Zinc rich spray paints on mild steel? 2

[DasKleineWunder]

I know they are nowhere near as hot or cold galvanizing, but

Are Zinc rich spray paints really better than regular paint in preventing corrosion?
Does it adhere firmly to bare metal (mild steel)?

What about using regular paint on top of Zinc rich paint?
Would it peel off?

Any other advice on Zinc rich paint?
Tnx

 

[CoryPad]

Zinc rich paints are among the best coatings for anticorrosion performance. They can have excellent adhesion. Topcoating is possible, though it may require special types. There is a lot of information on the Internet, look at PPG, International, Jotun, Hempel, and Carboline for product information. SSPC and NACE have specification information. Good luck.

 

[moltenmetal]

There are basically three types: inorganic zinc-rich primers, zinc-rich epoxies, and cheaper 1-component zinc-rich paints meant for repair and touch-up of hot dip galvanized surfaces.

Inorganic zinc-rich primers require a good surface preparation (at least a commercial blast and preferably a near-white blast) and they also require moisture to cure. We have noticed poor adhesion when these coatings are applied in the dead of winter where humidity levels are too low, but otherwise adhesion is excellent We have had no issues top-coating these materials.

Zinc rich epoxies have better, more reliable surface adhesion especially if surface preparation cannot be assured to be excellent. However, they provide poorer galvanic protection when compared against inorganic zinc which has been properly top-coated with an epoxy- but again only if the inorganic zinc is properly applied. We have trouble using zinc-rich epoxies here because we have several employees who are sensitive to either the vapour emissions themselves or to their odour- every formulation seems to contain butanol for some reason and it has a very pungent odour.

The 1-component zinc-rich primers and so-called "cold galvanizing" spray paints are better than nothing, but as coatings left for direct exposure to the elements, they're nothing to write home about. They need to be top-coated with something else to provide durable protection, but for aesthetic repairs to the cut ends of hot-dip galvanized items they do seem to do a pretty good job.

 

[TomDOT]

Agreed with moltenmental.

Talk to the industrial rep for any major paint manufacturer and they can recommend paint systems based on a zinc-rich primer.

Note that IOZ primers are typically less tolerant of over-thickness than epoxy zinc primers.

I'm not sure why producers would always include butanol. Do the formulations have PCBTF (Oxsol 100) for VOC reasons? The blending in butanol and/or acetone will help with solubility of epoxies more than straight PCBTF. Acetone flashes off REALLY fast, so you can't use much.

And speaking of smell - I find PCBTF much more pungent than butanol.

 

[CoryPad]

Tom,

Butanol is common to zinc-rich epoxy coatings from multiple suppliers. Dow touts that butanol has excellent solubility for organics and almost none for water. The lack of water can be good to avoid premature corrosion of the zinc. In the article SOLUBILITY OF EPOXY RESINS (Progress in Organic Coatings, 15 (1987) 33 92), Defrecheux et al. state:

Thus the resins are insoluble in the pure aromatics and in pure n-butanol but are soluble in a binary mixture of the two solvents over certain composition limits.

So it is common to mix, e.g., butanol and toluene.

 

[moltenmetal]

I'm puzzled why parachloro-trifluoromethylbenzene is considered to not be a VOC- it must be a regulatory loophole. The compound is certainly volatile...if it weren't, any product requiring solvent evaporation to form a film would not be able to use it.

The zinc-rich epoxies we use all contain butanol and are not California-regulated "low VOC" forumulations.

 

[TomDOT]

VOCs regulations are orthogonal to the name of "Volatile Organic Compounds"

Effectively, solvents are exempted from VOC by the US EPA because their contribution to ground-level ozone is below an arbitrary threshold. Some areas (SCQAMD) use actual ozone-forming activity for restrictions rather than the EPA pass/fail. Xylene is particularly active and thus further restricted in these areas. A gram of xylene produces 5x as much ozone as a gram of MEK, all else being equal.

Acetone, PCBTF and Tert-butyl acetate are common exempt solvents used in paints.

VOC is totally mis-used in the consumer marketplace as a substitute for "healthy" - paints are touted as "Low VOC" (and thus healthier) when really the classification for the solvents should be "Low HAP"* - since VOC is just ozone forming potential, while HAP is something directly dangerous to humans. On the other hand, IIRC the non-VOC solvents are also non-HAP. There are a bunch of solvents used in paints (particularly tail solvents, mostly glycol ethers) which are non-HAP but are a VOC.

*HAP = Hazardous Air Pollutant. Title III of the Clean Air Act.