Electric surface charge practically always appears on an object surface when it is placed into a fluid. All fluids contain ions, positive (cations) and negative (anions). These ions interact with the object surface. This interaction might lead to the adsorption of some of them on the surface. If the number of adsorbed cations exceeds the number of adsorbed anions, the surface would gain total positive electric charge.
There is another possible mechanism leading to surface charging. It is dissociation of the surface chemical group.
Details of the electric surface charge description and its relation to the surface chemistry are given by Lyklema in "Fundamentals of Interface and Colloid Science".[1]