Hard Chrome Plating: Thickness, Grinding & Equipment

Hard chrome plating is a functional coating. It is bought for wear resistance, low friction and the ability to bring a worn or undersized part back to dimension — not for appearance. That single fact changes almost every decision around it, including the ones buyers get wrong most often.

Three of those decisions do most of the damage. The thickness is written as one number when it should be a range with a grinding allowance. The grinding step is left out of the plan entirely, when it is the step that actually sets the final dimension. And the bath is assumed to plate evenly when it is the worst in common use at doing exactly that. This article takes them in turn.

What hard chrome actually is

Hard chrome — also called functional chrome, engineering chrome or industrial chrome — is chromium deposited directly onto the base metal. There is no copper or nickel undercoat. The deposit is semi-bright to matte, it is thick by plating standards, and it is applied because of what it does to the surface.

This is the opposite of decorative chrome, and the two are not variants of the same job:

Because a decorative layer is a fraction of a micron and a hard chrome layer is measured in tens or hundreds of microns, the two processes also demand different equipment. The chrome plating equipment page sets out both, and the comparison table there is the fuller version of the one above. Trivalent and hexavalent chemistry is a separate question again, covered in tri-chrome plating equipment.

"Hard" is a name, not a number

One caution before going further: hard chrome does not have a single hardness. The name describes the application — a coating hard enough to resist wear — not a fixed property of the deposit. Hardness is a product of the bath chemistry and the conditions it is run at, and the same bath run two different ways will not give the same coating.

That has a practical consequence: if hardness matters to your application, it is something to specify and to agree on with the plater, not something you get by ordering "hard chrome". Say what you need, ask how it will be verified, and agree a test method — a hardness figure with no test method attached is not a specification.

Thickness: specify a range, not a number

Thickness is the specification that decides whether the part works, and it is the one most often written as a single figure when it should not be.

Every surface the solution reaches gets coated, and it does not get coated evenly. Edges build faster than flats. Recesses build slower. Around a cylinder the deposit varies from point to point. So a drawing that says "25 µm" is asking for something that does not exist as a uniform layer — what exists is a distribution with a minimum somewhere on the part.

The useful way to write it is a minimum, a maximum and a grinding allowance, and to state which surface the number applies to. A hardness or thickness reading taken on a convenient flat face says very little about the inside of a bore.

These bands are industry starting points, and the governing figure is whatever your drawing or the OEM requirement calls for. Where a part is safety-related or a warranty item, that document decides — not a supplier's rule of thumb, and not this table.

Grinding is how the dimension is actually held

This is the step that surprises buyers most, and getting it wrong is expensive in both directions.

Hard chrome is normally applied oversize and then ground back to the finished dimension. The plater does not aim to deposit exactly 25 µm and stop; the plater deposits more than the minimum and a grinding operation removes the excess. That is how a tolerance is held on a chrome-plated part — not by controlling the deposit to a micron.

Four consequences follow:

• The thickness on the drawing has to be the thickness after grinding. A part specified at 25 µm finished may need noticeably more than that as plated, because some of it is going to the grinder. If the plater reads the number as an as-plated target and the grinder then removes material, the part ends up under the minimum.
• There has to be a grinding allowance, and the plater needs to know about it. Oversize is deliberate, not sloppy work. Say so on the order.
• Surfaces that cannot be ground need a different answer. A bore, a recess or a face the grinder cannot reach cannot be brought back to size this way. Those are the surfaces to mask, or to design out of the plated zone.
• Grinding through is a real failure mode. Where the as-plated deposit is thin at one point — an edge, a recess — grinding to size can remove the coating entirely and expose bare steel. This is why the minimum thickness matters more than the nominal one.

Hard chrome is also hard to grind, which is the point of it. Hard chrome that is genuinely hard will resist the grinding wheel; a deposit that grinds easily is often a sign the bath was not run where it should have been. If a shop reports that a chrome job ground like butter, that is worth a question rather than a compliment.

Why the bath plates unevenly

Chrome plating has poor throwing power. Of the common plating processes it is among the worst at depositing into recesses and around complex geometry. The deposit concentrates on high-current-density areas — edges, corners, the outside of a form — and thins sharply in low-current-density areas — recesses, deep bores, the inside of a shape.

For hard chrome this is not a minor cosmetic issue, because the parts it is used on are frequently the parts with geometry: a rod with a step, a mold with a cavity, a shaft with a keyway. The usual answers are practical rather than chemical:

• Anode placement and spacing. Distance and position between anode and part shape where the current goes. This is where a line's design work actually happens, and it is a large part of why a hard chrome line is not simply a nickel line run hotter.
• Conforming and auxiliary anodes for internal surfaces. Plating the inside of a bore is a specific problem with its own tooling — it can be done, but it is designed, not assumed.
• Racking and orientation, so that gas escapes and solution exchanges rather than being trapped.
• Masking, so that areas which cannot be plated or ground well are not plated at all.

If your part has critical surfaces in recesses, raise it before the order rather than after the first batch. The honest answer for a difficult internal surface is often that it should be plated by a different method or not at all — and a supplier who says so is more useful than one who does not.

Hydrogen embrittlement: the failure that arrives later

Hard chrome is frequently applied to high-strength steel — hydraulic rods, shafts and tooling among them — and that is exactly the combination where hydrogen is a hazard rather than a theoretical concern. Hydrogen is generated at the cathode during plating and during the acid cleaning and pickling before it. On ordinary steels it diffuses back out harmlessly. On high-strength steels it collects at stress concentrations and the part can crack and fail suddenly under load, sometimes days or weeks into service, below the material's normal yield and with no warning.

The countermeasure is a bake after plating, and the details that matter are the ones most often left off a purchase order: the bake must start promptly, it is a separate specified step rather than a warm dry, and the requirement applies even where plating current was brief, because pickling and cleaning generate hydrogen too.

The mechanism, the strength threshold above which this is treated as a genuine hazard, and what to put in writing are set out in nickel plating machined parts — the failure mode is the same regardless of which metal is deposited. For hard chrome the point to carry away is narrower: if the part is high-strength steel, ask what the drawing calls out and make sure the bake is in the order.

What a hard chrome line needs that a nickel line does not

Chrome is aggressive to equipment in ways nickel and zinc are not, and the differences are not optional. Most of them come down to the bath itself.

For line configuration — rack, hoist, tank layout and how much automation is worth paying for — the choice between a manual or semi-automatic line and an automatic gantry line usually comes down to part mix and volume, and it is covered in how to choose an automatic plating line.

Masking, and plating only what needs plating

Everything in the tank gets coated, and everything coated that has a tolerance has to be dealt with afterwards. Masking is usually cheaper than the alternative, and it is worth designing for rather than adding late:

• Mask the surfaces whose tolerance the coating would consume — press fits, sealing faces, threads, bearing seats.
• Mask the surfaces that cannot be ground back to size, because there is no recovery step available for them.
• Say why a surface is masked. "Protect the thread" is applied differently from "hold this pitch diameter".
• Check the mask survives the bath. Chrome runs hot and aggressive, and a mask that fails mid-process is invisible until after the fact.

What to tell a hard chrome supplier

The enquiry that gets a useful answer carries these:

• Base material and its strength, so hydrogen embrittlement relief can be specified rather than guessed.
• The finished dimension and which surfaces carry it — not just the part's overall size.
• Whether the part is plated-and-ground to print, and how much grinding allowance is acceptable.
• Minimum thickness after grinding, on the surfaces that matter.
• Any hardness requirement, with the test method.
• Whether the part may be stripped and re-chromed later, which affects how the base material is handled now.
• Part size, weight and quantity, which is what decides whether the work fits a given tank.

If you are buying a line rather than plating services, the same list becomes the brief the line is designed around — see contact us to work through it.

Frequently asked questions

How thick is hard chrome plating?

It depends entirely on the application, and the useful question is not "how thick" but "what minimum must survive after grinding". Decorative chrome is a fraction of a micron; hard chrome is measured in tens or hundreds of microns. The figure that governs is on your drawing or in the OEM requirement, and it should be written as a minimum with a grinding allowance rather than a single nominal number.

Is hard chrome harder than decorative chrome?

Yes, by a wide margin — but the more important point is that "hard chrome" does not describe one fixed hardness. Hardness comes out of the bath chemistry and the conditions it is run at, so the same process run differently gives different results. If hardness matters, specify it and agree how it will be tested.

Why does hard chrome have to be ground after plating?

Because the deposit is not uniform. Edges build faster than flats and recesses build slower, so aiming for an exact thickness everywhere is not achievable. Plating oversize and grinding back to the finished dimension is how the tolerance is actually held. That means the thickness you specify is the thickness after grinding, and the plater needs to know that.

Can hard chrome be plated inside a bore?

It can, but it is a designed operation rather than a routine one. Chrome has poor throwing power, so the inside of a bore is exactly where the deposit thins, and it needs auxiliary or conforming anodes and specific racking. Whether it is worth doing depends on the bore and the requirement — raise it before the order rather than after the first batch.

Do hard chrome parts need baking?

It depends on the steel's strength. Above roughly 1000 MPa tensile strength, hydrogen embrittlement relief is standard practice and belongs in the specification. Acid cleaning and pickling generate hydrogen as well as plating does, so the requirement is not limited to heavily plated parts. The exact bake is normally defined by the drawing or a referenced standard practice — confirm what applies rather than assuming a general-purpose bake covers it.

Can hard chrome be stripped and re-plated?

Often yes, and for expensive parts it is a normal repair route — which is one reason the grinding allowance and re-chroming question are worth settling at the specification stage. Stripping is a chemical operation that also affects the base metal, so it is a job to plan rather than to attempt on a part whose dimensions are already marginal.

Can hard chrome be barrel plated?

No. Barrel plating tumbles parts together, which damages the surface, and hard chrome is normally a ground finish on a part with a tolerance. Chrome is rack plated — parts are held separately throughout the process.