Hard chrome plating properties thickness
Hard chrome plating is an electrolytic coating of hexavalent chromium characterized by its high surface hardness, which can reach 1000 HV, and thicknesses typically between 10 µm and 500 µm per side. It is applied to base metals via a chromic acid bath at 60 °C and is mainly used to improve wear resistance, restore dimensions, and reduce friction in industrial components.
Manufacturing process
Section titled “Manufacturing process”Hexavalent hard chrome plating follows an electrodeposition sequence in which the part acts as a cathode and a lead-tin or platinized titanium anode supplies the chromium. The layer is built by reduction of hexavalent chromium ions (Cr⁶⁺) present in an electrolyte of chromium trioxide and sulfuric acid as catalyst. The typical steps are:
- Surface preparation: alkaline degreasing, pickling with hydrochloric, sulfuric or hydrofluoric acid depending on the substrate, and anodic electrolytic cleaning.
- Electrochemical activation in an acid bath (same electrolyte or separate tank) to remove oxides and ensure adhesion.
- Heating the workpiece to the bath temperature without applied current.
- Electrodeposition with direct current: average deposition rate is 25 µm/h (0.001 in/h). Time is adjusted to the target thickness.
- Rinsing to remove electrolyte residues.
- Post machining (grinding or lapping) when tight dimensional tolerances are required, since the freshly deposited surface has some roughness.
Operating parameters
Section titled “Operating parameters”The parameters of the hexavalent hard chrome plating bath are maintained in moderate temperature and current density ranges to ensure a uniform and hard deposit. Typical values are summarized below, and in a second table, the activation baths according to base material.
| Parameter | Typical range |
|---|---|
| Bath temperature | 50 – 65 °C (122 – 149 °F) |
| Cathodic current density | 30 – 60 A/dm² (195 – 390 A/ft²) |
| CrO₃:H₂SO₄ weight ratio | 75:1 to 250:1 |
| Deposition rate | ≈ 25 µm/h (0.001 in/h) at nominal current |
| Electrolyte pH | < 1 (strongly acidic) |
| Base material | Recommended activation solution |
|---|---|
| Carbon and low-alloy steel | Diluted sulfuric acid (5 – 10% vol.) or anodic activation in chrome bath |
| Stainless steel | Hydrochloric acid 10 – 20% vol. + reverse activation in chrome bath |
| Cast iron | Sulfuric acid 5 – 10% vol. with anodic etching |
| Nickel alloys (Nimonic, Inconel) | Ferric chloride or diluted hydrofluoric acid + anodic activation |
| Copper and copper alloys | Sulfuric acid 5 – 10% vol., brief anodic electrocleaning |
Coating properties
Section titled “Coating properties”Hard chrome plating achieves a surface hardness of 800 – 1000 HV, equivalent to 65 – 70 HRC and approximately 700 – 750 HB. The exact values depend on current density and bath temperature. The layer has a very low coefficient of friction (0.10 – 0.15 against lubricated steel) and excellent sliding wear resistance.
| Property | Typical value |
|---|---|
| Vickers hardness (HV) | 800 – 1000 |
| Rockwell C hardness (HRC) | 65 – 70 |
| Brinell hardness (HB) | ≈ 700 – 750 |
| Layer thickness (normal) | 10 – 250 µm (0.0004 – 0.010 in) |
| Maximum thickness (special cases) | up to 500 µm (0.020 in) per side |
| Corrosion resistance | Good in atmospheric and oxidizing acid environments (passivated layer) |
| Surface roughness post-deposit | Ra 0.5 – 2.0 µm (20 – 80 µin) before grinding |
| Coefficient of friction (lubricated) | 0.10 – 0.15 |
| Maximum service temperature | ≈ 400 °C (752 °F), above which hardness decreases |
Typical thicknesses and tolerances
Section titled “Typical thicknesses and tolerances”In hard chrome plating, thickness per side is specified. Final dimensional tolerances are achieved by subsequent grinding, with a typical precision of ±5 µm (±0.0002 in). The following table lists common thicknesses according to coating function.
| Application | Thickness per side (µm) | Thickness per side (in) |
|---|---|---|
| Light wear resistance (pistons, rods) | 10 – 50 | 0.0004 – 0.002 |
| General industrial use (hydraulic cylinders) | 25 – 125 | 0.001 – 0.005 |
| Dimensional restoration | 125 – 400 | 0.005 – 0.016 |
| Large thicknesses (mill shafts, compressors) | 250 – 500 | 0.010 – 0.020 |
| Tolerance after grinding | ±5 | ±0.0002 |
Applicable materials
Section titled “Applicable materials”Hard chrome plating can be deposited on most ferrous and non-ferrous metals, as long as the surface can be electrically activated. The most common are:
- Carbon and low-alloy steels (heat treated or not).
- Stainless steels (300 and 400 series, with special activation).
- Gray and nodular cast irons.
- Nickel base alloys (Inconel, Nimonic, Monel).
- Copper and brass (with careful preparation to avoid chemical attack).
- Titanium (requires prior base layer or strong anodic activation).
Not recommended on aluminum or light alloys without an intermediate layer, due to weak direct adhesion.
Applications
Section titled “Applications”Hard chrome plating is applied in sectors that demand high wear resistance and low friction. Some representative examples are:
- Hydraulic components: piston rods, cylinders, valves.
- Heavy machinery: shafts, liners, bearings, rolling mill rolls.
- Diesel engines and marine equipment: cylinder liners, piston rings, crankshafts.
- Plastic injection molds: cavities and cores where non-stick and abrasion resistance are required.
- Cutting and forming tools: gages, taps, dies (where the temperature limit is not exceeded).
- Dimensional restoration: restoration of worn parts to original dimensions.
Advantages and limitations
Section titled “Advantages and limitations”Advantages
Section titled “Advantages”- Exceptional hardness (800 – 1000 HV) providing high wear resistance.
- Low coefficient of friction, suitable for lubricated sliding pairs.
- Good corrosion resistance in atmospheric and many chemical environments.
- Possibility of controlled thicknesses from a few micrometers up to 500 µm.
- Mature industrial process, standardized (AMS 2460, AMS 2406) and widely available.
Limitations
Section titled “Limitations”- The process uses hexavalent chromium, a carcinogenic compound with strictly controlled handling.
- Poor penetration (low throwing power); recesses tend to receive less thickness than edges.
- Subsequent grinding or lapping is required to obtain fine tolerances and low roughness.
- Low cathodic efficiency (15-25%), which lengthens process times.
- The layer is brittle and does not tolerate repeated impacts; surface cracking (microcracks) is inherent and desirable for lubricant retention, but can initiate fatigue.
- Not suitable for continuous temperatures above 400 °C, as hardness decreases significantly.
Selection guide
Section titled “Selection guide”Hard chrome plating is recommended when:
- Surface hardness ≥ 800 HV is needed with controllable thicknesses between 50 and 300 µm.
- The part operates in sliding with lubrication and friction must be minimized.
- Restoration of dimensions on worn metal components is desired, without excessive weight addition.
- The working environment does not contain concentrated hydrochloric acid nor sustained temperatures > 400 °C.
Consider alternatives such as high-phosphorus electroless nickel when uniform thickness on complex geometries without grinding is required, or nitriding when hardening without significant dimensional increase is sought. For environments with high corrosion and abrasive load, tungsten carbide coatings applied by HVOF may be chosen.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the typical hardness of hard chrome plating?
Section titled “What is the typical hardness of hard chrome plating?”Hard chrome plating has a Vickers hardness of 800 to 1000 HV, equivalent to 65-70 HRC, measured directly on the coating without subsequent heat treatment.
What is the maximum thickness that can be applied in hard chrome plating?
Section titled “What is the maximum thickness that can be applied in hard chrome plating?”In industrial applications, thicknesses up to 500 µm per side (0.020 in) are achieved, although for reconstruction work 200-400 µm per side is common.
What is the difference between hard chrome plating and decorative chrome plating?
Section titled “What is the difference between hard chrome plating and decorative chrome plating?”Hard chrome plating is deposited with thicknesses greater than 10 µm and the objective is functional (hardness, wear), while decorative chrome uses very thin layers (< 1 µm) on a nickel base for brightness and corrosion protection.
On what materials can hard chrome plating be applied?
Section titled “On what materials can hard chrome plating be applied?”It can be applied on carbon steels, stainless steels, cast irons, nickel alloys, copper and brass, as long as appropriate electrochemical activation is performed before the bath.
Does hard chrome plating require post machining?
Section titled “Does hard chrome plating require post machining?”Yes, in most cases it is ground or lapped to reach the final tolerance of ±5 µm (0.0002 in) and a roughness Ra below 0.2 µm (8 µin).
What is the maximum operating temperature of hard chrome plating?
Section titled “What is the maximum operating temperature of hard chrome plating?”The hardness remains stable up to approximately 400 °C (752 °F); above this temperature, the layer progressively loses hardness due to microstructural transformation.
Sources Consulted
Section titled “Sources Consulted”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/bhn-brinell-hardness-number-d_1365.html
- efunda.com: https://www.efunda.com/forum/show_message.cfm?start=1&thread=17314&id=17316
- manufacturingguide.com: https://www.manufacturingguide.com/en/hexavalent-hard-chromium