Cooling Coils in the Anodize Tank

Question:

We are sulfuric anodizing in a plastic tank. Our old lead cooling coil has developed a leak, and we are replacing it with a coil made of 316L stainless steel. Does the new coil have to be lead coated? M.J.

Answer:

If the stainless coil could be lead coated, it would probably last longer than a stainless coil without a lead coating. I have never heard of lead coating on a stainless coil and would be surprised if it is viable. The life of the 316L stainless coil might be prolonged if you keep a trickle charge on it when you are not anodizing. If the coil is part of the cathode it will last longer than if it is isolated from the electrical power supply. Unfortunately, the life of even 316L in 15% sulfuric may be shorter than what you are expecting. The lead coils of times past worked well as far as corrosion resistance is concerned, but there are better ways to do the job these days.

The very best way to handle the anodize tank cooling is to design and install a system using chilled water as the cooling medium and to locate the cooling “coils” outside the tank in the form of a plate and frame heat exchanger. Use two separate pumps in the system—one to circulate the acid from the tank, and the other to circulate cold water from the chiller. With the right design and engineering the external cooling system will perform well and last perhaps 20 years.