You didn't answer this question: Did you frequently use acid to clean the cell?

Have you replaced the cell and did the problem go away?

CSI is calcium saturation index. It is a very good measure of the probability that calcium will come out of solution and scale things. The best way to determine CSI is to use the pool calculator: poolcalculator.com

For the longest life of the cell, target between -0.5 and -0.25.