Electroplating nickel versus chrome finish: how to choose

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  Electroplating nickel versus chrome finish: property comparison
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  What each bath actually deposits
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  Hardness, wear resistance and friction
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  Corrosion resistance and the duplex stack
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  Thickness control and what it does to your dimensions
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  Cost drivers, lead time and hydrogen risk
• 7
  When each finish is the wrong choice

Electrolytic nickel plates from a Watts bath at roughly 50–60 °C and 2–7 A/dm². The deposit runs 5–40 μm in normal shop practice. A matte or semi-bright bath gives a dull gray surface that still shows tool marks. Bright nickel adds organic levelers and brighteners, so the part comes out of the tank with a mirror look and no polishing step.

Electroless nickel is a different process, not a variant of the same tank. Nickel and phosphorus co-deposit by chemical reduction with no current, at 10–25 μm/h. Because there is no current density to chase, the coating follows complex geometry, threads and blind holes evenly. It costs more per part. The trade is coverage.

Chrome in this discussion means hexavalent chromium from a chromic acid bath. Decorative chrome is a thin topcoat, usually 0.1–0.8 μm, laid over bright nickel. Hard chrome is a thick functional layer, 20–50 μm, plated directly on steel and then ground to size. Treat decorative and hard chrome as two separate decisions.

One point catches buyers. Decorative chrome alone on steel is not a corrosion system. The thin chromium layer is porous and cathodic to nickel, so any pore pushes attack into the layer below. The nickel underlayer does the protecting, and the chrome mostly provides color and abrasion resistance at the surface.

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  Watts nickel50–60 °C, 2–7 A/dm², 5–40 μm typical
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  Bright nickelOrganic levelers give mirror finish straight from the tank
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  Electroless Ni-P10–25 μm/h, no current, even on complex shapes
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  Decorative chrome0.1–0.8 μm over nickel, cosmetic and thin

Microhardness separates the processes more sharply than any other property. Watts nickel sits at 150–250 HV. Bright nickel lands in the same range. These are soft coatings. They resist corrosion and look good, but a nickel surface will gall and scratch under sliding contact.

Electroless nickel-phosphorus reaches 500–700 HV as plated. A one hour anneal at 400 °C pushes it to 850–1000 HV. That heat treatment is the step most drawings forget to call out, and without it the wear number on the datasheet is not what the part delivers.

Hard chrome runs 800–1000 HV. Bath temperature matters here. Let the bath drift high and hardness falls toward 700 HV. For a shaft running in a bushing or a hydraulic rod with a seal, hard chrome at 20–50 μm is still the default answer.

Electroless nickel is the better choice when the wear surface is a thread, a spline or an internal bore. Hard chrome throwing power is poor, so those features come out thin or bare. Nickel also gives a lower, more consistent friction coefficient on sliding pairs than as-plated chrome, which needs grinding and honing before it runs clean.

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  Sliding shaftsHard chrome 20–50 μm, ground to Ra 0.1–0.2 μm
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  Threads and splinesElectroless nickel covers where chrome cannot throw
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  Annealed Ni-P850–1000 HV after 1 h at 400 °C

Decorative chrome over bright nickel is a corrosion system, not a single coating. The nickel layer, typically 10–30 μm, is the barrier. The chromium topcoat is porous and cathodic to nickel, so a pore becomes a small corrosion cell that eats the nickel beneath it. More nickel thickness buys more salt spray hours.

A copper underlayer of 15–25 μm improves leveling and spreads corrosion sideways rather than down through to the steel. On zinc die castings and on steel parts headed outdoors, the copper-nickel-chrome stack is the standard route. CASS and neutral salt spray testing to ASTM B368 and B117 is how the stack gets qualified.

Electroless nickel behaves differently. It is amorphous and free of the through-pores that plague thin electrolytic deposits, so a 15–25 μm layer holds up on its own. High-phosphorus grades resist chloride better than mid-phosphorus ones. For parts that see washdown or marine air, this is a real advantage.

Hard chrome is not a corrosion coating. It is porous by nature and cathodic to steel, so a pore concentrates attack on the substrate. On a hydraulic rod the chrome protects against wear and the seal keeps the environment out. Specify hard chrome for corrosion alone and the part will pit.

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  Outdoor steelCu 15–25 μm, then Ni 10–30 μm, then Cr
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  Washdown partsHigh-phosphorus electroless nickel, 15–25 μm
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  Die castingsCopper underlayer improves leveling and spread

Nickel thickness is set by time and current density. Expect about ±10% variation across a rack, and worse inside recesses where current density drops. If a bore has to stay within ±0.005 mm after plating, that variation is most of the tolerance. Plating allowance has to go into the drawing before the part is machined.

Electroless nickel holds ±10% or better over complex shapes as long as bath chemistry is maintained. That predictability is why it shows up on parts with tight bores and fine threads. It still moves the surface, so the same allowance rule applies.

Chrome throwing power is poor. On irregular geometry, expect roughly 50% thickness variation. Hard chrome is therefore plated thick and ground back to final size, which is why the process route usually includes a grinding operation and sometimes a masking step for areas that must stay bare.

Bright nickel delivers Ra 0.05–0.1 μm straight from the bath. Decorative chrome over leveled nickel gives the classic mirror at Ra under 0.05 μm. Hard chrome as deposited is rough at Ra 0.5–1.5 μm and needs diamond grinding or honing. Electroless nickel simply copies the substrate finish, so polish first if you want a smooth result.

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  Add allowancePut plating thickness in the drawing before machining
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  Hard chromePlate thick, then grind to final size
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  MaskingProtect threads and bores that must stay bare

Nickel is cheaper per part in volume. The bath is simpler, deposition runs at 20–30 μm/h, and bright nickel needs no post-polish. Hard chrome costs more because current efficiency is only 12–20%, throwing power is poor, and grinding is often required. That grinding step can add 30–50% to the process route.

Decorative chrome adds a second tank line and extra rinse stages. Rack plating lead times in our shop run in the range of several working days, and a duplex stack is longer than a single nickel layer. Build that into the schedule rather than assuming the finish is a same-week step.

Both processes charge hydrogen into steel. Hard chrome carries the higher risk because of its low current efficiency. Parts above roughly 32 HRC or 1000 MPa tensile should be baked within 1–4 h of plating at 190–230 °C for 4–24 h, depending on strength and thickness. Nickel carries a lower but non-zero risk.

Baking after plating slightly reduces chrome hardness and that trade has to be accepted. On high-cycle fatigue parts, the bake is not optional. If a drawing calls for hard chrome on a high-strength steel without a bake note, ask the question before the parts go in the tank.

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  Bake windowStart within 1–4 h of plating, 190–230 °C
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  Bake time4–24 h depending on strength and thickness
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  Trigger pointAbove roughly 32 HRC or 1000 MPa tensile

Do not specify decorative chrome where the part sees abrasion, impact or sustained salt exposure. The topcoat is too thin to carry wear, and the nickel underlayer corrodes at pores once the surface is breached. A mirror finish is not a wear rating.

Do not use hard chrome on complex internal geometry or threads. Poor throwing power leaves bare or thin areas that fail inspection and sometimes fail in service. Electroless nickel is the answer for those features, even at higher unit cost.

Avoid electroless nickel where the part runs above 400 °C in service unless the phosphorus content and heat treatment are matched to that temperature. Above the coating's stable range, hardness and structure change. Likewise, skip nickel on parts that need a bright cosmetic finish and see heavy handling, since soft nickel scratches.

One more boundary. If the part needs both a mirror look and sliding wear, that is usually a duplex stack plus a ground functional surface on a separate feature. Trying to get both from one thin layer is where finish specifications go wrong.

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  No chrome for abrasionDecorative chrome is cosmetic, not a wear layer
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  No hard chrome in boresThin or bare coverage on internal features
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  Watch service temperatureElectroless nickel has an upper limit around 400 °C