The wrong grade of sulphuric acid, caustic soda, or nickel acetate will ruin your bath — often without an obvious cause. Before you place the order, know exactly what you need to specify and how to verify it.
Every anodizing plant in India buys the same six chemical categories: sulphuric acid for the anodizing bath, caustic soda for etching, desmut acid for post-etch treatment, sealing salts for the seal tank, a degreaser for pre-treatment, and dyes for decorative work. These are commodity chemicals available from hundreds of suppliers across the country — but the grade, purity, and formulation that are correct for anodizing are not the same as the grades those suppliers sell to most of their other customers.
Plants that buy on price alone, without specifying the right grade, end up troubleshooting bath problems that have no obvious explanation — poor sealing, inconsistent colour, excessive smut, or early bath contamination. The root cause in many of these cases is a chemical that looked right on the invoice and was wrong on the CoA.
We are not a chemical supplier. We help anodizing plants in India specify the exact grade they need, qualify the suppliers who can reliably deliver it, set up incoming inspection procedures, and build batch-to-batch consistency checks into their QC system. This page describes what you need to know before you buy.
The table below gives the working specification for each chemical. These are not aspirational targets — they are the minimums below which process problems become likely.
| Chemical | Function | Correct grade / specification | Common sourcing mistakes |
|---|---|---|---|
| Sulphuric acid (H2SO4) |
Anodizing electrolyte (Type II: 15–20% w/v; Type III hard anodizing: 10–15% w/v at low temperature) | Industrial grade, min 98% purity. Iron <5 ppm. Chloride <5 ppm. Turbidity — clear, not yellow. | Battery grade (33–38%, impure). Electronic grade (unnecessarily expensive). Accepting deliveries without CoA. |
| Caustic soda (NaOH) |
Alkaline etch — removes native oxide and surface metal to create uniform matte finish | Technical grade, min 99% NaOH assay. Iron <10 ppm. Low carbonate content. No surfactant additives. | Food/cleaning grade with surfactant additives (causes foaming). High-iron batches that cause dark smut. Pearl/flake form without checking iron content. |
| Nickel acetate (sealing salts) |
Hot seal — nickel ions plug anodizing pores at 96–98 °C, pH 5.5–6.0, targeting 5–8 g/L Ni²⁺ in bath | Proprietary sealing salt blends from reputable suppliers, or pure nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O). CoA must state active Ni²⁺ content per kg of product. | Undisclosed blends with no CoA. Dosing by volume rather than verified Ni²⁺ concentration. No bath titration at shift start. |
| Desmut chemicals (nitric or ferric sulphate) |
Remove smut (alloying elements — silicon, copper, manganese — left on surface after caustic etch) | Either: nitric acid 15–30% v/v (technical grade, min 68% concentrated HNO3, low chloride); or proprietary ferric sulphate-based desmut (preferred for 2xxx and 7xxx series — less attack on base metal). | Using sulphuric acid rinse alone without desmut step. Wrong concentration of nitric (too dilute = incomplete smut removal; too strong = excess base metal attack). No control on ferric sulphate bath iron loading. |
| Degreaser / cleaner | Remove oil, machining lubricants, and fingerprint contamination before etch | Alkaline, non-silicated formulation designed for aluminium. pH 10–11 working solution. Must be rinsable without silicate deposit. Concentration as per manufacturer data sheet for aluminium. | Silicated cleaners (silicate residue interferes with anodizing). Generic industrial degreasers formulated for steel. Overdosing alkaline cleaner that attacks aluminium surface before etch. |
| Anodizing dyes | Colour decorative anodized aluminium in the post-anodizing, pre-seal dyeing step | Metal-complex acid dyes for maximum lightfastness (architectural, exterior). Reactive dyes for intense colour. Acid dyes for economical decorative. Working pH 5.5–6.0, temperature 50–60 °C. Dye supplier must state lightfastness (minimum ISO Blue Wool 5 for exterior use). | Textile dyes not formulated for anodized aluminium (bleed out in sealing tank). No lightfastness rating. No batch record — reordering a different dye lot that matches on name but not on shade. |
For deeper chemistry on each of these, see our reference articles: anodizing chemicals suppliers in India, anodizing bath chemistry reference, caustic soda in anodizing etch, desmut chemicals for anodizing, and nickel acetate sealing.
The problems we see repeatedly across plants are not random. They fall into four consistent failure modes:
We work with anodizing plants and procurement teams in India at four points in the chemical sourcing process:
If you are setting up a new anodizing plant, the chemical specification work connects directly to line design — the right acid concentration for your process, the etch tank sizing based on caustic soda consumption rate, and the seal tank design for nickel acetate. See our coating thickness calculator for related process parameters.
Ten minutes of specification work prevents months of bath troubleshooting. Tell us your process, alloy mix, and volumes — we will tell you exactly what to specify and what to ask for on the CoA.
No. Battery-grade sulphuric acid is typically 33–38% dilute and contains trace impurities — iron, chlorides, organic matter — that are tolerable for lead-acid cells but harmful in an anodizing bath. Iron above 5 ppm causes streaking and uneven coating growth. Chloride above 5 ppm causes pitting. You need industrial-grade sulphuric acid at minimum 98% purity (H2SO4 content), with iron below 5 ppm and chloride below 5 ppm. Electronic grade is overkill and not cost-justified for anodizing. Always request a Certificate of Analysis specifying purity, iron content, and chloride content before accepting a delivery. See our article on anodizing chemicals suppliers in India for more on how to evaluate supplier CoA data.
Technical-grade caustic soda (NaOH) at minimum 99% purity with iron below 10 ppm is the correct specification. High iron in caustic soda stains the aluminium surface during etch and creates dark smut that is harder to remove in the desmut stage. Avoid food-grade caustic sold for cleaning applications — it often contains surfactant additives that cause severe foaming in etch tanks. Also avoid pearl or flake form from an unknown origin without checking the iron content on a CoA. Carbonate content matters too: high carbonate (above 0.5%) means the caustic has absorbed CO2 and lost effective alkalinity — bath etch rate will be lower and less consistent. Full guide: caustic soda in anodizing etch.
Start with the CoA: nickel acetate sealing salts should state the active nickel ion content so you know the dose required to achieve 5–8 g/L Ni²⁺ in your sealing bath. In-house, perform a spot test using dimethylglyoxime (DMG) reagent on a bath sample — a pink precipitate confirms nickel presence. For quantitative verification, colorimetric titration measures Ni²⁺ concentration; if you lack in-house titration capability, an accredited metals testing lab can analyse bath samples for Ni²⁺ at low cost. Check bath pH (working range 5.5–6.0) and temperature (96–98 °C) at every shift start — both affect seal quality as much as nickel concentration does. A seal that appears complete but fails the admittance test is usually a temperature or pH problem, not a nickel dosing problem. Detailed background: nickel acetate anodizing sealing.
Shelf life depends on dye class. Acid metal-complex dyes — the most lightfast option for architectural and exterior aluminium — remain stable for 2–3 years in sealed, dry storage away from direct light and heat. Reactive dyes are more moisture-sensitive and should ideally be used within 12–18 months of manufacture date. Once dissolved into a working dye bath, degradation accelerates; most plants running continuous production replace dye baths every 3–6 months depending on throughput, pH control, and contamination level. Store dye drums upright in a cool, shaded storeroom — never in direct sun or near acid or caustic storage. If a newly prepared bath shows weak colour uptake, check pH (should be 5.5–6.0 at 55 °C), dye concentration by spectrophotometer or comparison panel, and water quality (high iron in process water can degrade dye baths). Faded colour on a fresh bath is almost always a pH, temperature, or dye quality issue — not simply a dosing issue.
We work with anodizing plants and procurement managers across India. A one-hour engagement can define the chemical specifications your plant needs for the next three to five years.