ChromeOxide coating

[raviswanadha]

Dear all,
We have recently replaced a chrome plating application with chrome oxide.
The application is coating on PISTONS of "Airless spray painting machines".
The pump is being used to spray coal tar epoxy. We found that the coal tar epoxy is sticking to the surface of the piston and creating trouble.
What is the best solution to this application. Earlier chrome plating did not create any problem. Will chrome carbide nickel chrome coating by HVOF work better than chrome oxide. Please advice.
regards
Ravi

[Simon Kim]

Hello Ravi,

I have been developed Chrome carbide coating for replace to chrome plating.

In case of high temperature erosion, CrC coating is better than WC coating.

Because of NiCr alloy, CrC binder metal, is stronger at high temperature.

And NiCr alloy has excellent corrosion resistance.

However at room temp. or under 300?, WC has better wear resistance.

If you need corrosion resistance, could choose Co-Cr alloy for binder metal.

You expected just same level of Chrome plating's wear resistance, choose WC17Co.

It will bring you good result.

In my experience, CrC coating is more sensitive and difficult to WC coating.

Maybe the reason is difference of solidification between binder metal and carbide's thermal characteristics, especially heat capacity and thermal conductivity.
Cobalt has very wide adhesive temperature range, and good strength and low coating stress.
So cobalt is very easy coating material.


I wish your success.

Simon

[Stephen Booth]

Hi Ravi

In my experience from the field, is that the surface condition of the coating might be the critical factor in the sticking of the TAR on the plungers. If this is indeed the critical issue, then many factors are brought into play, like porosity, and surface finish.

You might try the following:

1. apply teflon sealant on the chrome oxide
   
2. improve the surface finish of the chrome oxide by reducing porosity and better grinding
   
3. Switch to spray fuse coating of NiBSiCr material at HRc 60 plus
   

Hello Ravi,

I have been developed Chrome carbide coating for replace to chrome plating.

In case of high temperature erosion, CrC coating is better than WC coating.

Because of NiCr alloy, CrC binder metal, is stronger at high temperature.

And NiCr alloy has excellent corrosion resistance.

However at room temp. or under 300?, WC has better wear resistance.

If you need corrosion resistance, could choose Co-Cr alloy for binder metal.

You expected just same level of Chrome plating's wear resistance, choose WC17Co.

It will bring you good result.

In my experience, CrC coating is more sensitive and difficult to WC coating.

Maybe the reason is difference of solidification between binder metal and carbide's thermal characteristics, especially heat capacity and thermal conductivity.
Cobalt has very wide adhesive temperature range, and good strength and low coating stress.
So cobalt is very easy coating material.


I wish your success.

Simon

[MichiganMan]

Your issue is more than likely as Mr. Booth indicates a surface finish issue. Depending on the epoxy material that you are pumping it may bond tenaciously to any surface with a profile 1.5 mil (.0015") or greater. Grinding to a fine tolerance and addition of a sealer should improve the performance.

NOTE: Be very careful of your sealer selection. Many epoxies contain "hot" solvents that can be detrimental to sealers or have cleaning procedures that require aggressive solvents. In an ideal situation these Airless Spray Pumps will only be exposed to one paint, but many airless spray technologies are used by field contractors using a wide variety of coating products. I would suggest talking to your client VERY CAREFULLY about how the pumps are used and maintained, because in addition to the solvented coatings themselves there is very likely a cleaning procedure used where the system is purged with concentrated solvents like toluene or MEK or potentially even something as nasty as Methylene Chloride.