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January 27 - January 31, 2020

If I apply 140 microns of inorganic zinc to a steel substrate with a 75 micron profile, is there any concern about cracking of the primer during storage, shipment, and erection of the structure, or with field-application of a topcoat?

Selected Answers

From William (Willie) Mandeno of WSP NZ on February 7, 2020:
It depends on the formulation of the IOZ but mud cracking will be less of a problem with high-ratio waterborne type. Similarly, it depends on the type of field-applied topcoat, e.g., low VS could result in blistering due to solvent entrapment unless IOZ has weathered for several months to reduce porosity..

From HECTOR MEDINA of PEQUIVEN on February 2, 2020:
Good point from Larry Muzia. But I think that it's always important gto contact the coating manufacturer to assess the risk of mudcracking.

From Larry Muzia of Exceletech Coating & Applications, LLC on January 31, 2020:
To start, a 3-mil profile is excessive for an ioz application, but having this depth and insuring you have covered the peaks with sufficient film thickness has been successful on numerous occasions. Here in Florida, the Space Center has often gone with untopcoated ioz on carbon steel exposed to the salt environment at 3-5 mils as a specification requirement. I agree that, at the higher dft, there is both a chance of mudcracking or splitting of the zinc film, which has low tensile strength to begin with. Typically, the first location of mudcracking will be in the filet area for structural steel. When I have seen zinc applied at the higher end of the thickness range near or at 5 mils, I would suggest watching the film thickness of the overcoats and keep to the minimum recommended dft to minimize film stress to the zinc film. Topcoating a heavier ioz film is also more conducive to outgassing because the film of the zinc has more porosity because of the depth, unless it has weathered and the porosity has been reduced by the formation of zinc carbonate salts. Also, remember, for shop applications, the cure time must be extended before overcoating.

From David Lemke of Team Industries on January 30, 2020:
While most IOZ is recommended for application at  2-3 or 2-4 mils DFT, there some coating manufacturers that have statements on the Product Data Sheet, such as  "Do not exceed 6 mils DFT." Some manufacturers state a minimum profile of 2 mils and some 1-3 mils. In reality, this case would be at the outer limits for some manufacturers, but still tolerable. Put the material on in a hot 90° sun with 85% humidity, and it has a better chance of #1 happening. Slow down the drying speed with a lower temperature and RH, and it has better chance of not being a problem. The deeper profile might help for nicks and chipping during handling, provided a reasonable dry time was used. If a topcoat consists of heavy mil coats in a multi- coat system, the heavier IOZ could have a chance to stress the coating system. The heavier IOZ with a top coat of high-temp silicone designed for 1000° service could have chance to be a problem. A single top coat on a system not designed for high elevated temperatures, with a reasonable dry time for the IOZ, I  think, it would be satisfactory.

From Jon Cavallo of Jon R. Cavallo PE LLC on January 30, 2020:
Just a few items to clear up. 1. "Mud Cracking" is actually a defect phenomenon called "checking." The defects manifested in checking do not go completely through the affected coating film. 2. History shows that all brands of IOZ are prone to "mud crack" when applied in excess of 5 mils DFT. 3. The "mud cracking" phenomenon results from liquid silicate binder migrating toward the substrate during cure. When the IOZ film is built in excess of 5 mils, insufficient silicate binder is left at or near the film surface after curing; hence, shrink checking occurs (called "mud cracking"). Hope this helps.

From Josh Skinner of The Sherwin-Williams Company on January 30, 2020:
140 microns (5.5 mils) DFT for IOZ is excessive. At that thickness, you're at risk for mudcracking. Always consult the product data sheet for recommended film thickness.

From Ron Berry of NZ Corrosion Services Ltd on January 29, 2020:
There should not necessarily be a concern, particularly if no additional solvent is added If it does not mud crack within the first 24 hours at normal drying conditions, it is unlikely to do so later.  A  75-micron profile is high for IOZ. Why is it this high?

From Kalpesh Patel of Endura Manufacturing Co. Ltd. on January 28, 2020:
For IOZ coatings, mud cracking is very common problem, and the main causes of the problem are film thickness and insufficient  curing of the film. If you apply the right thickness and let it cure completely before you recoat or topcoat, you can minimize the problem. The thickness you are going to apply looks reasonable, but it needs to be consistent and make sure there is sufficient humidity to cure it completely.

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Tagged categories: Asia Pacific; Coating Application; EMEA (Europe, Middle East and Africa); Latin America; North America; Steel; Zinc-rich (inorganic)

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