Black Oxide plating or what is commonly called blackening is used to add mild corrosion resistance on iron materials. A quality heater will give you a 1 micron thick protective black finish on steel parts or ferrous metal.
Black Oxide immersion heaters should be 316 stainless steel so they can take the intense heat. In a hot black oxide tank the temperature gets to 290º F causing evaporation rates to increase. An L-shaped or Bottom immersion heater is suggested, but over-the-side stainless heaters can be custom made for your metal processing line.
Be sure to install a liquid level control for this application.