phosphate conversion coating for the cast valve bodies

[trentar]

Hay there!

has somebody some experinces with phosphate conversion coating for the cast valve bodies used for valve applied in oxygen process. To use it as corrosion preservation. Some info if according to wha, aiga, and so on.
any input is welcim!

Thanks

 

[btrueblood]

Cast what (what alloy)?

Oxygen in what form (gas, liquid) at what temperature and pressure conditions, at what velocity, in what service (hospital, chemical plant, rocket engines...)

Sounds like you are looking at iron/steel low-alloy valves...which I have never heard of in oxy service.

 

[TugboatEng]

Phosphate conversion coatings are meant to protect parts from corrosion in climate controlled environments until they can be assembled or installed in the field. Long term, phosphate conversion coatings are good for parts that will see service in oily environments or live their lives well protected under paint.

 

[btrueblood]

...and oil or paint plus oxygen = fire.

 

[TugboatEng]

Aware of that, I was just giving examples of where phosphated coatings are successfully used. They work well for parts inside of engines or under paint. They offer a trivial amount of corrosion protection in any environment otherwise.

 

[bcd]

It is common practice to use zinc or manganese phosphate coating on carbon steel valve parts. Black oxide is also common. The corrosion resistance is limited and is mostly achieved with the oil dip at the end of the process. Since phosphate coatings are used on carbon steel, it is not used for liquid oxygen because temperature is too low. Only gas. I have seen phosphate coatings used for many years on oxygen cleaned valves with no issues. However, keep in mind that carbon steel can only be used for low pressures and velocities with no impingement possibility. Also, the oil at the end of phosphate process is removed during Oxygen cleaning. And when cleaning involves scrubbing, some of the coating is removed as well. I cannot find any document or standard that states phosphate coatings are ok, but have seen them used on Oxygen clean valves for over 30 years without issue.

 

[Nick_Gray_88]

Run away if you’ve got an oxygen service there, too higher risk of something going wrong.

Go solid metal, stainless, brass, Monel, inconel etc. so long as meets the velocity limits etc.