metal chemical treatment
Chemical treatment of metals is an umbrella term for processes that change a metal surface by chemical reaction rather than by mechanical force. Under it sit four families that behave very differently: cleaning and pretreatment, conversion coatings, anodic films, and chemical etching. They are discussed together, which is why a buyer may pay for a coating when the drawing only needed a cleaned surface, or ask for a protective finish when the real requirement was to remove metal. This guide separates the families, explains what each one does, and shows how to choose by function.
1. A Classification Framework for Four Families
Before comparing processes, sort them by what they do to the metal. One axis is direction: remove material, convert the surface, or add a layer. Another is purpose: preparation, protection, appearance or shaping. The four families fall out of those questions.
Cleaning and pretreatment remove contamination and unwanted oxide. They are not meant to leave a functional finish, although passivation leaves a better passive film than it found. Conversion coatings, including phosphating and chromating, react with the basis metal to form a new layer. Anodising grows an oxide film on aluminium and, although driven electrically, belongs here because it changes the surface instead of adding a coating. Chemical etching removes metal in a controlled pattern and is the only family that changes the part’s shape.
That distinction matters. A treatment that adds or converts a layer is specified by coating weight, thickness, colour or corrosion performance, and changes the fit. A treatment that removes metal is specified by dimensions, tolerances and edge quality. Mixing the two vocabularies is a common source of a failed purchase order.
2. Cleaning and Degreasing
Cleaning is the least glamorous step and often skipped. Degreasing removes oils, handling residue, polishing compounds and shop dirt. If those remain, a conversion coating will not form evenly, a passivation bath cannot reach the metal, and a resist film will lift during etching.
The chemistry is only half the job. Mechanical action, temperature, concentration and contact time matter, and so does the rinse that follows. Contaminated rinse water carries dissolved salts into the next bath, where they can stain, spot or inhibit the reaction. A water-break test, flooding the surface and watching the film drain, tells more about cleanliness than a visual check.
Cleaning is not passivation, and passivation is not cleaning. A part can be visually clean and still carry free iron particles that will rust in service. Equally, a part can be properly degreased and still need pickling to remove heat tint or scale. For parts that will be photo-etched, cleaning also decides whether the resist bonds evenly, which is why pretreatment and etching are quoted together.
3. Pickling and Passivation
Pickling dissolves oxide scale, heat tint and the chromium-depleted layer left by welding or heat treatment. It attacks the surface with acid, so it removes a little basis metal too. That is acceptable when the drawing allows it and the bath is controlled; it is a problem when pickling covers up a defect that should have been ground out first.
Passivation follows pickling on stainless steel. It does not apply a coating. Instead it removes free iron contamination and allows the natural chromium oxide film to rebuild. The result is a chemically cleaner surface, not a new layer. A passivated part looks much like an untreated one, which makes the step easy to skip.
Neither step hides scratches, weld undercut or roughness. If the drawing calls for a cosmetic finish, that finish has to be produced mechanically or by etching, not by hoping the acid will blend it. Passivation does nothing for carbon steel; protection there comes from a coating or the alloy. On stainless steel the benefit is real, but depends on the surface being free of embedded iron.
4. Conversion Coatings: Phosphating and Chromating
Phosphating and chromating both convert the metal surface into a new layer. Phosphating produces a crystalline phosphate layer that anchors paint, holds lubricant and improves friction in sliding contacts. Chromating leaves a thin conversion film that slows corrosion and, depending on the chemistry, can preserve electrical contact resistance. Both are specified by coating weight or film thickness and appearance, not by a dimensional change.
Because the layer sits on the surface, it changes fit. Threads, bores, sliding fits and mating faces all have to be considered, and areas that must stay conductive or bondable are usually masked. Masking is labour and a hidden cost driver, along with bath control, chemical consumption and waste treatment.
The choice follows the substrate and the next operation. Steel that will be painted usually points to phosphating. Aluminium, zinc and cadmium parts that need short-term protection or a paint base often point to chromating. Restrictions on hexavalent chromium have pushed many supply chains toward trivalent alternatives, which behave differently and should be qualified, not assumed equivalent.
5. Anodising
Anodising is an electrolytic process that grows an oxide film on aluminium. The film is not applied; it forms from the metal itself, so an anodised surface keeps the texture underneath while gaining wear resistance and a porous structure that accepts dye and sealant.
The dimensional consequence is easy to miss. The oxide grows both outward and into the surface, so a tight bore or a fine thread can change size. Hard anodising, used where wear resistance matters more than colour, builds a denser film and changes dimensions further. Masking, rack marks and contact points need planning: the part must be electrically connected, and the contact area stays uncoated.
Anodising is not a repair process. It will not fill a scratch or hide a tool mark, and it follows the surface it is given. Colour matching across batches depends on alloy, temper and film thickness, so a colour reference should be a physical sample, not a screen image. We do not run anodising lines; where a drawing calls for it, we refer the work out rather than pretend the capability exists in house.
6. Chemical Etching and Material Removal
Chemical etching is the family that removes metal. Photochemical etching coats the sheet with a resist, exposes and develops a pattern, then passes the sheet through an etchant that dissolves the exposed metal. The result is a part cut from sheet without a die, mechanical force or the burr of a sheared edge.
Because it works by immersion, it can cut many features at once: holes, slots, channels and outlines difficult to stamp or machine. It also leaves the material stress-free, which matters for thin sheet. Etching is chosen when the drawing needs fine features, flatness or damage-free edges.
Etching is not one-sided. The etchant attacks from both faces and, once the cut breaks through, continues sideways under the resist. That undercut is predictable for a given material and thickness, but it must be allowed for in the artwork, so the etcher usually adjusts the tooling rather than asking the customer to redraw every feature. Half-etched features, such as recesses or witness lines, depend on the same balance of time and chemistry. We etch in house and prefer to review the drawing before the artwork is made.
7. Choosing a Treatment by Function
The practical way to choose is to state the function before the process. Cleaning answers a contamination problem. Pickling answers scale and heat tint. Passivation answers free iron and corrosion resistance on stainless steel. Phosphating answers paint adhesion or friction. Chromating answers short-term corrosion protection. Anodising answers wear resistance, colour or electrical insulation on aluminium. Etching answers shape, fine features and burr-free edges.
Once the function is clear, three inputs narrow the choice: alloy, surface condition and dimensional requirement. An alloy that cannot be passivated will not become corrosion resistant because a purchase order says so. A surface with heavy scale needs pickling first. A part with a tight fit may not tolerate a coating that adds thickness, and a sealing face may not tolerate a witness mark.
A supplier who offers only one treatment will describe every problem as that treatment. Ask which step the drawing needs, and what happens to surfaces that must stay clean, conductive or dimensionally tight. We etch in house; where a drawing needs a treatment we do not run, we say so and refer it out, rather than stretch the ones we have.
8. Common Misconceptions and Acceptance
Several beliefs cause avoidable failures. One is that chemical treatment is a single service bought by the kilogram. Another is that passivation will clean a part, or that anodising will hide a scratch. A third is that a conversion coating has no dimensional effect because it is thin. Each leads to a specification no competent supplier can meet.
Acceptance should follow what is actually measured. For cleaning and passivation, that may be a water-break check and a free-iron test. For conversion coatings and anodising, it is coating weight or film thickness, appearance and adhesion after forming. For etching, it is dimensional inspection, feature geometry and edge condition. Colour, gloss and witness marks need a physical reference, not an adjective.
The first article settles those points. Agree the test method, sampling and acceptance limit before production, and record them on the drawing or purchase order. A treatment that is not measured is not really specified; it is only requested.
