Dairy Homogenizer Plungers (pistons)
02-28-2009, 07:20 PM,
#1
Dairy Homogenizer Plungers (pistons)
Hello to all!
I am a registered user (just signed up) and have yet to receive my activation code, so I'm posting here. First off, I just happened on this site while researching a reason for/solution to a work related problem and I'm very impressed with this site. Indeed, what a gem it is!
I am an employee of a thermal spray job shop located in Washington State, USA. I'm posting to solicit learned feedback. Please forgive me for being long-winded, but I feel that a fairly detailed description is necessary.
A customer recently requested us to repair the OD seal surface of five 41.40mm plungers in Wc. Normally, we repair plungers with chrome oxide ceramic with excellent performance results. I've been involved in the repair of literally hundreds of plungers in this fashion over the years. However, in response to the customer's request, following is a synopsis of the repair:
Substrate: SS, 300 series. Coating selected: Wc Co12Cr4. Process:HVOF
Final coating thickness:.1778mm radial. Equipment: TAFA/Praxair ST 4000 (control console freshly overhauled & re-calibrated by Praxair).Surface Prep: grit blast post pregrind. Preheat:nominal. Sealer: Metcoseal AP. Finish grind : 150 grit diamond. Final finish: Diamond film superfinish. Final Ra: 1 or <.
The customer installed the plungers & reported back that after only three days of running in the pump, the coating was "failing" by abrading the seals, which were changed out a number of times in order to keep the pump running. Furthermore, it was reported that the coating had attained a dull gray & porous appearance in the reciprocating working area.
My instant suspicion was one of chemical attack, not by the product being processed, but by sanitization fluids used to clean the pump before daily production. So I requested information on the fluids & method used to sanitize the equipment, which I have received. The cleaning procedure is a four step process. 1) Caustic prewash circulating two gallons of 50% Sodium Hydroxide solution @ 180F for 20 minutes, followed by fresh water flush. 2) Caustic wash circulating 6 gallons of Sodium Hydroxide solution mixed with 8oz. "Domolish" (didn't get an MSDS for this, so I can't tell you what's in it) @ 180F for 60 minutes, followed by fresh water rinse. 3) Acid wash circulating a solution of 10% Phosphoric Acid/35%Nitric Acid @ 145F for 40 minutes, followed by fresh water rinse. 4) Circulate 240 gallons of water & .75 gallon SB-Paracetic Acid through the entire system for up to 5 minutes, followed by fresh water rinse.
Ready for production.
Thusfar, my internet research has provided some conflicting and non-conclusive information, but there IS a great deal of data pointing to the Cobalt binder in the coating being attacked by the Nitric Acid (at the very least).
So, does anyone here have any experience with or thoughts on this matter? Am I on the right track? I would welcome & greatly appreciate any feedback.
Finally, we are considering replacing the aforementioned coating with Wc Cr21 Ni6 C5 in the future, thinking that the increased Chromium content and the use of Nickel as the binder should provide the necessary corrosion resistance, although I have read that a Nickel binder is also subject to chemical degredation. Any thoughts please?

Thanks kindly!
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03-01-2009, 07:19 PM,
#2
RE: Dairy Homogenizer Plungers (pistons)
Hi Johnson E

Sign0016 to the Surface Engineering Forum.

I have moved your thread to this forum and have activated your account.

Quote:Am I on the right track?

Smile Yes, I think you are. A fairly common problem where a good coating system is employed and suited to a particular task, only to be destroyed by the use of aggressive cleaning chemicals that seem to be neglected/not mentioned at the time of formulating the coating system Sad

Quote:Finally, we are considering replacing the aforementioned coating with Wc Cr21 Ni6 C5 in the future, thinking that the increased Chromium content and the use of Nickel as the binder should provide the necessary corrosion resistance, although I have read that a Nickel binder is also subject to chemical degredation. Any thoughts please?

I think this will be better, but I have a feeling this will fail as well with this cleaning regime. I would feel happier with your chromium oxide ceramic system.
Reply
03-01-2009, 10:09 PM,
#3
RE: Dairy Homogenizer Plungers (pistons)
Thanks ever so much Gordon! Your input is very much appreciated!

JohnSmile
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