Expert Tip #19: High Potential and Standard Magnesium Anodes: The Difference
Physical Appearance
High potential (HP or M1) magnesium anodes and standard potential (AZ) magnesium anodes look alike. There is very little, if any, physical difference between the two anodes. While there is certainly a difference in the chemistry or alloy and their performance, you cannot tell the difference by just looking.
Most manufacturers stamp alloy, heat and other identifiers on each anode but in place of that information, the following explanation can help you identify the different anode alloy types.
Simple Test Method
A very simple and quick method to determine the anode difference is to put a drop of dilute phosphoric acid on the surface of the anode. If the anode is a "high potential" alloy, the anode surface will turn bright and shiny under the acid. If the anode is a "standard potential" alloy, the anode will turn black where the acid is placed.
Dilute phosphoric acid, or OSPHO, can be found at your local hardware store. If not, you can try certain cleaning solutions containing phosphoric acid, available at grocery or home improvement stores.