About hard chrome plating
The term “hard” chrome got due to the extreme hardness, which is equal to hardened tool steel, and to be distinguished from the bright chromium plating (decorative).
While hard chrome plating is applied when servicing machine parts and tools, bright chromium plating (decorative) is a very popular finish for many decorative applications and provides high corrosion resistance. The difference between them is also in the fact that hard chromium is applied directly over the base material, mainly steel, by an electrolytic process in layer thickness mostly up to 0.4mm (depending on the purpose of the component), while the decorative chromium is usually applied over the underlayers made from copper and nickel, with thickness of a few microns.
Due to its excellent mechanical characteristics, hard chromium has become very popular in a variety of industries. However, its popularity in our country is still in developing phase.
HARD CHROME PLATING CHARACTERISTICS
1) Chromium hardness is Rc 63-66 (Galvanic chromium is 5 to 10 times harder than metallurgical chromium). By the Mohs scale hardness of chromium is 9, which means that it has a hardness of corundum.
2) Specific weight of electrolytic chromium is 6.5gr./cm3.
3) Hard chromium has a low coefficient of friction due to its repulsion characteristics. Sliding resistance coefficients are: steel against steel = 0.20, chromium against steel = 0.16. It provides superb abrasion resistance, about 2 times better than the cemented parts (although they have a similar hardness), and up to 4 times better than nickel.
4) Chromium begins to soften above the temperature of 350ºC, it loses its color (oxidize) above 500 ºC, while its melting point is about 1600 ºC.
5) Adhesion for the base is very easy, what is ist main advantage in compare to other galvanic coatings, allowing its application for the most wear machine parts.
6) The chromium coating can be applied over steel in natural and heat-treated state, because the chromium plating process does not change the structure of the material.
7) Hard chromium has a high chemical resistance at high temperatures to most gases, alkalis, salts and acids, except halogen acids at all temperatures, and sulfuric acid at high temperatures.
Due to their chemical passivity chromium coatings are stable on air, they do not darken and therefore do not need to be cleaned (while nickel and silver coatings oxidize) so that objects coated with chromium looks like the precious metal.
APPLICATION OF HARD CHROME PLATING
The hard chromium is used for the reparation of worn and broken parts, to improve wear and abrasion resistance, reduce friction (because this feature depends mostly on the quality of material surface), for corrosion resistance, to extend the life of machine parts and tools, to maintain accuracy rate for measuring instruments etc.
Chromium with smaller layer thickness, decorative chromium, is applied to the parts exposed to weather conditions, for sanitary equipment, cutlery, etc. because it is more resistant to corrosion than nickel.
a) Layer thickness: In order to protect parts from corrosion, layer thickness when using in decorative chromium plating should be from 0.04 to 0.05 mm, if applying only chromium coating, which is rarely practiced (especially for brass) due to the porosity of the chromium layer. It is usually applied over metal surface “undercoated” with copper and nickel layers with thickness of only 0.002 mm so that the total thickness of all the layers with chromium coating is about 0.03 mm, which is more economical due to the slow deposition of chromium. It is more appropriate to work with surface “undercoated” with layers, especially when the surface has tiny pores (caused by casting or by corrosion), because the copper sediment has the special advantage to fill those gaps. Next, special attention should be paid to prevent copper electrolyte or accumulated hydrogen residues to retain in the pores, because they create stains, or blisters on the chrome layer. If chromium plating is not applied directly, nickel coating should be activated with the cathode before its deposition in dilute sulfuric acid or it should be polished. It is also recommended for copper coating as well, because tiny pores are filled with metal coating during the polishing process.
The thickness of the hard chrome plating for tools for cutting, drilling, pressing, grinding and scraping (rasps, drill bits), aluminum and aluminum alloys, copper, bronze, cast iron, brass, hard rubber and plastics is 0.003-0.05mm, for the tools for cold drawing of metal layer thickness should be 0.03-0.2mm, and for overhauling a layer thickness is up to 0.4mm, and sometimes more (up to 1mm) which depends on the function of the part and its application (see the table).
|Chromium layer thickness|
|Parts||For new part||For repairs|
|1. Cutting tools (knives, blades)||0.003-0.008mm||0.003-0.008mm|
|2. Molds for metal casting under pressure||0.01-0.2mm||0.05mm|
|3. Measuring and control tools (gauges, prongs)||0.01-0.05mm||0.12mm|
|4. Molds for molding plastics etc.||0.04-0.06mm||0.10mm|
|5. Machine parts and devices exposed to friction (pins, bearing cups, crank, pistons, pins gears, rollers)||0.01-0.05mm||0.50mm|
|6. Cylinder liners||0.05-0.50mm||0.75mm|
|7. Chrome plating of parts due to no-load rate (pressed parts), nests ball bearings, bushings||–||1.0mm|
|8. Piston rings and cylinders||0.12-0.25mm||do 0.3mm|
For parts that are particularly subject to wear, chrome layer of 0.1mm is recommended if the substrate (base material) has a high hardness (about 60 Rc), and if the substrate is soft (hardness 30-38 Rc), then the layer thickness should be from 0.3-0.4mm.
MATERIAL FOR HARD CHROME PLATING
As a base material for chromium coatings, besides steel, can be used also cast iron, copper, brass and aluminum bronze, but the power of penetration of chromium is the best on steel.
Recommended structural steel for chrome plating is:
St.3411, St.3711, St.6011, St.7011.
St.1061, Stc.1661, Stc.2561, Stc.3561, Stc.4561, Stc.6061.
En15, ECN25, ECN35, ECN45.
VCN15, VCN25, VCN35,VCN45.
The new part that is to be chrome plated, which is exposed to impact or pressure, should correspond to the purpose, and should be applied as without chrome plating, while the parts that are exposed to the friction only, should not be thermally processed.
PARTS FOR HARD CHROMIUM PLATING
Hard chrome plating improves the performance of structural parts such as various shafts and pins, plates, gears, bearing cups, different bushings, pump components, levers, caps, frames, crankshaft journals and the camshaft, cylinders, piston rings (compression rings, press clips, various pins, rollers in printing and textile industry etc.).
A wide variety of tools can be coated: cutting tools, cold drawing tools, measuring and controling tools, (such calipers, micrometers, depth meters, gauges, various pins, etc.) dies, vulcanization tools, tools for production of plastic parts and others. (Chrome is resistant to sulfur, phenol, vinyl chloride, vinyl acetate, nitric acid from nitro cellulose, etc.).
Chrome behaves good in conjunction with cemented and nitrided (the best) parts, with steel with increased hardness of 57 Rc, as well as with higher hardness bearings (lead bronze, aluminum bronze, cast iron, artificial prressed masses, etc.), while with silver does not give satisfactory results. Chrome over chrome does is not applicable because of the resulting fraying.
The advantage of the chrome plating layer is in the fact that it can be replaced with the new coating in case of fraying.
VIABILITY OF HARD CHROME PLATING
Hard chrome plating is quite a long process and a layer of chromium 0.3 – 0.4mm can sometimes be uneconomical. In addition, thicker layers are less resistant to wear as they have a rougher structure so the maximum chromium layer of 0.25 – 0.30mm for weared parts is recommended. Depending on a size and profile of the part, it should be left an excess layer of chromium of 0.05 – 0.15mm for grinding.
In the case of recuperation of very expensive (often imported) parts, the question of profitability is not an issue.
Restrictions in the application relate to the complicated parts, heavily loaded in operation, in particular where there are weaker sections, especially if they are thermally or mechanically strained (milled) prior to chrome plating, because in this case the tensions are compressing.
Before and after the hard chrome plating, or final treatment, responsible parts must be sent to the magnetic inspection for possible cracks, which are a source of fracture due to fatigue. Chromium is not recommended for areas that receive concentric shocks in one place (eg. spring).
Chrome plating can not be applied over magnesium, aluminum, zinc, cadmium, lead and their alloys.
When chrome plating steel parts within the parts of the electrons (magnesium alloy) and brass, special attention should be paid to the occurrence of their dissolution in the chromium electrolyte, especially on textured surfaces due to the emergence of the “shadow” or from interruption in electricity. If a chrome coated part during operation is exposed to increased temperature, to the differences in temperature or to local cooling, then it is advisable to take a smaller layer of chromium, because of various thermal expansion of steel and chromium (chromium has a lower coefficient of thermal elongation of the cast iron and steel, and significantly less than copper and nickel) can occur separation of chrome layers from the base material. In contrast, in the case of a steel base, in particular in the cases of large loads, a thin layer of chromium may be pressed into the base, so that occurs a wear of chromium, peeling off, etc. The irregularities in the material (cavities, slag, etc.) to a depth of 0.5mm affects the quality of the chrome layer, and even stronger if the layer thickness is higher.
Feature of chrome coating is that has poor adhesion to other materials, which is particularly important in the case of lubrication. Therefore, in order to maintain the oil film on rubbing surfaces, so called porous chrome plating will be applied (stubline etc.) because an ordinary chrome plating requires good lubrication.
As speaking of corrosion protective properties, because the chromium oxide, which is produced in chrome plating process, and surrounds the grains of iron, is electrically more positive than iron, this protection is mechanical. It is necessary to completely cover the surface with chrome layer which need to have a sufficient thickness, if we dont want to have even greater corrosion. If there are holes in the material, due to corrosion between the base and chromium, occurs separation-peeling off the coating.