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Ted Mooney, P.E. RET
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Cast Iron Dies Won't Re-Chrome
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Q. We recently received a press part die from our customer to be hard chrome plated. The die had existing chrome plating on it, so we de-chromed it using Hydrochloric acid. Due to higher concentration minor pitting has occurred on the casting and it turned blackish.
We have tried to plate is multiple times now, however hard chrome is being deposited on high current density areas (10% of the entire die). What can be done to revive the die and re-chrome plate the same.
Kalpesh Dagliya- Mahasrashtra Pune
May 7, 2024
for Shops, Specifiers, & Engineers
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"Hard chromium plating: A Handbook of Modern Practice"
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"A Chromium Plating Bath With The Fluoride Ion"
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A. Hi Kalpesh.
You probably know 10X as much about hard chrome plating as I do, but without understanding what you know and what you don't know, I don't want to miss the obvious :-)
You do already realize that it is entirely normal for chrome to deposit only at high current density? Are you able to estimate the current density on the areas that are not plating?
Is it possible that these components simply have more surface area than other items you plate and your rectifiers cannot provide sufficient current to reach the necessary threshold in the low current density areas?
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
A. Kalpesh,
Check chrome tank concentration and temperature and rectifier.
Analyze chrome thickness at dark spot against bright spot.
Maybe need to change rack design or too many parts on rack-reduce parts on rack.
Popatbhai B. Patel
electroplating consultant - Roseville, Michigan
Q. Hi,
So nice to hear from you almost immediately. Yes, I am aware that plating chances are more on HCD areas. However generally we are able to plate around 95 percent of the dies with some female radius left without plating. In this case, because of over degradation of the casting due to high exposure in hcl, some inherent properties of the casting have changed which is giving us issues. The rectifiers are of 15000 amps, so I am pretty sure that's not a problem. Sharing some dies that we have chrome plated recently for your reference.
Now let's talk about how to prevent this going forward. Do you know a better solution to strip chrome rather than using hcl. If we use hcl, what is the concentration that should be used so that it does not affect the die casting.
One of my operators had an interesting observation. When we are stripping the chrome off a casting using low concentration HCl, we have to increase the time required to strip the chrome. But this is more dangerous as low concentration of HCl also affect the inherent base material faster.
Instead when higher concentration when used, strips the chrome plating faster and there is less damage to the casting as the time of exposure is minimized.
I still have to test this hypothesis, but could you validate or add your valuable inputs on this ?
- Maharashtra India
May 8, 2024
A. Hi again,
I think most shops strip hard chrome from iron and steel in an anodic alkaline stripping tank (essentially an anodic electrocleaner). This has the disadvantage that it creates hexavalent chromium, but the advantage that it does not etch the iron or steel. What is your experience with that?
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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