Where the
world gathers for metal finishing
Q&As since 1989


-----

Phosphating salt spray life

Q. We have been asked by a client to zinc phosphate a steel bar that should at least resist 72 hours of salt spray. The process we have at our facility is as follows: soak clean, rinse,electroclean, rinse, zinc phosphate (45 minutes), rinse, chromic seal (30 seconds), rinse and soluble oil (1 minute) as a final step. With this process we have no problem getting 72 hours of salt spray, but now our client wants that the parts not to feel greasy or oily to the hand touch, and this can be achieved in two ways: by spinning the parts or baking them, but after either of these two processes the salt spray resistance drops below 72 hours.

Do you have any idea of what are we doing wrong, what can we do to get the correct resistance?

Thanks in advance

Guillermo Luna
- Mexico City, Mexico
1996

---

---

Multiple threads were merged: please forgive repetition, chronology errors, or disrespect towards other postings [they weren't on the same page] :-)

---

---

Phosphate coating salt spray specification

Q. I would appreciate if anybody can suggest a salt spray specification for an iron phosphated component having a coating weight of 30 mg/sq. ft to 60 mg/sq.ft.

This will be a great help to me, thanks.

Sivakumar Balasubramanian
- Hesston, Kansas
2002

---

"Phosphating & Metal Pretreatment" by D.B. Freeman on AbeBooks or eBay or Amazon (affil links) A. Hi S.B., Are you talking about the salt spray resistance of phosphated, unpainted metal? If so, then the expected exposure time before the entire surface is completely corroded will be on the order of minutes. The amount of coating does not matter. George Gorecki - Naperville, Illinois 2002 A. The more important factor is what type of coating will be going over the phosphate. With a chromic seal and a good powder coat, you can get 1000 hrs. Dan Brewer chemical process supplier - Gurnee, Illinois 2002

---

---

Phosphate & oil salt spray performance

Q. Having trouble with corrosion protection with phos/oil. Coating Gr. C prevailing-torque locknuts and nylon insert locknuts with .00025 zinc then 1000 mg/sq ft phos oil but parts will not pass ASTM 72-hr salt spray. Is phos/oil supposed to give that strength of corrosion protection? Past experience in locknuts industry (40 yrs) have had OEMs use phos/oil as lubricant on bearing retainer applications for transmissions, but now having trouble getting parts to pass salt spray test. Would appreciate info as to whether this type of coating is supposed to withstand 72-hr salt spray or is spec asking too much?

Bob Jarman
- Niles, Illinois
2003

---

---

Salt spray test of CR steel with only iron phosphate coating?

Q. Could anyone tell the normal hrs of salt spray test on CR steel with only iron phosphate coating? We are producing steel tubes where I.D. could not be powder coated.

Stanley B
- Hong Kong
2005

---

---

Salt spray life of manganese phosphate

Q. If we do 5 micron thick Mn-Phosphating on steel part, what should be the Salt Spray Test Life in hours for the part ?

What is the alternate method to check whether the Mn-phosphating is Okay?

Tarun Jagga
auto industry - Noida, UP, India
2006

---

A. Tarun, You do not say what supplementary treatment you are using. If you are not using one then the salt spray resistance will be virtually nil. If using a chromate wash after phosphating then you may get 1-2 hours salt spray life. If you want longevity for your part you need to use a good supplementary treatment. This may be a corrosion preventative oil, a lacquer or even a paint coating. It will depend on what the final use is before anyone could make a sensible suggestion as what to use and what to expect performance-wise. Brian Terry Aerospace - Yeovil, Somerset, UK 2006 A. Dear Sir, Just 5µ Manganese phosphate coating is not good enough for better corrosion protection. You should apply at least 15µ for good protection, then apply good quality rust preventive oil for sealing of phosphate surface. Please note: phosphate coating is not a rust proofing in itself. Apply 108 (Min.) hours salt spray test. Thanks, Navin Maheshwari - New Delhi, India 2006

---

Q. Dear Sir,

What is relation between coating thickness and surface finish in Mn Phosphating? Also for 10-20 micron thickness, what would be coating wt (grams/square meter)?

Prashant H Patil
- Mumbai, Maharashtra, INDIA
2006

---

---

Phosphating Salt spray test vs. actual performance

Q. One of our customer end OEM's, found the phosphate coated clip rusted on vehicle. The salt spray test requirement given by customer is 6 hours to red rust . The supplier says phosphate coating can never give assurance more than 6 hours.

So my Question is, is it true that salt spray test and actual rust life are anyhow related; and what can be the reason behind such rust?

deepa F
- pune Maharashtra India
July 8, 2015

A. Hi Deepa, There are several good answers in this thread already addressing the level of corrosion resistance and phosphating. On its own, with no supplementary treatment there will be virtually no corrosion resistance, either in salt spray or in real life. Supplementary treatments, including oiling, waxing, painting, powder coating etc can give many hours of corrosion resistance. Now, the problem of comparing salt spray life and real time life has been the subject of many a comment on this forum. Whatever you get out of salt spray for corrosion resistance will bear no relation to what you get in real life. It is really just a quality control check, like so many other tests that are carried out. Even if your phosphate coating was to survive 6 hours to red corrosion product this does not relate to your product surviving once in service. Brian Terry Aerospace - Yeovil, Somerset, UK July 8, 2015 A. Deepa, Accelerated corrosion tests like salt spray are designed primarily to be repeatable and consistent so as to be useful as a benchmark. There is no way to say that X hours survival in salt spray is equivalent to Y months or years in the real world, mainly because real world conditions vary wildly depending on the ambient environment. At best you can compare one salt spray result to another. This alloy is better than that one, this treatment or coating is better than that one, etc. Ray Kremer Stellar Solutions, Inc. McHenry, Illinois July 8, 2015