Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
Doís and Doníts of Hard Chrome Plating Part 2
Hard chrome plating solutions
There are many hard chrome plating solution formulas and operating conditions that produce different hard chrome deposit characteristics. There is a wide spread notion that chrome electroplated deposit are always 1000 Vickers hardness number. Actually the hardness can vary from 450 to 1200 depending on operating temperature, current density and formula and how the Vickers hardness tests is performed using the correct indenter and gram load. One of the most used formulas for hard chrome plating uses 30 to 33 oz/gal. (225 top 248 g/L) chromic oxide and 0.3 to 0.33 oz/gal (2.25 to2.47 g/L) sulfate, usually added as sulfuric acid in a ratio of 100 to1. The ratio is sometimes changed to alter the properties if the chrome deposits to provide special needs such as increasing the covering power and throwing power. Silicofluoride can be used as the catalyst, alone or with sulfate to provide a self regulating catalyst. Silicofluoride is partially soluble, so that there is a reserve of catalyst that dissolved as needed. Fluoride can also cause etching if the work is left in the solution without power, on or if items fall from the rack .And or on low current density (CD) areas of the work items where no chrome is deposited.
Operating temperature must be controlled within a narrow range. Hardness of the chrome deposit increases as the temperature is increased. Also important is the current density, since the hardness of the deposit is generally increased with increasing CD. However at high temperature, i.e. 150F (65.5C) and a current density of about 300 A/sq ft. the hardness drops to 454 Vickers hardness number or lower. Some examples: At 100F (37.8C) and a CD above 500, the hardness will be approximately 850. At 600 ASF and 100F the hardness would expected to be 1200VHN. As the solution temperature rises above 110 F (43C) the hardness of the deposit lowers.
The type of catalyst and concentration of the catalyst plays a role in deposit hardness, smoothness, cracked or not cracked, bright or dull appearance, high stress or low stress deposits. Deliberate cracking is useful where lubricants are used by entrapping lubricants thus lowering the friction and maintaining lubricity.
Common impurities in chrome plating solutions are iron, trivalent chromium,
nickel, copper and chlorides. Of all of these only trivalent chrome can be
removed by using the porous pot.
Trivalent chrome lowers current efficiency. To keep trivalent chrome low, use active anodes with large anode area to work area. Chlorides can be easily removed by adding an equivalent amount of silver oxide. (Silver chloride is insoluble and precipitates out of solution.) Chloride contamination causes low current efficiency and loss of throwing power. Sources of chloride can be city water. (Most city sources contain chlorides) drag-in from preparation solutions.
Select chrome solution operating temperature required for the hardness desired and hold it steady while plating.
Keep the current density constant.
Keep the solution level constant.
Analyze for chromic acid, sulfate and keep the ratio steady.
Keep the anodes a chocolate-brown appearance (lead peroxide).
Check anodes for orange- yellow color. Remove and electrolyze to the correct color. Orange-yellow anodes are not working.
Remove all loose parts from the bottom of the tank during down periods.
Use lead-7% tin anodes.
Use a rectifier that has more than 5% ripple
Use water containing chlorides for make and maintenance.
Allow current interruption. Interruption of current results in "white-out"
Use hard chrome for applications that mate it with soft materials, Excessive wear of the soft material can occur.
Ignore proprietary hard chrome processes. Several formulas are available that use proprietary catalyst and specific operating instructions. These systems offer formulas that are easy to use and control and can target specific properties such as high hardness, porous, cracked , smooth, non- cracked, lower internal stress and other specific characteristics to meet special situations and every day needs.
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