Cavitation

[Joris Kraak]

What is the best coating to resist cavitation?!

Currently WcCoCr coatings are used since there is a combination of heavy abrassive wear and cavitation. I am not sure if this is the best solution.

The hardest the better is not alway the solution.

[Gordon]

Hi Joris

to the Surface Engineering Forum.

Quote:What is the best coating to resist cavitation?!

Good question, unfortunately not easy to answer.

Generally it is recommended that a coating should be harder than the original surface that suffered cavitation, but should also be tough, ideally with ability to work harden. Coatings that are good with fretting wear involving surface fatigue are said to be good with cavitation.

Quote:Currently WcCoCr coatings are used since there is a combination of heavy abrassive wear and cavitation. I am not sure if this is the best solution.

The hardest the better is not alway the solution.

I think you very rarely get a situation where the only problem is cavitation. It will almost always go hand in hand with corrosion, erosion, abrasion etc. So, I think when making a coating selection, it is best to start by elimination of coating types not suitable to the other environmental factors like corrosion and erosion first. Narrow the choice down and then select for likely cavitation resistance. Hardness, toughness, work hardening properties (trouble is these properties do not usually go hand in hand) along with all the other considerations like ease of application, finishing, economics etc... Best of all feedback and past experience from field usage.

WcCoCr coatings have become a bit of a first choice favourite for many applications and in many cases deservedly so. Is it specifically good at resisting cavitation or is it just a good all-rounder?

[Vijay Deshpande]

Dear Joris,
There was a thought in between that polymer coatings which will absorb the shock of exploding water bubbles will reduce cavitation problem.But I do not know further developments.
Please coment.
Regards
Vijay Deshpande

(08-13-2011 02:37 PM)Gordon Wrote:  Hi Joris

to the Surface Engineering Forum.

Quote:What is the best coating to resist cavitation?!

Good question, unfortunately not easy to answer.

Generally it is recommended that a coating should be harder than the original surface that suffered cavitation, but should also be tough, ideally with ability to work harden. Coatings that are good with fretting wear involving surface fatigue are said to be good with cavitation.

Quote:Currently WcCoCr coatings are used since there is a combination of heavy abrassive wear and cavitation. I am not sure if this is the best solution.

The hardest the better is not alway the solution.

I think you very rarely get a situation where the only problem is cavitation. It will almost always go hand in hand with corrosion, erosion, abrasion etc. So, I think when making a coating selection, it is best to start by elimination of coating types not suitable to the other environmental factors like corrosion and erosion first. Narrow the choice down and then select for likely cavitation resistance. Hardness, toughness, work hardening properties (trouble is these properties do not usually go hand in hand) along with all the other considerations like ease of application, finishing, economics etc... Best of all feedback and past experience from field usage.

WcCoCr coatings have become a bit of a first choice favourite for many applications and in many cases deservedly so. Is it specifically good at resisting cavitation or is it just a good all-rounder?

[jasonpbond]

Vijay has a good point. And harder is not always better. In fact in recent years we have complemented metal spraying with a wide range of polyurethane coatings to resist corrosion, abrasion, erosion. Apart from the obvious cost advantages, these coatings can also have advantages in weight reduction compared to metallic coatings. Different types of urethanes are designed for different hardness and different properties - e.g. impact resistance, hydrophobicity (corrosion resistance), sliding particle wear resistance. The results in the last few years of these coatings has in some cases been similar or even better than some metal spray coatings.

Of course polymer coatings do have a few disadvantages - the largest being the operating temperature range - significantly lower than most metal spray applications!

(08-14-2011 07:23 AM)vijaydeshin Wrote:  Dear Joris,
There was a thought in between that polymer coatings which will absorb the shock of exploding water bubbles will reduce cavitation problem.But I do not know further developments.
Please coment.
Regards
Vijay Deshpande

(08-13-2011 02:37 PM)Gordon Wrote:  Hi Joris

to the Surface Engineering Forum.

Quote:What is the best coating to resist cavitation?!

Good question, unfortunately not easy to answer.

Generally it is recommended that a coating should be harder than the original surface that suffered cavitation, but should also be tough, ideally with ability to work harden. Coatings that are good with fretting wear involving surface fatigue are said to be good with cavitation.

Quote:Currently WcCoCr coatings are used since there is a combination of heavy abrassive wear and cavitation. I am not sure if this is the best solution.

The hardest the better is not alway the solution.

I think you very rarely get a situation where the only problem is cavitation. It will almost always go hand in hand with corrosion, erosion, abrasion etc. So, I think when making a coating selection, it is best to start by elimination of coating types not suitable to the other environmental factors like corrosion and erosion first. Narrow the choice down and then select for likely cavitation resistance. Hardness, toughness, work hardening properties (trouble is these properties do not usually go hand in hand) along with all the other considerations like ease of application, finishing, economics etc... Best of all feedback and past experience from field usage.

WcCoCr coatings have become a bit of a first choice favourite for many applications and in many cases deservedly so. Is it specifically good at resisting cavitation or is it just a good all-rounder?

[MichiganMan]

Cavitation is a tricky devil. Hydroelectric turbines have had Stellite alloys applied with good success. I have had good success in some centrifugal pump applications with WC-Co-Cr. I suppose the nature of the cavitation and the operating system would have some bearing on the ideal coating for the application that you are considering. One thing that is often lost in cavitation applications is that the cavitation can sometimes (I'd even say often) be eliminated by making a minor change in operating parameters of the system (for example in the case of centrifugal pumps look for partially closed valves or plugged filters on the suction side of the pump). Even a minor change that partially mitigates cavitation can make a major difference in time between maintenance because the forces generated are so severe.

[a.termika.ltd]

(08-13-2011 06:56 AM)Joris Kraak Wrote:  What is the best coating to resist cavitation?!

Currently WcCoCr coatings are used since there is a combination of heavy abrassive wear and cavitation. I am not sure if this is the best solution.

The hardest the better is not alway the solution.

Dear Joris
From our experience better of all Self-fluxing coatings NiCrBSi possible add WC
Rgrds
Dr.Boris A.Termika Ltd

[Joris Kraak]

Dear all,

Many thanks for all the answers. Cavitation is indeed a tricky bastard!
Vijay, good comment, I have been studying these polymer coatings too but in the end they will wear out and the cavitation attack continues, They also cause some balancing problems so it is not the ideal solution.

[Vijay Deshpande]

Dear Joris,
I have seen the state of hydro-turbine runners, guide vanes reduced to razor blade thin due to silt erosion in one of the hydro power station in Himalaya Range. The same phenomena is not there in Sahyadri Range. In one of the Mumbai's Pumping station they have installed a compressed air line in suction side to reduce cavitation.
Regards.
Vijay

[pifalert]

Maybe this article will help (link dead) let me know what you think.

Alex

[Vijay Deshpande]

Dear All,
Is there any rough estimate that cavitation and silt erosion costs Hydro Power Industry
Vijay