Chromium Oxide Coating - Surface Finish Ra values

[Ian Mc]

Afternoon to all, I have read through many previous threads and have been able to gather most of the information I need but still have a couple of queries I hope someone might be able to assist with - My questions centre around the achievable surface finish of a Chromium Oxide Coating on a 316L (1.4404) or Duplex (1.4462) turned surface. We are having a min thickness of 0.15mm applied to a substrate with a surface of 0.1-0.12um. We need to have a finished surface of 0.1um or better post coating - Q1 is this possible ? - Q2 Is there an 'industry std' method for taking Ra readings on a dia coating ? - Q3 We have the surface burnished with PTFE stick post treatment to seal, should we consider other methods ?.... FYI application is for Food/Drug machine applications so this is No.1 consideration - Would appreciate any thoughts or comments and thanks for taking time to read my questions

[djewell]

Q1. You will have a hard time achieving a 0.1 um finish on a chrome oxide coating. How porous is your coating?

Q2. I have a hand held profilometer which is small enough to sit on the surface of a cylindincal part and take measurements. The software can compensate for the curvature when calculating surface roughness.

Q3. I would definitely look at another way of sealing the coating. Burnishing cannot get any sealant into the coating to seal it. Whatever material is applied to your coating remains on the surface. You should look at ways to get the sealant all the way into the coating to fill the porosity. There are many liquid sealers on the market. You should be able to find one suitable for pharmaceutical applications.

[Gordon]

Hi HVoilF

Have made a new thread for your post repair -to-hd-vertical-pumps as it is off-topic in this thread.

Hello T/S colleagues,

we do need to repair HD vertical pumps(1200HP). The fluid being pumped is a salt water slurry mixed with heavy oil and sand so the abrasion is an important technical factor to be considered. The OEM pump has duplex(stainless) shafts and carbon bearing and obviously the pump must be serviced every 4 to 6 months to replace the weared bearing and the shafts. So our doubt is ,if in this working condition, being the bearing water cooled and lubricated ,could we manufacture stainless steel bearings and coat them in it I.D. with HVOF Wc Co Cr as well as the shaft in the bearing working area,resulting in a mating Wc/Wc pair bearing-shaft without any adhesion (galling) between the sprayed Wc coating. Would it work? Is it a good mating pair the Wc against Wc material? Any suggestion is welcome. Thanks

[Gordon]

Hi Ian

to the Surface Engineering Forum.

Generally agree with djewell post. With high quality dense chromium oxide coatings of 0.1 micrometre Ra and below are possible with effort using super-finishing and lapping techniques.

[Ian Mc]

Thanks for the welcome Gordon and thanks also to djewell for your informative response.
You have really confirmed my comcerns that we are chasing nirvana on the 0.1 Ra or better, when I view the surface using x10 loop it is like looking at a tarmac road.
Ref: post treatment (polish/lapping) I assume I must use special compounds due to brittle nature of coating, has anyone found a reliable way to do this ?
Ref: sealing - can anyone point me towards suitable products or treatments that would comply with FDA regs.
Thanks once again for your input, all info greatfully received.

[Gordon]

Hi Ian

Ref: post treatment (polish/lapping) I assume I must use special compounds due to brittle nature of coating, has anyone found a reliable way to do this ?

This may be helpful: (link dead)

As with finishing technique, the quality/type of chromium oxide coating will be very important.

Ref: sealing - can anyone point me towards suitable products or treatments that would comply with FDA regs.

There are a number of commonly used sealers based on wax, phenolic resins, linseed oil, epoxy resins and others, but as to complying with FDA regs, I'm sorry I don't know.