Chemicals for metal finishing and surface treatment

Metal finishing is a sequence of baths, and each one sets up the next. Parts are cleaned of oils in a hot alkaline soak built on sodium hydroxide or in a solvent wash, then pickled in hydrochloric or sulfuric acid to remove rust and mill scale. Etching and bright dipping in phosphoric, nitric or fluoride chemistry texture the surface and remove smut. Stainless steel is passivated in nitric or citric acid to remove free iron, aluminum is anodized in sulfuric acid, and some parts receive a chromate conversion coating or seal.

Weak cleaning shows up later as poor adhesion, blistering or plating that peels under stress. Bath chemistry drifts as parts consume acid and carry solution out, so shops titrate and replenish on a schedule. Rinse water is neutralized before discharge, which also drops dissolved metals out as hydroxide sludge. The chemicals you buy decide corrosion resistance, appearance and scrap rate across the whole line.

Technical grade is the norm for pickling, cleaning and neutralization, but "technical grade" is a description, not a specification, and impurity levels vary between sources. Iron, chloride and heavy metals in a low-cost acid or caustic accumulate in the bath and carry into the plating tank. Electropolishing and bright dipping of stainless steel and aluminum are more sensitive, and impurities there can stain or dull the surface. Paying for a reagent grade where technical grade is enough adds cost without improving the parts, so match the grade to the step. Lot-to-lot swings in assay also force constant retuning of automatic dosing systems. Check concentration and the impurities that matter for your bath on the COA for each lot before it goes into a tank.

Discharges from finishing lines fall under EPA effluent guidelines for electroplating (40 CFR Part 413) and metal finishing (40 CFR Part 433), enforced through local pretreatment permits that cap metals, cyanide and pH. Hexavalent chromium from sodium dichromate brings the OSHA chromium (VI) standard, with exposure monitoring and medical surveillance, and chromium electroplating and chromic acid anodizing tanks fall under EPA air rules (40 CFR Part 63, Subpart N). OSHA exposure limits for MEK, acetone and acid mists set tank ventilation. EPA rules finalized in December 2024 prohibit most uses of trichloroethylene, with time-limited exemptions that include batch vapor degreasing of essential aerospace parts. Perchloroethylene vapor degreasing can continue under EPA workplace exposure controls. Both rules are under legal challenge and EPA review, so confirm the current status before you order. Keep an SDS for every chemical in your hazard communication program, and confirm current requirements with your environmental team.

• Passivating stainless steel with hydrochloric acid. Chloride attacks the chromium oxide film on 300-series stainless and leaves pitting and flash rust. Use nitric or citric acid as your specification calls out.
• Skipping non-volatile residue. A technical-grade solvent used for precision cleaning can leave a thin film that interferes with adhesion or electrical contact. Check NVR before you use it for that job.
• Buying caustic on price alone. Sodium hydroxide high in iron can drop iron into the cleaner and carry it to the nickel tank, where it shows up as rough plate and reworked parts.
• Letting free acid or free caustic drift. An etch bath left untracked can take parts below drawing thickness and scrap the batch.