Standard for number of hours in salt spray test for HDG

A. At first: there is no such thing as "this treatment gives so many hours NSS"; its dependent from many, many variables.

1) If there's no passivation product applied white rust will in general occur in less then 24 hours.
2) With a passivation it can go up to 72 hours with less then 5% white rust, depending on the type of passivation product and a lot of other variables in the total process.
3) With an anti-fingerprint coating (also called T.O.C.) protection can last 200 - 500 hours.
4) Painted (considering a good pretreatment has been applied prior to painting) salt spray test can be up to 1000 hours, again if all other variables are optimized.

A. Salt spray is in some ways an unfortunate test to use for galvanized steel.
There are two types in common use, neutral and acetic acid ⇦ this on eBay or Amazon [affil links] . Neither of these does what we want in a test - that is to replicate nature on an accelerated time basis.

Further: the mechanism of protection in galvanizing is via the oxide layer that forms on the surface of the zinc coating. That oxide is removed in salt spray testing, exposing fresh unoxidized zinc which in turn corrodes and is removed.
So salt spray is a means of removing the coating in an accelerated way, but not really a replication on how galv lasts in the environment.

On the plus side, its a test that can be repeated with consistent results, and its widely used in other coatings.
Galv doesn't compare well with many coatings in salt spray, but does compare well in real life.

A. Hi Vijay. Barry tells us to expect white rust in less than 24 hours, but your question is too lacking in context to say much else. Give us some details please. Good luck.

Regards,

A. There cannot be a good reason to put galvanized material is a salt spray cabinet. The test is meaningless.
Why do this?
Salt spray is a good test for consistency between organic coatings, and gives an indication of life expectancy (though is not accurate). But it is a wholly inappropriate test for galvanizing. The salt spray destroys the very mechanism that galvanizing uses to give a good life in protecting the steel. Galvanizing protects by providing a barrier (as most organic coatings also do), but also, and more importantly an electrolytic protection as well. But being more reactive (the zinc) than the steel it is protecting, it needs some way to last longer itself. It does this by forming a layer of carbonates and oxides on the surface of the zinc. This tightly adherent layer preserves the zinc, and the zinc preserves the steel.
Put a galvanized item is a salt spray cabinet (conditions almost never experienced in actual life), and this layer is quickly eroded, allowing the zinc to be rapidly depleted. This gives a false idea on how the galvanizing will perform, rendering the test meaningless.
If someone asks you for salt spray hours for a galvanized coating, it shows they don't understand that test.

A. Hi Ranjit. If your interest is long service life in the real world, hot-dip galvanizing is an excellent finish. All around the world electrical transmission towers are hot dip galvanized because no other finish approaches it for decade after decade of low-maintenance/no-maintenance installation.

But if passing a salt spray test is your objective, paint the panels. I'm not sure that any standard thickness of galvanized sheet will achieve even 216 hours let alone 1000. Good luck.

Regards,

A. Following on from Ted's comments about salt spray testing.
People get obsessed about passing a certain test with a coating or product, sometimes failing to recognize that accelerated testing like salt spray doesn't represent real life. In few cases can an accelerated test be extrapolated to get a predicted actual life.
In the case of salt spray testing on zinc coated products a salt spray test gives no useful information.
Galvanizing for example, will appear to perform very poorly in a salt spray test, but we all know that as a corrosion protection system is performs really well in real life.

The best use of salt spray testing is to predict the life expectancy of the lining of a salt spray cabinet!

 Three Cheers for Mr Crowley!