What are the surface treatments for stainless steel parts?

Stainless steel is widely used in our daily lives. With so many metal surface treatment methods available on the market, which ones are suitable for stainless steel? The first step is to identify the core objective: is it to enhance the appearance and texture, improve corrosion resistance, optimize functional properties (such as wear resistance and anti-static properties), or meet industry standards (such as those for food and medical industries)? Based on the purpose of the treatment and the principles of the process, surface treatments for stainless steel can be categorized into four major types: surface smoothing, chemical conversion treatment, coating/plating treatment, and functional surface modification.

Surface defects (such as burrs, scratches, and oxide scale) are removed through physical or mechanical means to optimize surface roughness (Ra). This treatment is divided into two main directions: "matte/brushed" and "mirror finish," and it is the most basic and widely applied method.

A dense oxide film/passivation film is generated on the surface of stainless steel through chemical reactions. This enhances corrosion resistance without the need for an additional coating and without changing the part's dimensions (film thickness is typically 0.1-1μm), making it suitable for precision parts.

• Passivation Treatment (the core chemical treatment)

Stainless steel is immersed in a nitric acid solution (or citric acid, chromate solution, which are environmentally friendly) to oxidize the Cr element on the surface and form a Cr₂O₃ passivation film (thickness of about 2-5nm). This film prevents the base material from coming into contact with air and moisture, significantly improving corrosion resistance.

• Traditional Passivation: Using a 65%-85% nitric acid solution, suitable for common stainless steel grades (such as 304, 316), but the chromium-containing wastewater needs to be treated.
• Eco-friendly Passivation: Using chromium-free solutions such as citric acid and phosphoric acid, which comply with RoHS and food-grade standards (such as FDA), and are widely used in the medical and food industries.

• Coloring Treatment

A colored oxide film is generated on the basis of the passivation film through chemical oxidation (such as alkaline oxidation solution) or electrochemical oxidation. The color of the film is determined by its thickness (blue, purple, red, green, etc.), offering both decorative and corrosion-resistant properties (film thickness 5-20μm).

When the inherent corrosion resistance and wear resistance of stainless steel are insufficient, functional layers are added through "coating" or "deposition" methods to meet the demands of extreme environments (such as high temperature, strong acids, and high wear).

• Physical Vapor Deposition (PVD Plating)

In a vacuum environment, metal target materials (such as Ti, Cr, Zr) are deposited onto the surface of stainless steel through evaporation, sputtering, or ionization to form hard films (such as TiN titanium nitride, CrN chromium nitride).

• Applications: Cutting tools (surgical knives, craft knives), molds, watch cases, and automotive decorative parts.

• Chemical Vapor Deposition (CVD Plating)

Ceramic films such as silicon carbide (SiC) and aluminum nitride (AlN) are generated through the reaction of gaseous reactants with the surface of stainless steel at high temperatures (800-1200℃), with a film thickness of 5-20μm.

• Applications: Corrosion-resistant components in the chemical industry, parts inside high-temperature furnaces, and semiconductor wafer carriers.

• Organic Coatings (Spraying/Electrophoretic Deposition)

Organic resins (such as epoxy resin, polytetrafluoroethylene PTFE, fluorocarbon paint) are applied to the surface through spraying or electrophoretic deposition to form insulating, weather-resistant, or non-stick layers.

• Epoxy Resin Coating: Good solvent resistance and insulation properties, used for electrical equipment casings and circuit board supports.
• PTFE Coating (Teflon): Non-stick and temperature-resistant (-200℃ to 260℃), used for non-stick pans and food molds.
• Fluorocarbon Paint: UV-resistant and outdoor aging-resistant (service life over 15 years), used for outdoor stainless steel facades and billboards.

• Graphene Composite Nanoceramic Coating

This coating uses a nano-deposition process that combines liquid-phase and vapor-phase deposition, resulting in ion-level density. It significantly improves thermal conductivity and heat dissipation, is suitable for long-term use between -120°C and 300°C, and has a stable and controllable thickness of ±1 micron. It prevents low-temperature condensation and frosting, is anti-static, and corrosion-resistant.

• Applications: Digital 3C products, mechanical equipment, data centers, biomedicine, smart home appliances, transportation, and precision devices.

To meet special needs (such as antibacterial, conductive, or hydrophobic properties), the surface microstructure or composition is altered through physical or chemical means to achieve "functional customization."

• Antibacterial Treatment

Silver ions (Ag⁺), copper ions (Cu²⁺) are deposited on or doped into the surface, or antibacterial resins (such as silver-loaded epoxy resin) are applied. These metal ions disrupt bacterial cell membranes, inhibiting the growth of E.coli and Staphylococcus aureus.

• Applications: Medical equipment (bed railings, infusion stands), public facilities (elevator buttons, handrails), and children's tableware.

• Hydrophobic/Superhydrophobic Treatment

Microscopic concave-convex structures are created on the surface through laser engraving or the application of low-surface-energy materials (such as polydimethylsiloxane PDMS). This results in a contact angle greater than 150°, causing water to form droplets and roll off, achieving a "self-cleaning" effect.

• Applications: Outdoor surveillance camera casings, solar photovoltaic panels (stainless steel frames), and car rearview mirrors (stainless steel edges).

• Conductive/Magnetic Treatment

Copper, nickel, silver (for conductivity) or Permalloy (for magnetism) are electroplated onto the surface of stainless steel to compensate for its inherently poor conductive/magnetic properties.

• Applications: Electronic connectors (stainless steel base material+silver plating), electromagnetic shielding covers (stainless steel+nickel plating).