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PRINCIPLE
This electrolytic coating process is one of the best known, although this name also improperly designates multi-layer protections, such as nickel + chromium. Its industrial importance is justified both by its decorative function (non-tarnishing), with thicknesses of less than or equal to one micrometre, and by its protective and mechanical function (hard chromium plating), where the same chromium deposit is made on the base metal, with thicknesses of between ten and several hundred micrometres.
The performance of electrolytic chromium, for such applications, is due to its hardness, its resistance to abrasion, the passivity of the surface and the frictional qualities of counteracting parts, in sliding or rotation
REACTION MECHANISMS IN CHROME PLATING
For electrolysis to result in chromium deposition, a certain proportion of a catalyst (SO4--) is required (≈ 1/100 by mass of chromic anhydride). The phenomena are schematised in the figure below and correspond to the electrochemical reactions at the cathode reactions at the cathode and anode.
CHROMAGE DECOR
This name in fact covers a multi-layer corrosion protection system: Ni + Cr or even Cu + Ni + Cr, with chromium providing passivity, surface hardness and resistance to tarnishing, but a generally cracked structure; nickel, which is thicker, ductile and non-porous, isolates the base metal from the surrounding environment. Anodic to chromium, nickel will corrode at cracks or porosities in the chromium deposit. A wide variety of nickel + chromium systems exist: The main ones are, in increasing order of protection:
● bright nickel plating + standard chrome (e.g. interior objects)
● bright nickel plating + micro-cracked chrome
● bright nickel plating + duplex chrome (non-cracked + micro-cracked),
● duplex nickel plating + uncracked chrome
● duplex nickel plating + micro-cracked chrome.
These deposition systems have been widely used in the automotive industry, where the thicknesses required range from 10 to 50 µm of nickel and 0.1 to 1 µm of chrome. Decorative chrome applications are found in many industries, particularly:
● automotive industry and equipment,
● optical, precision and medical instruments
● telephone and electrical equipment
● metal furniture,
● sports goods and domestic appliances,
● etc...
The quality controls concern adhesion, porosity, thickness, corrodibility. The standard NF A 91-119, can be usefully consulted.
HARD AND THICK CHROME PLATING
This chromium plating technique for industrial uses concerns thicker deposits than for decorative uses, and where one or more of the following properties are taken advantage of: ● low frictional consistency,
● non-stick properties,
● wear resistance and high hardness,
● corrosion resistance,
● load bearing qualities.
FEATURES
Hard chrome coatings are usually 8 to 250 µm thick. The baths used are quite similar to those used in decorative chrome plating, except for the lower Cr3 concentration (150 to 300 g/L) and the temperature at 50°C, which allows current densities up to 80 A/dm2 .
To obtain the highest hardnesses (≈ 1000HV), temperature and cathodic current density must be considered together (figure above). As thick chromium plating involves high cathodic current densities, and therefore a high current per unit volume, it will often be useful to provide a bath cooling system adapted to the power dissipated by Joule effect.
APPLICATION TECHNIQUES
Shape and surface condition The final quality of the thick chromium deposit is related to the surface condition and shape of the part to be coated.
Thick chrome amplifies the defects of the base metal, the thicker it is. The surface must therefore be free of pits, occlusions, work hardened areas, metal tears, drawing stripes, hardening taps, blowholes, etc…
It is also necessary to take into account the amplification of the surface roughness by providing for prior lapping, polishing or grinding operations (total roughness R t ≤ 1 µm and R a ≤ 0.5 µm).
Machinable by grinding or honing, electrolytic chromium allows very low roughnesses to be obtained by superfinishing: R a from 0.1 to 0.02 µm. Choice of deposit For a toleranced cylindrical part, it is possible to obtain a surface by chromium plating that can be used without retouching, taking into account that the tolerance on the chromium-plated diameter is the algebraic sum of the machining tolerances of the support and the thickness tolerance of the chromium plating.
Chromium plating to dimension
- Chromium thickness ≤ 12μm: dry friction, e.g. cutting tools;
- Chromium thickness 10 to 25 μ: moulds,
- Chromium thickness 20 to 60 μm: sliding friction, light wear or corrosion prevention. Thick chrome plating
- Thickness > 50μm, sometimes up to 500μm to ensure accurate dimension after grinding on parts: - New: protection, important against wear (abrasion and corrosion);
- Defects in machining or worn: contribution > 50 μm and diametrical tolerance ≤ 40μm after grinding Pre and post-treatments The treatments prior to chromium plating will remove dirt, oxides but also the hardened surface layer, and possibly lower the roughness.
In order to improve the fatigue life, shot peening can be specified:
- Shot peening intensity 0.3 mm arc, steel R m < 1,100 MPa, - Shot peening intensity 0.4 mm arc, steel R m ≥ 1,100 MPa, After electrolytic chromium plating, high-strength steels, due to the intense release of hydrogen, undergo embrittlement.
The properties of such parts are restored by a degassing heat treatment which favours the diffusion of hydrogen towards the outside, immediately after chromium plating (less than 4 hours) and before any mechanical finishing. Parts that must resist fatigue and are not pre-stressed will undergo the following heat treatment: 400 to 480°C for a minimum of 1 hour; but this may lead to a reduction in the hardness of the chrome and steel. A stress relieving treatment at 135 to 150°C for 2 to 5 hours may be prescribed, after rectification of the chromium deposit. In all cases, magnetic particle or dye penetrant inspection is recommended before and after chromium grinding, as poorly conducted grinding results in increased deposit cracking.
STRUCTURE AND PROPERTIES OF HARD CHROME
Even at small thicknesses of 0.5 to 1 μm, the electrolytic chromium deposit cracks because the internal stresses of the deposit (≈ 100 MPa) are greater than the toughness of the chromium (≈ 15 MPa).
Microscopic examination thus shows characteristic crack arrays, arrays that are superimposed without continuity as the deposit increases A relative tightness of the deposit is only obtained from 30 to 50 μm thickness.
In contrast, in other electroplated deposits, the grain size is excessively fine, 0.008 to 0.12μm; it is revealed only by X-rays and is only modified by heat treatments exceeding 400°C and lasting more than one hour, a modification which then coincides with a decrease in hardness.
Hardness and wear resistance are two essential properties of electrolytic chrome. The parameters seen in the paragraph "characteristics of hard chromium plating" will enable this value to be fixed most often, at best, between 800 and 1,000 HV.