Protective Coatings

 

Coating Test Chamber
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ERCo, with assistance from Brookhaven National Laboratory and funding from DOE and the New York State Energy Research and Development Authority (NYSERDA), has developed ceramic coatings for high temperature corrosion resistance. The coatings are designed to solve a specific industry problem: many metal process industry plants discharge high temperature flue gases to the atmosphere wasting a large percentage of their process thermal input. Additionally, some industries have corrosive flue gases that do not allow the use of recuperators.

The secondary aluminum industry is such an industry, and it has received considerable attention concerning flue gas energy recovery. Typically, a secondary aluminum smelter will exhaust its flue gases at 2200oF wasting over 50% of the total thermal input. Several attempts have been made to recover this energy. However, metallic recuperators are quickly attacked and destroyed by the corrosive gases and alumina coated metallic recuperators fail due to the thermal expansion differential between the coating and base metal leading to coating spalling and subsequent base metal attack. Ceramic recuperators have also been tried but suffer from combustion air leakage and high cost. Novel fluidized beds have been field tested, but invariably the distributor plate fails due to high temperature stress corrosion or fouling. 

Our concept uses a unique ceramic material, never used in this application, to coat and protect a custom made radiant tube metallic recuperator. In this concept, the flue gases from the furnace pass through a 1.5 feet diameter Inconel tube with the ceramic coating. Combustion air passes through an annulus surrounding the inner tube. The flue gases heat the combustion air to 1000oF which results in an energy savings of 33%. 

The coating has the corrosion and temperature resistance of a typical ceramic, yet with the thermal expansion coefficient of a metal. This allows the coating to protect the underlying metal (typical of a ceramic) yet retain excellent thermal spalling characteristics to assure a long life even in a thermally cycling environment. 

Coated Recuperator 
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The results of our coating have been quite successful. Coupon tests with a bare metal and with metal samples coated with our ceramic coating have been tested for over 1700 hours in secondary aluminum furnace flue gasses during commercial production (see the above figure). The bare metal was completely corroded while the coated materials were untouched. 

We next tested the coating on the recuperator shown in the figure. After six months of continuous operation, the recuperator showed no visible signs of damage, whereas a stainless steel pipe used as a control was completely destroyed from corrosive attack. 

In 1999, Alcoa was awarded a DOE NICE3 grant for installing recuperators coated with the protective coating. ERCo is serving as a subcontractor to Alcoa, and will be responsible for supplying and applying the coating to the recuperator tubes. 

ERCo is actively seeking licensing partners for this technology. In addition to recuperators, areas for potential application of the coating include protectively coating turbine blades and other metal surfaces in harsh environments, and sealing porous ceramic materials.