Chrome Oxide Coating Failure

[M Brecker]

We are looking at a Chrome oxide (Metco 106F) coating with a nickel bond coat and a 316 stainless base. Two 1/4" circular areas of the coating broke off after the part was in use for 6000 hrs. We did metallography and found that:
The coating broke off at the interface between the 316 stainless base metal and the nickel bond coat.

The nickel bond coat was very uneven and didn't cover the base metal completely. It appeared thicker in the valleys of the surface and thinner at the peaks.

The 316 base metal was chilled cast iron grit blasted. The stainless surface had what looks like laps where the surface was pushed back on itself. The surface also had what looked like mechanical twinning from the grit blasting.

Would any of these issues affect adhesion? Anything else we should be looking for. Thanks very much!

[Gordon]

Hi M Brecker

to the Surface Engineering Forum.

A few questions:

"a nickel bond coat" is this pure nickel or maybe NiAl or NiCr?

What are the service conditions for the coating?

Can you tell if coating failed purely mechanically or via corrosion?

Was the coating sealed?

Can you show us metallography images?

[M Brecker]

is this pure nickel or maybe NiAl or NiCr?
It's NiAl.

What are the service conditions for the coating?

The coating is on a charging pump plunger that moves vertically. The problem is in the area where the plunger is in contact with rubber seals.

Can you tell if coating failed purely mechanically or via corrosion?

I didn't see any signs of corrosion or discoloration however I couldn't rule it out.

Was the coating sealed?
No sealant was used.

Can you show us metallography images?

Yes, I am attaching 2 photos of the defect and 1 cross section. The cross section has are pullouts and cracking that are artifacts from the polishing.

Thanks!            

[Gordon]

Hi M Brecker

I would avoid drawing to many conclusions from the metallography, as these coating are difficult to prepare and some of the cracks/layer separation may be induced or made worse by metallographic sample preparation. The interface between substrate and bond coat on the first image is possibly not that bad.

By shape and size of failure my gut feeling is that it is caused by crevice corrosion or mechanical dints (rough handling). Crevice corrosion can be a problem with stainless steel substrates covered with ceramic. My advice here would be to seal coating after spraying and then to seal again after grinding (making sure cutting fluids are removed and coating is dry). I would also consider using a NiCr alloy base coat instead of NiAl or better still a HVOF applied hastelloy C or inconel 625 type coating. These may not provide the same level of bond strengths, but should provide a much better buffer to crevice corrosion.

Is there any evidence of coating blisters, or slightly raised coating areas on the rest of the surface? or possibly slight recesses/dents indicating mechanical damage? Wear marks/polishing effects/cracks may highlight these areas.

Hope this gives a few pointers in your failure investigation, good luck

[KevinT]

Hello MBecker,

I am in total agreement with Gordon.

The photo of your pump plunger appears to have mechanical damage, however the spalled coating may have occurred from simple disbondment.

The working environment such as temprature, chemistry, hours of use, is very important to analyzing this problerm.

The Mechanical bond strength of the flame sprayed ceramic is not as strong if not sealed. There is a chrome oxide ceramic that is thermo-cheimcally bonded as opposed to the mechanically bonded flame spray coating.

Look for http://www.bodycote.com. On this website query for KTech ceramics. There is a facility in the UK. Should you have problems with that try http://www.ceramiccoatings.com which is the website for Ktech in North America. Ktech can solve this problem for you. We do it everyday.

[M Brecker]

This is great information. I appreciate it very much. I understand the the sealant penetrates through the porosity in the coating. Our chrome oxide is machined to specific dimensions. Won't most of the sealant be removed? How does a soft sealant provide strength in wear type applications?

[KevinT]

(03-10-2009 04:29 PM)M Brecker Wrote:  This is great information. I appreciate it very much. I understand the the sealant penetrates through the porosity in the coating. Our chrome oxide is machined to specific dimensions. Won't most of the sealant be removed? How does a soft sealant provide strength in wear type applications?

MBecker,
The saelant is applied to the flame srpayed coating , in the "AS SPAYED" condition, or prior to the grinding operation. This allows for the maximum sealing capabilities of the ceramic solution. After grinding to size the ceramic solution is applied 2 to 3 more applications. If you grind the part first you take a huge chance of smearing the coating, thus covering porosity or cracks preventing the ceramic solution form penetrating and sealing the flame sprayed coating. As the coating wears, this porosity can be opened to the environment and the failure process starts again.

Specifc to your question the sealant provides minimum wear characteristics in relationship to the flame sprayed coating. The sealant main purpose is to prevent a corrosive environment from penetrating into the coating and attacking the substrate, which ultimately undermines the flame sprayed coating and then failure occurrs. Secondly, you will recognize an increase in bond strength as the sealant will bond to the flame Sprayed coating as well as the substrate. You will recogognize a reduction in friction. Tests on Tungsten carbide have shown a 30% reduction in weight loss in ASTM G 65 wear testing. So there is some wear protection.

Kevin.tackett@bodycote.com. Send me an email anytime. I like to help.

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(03-11-2009 01:36 PM)KevinT Wrote:  

(03-10-2009 04:29 PM)M Brecker Wrote:  This is great information. I appreciate it very much. I understand the the sealant penetrates through the porosity in the coating. Our chrome oxide is machined to specific dimensions. Won't most of the sealant be removed? How does a soft sealant provide strength in wear type applications?

MBecker,
The saelant is applied to the flame srpayed coating , in the "AS SPAYED" condition, or prior to the grinding operation. This allows for the maximum sealing capabilities of the ceramic solution. After grinding to size the ceramic solution is applied 2 to 3 more applications. If you grind the part first you take a huge chance of smearing the coating, thus covering porosity or cracks preventing the ceramic solution form penetrating and sealing the flame sprayed coating. As the coating wears, this porosity can be opened to the environment and the failure process starts again.

Specifc to your question the sealant provides minimum wear characteristics in relationship to the flame sprayed coating. The sealant main purpose is to prevent a corrosive environment from penetrating into the coating and attacking the substrate, which ultimately undermines the flame sprayed coating and then failure occurrs. Secondly, you will recognize an increase in bond strength as the sealant will bond to the flame Sprayed coating as well as the substrate. You will recogognize a reduction in friction. Tests on Tungsten carbide have shown a 30% reduction in weight loss in ASTM G 65 wear testing. So there is some wear protection.

Kevin.tackett@bodycote.com. Send me an email anytime. I like to help.

PS. I forgot to add the most important part. The sealant has a Microhardness value of 2850 HV and a macro hardness in the 1000 to 1500 HV range. This is not a soft sealant as some use. Also, this sealant can and is working in temperatures +2000F with no deleterious results