Potassium Dichromate

Product Overview

Potassium Dichromate (K2Cr2O7) is the classic hexavalent-chromium salt of the conversion-coating world - bright orange-red crystals that give yellow (iridescent) chromate finishes on zinc and cadmium, the traditional dichromate seal for anodized aluminium, and tarnish-retardant dips for copper alloys. It is a strong oxidizer and, in dilute acid solution, it reacts with zinc or cadmium surfaces to grow a thin, self-healing chromate film whose yellow iridescence has meant "corrosion protected" to generations of inspectors. Plating-grade technical salt is sold with tight chloride, sulfate and iron limits and a guaranteed assay; lower grades go to pigment, leather and reagent markets. Compliance first: hexavalent chromium is restricted in the European Union (REACH - among the most heavily regulated substances in finishing), in automotive ELV and RoHS electronics, and in many export markets. Where the customer's specification allows it and local law permits, conventional yellow chromate still runs - but the responsible supplier states clearly what the salt is for and what it is not, and supports trivalent or Cr-free alternatives wherever regulation demands them.

Product Identity (Encyclopaedia Card)

What it is and what it does. Potassium dichromate is the traditional chromate and passivation salt used for the chromate conversion coating of zinc and cadmium, and for the colouring and passivation of copper alloys.

• Orange-red crystals; the dichromate ion forms the protective chromate conversion film on zinc.
• Conventional hexavalent chromium chemistry, now increasingly replaced by trivalent passivates because of its carcinogenicity.
• It is an oxidiser as well as a chromate; storage must be separate from acids and combustibles.

Typical Use in a Plating / Finishing Line

Safety, Handling & Storage

Quality Documents Shipped With Every Batch

Buyer Questions We Answer Every Week

How to Check Quality Before Loading a Batch

How QLQ controls it: because this is a regulated substance, control starts before the salt arrives - every order is checked against end-use compliance, every lot is quarantined, assay-verified and panel-tested, and the COA plus batch record are kept for full traceability. QLQ also documents exactly what the material may and may not be used for under current regulations.

How It Works in the Line (Conversion, Sealing, Tarnish Control)

In an acidic dichromate dip the hexavalent chromium oxidises the zinc (or cadmium) surface while being itself reduced to a gel of trivalent chromium oxide containing residual hexavalent chromium - that mixed film is what gives yellow chromate its colour and its self-repairing corrosion behaviour: if the film is scratched, soluble hexavalent chromium in the film re-passivates the exposed metal. On anodized aluminium the dichromate seal works differently: it is a pore-closing hydration step in which dichromate is adsorbed into the porous oxide, giving the classic green-yellow tint and the best corrosion results of the classical seals. Baths are made up from the salt, acid and water; working data below are published ranges that must be tuned to the substrate, the required film weight and the applicable spec (for example the classical yellow chromate range on zinc).

Common Problems & Fixes (Workshop Table)

Handling & Safety (Cr(VI) - Read This)

• Health: hexavalent chromium compounds are carcinogenic by inhalation and cause skin ulcers and allergic dermatitis. Never touch crystals with bare hands, never create dust, and keep all chromate tanks under extraction. Respiratory protection is mandatory where mist or dust is possible.
• PPE: dedicated gloves (nitrile/PVC), goggles or face shield, apron, and a respirator with P3/HEPA filters for powder handling. Change and decontaminate gloves after every shift; wash hands before eating.
• Dissolving: dissolve slowly in water with stirring; the solution is acidic and stains everything it touches. Use dedicated plastic tanks and tools - never wooden or woven materials that soak up chromium.
• Mixing: add acid to the dichromate solution carefully - the mixture heats; never add dichromate to hot concentrated acid.
• Waste - non-negotiable: hexavalent chromium must be reduced to trivalent (ferrous sulfate or metabisulfite at low pH) before precipitation, and the sludge handled as hazardous waste. Never discharge Cr(VI) to drain. Rinse water needs its own treatment loop and monitoring.
• Regulatory: confirm before ordering that the end use is lawful in the destination market (REACH authorisation/restriction, RoHS, ELV, defence exemptions where they apply). QLQ supplies documentation and restricted-use statements and will always propose trivalent or Cr-free systems where the spec allows.

What It Does on Iron, Zinc Alloy, Copper and Stainless Steel Parts

FAQ

Chromating or sealing questions? Tell QLQ your substrate, finish spec and destination market - engineers recommend the compliant route (hex or trivalent) and help tune the bath. Contact us.