Thiourea

Product Overview

Thiourea (CH4N2S) is a white crystalline sulfur compound with three real jobs in metal finishing: it is the ppm-level brightening and leveling additive in acid copper plating, a classic inhibitor in acid pickling of steel, and the sulfur building block from which many proprietary brightener and leveler molecules are made. In QLQ's finishing plants thiourea chemistry appears in the acid copper stages that underlay the decorative nickel-chrome finishes on zipper sliders and hardware, and in the inhibited pickles that protect spring steel from acid attack and hydrogen embrittlement. The commercial market sells industrial and premium grades; for plating work the specification that matters is assay and insolubles - thiourea is dosed in milligrams per liter, so a cheap lot carrying dirt or insoluble residue can drop straight into the deposit as roughness. Two related facts keep thiourea honest: it is not a brightener that can be dumped in by the gram - modern acid copper systems use it or its derivatives at levels close to the detection limit of the lab - and heating it in solution slowly decomposes it, which is why dosing is done cool and often.

Product Identity (Encyclopaedia Card)

What it is and what it does. Thiourea is the grain refiner and brightener used in acid copper, in electroless nickel (as a stabiliser) and in the colouring and etching of copper alloys.

• White lustrous crystals; added at very low concentration in plating baths because it is a powerful brightener.
• It is also the classic accelerator in some phosphating and the complexant in electroless nickel stabiliser systems.
• Contains sulphur, so overdosing increases the deposit stress and can make the layer brittle.

Typical Use in a Plating / Finishing Line

Safety, Handling & Storage

Quality Documents Shipped With Every Batch

Buyer Questions We Answer Every Week

How It Works in the Bath (Why Dosage Is in Milligrams)

Thiourea works by adsorbing on the growing copper surface and slowing deposition locally, which is exactly what a leveler must do: where the deposit grows fastest (peaks, edges, high current density) it is inhibited most, so valleys catch up and the surface levels and brightens. The effective band is narrow - studies and production practice place working concentrations from a few to roughly 60 mg/L depending on the bath chemistry, with excess thiourea dulling the deposit and eventually roughening it as decomposition products build. Because the working dose is so small, thiourea is always predissolved as a dilute stock solution (1 g/L or weaker) and added by measured volume against amp-hours, never scooped. In acid pickling the mechanism is different but equally simple: thiourea adsorbs on bare steel and blocks the acid attack on the metal surface while still allowing scale and rust to be loosened, cutting both metal loss and the hydrogen that causes embrittlement of hardened springs and fasteners.

Contamination control belongs in the same paragraph as dosing: copper baths hold thiourea species in balance with chloride and the main brightener, so adding thiourea of unknown purity or history is a good way to spend a weekend carbon-treating.

How to Check Quality Before Loading a Batch

How QLQ controls it: thiourea and its derivatives are bought against assay and insolubles limits, checked by melting point and clarity on arrival, and stored as dilute, dated stock solutions rather than bulk powder beside the plating tanks. Lot traceability on the additive log is what lets the lab explain a sudden haze - or rule the chemical out in an hour.

Common Problems & Fixes (Workshop Table)

Handling & Safety for the Plating Line

• Health: thiourea is an irritant to eyes and skin and is classed as a suspected carcinogen in some jurisdictions; avoid inhaling dust and wash hands after handling. Use gloves, goggles and a dust mask when weighing.
• Dust control: fine powder disperses easily. Empty bags slowly in a ventilated area and keep containers closed between uses.
• Solution preparation: predissolve as a dilute stock (1 g/L or weaker for plating doses) in cool water; do not boil or hold stock hot, because thiourea decomposes with heat.
• Storage: keep dry, cool and sealed; thiourea is stable for years if dry but hydrolyzes slowly in damp air. Separate from oxidizers and acids in the store.
• Spills: sweep dry for disposal; small residues rinse away with water. Do not flush concentrated material into acid waste without dilution.

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

FAQ

Acid copper hazing or springs failing after pickle? QLQ supplies thiourea with COA and engineers who help customers in 10+ countries balance copper additives and inhibited pickling. Contact us with your Hull cell panels.