Measuring pH on metallic planar surface

We are currently trying to measure pH levels on the (wet) surface of metals as part of a study about corrosion. The surface may have a non-conductive protective layer.

  • is it feasible to measure pH on a planar surface (dry or wet)? Which kind of sensor probe can be used for this task?

  • which order of magnitude will be the measure? (precision of the sensor)

Any advice would be greatly appreciated!