Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Microcrack density in hard chrome
Q. We are carrying out harding chrome plating on Internal combustion valves to thickness of 5-9 microns. We obtained a crack density of 661 cracks per linear cm. Could you please tell us the relation between crack density and the oil retention property.
Arumugam RKarmobiles Ltd - Peenya, Bangalore
2005
Can micro-crack density be too high?
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Q. We manufacture hard chrome plated rods for shock absorbers. We are having a corrosion issue and would like to know if there is a point where micro-crack density can be too high and contribute to corrosion rather than prevent. Our standard has always been 1500 cracks per inch minimum and we normally are in the 2500 to 3000 cracks per inch range.
Tim HamiltonProduct/Process Engineer shock rods - Pulaski, Tennessee, USA
December 13, 2008
A. Hi,
The more microcracks / not macrocracks the chromium deposit has, the better it will be to retain oil in use. But remember that many people do not recognize the difference about macro and microcracks.
Regards
Anders Sundman
4th Generation Surface Engineering
Consultant - Arvika,
Sweden
December 21, 2008
Q. Our corrosion issue is in salt spray testing so there is no oil on the rod or in the cracks. All of our testing, also by our chrome supplier, shows that we have micro cracks.
Timothy Hamilton [returning]- Pulaski, Tennessee, USA
January 26, 2009
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A. How thick is your coating? Generally to pass salt spray with chromium plating it needs to be at least 2 mils (50 microns) thick, or have an underlayer like nickel.  Lee Gearhart metallurgist - E. Aurora, New York January 28, 2009 A. Theoretically, the more cracks the better for they will spread the potential for corrosion over a larger area, thus, reducing the possibility of a large single point of severe attack. But, on the other hand, if you eliminate every crack and produce an impervious coating, it could protect even better. That is why electroless nickel has gained acceptance as a good substitute to chrome against wear and corrosion. When properly applied, it is crack and pore free. Guillermo MarrufoMonterrey, NL, Mexico January 28, 2009 |
A. Tim,
I have never had a lot of faith in hard chromium coatings imparting any significant level of corrosion protection. Even very heavy coatings of 4-6 mils will still corrode eventually, although the chances of a direct route to the basis metal reduces with thickness.
If you want a proper corrosion resistant coating you do need an undercoat of 0.5-2 mils of nickel.
I have seen too many problems with hard chromium being used as a corrosion resistant coating and spent many an hour recovering parts because of this belief.
Aerospace - Yeovil, Somerset, UK
January 30, 2009
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A. Tim, - Emmen, Netherlands January 30, 2009 A. Hi, Anders Sundman 4th Generation Surface Engineering Consultant - Arvika, Sweden January 30, 2009 |
August 19, 2012
Q. Here are a few questions:
1) After analysis I am getting 300-320 cracks per linear cm; I want result of 600-650 cracks per linear cm. (I did sound baking at 170 °C for 2 hours for increasing Microcracking; maintained Chromic, Sulphate, Temperature).
2) Copper content in my plant is on high side as per analysis, What can be done for it?
Dummy is of no use (already tried). Any other solution to reduce Cu (except porous pot)?
3) How to reduce Ripple Effect?
(As the plating draws DC current from Rectifier, which converts AC current to DC current. We had done observation of the same which comes to 5.3% at 1400 Amp out of 3000 Amp capacity of rectifier. 1000 Amp used for bath, out of 6.9% observed non converted DC power.) This leads to deterioration in all plating properties).
4) How to clean hole which is less than 1 mm, or how to prevent it from rusting? (I tried electrolytic cleaning, chromic etching, alkaline cleaner).
- Mumbai, Maharashtra
A. Hi,
You have too much ripple in the rectifier. A rectifier should use the Amperage they are built for. You say yours is for 3000 amps and you are plating at 1400 amps; that will cause ripple.
Regards
Anders Sundman
4th Generation Surface Engineering
Consultant - Arvika,
Sweden
October 3, 2012
A. If your power supply is single phase, you will have a problem with it!
For copper, why do you exclude porous pot? It is the most cost effective way to keep your chrome tank in great operating shape that there is.
An old ADV in the USA was a car mechanic saying "you can pay me now or you can pay me later". It referred to proper maintenance of the car. The same goes for your tank.
- Navarre, Florida
October 4, 2012
Q. Hi Ted and the community,
Is there a way to make a HEEF25 chrome plating tank that claims greater than 1000 cracks per linear inch into one with less than 500 cracks per linear inch?
Best regards
Mark
Industrial Chemist - Flippin' freezin' rock in the Irish Sea
February 13, 2018
A. Hi Mark. My knowledge of chrome plating chemistry is almost solely book knowledge, and most of the books largely predate HEEF25. But Dubpernell claims that chlorides greatly increase cracking, so maybe the addition of a little silver or dummying under the right conditions could reduce the crack density? Topic 56140 is a very good discussion of chlorides in HEEF25 baths and these treatments. Good luck.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
April 2018
Thanks Ted,
The customer who asked for this had failed to achieve the requirement themselves and were trying to pick our brains it turns out. When I contacted the HEEF25 supplier they said that in order to achieve this you would in effect have to contaminate the bath, you are trying to get the bath to run counter to its design. So such a request could not be fulfilled without destroying the basic nature of the bath, that if the low crack density was achieved, it could not be reversed and would render the bath useless for a return to normal high crack work.
So we will not be pursuing this further.
Best regards
Mark
- A cloudy rock in the irish sea
April 10, 2018
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