Thickness is a compromise, not a quality grade
A thicker chrome layer is often marketed as a better rod. In service the relationship is more awkward. Chrome is hard and wear resistant, and it is also brittle and stressed. As the layer grows, the internal stress grows with it, and the micro-crack network that runs through all hard chrome becomes coarser. Past a certain thickness the layer starts to work against the rod rather than for it.
Two failure modes set the practical ceiling. The first is spalling, where a stressed thick layer breaks away in flakes under point loading. The second is fatigue, because a thick brittle coating on a ductile core crack-initiates earlier under bending. Thickness selection is therefore a balance between wear allowance and coating integrity.
Commonly specified bands
| Layer thickness | Typical use | Trade-off |
|---|---|---|
| 10 – 20 µm | Light duty, indoor cylinders, short stroke, low cycle count | Lowest cost. Little wear allowance after grinding and polishing. |
| 20 – 25 µm | General industrial and mobile hydraulics, the common default | Wear allowance with manageable stress. Most catalogues specify a 20 µm minimum. |
| 25 – 30 µm | Abrasive or corrosive environments, mining, marine, long-stroke | Better corrosion barrier. Requires tighter process control. |
| 30 – 35 µm and above | Rods specified for re-grinding and re-plating in service | Allows future rework. Higher stress, longer plating time, higher cost. |
One practical note on how that number is produced. A rod is plated oversize and then ground and polished back to final diameter, so the figure on the drawing is the finished thickness after grinding. A supplier quoting as-plated thickness is quoting a different thing. Ask which one you are being sold.
What sits under the chrome
The coating only performs as well as the substrate and the preparation allow. Three substrate routes dominate.
- C45 / CK45 as supplied or quenched and tempered. Core hardness in the region of 190 to 220 HB. Adequate for moderate duty at low cost.
- Induction hardened then chrome plated. Surface hardness commonly quoted at 55 to 60 HRC with case depths around 1.2 to 2.5 mm. This is the route for impact loading and for rods that take side knocks in service.
- 42CrMo4 quenched and tempered. Through-hardened alloy core for high-strength and high-fatigue applications, used where rod diameter is driven by buckling rather than wear.
Chrome surface hardness is typically quoted between 800 and 1000 HV, with some suppliers claiming up to 1150 HV. Adhesion depends on cleaning, activation and current density control. Poor preparation produces a layer that passes a thickness gauge and then peels in the field, which is why adhesion testing per ASTM B571 or ISO 2409 belongs in a serious incoming inspection plan.
Corrosion: what the salt spray number means
Hard chrome is porous by nature. The micro-crack network that makes it a good bearing surface also gives moisture a path to the steel underneath, so corrosion protection depends on sealing those paths. Neutral salt spray testing per ASTM B117 or ISO 9227 is the common yardstick, and published figures vary widely, from 96 hours at NSS rating 9 up to 500 hours and beyond for multi-layer systems.
Read the test conditions before comparing numbers. Hours to first red rust, rating number, layer system and test standard all change the meaning. A 500 hour claim on a triple-layer system says little about a single-layer 20 µm rod. If corrosion is the driving concern, specify the test standard, the rating required and the hours in one line, and ask for the report rather than the brochure figure.
Note also that hexavalent and trivalent chromium processes differ in environmental compliance and in deposit properties. If your market restricts hexavalent chromium, state it at enquiry stage, since it changes which plating line can run the job.
How we specify and control it
Our routine chrome layer on piston rod is [ORISE: confirm standard layer thickness, for example 20 µm minimum or 0.03 mm as used on the hollow chromed rod sample]. Custom thickness is available within [ORISE: confirm achievable range]. Routine surface hardness target is [ORISE: confirm HV target]. Controlled values are recorded per plating batch; anything we have not measured on our own line is marked not published.
Full product scope is on the piston rods page. Certification documents we issue are described on the certifications page, and the meaning of EN 10204 document types is covered in our article on 3.1 versus 3.2 certificates under resources. Send thickness and corrosion requirements through the RFQ page, quoting test standard and hours.