Zinc phosphating for 72 Hour salt spray test endurance

A. Salt Spray life is mostly dependent on the rust preventive which is applied over the phosphate. Any of the major suppliers can give you a heavy wax or oil which will meet 72 hours.

2005 Mr Mohanan It is possible to examine this approach. Plate 7 microns Alkaline Cyanide free zinc. Chromate. Paint . SST will exceed your needs -- 120 hours. Regards Asif Nurie [dec.] - New Delhi, India With deep regret we sadly advise that Asif passed away on Jan 24, 2016

A. Hello Mohann;

You need to use a water soluble topcoat, the kind they use over zinc plating, over your phosphating, it may work.

Khozem Vahaanwala
Saify Ind
Bangalore, Karnataka, India

A. The phosphate coatings are basically a conversion coating. Your chemical supplier can give a chemical to get a coarse crystal conversion or a fine crystal conversion coating. If you go for a coarse crystal formation, the coating is porous, which will corrode fast. This type of coating is generally preferred for preparation before painting, so that the paint can adhere well with the pores.

For a plain phosphating you should go for a fine crystal formation. You need to convert these phosphate coatings with an additional chromating conversion, which is very simple dip in a chromate solution. This fine crystals will have very low porosity, and can have about 12 to 24 hours salt spray resistance without oiling.

An additional good quality of rust preventive oil in combination with zinc phosphate and chromate can give up to 96 hours salt spray resistance.

A. Hi Dharmaraj. Unfortunately there is simply no way to approach the problem from that direction because failure can come from numerous problems with the substrate, the pretreatment, the phosphate process per se, the post-treatment, handling & packaging, shipping, exposure conditions, or mis-specification. Instead you must describe what you are doing and exactly what you mean by "corroding only later stages". Thanks, and sorry.

Regards,

A. A phosphated panel that has not been painted or does not have a protective coating applied will corrode very quickly in a salt spray chamber, probably less than three hours.

A. Phosphating is a pre-treatment and not a finish. Hence, it cannot offer much resistance to corrosion in salt spray test.

One of the important attributes of phosphate conversion coating is its ability to prevent underfilm corrosion.

Measurement of the extent of spreading of corrosion from the scribed region of the phosphated and painted panel compared to its non-phosphated counterpart will give an idea about the quality of the phosphate coating.

A I have done the salt spray test on a tri cationic cold zinc phosphating, and have the certificate from a testing body that my panel lasted 23 hours under salt spray chamber.

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! Hi, Shankar. Thanks!

This then begs the question of what wax or oil was applied to the phosphate, either deliberately or inadvertently, which may account for the salt spray survival time? I think if you degrease the panel after phosphatization to remove those greases and oils you may find that you won't even get 23 hours in normal atmosphere, let alone in a salt spray chamber :-)

Regards,

Ted Mooney, P.E.
finishing.com - Pine Beach, New Jersey
Striving to live Aloha

A. Hi, Sathis.

I'm not sure if I understand the question in light of the previous responses, where you heard estimates from 3 hours for zinc phosphate to 23 hours for tricationic. If you can phrase your question in terms of the previous responses, with what type of phosphating you are applying, and additional details, maybe an estimate can be offered -- but I personally think that Mr. Sankara offered a very insightful answer, i.e., that testing a scribe mark in a phosphated and painted panel is the proper way to judge the quality of the zinc phosphate pretreatment, and salt spray testing at an intermediate stage in the process is not particularly useful. Good luck.

Regards,

A. Hi, Vijay.

George Goreki estimated 3 hours in salt spray. If all of the oil has been stripped, I'll estimate a day in normal atmosphere; I've seen flash rusting much sooner than that.

As noted, phosphatization is not a final finish; because of that, arbitrary and inconsistent factors may have more influence on the corrosion resistance than the quality of the phosphatization. This frames the question as whether there is any value at all in such testing or you are just throwing away money and resources by conducting such a test. :-(

Please try to tell us your situation, and please very slowly re-read Mr. Sankara's response and try to profit from what I feel is its wisdom. Thanks and good luck!

Regards,

A. Hello Duygu,

The panels were put in a salt spray chamber AND transported or the panels you put in the salt spray chamber are a sample of the ones that you transport?

I agree with Ted, phosphating is a pretreatment, a finish could be if you treat it with some anti-corrosive product (water-displacement oil, for example), or paint it (epoxy, KTL).

If your panels last 5 hours in salt spray chamber, I suppose they can last several hours in normal (dry) atmosphere.

Please tell us some more details :)

Regards,

A. Hello Alok,

You can use some heavy water displacement oil, there are many vendors capable of help you out of this requirement. Only, as I recall in my process, you must have more than 10 g/m2 of phosphate in the surface you want to protect. If you can use hexavalent chromium, I recommend a phosphate sealer before you apply the oil. You can't lose!

Hope I could clarify something! Best of lucks,

Daniel

A. Hi Harish. The way I read the helpful answers offered, 72 hours is a good target if you are doing the phosphating correctly with quality materials and are applying a proper rust-preventative. How many hours are you actually getting? Remember that the purpose of a salt spray test is to learn whether you are currently doing it right, not to extrapolate anything.

Regards,