Introduction to Trivalent Chromium Finishing
Trivalent chromium finishing, also known as Chromium III finishing or CrIII finishing, is a metal finishing process that uses trivalent chromium instead of the more toxic hexavalent chromium. Trivalent chromium provides corrosion resistance and durability to metals like steel and aluminium, but is less hazardous to human and environmental health.
Chemistry Behind Trivalent Chromium Finishing
In Trivalent Chromium Finishing, the metal surface is treated with a solution containing trivalent chromium ions (chromium with 3+ charge). When these chromium ions come in contact with the metal surface, they get reduced to form a thin CrIII layer either by adsorption or an electrochemical process.
This CrIII layer bonds strongly with the metal surface via chemical or electrostatic bonds. Being an inert and stable oxidation state of chromium, the CrIII layer protects the base metal from corrosion very effectively. It seals the pores on the metal surface and forms a barrier against corrosive elements like water, oxygen, acids etc.
Compared to hexavalent chromium, trivalent chromium is less reactive and less mobile in aqueous solutions. It does not readily dissolve, migrate or leach out from the protective chromium layer. This makes trivalent chromium finishing more environment-friendly with reduced health and safety risks.
Advantages of Trivalent Chromium Over Hexavalent Chromium
Some major advantages of using trivalent chromium instead of hexavalent chromium in metal finishing processes are:
– Lower Toxicity: Trivalent chromium is significantly less toxic than hexavalent chromium. It does not cause asthma, dermatitis or cancer even after prolonged exposure.
– Improved Performance: Modern trivalent chromium coatings provide equivalent or better corrosion resistance compared to hexavalent chromium coatings. Their performance has improved over the years.
– Regulatory Compliance: Trivalent chromium complies with various environmental regulations restricting the use of hexavalent chromium worldwide. It offers a compliant alternative.
– Cost Savings: Shifting to trivalent chromium eliminates costly hexavalent chromium management requirements like wastewater treatment, personal protective equipment etc. This lowers overall processing costs.
– Sustainability: Being environment-friendly, trivalent chromium finishing supports companies’ sustainability goals and improve their green credentials. It causes less harm to the natural environment.
Process Steps Involved in Trivalent Chromium Finishing
The basic steps involved in a typical trivalent chromium finishing process are:
1. Surface Pre-treatment: The metal parts are first cleaned thoroughly by alkaline/acid cleaning to remove grease, oil and dirt.
2. Activation: A mild acidic solution is used to activate the metal surface for chromium deposition by micro-etching.
3. Chromium Plating: The activated parts are electroplated with trivalent chromium ions from a trivalent chromium plating bath containing chromic acid.
4. Rinsing: After plating, parts are rinsed thoroughly with deionized water to remove any residual plating chemicals.
5. Sealing: Optionally, a post-seal may be applied to enhance corrosion protection by filling any minor scratches or pinholes.
6. Drying and Quality Checks: Parts are dried then checked to ensure a defect-free chromium finish meeting thickness specifications.
Quality Performance of Trivalent Chromium Coatings
High-quality trivalent chromium platings provide matchless protective performance equivalent to hexavalent chromium coatings. Some key quality aspects are:
– Thickness Uniformity: Consistent chromium plating thickness of 2-5 microns ensures uniform protection.
– Adhesion: Excellent adhesion to the substrate measured over 500 hours of exposure in ASTM B-571 and ASTM D-3359 tape tests.
– Corrosion Resistance: Passes over 1000 hours of salt spray testing per ASTM B-117 and humidity testing per ASTM D-2247.
– Flexibility: Withstands flexible tabbing and bending per ASTM D-522 method without cracking or flaking.
– Aesthetics: Provides a bright, reflective finish similar to hexavalent chromium for an attractive appearance.
Applications of Trivalent Chromium Platings
Trivalent chromium platings are most commonly used in:
– Automotive Components: Engine covers, brake calipers, chassis etc. provide long-lasting corrosion protection.
– Architectural Hardware: Door handles, cabinet pulls, hinges resist harsh exterior environmental conditions.
– Appliance Parts: Refrigerators, washing machines, air conditioners require rust-resistant internal mechanisms.
– Industrial Equipment: Valves, pumps, machinery used in chemical plants need durable chromium coatings.
– Aerospace Applications: Fastener assemblies, hydraulic cylinders used in aircraft face salty, humid conditions.
– Decorative Plating: Available as sulfamate and sulfate chromates for a very thin, highly reflective finish.
With its enhanced sustainability profile and growing regulatory acceptance, trivalent chromium is positioned as the future-ready substitute for hexavalent chromium plating. It combines eco-friendliness with the proven corrosion resistance of conventional chromium coatings. As trivalent chromium technology progresses further, it is gaining wider acceptance across various industry sectors for critical metal finishing applications.