STELLITE OR ELECTRO NICKEL PLATED 10

[GabrieleB]

I note that on globe and check valve the use of stellite as hard facing and corrosion resistant layer on trim parts is the best solution for many vendors.
For ball valve I see that a lot of vendor use carbon steel with Electro Nickel Plated as trim material as corrosion and hard facing layer.
Someone knows the reason of this distinction. Which is the main difference beetween this two surface treatment.

 

[rneill]

Stellite hardfacing consists of a weld overlay (so a very thick coating) of cobalt or some variation thereof whereas ENC/ENP is just a very thin coating of nickel (between 1 and 3 mils) that is applied to the ball valve ball.

It would be very expensive to provide a weld overlay on a ball valve ball and while I think there might be one or two vendors who offer weld overlays (I think Grove offers a 625 weld overlay as an option?), I've never actually seen or purchased one.

ENC/ENP coatings will be okay in many services but because they are such a thin coating, they do have a tendancy to develop scratches or other minor damage which then results in corrosion creeping back under the coating causing further damage. This usually leads to the valve weeping and allowing some product to pass when closed. You won't get 100% shutoff.

My general rule is to not accept ENC/ENP coatings for critical valves (or in sour service) but to accept them in more routine or low risk applications. This is a personal preference and many people routinely specify them for almost all service conditions. It may have something to do with whether you have to deal with the consequences or whether you're just executing the project and handing off

The usual preference for ball valves, something you didn't mention, is to purchase with solid stainless steel balls. This provides good corrosion resistance and performance without the risk of scratches or other minor damage leading to more extensive problems. Ball valves with solid stainless balls are commonly available and in fact may be the default product and less expensive product in some sizes and designs (e.g., small floating ball valves).

Attached is a link to a document with a good description of stellite hardfacing:

http://www.stellite.co.uk/Portals/0/Guidelines for hardfacing onto steels.pdf

For further information on ENC/ENP you could look at ASTM B733 or NACE 6A287.

I've attached a link to a picture of an ENC/ENP ball with corrosion damage.

http://files.engineering.com/getfil...6-a4e0-4d3170d402c0&file=ENC-Failure-Corr.jpg

 

[bcd]

The main difference is cost.

ENP is cheap as compared to Stellite. If ENP is good enough for the application conditions, then use it. If high temperature, if corrosive media that attacks ENP, or heavy metal-to-metal contact that must slide with minimal wear ans shearing of the protective layer, then Stellite.

 

[rneill]

We have seen scratches and other damage on ENC coatings where there is no metal to metal contact (valves with soft seats) but where there are small amounts of abrasive materials in the production stream (e.g., oil and gas production pipelines) which then grind into the ball during closure.

 

[JimCasey]

Yep, I agree with everybody on this one. Additionally I will offer that ENP has a low coefficient of friction and helps to keep the valve torque low. I'd probably ask for ENP to be applied over stellite, with the stellite being there to provide for burly damage resistance, and the ENP to give the nice slick finish.

 

[GabrieleB]

All your answers are clear and confirm what I expected about the argument.
Is clear that the ENP film have a partial action against the corrosion and that for high corrosive environment a thin thickness of 50-75 micron is not sufficient and that in some application is better to use inox or duplex material.
Thank you for your collaboration.

 

[gerhardl]

A late comment, as an addon:

A certain factory can and will do and have supplied a welded on overlay on ball surface for SS ballvalves class 1500 ANSI, 8" to my knowledge, to satisfy North-sea sour service.

At the moment I am not sure if this is upheld to recent times, and I am not sure of overlay material, but seem to remember this is stellite.

Obviously the weld process is very specialized and have to be checked 'all way, before the finalized surfacing process, but ending up cheaper than exotic materials.

 

[sanjay300864]

My submission is as follows:-
Stelliting is used for getting harder seating surfaces.
ENP is used as cost saving to substitute solid stainless balls of ball valve by CS-ENP balls.

 

[steamdog]

One more note, the major control valve manufacturer I work with does not recommend stellite when there are amines present (like in boiler feedwater), as the amines degrade the stellite.

 

[Sweguy]

Hi

Where I work we have Crome as standard, Stellite is rather common, We also use Chemical nickel plating when the valve is made of some special materials.

The stellite trims are usually casted stellite, on bigger sizes we weld the stellite.

We use Chemical nickel plating when the pH is low.
Chrome can`t be used for low pH applications.

Stellite works fine for all pH values that we have come uppon.

All seatrings has welded stellite.

Regarding scratches
Stellite
Is the softest of these three materials but it has a thick layer when it is welded and this makes it very reliable.

Chrome
The structure of chrome can be described as many needles that are standing beside each other, each needle is bounded to the ground material. An injury will not automaticly get larger.

Chemical nickle plating
The structure can be described as a thin foil that are surrounding the ground material. If there will be any damage here the rest of the foil will easily disappear. It is very important that the Chemcial nickel plating process works perfectly otherwise there will be an dissaster.

 

[JLSeagull]

Many valve companies avoid the trade names, instead prefering the generic UNS designations. Stellite is a trade name that applies to the Deloro Stellite family of alloy materials including cobalt and nickel alloys. Stellite alloy 6 is probably the most commonly used alloy for hard facing valves. A dozen other Stellite cobalt alloys exist plus several stainless, tungsten and chrome materials. Consider more specific descriptions of the hard facing materials.

OK, I got off my high horse. ...

 

[302Hugo]

I have a question regarding ENC plated and SS trim. Why do most specifications call for SS trim in low temperature applications instead of ENC plated?

 

[GabrieleB]

In my opinion SS material, mainly austenitic stainless steel as AISI 316, has a good behavior at low temperature. In fact austenitic SS is used in many low temperature application, beacuse of its high tenacity and low hardness.
Is not the same for martensitic SS as AISI 410 that has high hardness and strenght, so no good behavior at low temperature.
So ENC is harder than SS (austenitic) and for low temperature application is not recommendable, I preferring SS (as AISI 316) because of its tenacity.

 

[nctexan22]

What kind of reduction in coefficient of friction can be achieved with the use of ENP against stainless?

 

[unclesyd]

Here is general description of EN.

http://www.thelindgrengroup.com/Benefits_electroless_nickel_plating.pdf

One thing to remember is that most of the Stellite Alloys are not very hard. Their claim to fame is anti galling and anti-wear under very sever conditions.

As been alluded to above it is very hard to compare them head to head as they are used under different conditions.