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, chromium-to-catalyst ratio, plating current-density, bath temperature, concentration of bath impurities (iron, copper, zinc, nickel, trivalent chromium, etc.) 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.
How much chromic acid will be consumed to deposit say one gram of chromium or if you can tell me how to calculate it?
To calculate this, look at the molecular weight of chromic acid (CrO3). It is 100 and 52% is chromium. So, 1 gram/(0.52) = 1.92 grams of chromic acid to plate 1 gram of chromium.
Q3. What is thin dense chrome plating? What I have gathered is that it is a thickness of 5 to 7 microns which is the thickness before the internal stress in plating builds up to form micro-cracks and hence has extremely good anti-crrosive properties and hardness. What is thick dense chrome? Is the bath composition different from conventional bath?
A3. You are generally correct in your assumptions regarding "thin dense hard chrome". It is uniform, highly corrosion resistant, hard, low fatigue layer of chromium, used most commonly on copper alloys, low alloy steels and stainless steels.
Thin Dense Hard Chrome - Provides a thin chromium coating with no cracks and improved fatigue life. Thin dense Hard chrome thickness ranges from 0.0001 to as much as 0.0008.
The process is very similar to standard hard chrome plating with the exception the bath requires the addition of a catalyst additive (most commonly crack-free plating salts) and operates at an elevated temperature (generally 145 - 150F)
Agitation is important to keep the bath properly mixed. Racks, anodes and tooling are similar to standard hard chrome. No grinding of excess plating is permitted in this type of coating.
A successful process is determined by the same test methods common to standard hard chromium coatings, i.e., visual, thickness, adhesion, ferroxyl and embrittlement testing. There is an additional visual test under 150X magnification to determine the absence or presence of cracking characteristics.
I hope this helps answer your questions.