Electric heater problems, and zinc/cobalt plating vs. galvanizing
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Q. Hi. There are a few problems that I was hoping you might be able to help us with.
1) We use Sulphuric Acid Pickles which operate at around 40-50 °C. We use 415 V (2 active phases) 5 kW Quartz sheath Heaters & are experiencing failures of the ceramic bobbins. What do other people use to heat their pickles, (10-15%v/v)? We have been advised that lead heaters have a limited life, but something has got to be better than heaters lasting a fortnight! The element length in the heaters is 26", and the sheath length is 32".
2) We are also experiencing failures on titanium elements (over the side type), also 5 kW, which are in service on our M&T (ATOTECH) proprietary electrocleaners. The titanium is being etched - the heaters are insulated from the tank walls which are used as cathodes. In the past 10 years we have used both Ti and mild steel elements without incident. The manufacturer suggests that Ti should not be used, and recommends Incoloy(R) 825 instead.
3) We would like to hear from anyone who has tried to replace galvanising of fasteners with Zn/Co plating. Local manufacturers are desperate for an alternative to galvanisers, and are keen to know if the corrosion properties of Zn/Co are comparable. We'd like to hear from anybody who has been successful in displacing galvanising. The prices of plating vs. galvanising are comparable in Australia, but authorities specify gal purely from force of habit! Any experience here?Andrew R [last name deleted for privacy by Editor]
- Quartz heaters seem to be getting rarer over here (just based on my impression), due to their fragility and an apparent difficulty in making them as safe as metal heaters. Teflon coated heaters would probably be the ideal solution for your sulfuric acid pickle, but other possibilities might include nickel or titanium. Titanium is tricky stuff in a plating plant, and you should get the supplier to tell you exactly what to do in terms of anodic protection, etc.
- Incoloy in an alkaline solution strikes me as an expensive, exotic, answer. It may work, but your competition is probably using steel or stainless steel, like virtually every shop I visit. What are the tank walls made of? I'd guess steel, since you say they are used as electrodes.
- Suppliers can give you case histories for Nickel-Cobalt; some suppliers include CST-SurTec and Enthone. It is corrosion resistant but galvanizing is so much thicker that I'd bet on the galvanizing if corrosion resistance is the major criteria.
Ted Mooney, P.E.
Pine Beach, New Jersey
A. We have seen significant failure of Titanium heaters lately, in simple caustic cleaners. The failures seem to principally at the welds. Since most titanium is rarely pure, and usually alloyed with some level of Aluminum, we have theorized that it is due to high Aluminum content welding rod. We know what ingredients are in our cleaners, and there is no way that these things can attack Titanium. We have also done testing, and observe how the cleaners attack most Titanium, until the Aluminum in the surface has been eaten away, then all attack ceases.. Hope this helps.Rudy Sedlak
A. This letter is a response to the person who was having a problem heating a sulfuric acid pickle tank because of the corrosive effect of the solution on the heater.
Where I currently work, we utilize an over-the-side heater that is essentially a zirconium heat exchanger with hot water as the heat source. The zirconium holds up very well to the environment, but I would not recommend it in an application where the heater can be subjected to any type of shock. We had to replace a heat exchanger after a jam-up in the processing line subjected it to a shock. The zirconium cracked at the weld connecting the tubes to the manifold. The only way we found it was by noticing an unexplainable increase in tank volume and related decrease in concentration. (This took quite some time to find.)
Fabrication of a zirconium heat exchanger can be rather expensive due to the welding and availability of a supplier. A small exchanger (5 feet long by 2 feet wide with 1/2" tubes) can cost several thousands of dollars. You have also have to be extremely careful when installing it. However, if the exchanger will not be subjected to any potential shocks, it might be a good alternative. We've had heaters that have lasted for years in a 10-15% sulfuric acid pickle tank.Christian M. Restifo
- Pittsburgh, Pennsylvania
December 26, 2012
A. PVDF heat exchanger with hot water (or steam) circulation is a better way that's very simple. You can make a heat exchanger with PVDF tubs and using hot water or steam for heating. We use this method in hot dip galvanizing pickling process.MOHAMMAD JAVAD HEYDARY
- QOM, IRAN
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