Electroles Nickel Plating Is it prohibitted in H2S service

[bmoorthy]

We have many valve stems that are plated with Electroless nickel plating.

I was given to understand by one of th reputed companies that Electroless nickel plating is not suitable when the fluid contains NACE levels of H2s.

Is it correct, if so what should be the alternative plating one should consider.

Firstly why would the valve manufacturer perform plating on a High alloy stem especialy Inconel and Incolloy and 316 material?

 

[Sircrashalot]

Manufacturer's that I have used do this to:
1) improve the surface finish of the stem (better sealing and increased seal life)
2) increase the surface hardness of the stem to minimize the potential for galling with other like materials in the gland or ball.

I've had this coating in NACE qualified valves too I believe.

Andy

 

[vpl]

It seems to me that, since a company representative told you that Electroless nickel plating is not suitable, he should be able to provide you with the reasons why, the pertinent code requirements and a suitable replacement material.

Then you'd be able to contact your current valve vendor and ask his opinion. Or you'd be able to check the code requirements.


Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[Ashereng]

I just bought a few trunnion ball valves with 3mil ENP on CS on the trim. The valve complies with NACE MR0175-2002.

To the best of my knowledge, ENP by itself may be used in NACE required service.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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[kenvlach]

A general article on electroless nickel plating says:
"Electroless nickel..Due to its exceptional corrosion resistance and high hardness, the process finds wide application on items such as valves, pump parts etc., to enhance the life of components exposed to severe conditions of service, particularly in the oil field and marine sector...High phosphorous electroless nickel is standard in industries that require resistance to strongly acidic corrosive environments like oil drilling and coal mining."

http://www.corrosion-doctors.org/MetalCoatings/Electroless.htm

High phosphorus electroless nickel is 9 or 10 to 14 wt% P.

ASTM B733, 'Standard Specification for Autocatalytic (Electroless) Nickel-Phosphorus Coatings on Metal,' describes SC4 Severe Service as including "acid solutions, elevated temperature and pressure, hydrogen sulfide and carbon dioxide oil service..."
The appropriate EN thickness based on service condition is 75 [μ]m.

There are some limitations on EN involving heating:
1) Crystallization and precipitation shrinkage. The EN deposits amorphously and upon heating, crystallization and Ni₃P precipitation occur. The heat treatment is often done intentionally for increased hardness & better adhesion. But, it results in an EN volumetric decrease of 1-3%, which can result in cracking & reduced corrosion resistance. Much less a problem on convex surfaces such as valve stems (think of EN as heat shrink tubing). The crystallization temperature varies with %P but usually > 200 ^oC. Thicker coatings (> 25 [μ]m) should be used where heated coatings are exposed to corrosive environments.

2) S-containing eutectic at 637 ^oC. For all types of nickel plating & nickel alloys in general, hot corrosion can occur in S-containing atmospheres due to the eutectic between Ni & Ni₃S₂ at 637 ^oC. In alloys exposed to H-O-S gases, the liquid may occur beneath an outer Cr₂O₃ layer.

3) There is also a eutectic between Ni & Ni₃P at 870 ^oC but perhaps irrelevant for ordinary uses.