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Electroless nickel plating on cast iron parts
January 29, 2010
My name is George Ivanov and I'm heat treatment and coating technologist in company in Bulgaria. Our company manufactures hydraulic motors and steering units - www.ms-hydraulic.com . I suggest to my managers to put electroless nickel plating 20-25 microns onto our hydraulic motors.
After coating we carried out Salt Spray test (NSS - 400 hours) and the results were not so good. I have found that all steel and cast iron parts, which were machined (by turning or milling, they were perfect (without any corrosion). But the cast surfaces, which are not machined were coated with corrosion. Nickel plating coating is very uniform and without any cracks and defects.
That is my problem and I ask you to advise me :
1. How I can coat non-machined cast surfaces with nickel for corrosion protection ?
2. What do I have to do before coating to improve corrosion resistance of the nickel coating ?
engineer - Kazanlak, Bulgaria
January 30, 2010
Oxides from the casting process are thick and tenacious. An initial mechanical cleaning (sand blasting) before machining sounds like a good start. Also, find out what lubricants or coolants are being used during machining. Your pre-cleaners must be able to handle them. Good luck.Guillermo Marrufo
Monterrey, NL, Mexico
First of two simultaneous responses -- February 2, 2010
On the cast surface, which is not machined, there is no rust or oxides, it seems to be sand blasted after casting process, but I will try sand blasting or shot peening again. About lubricants and cooling liquids during machining we have been using semi-synthetic liquids - Houghton products and after machining we clean these parts by alkaline solution in spraying washing machines anad before assembling we have manganese phosphating process.
I find out that maybe roughness of casting non-machined surfaces seems to be most important reason for this problem.
Because where roughness is very high, there is rust on the nickel layer. I red in some articles that the roughness influences in opposite way, compared with painting process, where the adhesion is much better when the roughness is higher.
- Kazanlak, Bulgaria
Second of two simultaneous responses -- February 2, 2010
Some basic questions first:
What phosphorus content is in your Electroless nickel (low, e.g. 5% and below, medium, e.g. between 5 and 10% or high, e.g. 10-14%)
What are your cleaning stages prior to plating?
What does the salt spray failure look like (discreet failure points or a general covering of corrosion products)?
My first thought is that your thickness of nickel is not sufficient to withstand 400 hours of salt spray. I would suggest a minimum thickness of 0.002" (50 microns) of high phosphorus nickel.
aerospace - Yeovil, Somerset, United Kingdom
February 16, 2010
Sorry for this late answer, but I was in holiday last weeks.
About your questions:
1. We use middle phosphorus(5%)nickel plating process
2. prior stage of plating is alkaline degreasing by immersing - temperature 60°C
3. The rust looks like as many spots and small lines, there is no full surface coverage on the cast part. It mean that the rusted surface is about 15%.
Thank you for your advice, but we make this process in other company I have to ask this company to implement your suggestion.
If they couldn't make this process what is your advice ?
- Kazanak, Bulgaria
February 17, 2010
My first advice would be to change to a high phosphorus coating (10-14% P). Lower than this means that you may not get the desired level of corrosion resistance.
Secondly, I would certainly consider using a thickness of 50 microns if want a good corrosion resistant coating.
There is one other thing you could consider doing (but don't do this if you need to be RoHS compliant) and that is to carry out a 5% chromic acid wash (CrO3). This will impart a certain amount of corrosion resistance, not a great amount, but it might be the difference between passing and failing.
Finally, your description of the failure mode certainly indicates to me that the failure is more likely due to thickness issues rather than pre-treatment issues. I think that your pre-treatment is probably sufficient for what you are doing, although the surface can never be "too clean".
Hope this helps on a way forward.
aerospace - Yeovil, Somerset, United Kingdom