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Ask the Expert Question-and-Answer Archive by Ron Joseph October, 2004 Inorganic Zinc Paint Q. We are using inorganic zinc paint as a primer under a temperature indicating topcoat (Paints are Finnaren & Haley Indurazinc FB-Zinc primer, and F&H HT 800 high temperature topcoat) for some equipment operating at normal temps up to 500F. It has been suggested by some people that the zinc in the primer will melt at about 780F if the equipment overheats due to a malfunction. Is this true? If true, what will happen to the zinc, just drip-off, or volatilize? or ? Is this a true problem, or just an unwarranted concern? I have never heard of this problem. Inorganic zinc-rich primers are used for high temperature applications and I have specified them for furnace ducting operating in excess of 500oF. If melting is a problem the coating manufactuer's data sheet should state the maximum operating temperature at which the coating can be used. I have looked at three coatings advertized on the Internet (see URLs below) and the manufactuers differentiate between intermittent exposure to high temperature and continuous exposure. The three data sheets represented by the URLs give different temperature ranges, but for intermittent exposure, such as you referred to in your email, the coating can be expected to remain functional. The melting point of pure zinc is 787.15oF, but for some reason the inorganic silicate zinc-rich coatings can apparently withstand higher temperatures for short periods. In any case, I've never heard of a case in which an inorganic zinc-rich primer melted and dripped. Carboline is one of the most reputable manufacturers of inorganic zinc-rich primers, and on the data sheet for Carbozinc 11- Weld, they state that their coating can withstand an intermittent temperature of 800oF and a continuous exposure up to 750oF. Be aware that the topcoat will probably fail long before the substrate reaches these high temperatures. I suggest that for liability reasons you should discuss this with several manufacturers and then make a decision as to how you want to proceed. http://www.mercurypaintcorp.com/611.htm Best wishes, Ron Joseph
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