Sunday, June 27, 2010

Physical Properties of Chromium Plating

Hardness and Wear Resistance
Hardness and wear resistance of chromium deposits are generally quite good in service, as long as satisfactory thickness is applied for the intended use and for best coordinates with the hardness for the basic metal and the pressures encountered. Frequently service is at elevated temperatures and same of the as plated hardness is lost. On the other hand, relatively soft deposits produced from hot solutions have often given especially good service, possibly owing to freedom from cracks or to greater strength and cohesion of this type of plate.

Generally hardness of bright chromium plate is given as about 900 to 1000 kg/cm2, or just this many hardness units. Microhardness number depends to some extent on the test load used, and the deposit must be thick enough to withstand it. On a soft basis metal, a thin chromium deposit will show only the hardness of the basis metal, it would have to be extremely thick to indicate its full hardness. Chromium deposit 25 μm or more thick appeared satisfactory on a hardness steel base using 50 to 1999 g-loads; it is recommended that the plate thickness should be at least 14 times the depth of penetration of the indenter. Low loads often result in considerable increase in the hardness number, and greater accuracy. Test are sometimes made on the across section of the deposit if it is thick enough and well supported.
Coefficient of Friction
The low coefficient of friction of chromium plate against other metals is an important factor in its use of shafting, piston rings, internal combustion engine cylinders, and similar applications. On the table below illustrate the superiority of chromium in term of this property.
 
Metal
Static

Coefficient
Sliding

Coefficient
Chromium Plated Steel on

Chromium plated Steel
0.14
0.12
Chromium Plated Steel on

Babbitt
0.15
0.13
Chromium Plated Steel on

Steel
0.17
0.16
Steel on Babbitt
0.25
0.20
Babbitt on Babbitt
0.54
0.19
Steel on Steel
0.30
0.20
Bright Chromium plate

On Cast Iron

0.06
Bright Chromium plate

On Bronze

0.05
Bright Chromium plate

On Babbitt
0.08
Hardened steel on cast iron
0.22
Hardened steel on bronze
0.11
Hardened steel on Babbitt
0.19

Bright chromium plate against cast iron gave a lower coefficient of friction than mat or burned plates. The coefficient of friction of chromium plate against steel or cast iron increase rapidly with temperature; the increase may be avoided by final polishing after heat treating.

Coefficient of expansion
The average coefficient expansion of annealed electrolytic chromium in the temperature range of 20 to 100oC about 6.8 x 10-6 /oC. The expansion coefficient for another temperature between -75 and 650 oC will follow the formula σt = (5.88 + 0.01584 t – 0.00001163 t2) . 10-6.

Melting Point
Melting point for chromium metal is about 1560 to 1920 oC. The American Society for Metals has adopted the value of 1875oC, but that value still close to general statement of chromium melting point to 1878 ± 22oC that to be the most likely.

Density
The density of chromium plate varies according to the amount of inclusion in the plate, the number and size of cracks, and the magnitude of internal strain. The density of chromium deposited under various conditions, the value from 6.90 to 7.21 g/cm3 were obtained. The oxide content decreased as the density increased. After annealing at 1200oC, the density of the deposits increased to within the range 7.09 to 7.22 g/cm3. The density of pure chromium is 7.20 g/cm3, as calculated from its lattice parameter.

Reflective Power
The reflectivity of chromium plate over a light range from ultraviolet to infrared. For the visible range of light, 4000 to 7000 Å in wavelength, they obtained reflectivity value between 62 and 72%. For ultraviolet light, the reflectivity range from 55 to 70%, and for infrared from 62% at 7,000 Å to 88% at 40,000 Å. The reflectivity may be seriously reduced when the plate is exposed to highly corrosive atmosphere.

Electric Resistance
The electrical resistivity; like the density, is a measure of the continuity, purity, and general soundness of metal. The number, distribution and size of the inclusion filled cracks in chromium are related to the plating conditions. Therefore the electrical resistivity varies according to these conditions.

The resistivity is about 50 – 60 microohm-cm at 25oC may be expected for conventional chromium plate, with much lower value down to 14 microohm cm for deposits from hot solutions. After annealed at 1200oC, the oxide film inclusions are continuous. As a result, the resistivity of annealed electrolytic chromium approaches a common value of 13 microohm-cm at 28oC, regardless of conditions of deposition.

Internal Stress
On conventional plating practice produced thicker cracked plates have internal stress of about 12 kg/mm2. Plates from a dilute bath at 85oC were crack free but contained stresses of 45 kg/mm2.

Cathodic treatment of ordinary bcc deposits in N Na2SO4 at 40oC and 1 A/dm2 decrease the stress in less than 1 hr to less than that of the cold chromium. Additional work on stress in chromium deposits has been reviewed.

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