Black spots after metal spray

[turbo]

Dear members,

We are rotary equipment service center in Karachi, Pakistan and in the process of adding thermal coating facility in our workshop. We have no previous experience of metal spray. To start with we have ordered one combustion wire 16E gun but not received yet. In the meantime we found one old 10E gun and also combustion wires Metcoalloy 405 and 2, to get some experience, we tried to carryout refurbishment of 3 wornout shafts. For surface preparation, we carryout grit blasting and sprayed 405 bond wire and than top coat using Metcoalloy 2. After grinding and polishing, we found minute black spots on the whole sprayed area and want to know the reason of these black spots. We used acetylene and oxygen gases. Compressed air used was not 100% moisture free as air drier was not working properly at the time of spray.

Further we want to know what test equipment we must have to check the bond strength and quality control.

Hope to receive technical support from the members.

Best regards,
Zubair Ghani
Email: turbo@cyber.net.pk
rotorbalancing@gmail.com

[KevGarty]

It depends what you mean by black spots, it could be porosity(small holes) in the coating due to wrong parameters.

Check have your gas flow set correctly, wire speed and spray distance etc

How much bond coat did you apply and how much top coat could it be the case that you are breaking through the top coat into the bond coat ?

Kev

[Gordon]

Hi turbo

It depends what you mean by black spots, it could be porosity(small holes) in the coating due to wrong parameters.

The coating will naturally contain a relatively large amount of porosity and oxide inclusions, so its possible you are looking at the normal finish for this coating. It will not finish as brightly as a wrought steel.

If what you are describing is worse than a known good coating of the same type, then I agree with Kev's comments. Your machining technique may also effect the way the finish appears.

Further we want to know what test equipment we must have to check the bond strength and quality control.

Well, it would be good to have a laboratory set-up with metallography, hardness, tensile testing etc. and not forgetting well trained and skilled staff to operate it. This type of quality control does not come cheap. Getting happy, skilled observant operators committed to producing quality work and checking/maintaining parameters/equipment is definitely a must have