Best final rinse after phosphating?
A discussion started in 1996 but continuing through 2017(1996)
Q. Just returned from Powder Coating show in Indianapolis. Still haven't had a consensus opinion as to the "best" final stage in a five stage multi metal iron phosphate dip tank line for surface prep for a powder booth. Some say sealer rinse is better, others insist on deionized water.
I'd appreciate any opinions out there.
A. I am a believer in final seals. The phosphate coating is not a perfect coating nor 100% reacted. The final rinse will fill in the voids. They will also react unreacted phosphate salts. If your interest is in salt spray then a final seal is a must.
There are many new non chrome final seals on the market. Many are alkaline. Most are very good. Pretreatment suppliers will do panel tests for you with and without final seals for you to see the difference.Dan Zinman
A. Sorry I didn't talk to you at the PCI show! In my opinion, sealers are NOT a must for a decent powder system. First of all, what is your product? How will it be used? When talking about final seals, you must remember that most were created to replace Hex chrome. Most if not all, fall short of that as Chrome did a number of things for the substrate and coating. (take a look at the responses to salt spray questions under the CCAI under finishing.com) Many of my customers have decided that a purified water rinse (DI or RO) is adequate for *their* needs.Dave Wright
- Mequon, Wisconsin
A. The answer is that there is no "ONE" perfect final seal for any application, be it powder or liquid finish. Understanding your product and the performance you expect from it in use is the most important factor here (i.e. salt spray, impact resistance, adhesion, etc...). Remember that the pretreatment and paint work together to provide that performance. Selecting the wrong coating material can limit your performance.
Only after a detailed study of various types of seals: chrome, non-chrome, organic, reactive, and wash primers can you select the one that provides the performance you are looking for. Also keep in mind that if you paint with a lot of different coatings that you may not get the same performance out of those coatings, because the seal you have selected may not be optimum for them.
Making a good decision regarding a final seal takes a lot of data. First, determine your performance requirements for now and for the future(where are you going to go with performance). Then, test various coatings versus each other with the current sealer to determine any variations. Then, test a couple of representative coatings with a wide variety of sealers to determine a relative ranking. Finally, test all coatings with the selected sealer to determine any variations in performance.
A. If your ground water is good to begin with, you'll get less benefit from DI or RO water than from a good reactive final rinse. If your water is bad you'll normally get as much or more improvement from pure water for rinsing as from a 'seal rinse'.
The main reason seal rinses are in widespread use is that they don't require the $20 - $60,000 capital equipment like purified water does, nor do they require the hazardous chemicals like DI, or the special care of the membranes like RO. In short, both is best, but if I had to choose, based on average ground water I'd choose a reactive final rinse using exactly the procedure outlined by Craig Burkart in the previous response.
Hope all this helps!
- Pearland, Texas
Thanks folks. All the answers are great and appreciated, but I especially liked Jeff's succinct first paragraph which may partially explain why Dan's and Dave's answers were opposite :-)
Ted Mooney, P.E.
Pine Beach, New Jersey
July 31, 2016
Q. Respected sir, please tell me can we use RO/soft water for post pretreatment process?
We are phosphating wheels of cars, and after that CED process.
DM water dip
before DM rinsing; all processes use soft water/RO.
sswl,dapper,punjab - hisar,haryana,india
A. Hi Rajesh. RO water throughout the process, with a final DI rinse sounds fine to me. I don't think softened water is very good though, because it has just as many contaminating ions as the hard water.
Pine Beach, New Jersey
Striving to live "Aloha"
August 2, 2016
A. RO or DI water is best for the final rinse, since it will leave little or no salts or "water spots" on the surface. If these salts are present then they can cause filliform corrosion, particularly in outdoor exposure.Lyle Kirman
consultant - Cleveland, Ohio
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