Aluminium oxide surface and conductivity?

[spareine]

Aluminium surfaces are protected by a thin aluminium oxide layer. The oxide layer supposedly does not conduct electricity. However, when measuring the contact resistance between the test pen of a multimeter and the aluminium, this resistance is always small. Why? Is the oxide layer easily punctured?

 

[Vagn]

Aluminium surfaces are protected by a thin aluminium oxide layer. The oxide layer supposedly does not conduct electricity. However, when measuring the contact resistance between the test pen of a multimeter and the aluminium, this resistance is always small. Why? Is the oxide layer easily punctured?

The oxide layer is extremely thin, typically up to about https://www.surrey.ac.uk/mes/files/Castle_paper2.pdf. For such thin layers, the breakdown voltage is well within the voltage range that a multimeter can reach (for example a breakdown voltage of 6V is reported http://photonics-benelux.org/proc08/s08p195.pdf for a 10nm crystalline sample)

 

[NascentOxygen]

Aluminium surfaces are protected by a thin aluminium oxide layer. The oxide layer supposedly does not conduct electricity. However, when measuring the contact resistance between the test pen of a multimeter and the aluminium, this resistance is always small. Why? Is the oxide layer easily punctured?

What is the smallest current range on your multimeter?

If you use a1.5v battery and a series resistor (for example, 150k ohm to limit current to 10uA), then connect these in series with the aluminium and your meter (set to measure current, 10uA FSD) you can test its resistance at this lower voltage. Let us know what you find.

 

[spareine]

I examined the contact resistance of aluminium foil on aluminium foil (4 cm²) with a voltmeter, an amperemeter, and a variable voltage power supply. Results: the contact resistance was 2Ω for the entire current range from 1 to 100 mA (voltage range 2 - 200 mV). Linear behavior, constant resistance value, no threshold ('breakdown voltage') or hysteresis.

I used identical electrodes of aluminium foil to exclude electrochemical effects. (Anyway, repeating the experiment with a steel point on aluminium foil resulted in a similar ohmic contact resistance, of the same order of magnitude as 2Ω.)