7 Questions to Ask Before Choosing a Metal Finish
What is the part made of?
Steel, stainless steel, and aluminum call for different processes.
Where will it be used or stored?
Humidity, outdoor exposure, and storage time affect corrosion requirements.
Will it slide, thread, or bear a load?
Moving surfaces may need wear protection or lubrication.
Will it be painted later?
Zinc phosphate can provide a suitable base for a topcoat.
How tight are the tolerances?
Coatings add thickness; passivation does not.
Is the part hardened?
Tell the finisher its hardness so the proper embrittlement controls can be applied.
What does the drawing require?
Check the full specification, including type, class, post-treatment, and masking instructions.
01.
What Should Connecticut Machine Shops Include on a Metal Finishing Purchase Order?
Many finishing decisions start with a name. Someone asks for black oxide because the last job used it. Someone else writes "black finish" on a drawing and leaves the rest to the shop.
That approach works until it doesn't. A finish chosen by habit can rust in storage, gall in assembly, push a bore out of tolerance, or peel off under paint.
The better starting point is the part itself. What does it need to do once it leaves your building? Where will it live? What will touch it?
This article walks through that decision. It is written for Connecticut manufacturers, machine shops, engineers, and buyers who know their parts well but may not work with finishing chemistry every day. It draws on the processes J.H. Metal Finishing runs at its New Britain shop: black oxide, zinc and manganese phosphate, passivation, dry-film lubricants, Cerakote, and the cleaning and preparation steps that sit underneath all of them.
02.
Why Start with the Application Instead of the Finish?
Every finishing process is a trade-off. Each one does a few things well and other things poorly.
Black oxide looks clean and adds almost nothing to dimensions, but it offers little corrosion protection on its own. Manganese phosphate holds oil and resists wear, but it adds measurable thickness. Cerakote gives strong barrier protection and color choices, but it is a true coating with real film build.
None of these is "better." Each is better for something. Start with a finish name, and you inherit its weaknesses without noticing them.
03.
What Questions Should You Answer First?
Before comparing processes, answer a few questions about the part.
What Is the Part Made Of?
Material narrows the options faster than anything else.
Black oxide and phosphate coatings are conversion coatings for ferrous metals. They chemically convert the surface of carbon steel, alloy steel, and, in some cases, stainless steel or iron. They are not the answer for aluminum.
Passivation is specific to stainless steel. It does nothing useful on carbon steel.
Copper alloys have their own black chemical finish, covered by MIL-F-495. Aluminum parts that need anodizing or a chemical film require different processes than those covered here.
How Much Corrosion Protection Does It Need?
Ask where the part will spend its life. A gauge that stays in a climate-controlled inspection room has very different needs than a bracket on outdoor equipment or a component stored in a humid warehouse for a year.
Also ask whether the part will be oiled, painted, or assembled into a sealed housing. A finish that performs poorly bare can perform well once it is sealed or topcoated.
When a customer specifies corrosion performance, it is often expressed as hours of exposure under ASTM B117, the standard salt spray test. If your drawing or purchase order includes a salt spray requirement, that requirement should drive the choice of finish and any supplementary oil or topcoat.
Does It Slide, Thread, or Wear Against Something?
Gears, cams, splines, threaded fasteners, slides, and pins all see contact. Some need to break in smoothly. Some need to resist galling, where metal surfaces tear and weld together under load. Some need lubrication in places where oil can't be reapplied or would attract dirt.
If friction, wear, or seizing is a concern, the finish has to address it directly. Appearance comes second.
Will It Be Painted or Coated Later?
If the part is headed to a paint line or powder coater after finishing, adhesion becomes the priority. Some finishes are designed to be a base for paint. Others make paint adhesion worse.
How Tight Are the Tolerances?
Every finish except passivation adds something to the surface. The question is how much, and whether your tolerances can absorb it.
As a rough guide:
Black oxide typically builds around a micron, often less. On most parts, dimensions are effectively unchanged.
Phosphate coatings typically build several microns, and heavier manganese coatings can reach 15 to 20 microns or more.
Heat-cured dry-film lubricants under MIL-PRF-46010 are specified at roughly 0.0003 to 0.0005 inch after cure.
Cerakote is commonly applied around 0.001 inch.
Passivation adds nothing. Treat dimensional change as a defect.
On a threaded feature or a close bearing fit, these differences matter. The time to account for them is at the design stage, not after the first lot fails inspection.
Does Appearance Matter?
Sometimes the look is functional. Black finishes reduce glare on optical and sighting components. Color coding helps assemblers and field technicians.
Sometimes appearance doesn't matter at all. Choosing a finish for looks then adds cost without adding performance.
Is the Part Hardened?
High-strength steel needs extra attention. Some finishing steps can introduce hydrogen into hardened steel, which can lead to delayed cracking in service.
Finishing specifications address this with baking requirements above certain hardness levels. MIL-DTL-13924, for example, calls for embrittlement relief on steel parts above 40 HRC. If your part is hardened, state the hardness on the drawing or purchase order so the finisher can apply the right controls.
04.
Which Finishing Process Fits Which Need?
When Does Black Oxide Make Sense?
Black oxide converts the steel surface into a thin layer of magnetite. The result is a uniform matte black with virtually no dimensional change.
It fits well when:
- Tolerances are tight, and there is no room for film build
- A black, low-glare appearance is needed
- The part will be oiled or used in a mild, indoor environment
It is a poor fit when the part needs meaningful corrosion protection on its own. The military specification for black oxide, MIL-DTL-13924, states plainly that the coating provides only limited protection under mildly corrosive conditions. Most of its corrosion performance comes from the supplementary oil or wax applied afterward, so specify post-treatment, not assume it.
Black oxide is common on precision tooling, gauges, fasteners, and small mechanical components. It is also covered by AMS 2485 for aerospace work.
When Does Zinc Phosphate Make Sense?
Zinc phosphate forms a fine crystalline layer that gives paint and powder coat something to grip. That is its main job.
It fits well when:
- The part will be painted or powder coated
- Moderate corrosion protection is needed under a topcoat or oil
- A consistent base is needed before a downstream coating
It is less suited to heavy sliding wear, where manganese phosphate usually performs better.
Zinc phosphate is typically called out as MIL-DTL-16232 Type Z or AMS 2480. The type sets the chemistry. The class, which is easy to leave off a drawing, sets whether a supplementary oil or preservative goes on afterward.
When Does Manganese Phosphate Make Sense?
Manganese phosphate builds a heavier, darker crystalline coating. Its porous structure holds oil and supports break-in on moving parts.
It fits well when:
- Parts slide, mesh, or bear load against each other
- Galling or seizing is a risk during break-in
- The part needs to hold oil or serve as a base for a dry-film lubricant
It is a common choice for gears, cams, pins, and heavily loaded fasteners, especially on defense hardware. It is typically specified as MIL-DTL-16232 Type M or AMS 2481.
The trade-off is thickness. Manganese phosphate adds more than black oxide, so close-tolerance threads and fits need to be reviewed before it is specified.
When Does Passivation Make Sense?
Stainless steel resists corrosion because of a thin, chromium-rich oxide layer on its surface. Machining, handling, and contact with steel tools can leave free iron and other contaminants on that surface, which can rust even when the base alloy should not.
Passivation removes that contamination and allows the protective layer to reform. It is a chemical treatment, not a coating, and it does not change dimensions.
It fits well when:
- Parts are machined from stainless steel
- Corrosion resistance needs to match what the alloy is designed to deliver
- Appearance and dimensions must stay unchanged
J.H. performs passivation to ASTM A967, which allows both nitric and citric acid methods. If your application requires one chemistry over the other, say so on the order.
Passivation will not remove heat tint, weld scale, or mill scale. Those need descaling first, which ASTM A380 addresses.
When Does a Dry-Film Lubricant Make Sense?
A dry-film lubricant is a bonded solid film, often based on molybdenum disulfide or graphite, that reduces friction without oil or grease.
It fits well when:
- Wet lubricants would attract dirt, dust, or grit
- Lubricant can't be reapplied over the life of the part
- Parts must resist galling on threads, splines, or sliding surfaces
- Operating temperatures are beyond what oil or grease can handle
Heat-cured films are covered by MIL-PRF-46010. Air-cured films are covered by MIL-PRF-46147, which matters when the part or an assembly can't tolerate a heat cure. The heat-cured specification calls for curing at 204°C for one hour or 150°C for two hours, so temperature-sensitive parts need to be flagged early.
On steel, dry films often go over a manganese phosphate base. The phosphate anchors the film and extends its life.
When Does Cerakote Make Sense?
Cerakote is a thin-film ceramic coating that is sprayed and oven-cured. It provides barrier protection against corrosion and chemicals, adds wear resistance, and comes in a wide range of colors.
It fits well when:
- A durable colored finish is required
- The part needs barrier protection in a harsher environment
- Color coding or a specific appearance matters
Because it is a true coating, film build has to be planned for. Threads, bores, and mating surfaces may need masking. It is also oven-cured, so the part must tolerate the cure temperature.
05.
Why Does Cleaning and Preparation Matter So Much?
A finish is only as good as the surface under it.
Machining oils, coolant residue, fingerprints, and shop soils all interfere with conversion coatings and bonded films. A phosphate coating will form unevenly on an oily surface. A dry-film lubricant or Cerakote coating can lose adhesion and flake off.
That is why preparation is part of the process, not a separate afterthought. J.H. runs vapor degreasing to remove oils and contaminants before finishing, and abrasive blasting where a surface profile is needed for adhesion.
Cleanliness can also be verified. ASTM F22, the water-break test, checks whether a surface is free of films that repel water. A clean surface holds an unbroken sheet of water. A contaminated one breaks it into beads.
If your parts arrive heavily oiled, carry heat-treat scale, or have been stored for a long time, say so up front. It affects how they need to be prepared.
06.
What Does This Look Like for Connecticut Manufacturers?
Connecticut's manufacturing base leans heavily toward precision work. AdvanceCT, the state's economic development organization, counts more than 250 aerospace and defense manufacturing establishments, supported by a deep network of machine shops, heat treaters, and special-process suppliers.
In aerospace and defense work, the customer often decides the finish. The drawing calls out a specification, the purchase order flows down its requirements, and the finisher's job is to meet it exactly. In those cases, the most important thing a supplier can do is make sure the callout is complete: specification, type, class, supplementary treatment, and any hardness or masking notes.
Job shops and industrial manufacturers often have more freedom. A drawing might say "black finish" or "corrosion protection" and nothing else. That is where the application-first questions above earn their keep. Two finishes can both satisfy "black" and behave very differently in service.
A few common scenarios illustrate the point:
A machined stainless housing for industrial equipment usually needs passivation, not a coating. The goal is to restore the alloy's own corrosion resistance without touching dimensions.
A hardened steel gear or cam for a defense assembly often calls for manganese phosphate, sometimes with a dry-film lubricant on top. Hardness should be noted so embrittlement controls are applied.
A steel bracket headed for paint is a natural fit for zinc phosphate. Black oxide would look fine but give the paint little to hold.
A component that needs a durable color or barrier protection in a harsher environment may be a candidate for Cerakote, with film build planned into the tolerances.
In each case, the answer came from the part, not from the finish name.
07.
What Should You Do When the Right Process Isn't Obvious?
Some parts check several boxes at once. A stainless part may need lubricity. A hardened steel part may need both wear resistance and corrosion protection. A drawing may reference a specification that no longer fits how the part is actually used.
When the answer isn't clear, the most useful step is a short review before production begins. An experienced Connecticut metal finisher can review the material, the drawing, the specification, and how the part will be used, then help you settle on a process, a post-treatment, and callouts that match. That conversation costs little. Reprocessing a lot, or discovering the wrong finish in the field, costs far more. Don’t hesitate to call us with any questions about how JH can help make your job easier.
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