protective coatings, enhancing their adhesion and overall effectiveness. This type of conversion coating is typically used in applications where conductivity isn't paramount but where cost and protection are a balance. The process requires strict control of reaction conditions to ensure uniform film thickness and zero gaps, preventing corrosion risks caused by incomplete coatings.
Organic coatings are applied to the contact plates of switch series with a highly water-resistant organic polymer material, creating a flexible protective film that both blocks moisture and adapts to the slight elastic deformation of the contact plates during switching. Commonly used organic coatings include epoxy resin and polytetrafluoroethylene (PTFE). Epoxy resin coatings offer strong adhesion and excellent chemical resistance, forming a continuous protective film on the contact surface, preventing moisture from penetrating. PTFE coatings, on the other hand, possess extremely low surface energy, making them not only water-resistant but also reducing the adhesion of dust and impurities to the contact surface, preventing the combined effects of impurities and moisture from exacerbating corrosion. However, it's important to note that organic coatings shouldn't cover the conductive contact areas of the contact, as this could affect the switch's electrical performance. Therefore, precise coating coverage is crucial, ensuring protection is applied only to non-contact areas to ensure that the protective and conductive functions don't interfere with each other.
In addition, surface sealing can complement plating or conversion coating treatments by sealing any tiny pinholes or cracks in the plating or film, further enhancing the integrity of the protective layer. For example, a very thin layer of organic sealant can be applied to the surface of the metal coating. This sealant penetrates the tiny pores of the coating and solidifies, forming a three-dimensional protective network that completely blocks moisture from reaching the substrate through these pores. Sealing is particularly essential for porous coatings like phosphate films to prevent moisture accumulation within the pores and localized corrosion. This supplemental protection is particularly suitable for environments with extremely high humidity or the presence of small amounts of corrosive gases, significantly extending the corrosion life of switch series contact pieces.
When selecting protective measures, the switch's usage scenario and performance requirements must be considered comprehensively. If the switch requires frequent switching and has strict contact resistance requirements, gold or nickel plating is preferred. If the switch is used in a moderately humid environment and cost control is a priority, tin plating or chemical conversion coating is more suitable. In environments with extremely high humidity and the presence of impurities, a combination of "plating + sealing" or "conversion coating + organic coating" can be used. At the same time, no matter which protection method is adopted, subsequent storage and installation must also pay attention to moisture-proofing to prevent the protective layer from being damaged by the humid environment before use, ensuring that the switch series contact pieces can resist oxidation and rust throughout the entire cycle from production to use, and maintain stable conductivity and switching performance.
