Surface Treatment in Metal Finishing
End-to-end Surface Treatment programs tailored to Metal Finishing operations — from coagulation and pH control through corrosion management. Backed by on-site lab support and process expertise built since 1977.
About Surface Treatment
Specialized chemicals for metal finishing, etching, cleaning, and surface preparation across automotive, aerospace, and electronics industries.
About Metal Finishing
Etching solutions, passivation chemicals, cleaning agents, and surface activators for precision metal treatment.
What is chemical treatment in metal finishing?
Chemical treatment in metal finishing is the sequence of wet-chemistry stages that prepare a metal surface before a coating is applied — alkaline degreasing, acid pickling, rinsing, fluxing or activation, and passivation — so that zinc, paint, enamel or a phosphate layer bonds to clean, oxide-free metal. Every coating defect that appears at the end of the line was usually decided in one of these baths at the start.
The chemistry is unglamorous and decisive. Oil left by the degreaser reappears as bare spots and blisters; scale left by the pickle becomes a rough, uneven coat; iron carried from the pickle into the flux becomes dross in the zinc kettle. A finishing line is only as good as its worst bath, which is why bath control — not brand — is where quality is won.
The pretreatment sequence: degrease → pickle → rinse → flux or activate
Degreasing comes first: a hot alkaline bath (ART CLEAN 15 runs at pH 11.0–13.0, in soak or electro-cleaning duty) lifts drawing oils, lubricants and shop soil so the acid that follows can reach the metal. Pickling dissolves rust and mill scale — hydrochloric acid at ambient temperature is the galvanizer's standard; sulphuric acid, heated, is used where the work or the effluent plant favours it. Rinsing between stages stops drag-out of one bath poisoning the next. For hot-dip galvanizing the final chemical stage is the flux — a zinc ammonium chloride double salt (ZnCl₂·2NH₄Cl, supplied as CHIMI FLUX Powder or as CHIMI FLUX Solution at 25–30 % active content, pH 4.5–5.5) that keeps the pickled steel active and oxide-free until it enters the molten zinc. For stainless steel the equivalent last step is passivation, classically with nitric acid, which strips free iron and lets the chromium-oxide layer re-form.
What the standards ask for — and what a good line measures
The coating standards define the result, not the recipe. ISO 1461 and ASTM A123/A123M specify hot-dip galvanized coatings on fabricated steel: ISO 1461, for example, sets a minimum mean coating thickness of 85 µm (local minimum 70 µm) on steel thicker than 6 mm, with lower minima for thinner sections, plus requirements on appearance and adherence. ISO 8501-1 gives the visual preparation grades for steel before painting. Meeting them consistently depends on a handful of bath measurements taken on schedule: free alkalinity and oil loading in the degreaser; acid strength and dissolved iron in the pickle; density, ammonium-to-zinc ratio, iron content and pH in the flux; rinse-water conductivity; and the water-break test — a clean surface holds an unbroken film of water — at the end of cleaning.
| Stage | Typical chemistry | What it does | What to measure |
|---|---|---|---|
| Alkaline degreasing | Phosphate-based alkaline cleaner, pH 11–13 (e.g. ART CLEAN 15), soak or electro-clean | Removes oils, drawing compounds and shop soil | Free alkalinity, oil loading, temperature, water-break test |
| Acid pickling | Hydrochloric acid (30–37 % as supplied, diluted) or heated sulphuric acid | Dissolves rust and mill scale to bare steel | Acid strength, dissolved iron, temperature, immersion time |
| Rinsing | Counter-flow water rinses between every chemical stage | Stops drag-out poisoning the next bath | Rinse conductivity / pH, flow rate |
| Fluxing (hot-dip galvanizing) | Zinc ammonium chloride double salt (e.g. CHIMI FLUX Powder or Solution, pH 4.5–5.5) | Keeps pickled steel active and oxide-free until it meets molten zinc | Density, NH₄Cl:ZnCl₂ ratio, iron content, pH, temperature |
| Passivation (stainless steel) | Nitric acid (68–70 % as supplied, diluted) | Strips free iron; lets the chromium-oxide layer re-form | Acid concentration, temperature, time; water-break / free-iron test |
Standards & references
- ISO 1461:2022Hot dip galvanized coatings on fabricated iron and steel articles — specifications and test methods; minimum mean coating thickness 85 µm (local 70 µm) on steel thicker than 6 mm.
- ASTM A123/A123M-24Standard specification for zinc (hot-dip galvanized) coatings on iron and steel products — the North-American counterpart to ISO 1461.
- ISO 8501-1:2007Preparation of steel substrates before application of paints — visual assessment of surface cleanliness; rust grades and preparation grades (Sa, St, Fl).
Chemical treatment for metal finishing — frequently asked questions
What is the difference between degreasing and pickling?
Degreasing removes organic soil — oils, drawing compounds, lubricants — with a hot alkaline bath; pickling removes inorganic scale and rust with acid. The order matters: acid cannot reach metal through an oil film, so a pickle that follows a poor degreaser pickles unevenly and the defect surfaces in the coat. Rinse between the two.
Why does flux matter in hot-dip galvanizing?
Pickled steel re-oxidises in seconds. The zinc ammonium chloride flux film protects the active surface until it enters the kettle and then cleans the steel–zinc interface at the moment of immersion, so the zinc–iron alloy layers form evenly. Weak or iron-contaminated flux shows up as bare spots, black spots and excess dross.
How do I know my pretreatment is working?
Measure the baths, not the mood: free alkalinity and oil loading in the degreaser, acid strength and dissolved iron in the pickle, density, salt ratio and iron in the flux, rinse conductivity — and the water-break test on cleaned parts. A surface that holds an unbroken water film is chemically clean; one that beads is not.
What causes bare spots and black spots after galvanizing?
Usually something upstream: oil or paint that survived degreasing, scale left by under-pickling, flux that was too weak, too iron-rich or dried unevenly, or rinse water that carried one bath into the next. Trace the defect back bath by bath with the measurements above before changing the zinc kettle.
How is metal-finishing rinse water treated before discharge?
By neutralising and precipitating: acid rinses are brought to a controlled pH with sodium hydroxide, dissolved metals drop out as hydroxides, and a coagulant plus flocculant settles the fine solids before discharge. Phosphate from alkaline cleaners adds to the phosphorus load. The same water-treatment chemistry a plant uses on its effluent line applies here.
Does CHIMI ART manufacture metal-finishing chemicals?
Yes. CHIMI ART Industries manufactures ART CLEAN 15 (alkaline degreaser) and CHIMI FLUX Powder and Solution (galvanizing flux) in 6th of October City, Giza, and supplies the pickling and passivation acids and the caustic soda a finishing line consumes, with technical and safety data sheets for every product, from a manufacturer operating since 1977 under ISO 9001, ISO 14001 and ISO 45001 management systems.
Surface Treatment product lines for Metal Finishing
Drill into each product family used by this industry, with full specs and direct quote paths.
Galvanizing Flux for Metal Finishing
High-speed galvanizing flux solutions in powder and liquid forms. Ensure uniform zinc coats with low fuming and minimal ash.
Degreasers for Metal Finishing
Heavy-duty industrial degreasers for metal surface preparation. Essential for enameling, electro-cleaning, and galvanizing processes.
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