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July 7 - July 18, 2010

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

In general, the greater the surface profile, the better the coating adhesion will be. One exception to this rule appears to be inorganic zinc silicate primers, which tend to split (lose cohesion) when the anchor profile exceeds about 67 microns (2.5 mils). This may be caused in part by attempts to increase coating thickness to cover profile peaks. A 100-micron DFT is enough to get very good corrosion protection, and the top coat will need a mist coat to avoid pop or trapped air onto the surface which may be the cause of pinholes. Inorganic zinc silicate primers are well known to be sensitive to excess thickness. Follow the guidelines on the manufacturer's technical data sheets.

From Marco Antonio Alvarado Meneses of Sherwin Williams Perú on August 13, 2010:
It depends on several things. The most important, I guess, is the manufacturer's recomendation about this issue. Environmental conditions are also very important, including maintaining appropriate minimum relative humidity. Check all these. Geometry of the surface is very important because DFT may be higher than data sheet recommendations in corners, angles, etc. A good painting system is inorganic zinc rich, followed by an epoxy two pack and an aliphatic acrylic polyurethane.

From Tolga DIRAZ of CARBOLINE on July 16, 2010:
There are two critical questions that should be asked? - What type of inorganic zinc coating, i.e., alkali, alkyl, or reinforced, is to be applied to a steel surface? - What are the enviromental conditions for proper cuirng? - What is the geometry of the surface, i.e., flat or with some irregularities such as corners and sharp edges? As most of you may agree with me, painting an irregular surface can definitely create some problems that may yield cracking of inorganic zinc type coatings.

From Suresh Chandra Gupta of Bharat Heavy Electricals Limited on July 9, 2010:
Application of an inorganic ethyl zinc silicate coating of 140 microns to a substrate with a 75-micron surface profile would not pose any problem during storage, transportation, and erection, provided the substrate is free from grease, oil, and dust, and is shot-blasted to Sa 3 or Sa 2.5 minimum before application of the coatiing. (Eds. Note: Sa 3 is approximately the same as SSPC-SP 10, and Sa 2.5 is about the same as SSPC-SP 10.) The paint should have been properly mixed and applied by air spray, with recommended RH of 65% minimum. The curing of the coating must be checked by MEK rub test as per ASTM before dispatch. If these conditions are observed during application, no problem would be expected even on prolonged storage. However, if it is required to be top- coated, then a mist coat before application of the top-coat is recommended to enhance the adhesion between the coats as well as life of the coating. The most important parameters during the application are surface preparation and the application method.

From Pedro L Castillo M of Ancel C.A. on July 7, 2010:
The profile is not the concern. It is possible that high DFT of IOZ can cause cracking, because the Zn content in the film may be too high. We hope that a DFT of about 100 microns is enough to get very good corrosion protection and the topcoat will need a mist coat to avoid pop or trapped air.

From Jeff Croll of Northern Corrosion Technologies on July 7, 2010:
It depends upon the "inorganic zinc" as well as the conditions of application and the recommended film thickness by the manufacturer. Some products labelled inorganic zincs are not silicate inorganic zincs but are called inorganic due to certain modifications that allow for fast recoat when the resin or binder is actually an epoxy. The environmental conditions play a part in that too little moisture in the air will prevent the zinc from fully curing. Generally, most manufacturers' inorganic zincs are not high builds and thus there is the possibility of splitting when topcoated if the thickness of the zinc is too high. It is very important to follow the recommendation of the manufacturer as regards environmental conditions, film thickness, and profile depth and angularity when applying inorganic zinc.

From Tom Schwerdt of Texas Department of Transportation on July 7, 2010:
Properly applied, most IOZ coatings on steel will have no cracking problems associated with a DFT of 140 microns over a 75 micron profile. Our specifications allow nearly twice that DFT for shop-applied IOZ, and we field-apply a topcoat an acrylic topcoat a year or more later with no issues.

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