Let's start with what Stopcor is.
It is a galvanic battery, nothing more, nothing less:
A piece of magnesium in an electrolytic solution and a cathode contact (a screw, nothing special).
Specifications as measured by me:
Vol = 1.475V
Isc = 20mA
Vn = 1.470V
And here comes the problem with these imposed voltage devices:
What is the equivalent resistance of the circuit they will be called to protect?
We know from the literature that steel, with water as the electrolytic medium, requires at least 5mA per square meter of interface area. That is, under ideal conditions (protection circuit resistance 0Ω), the A1 Plus could protect 4m² of surface area).
However:
The anode resistance they give you, yes, that little "oops," is huge. This means that the current that can pass is minimal.
With a poor measurement method, it was found that in my installation the total loop resistance is 20kΩ.
Applying Ohm's law, we find that the current that can pass is:
1.5 / (20 × 10³) = 75μA. Yes, you read that right, microamperes.
When actually measured on the system, I found 89μA. Therefore, the current density is insufficient to do anything; it can do nothing and does nothing.
The only way for it to work is with a series of anodes locally interchangeable in the boiler room space, so that the loop resistance drops low enough, to tens of Ω.
If you want to apply cathodic protection, the only solution is:
With a rectifier, or
With anodes in the electrolyte (water), not with the exchange of three media.