Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)
by Randy Taylor, Advanced Tooling Corporation.
Micro Cracks in Hard Chrome
Q. How will the number of micro cracks per inch effect the properties of hard chrome plating?
A. Yes, the number of "cracks" in a given distance affects the properties of chrome plating. Almost all electroplated hard chromium deposits are cracked. Cracking occurs during the plating cycle when internal stress exceeds the tensile strength of the chromium, which is hard and brittle. The width, depth and population density of these microcracks varies widely and is influenced by the following:
the type of plating chemistry used (single-catalyst, mixed catalyst, proprietary),
chromic acid concentration,
type and concentration of catalyst,
concentration of bath impurities (iron, copper, zinc, nickel, trivalent
chromium deposit thickness
surface condition of substrate
Generally speaking, a microcrack structure which is comprised of a high population density of narrow, shallow cracks is desirable, because the deposit tends to have a lower stress, higher lubricity, good wearability and better corrosion resistance.
If the conditions during plating cause the cracks to be coarse in nature, they are often referred to as macro-cracks, which may be visible to the naked eye. Usually, chromium with this type of microstructure exhibits less desirable properties in service. It should be noted that macro-cracking can occur in chromium deposited over any type of substrate, not just those that are already stressed in tensile.