Previously, a cyclic heating and quenching process for developing IrO_x electrodes is proposed. The fabricated IrO_x electrodes show excellent comprehensive properties including wide E-pH range, near Nernst pH sensitivity, fast response rate, and long term stability etc. In this work, the IrO_x electrodes are applied in the detection of surface pH changes during galvanic corrosion process of Zn/Steel couple in thin electrolyte film and bulk solution respectively. This detection is realized by integrating the self-fabricated IrO_x pH sensor and self-built moving platform together. Before the application of the IrO_x electrode, the appropriate way of scanning the surface of the Zn/steel galvanic couple is discussed. Comparing with the case in bulk solution, the Zn/steel couple corrodes faster in 1mm electrolyte layer, the surface pH of which increases to a high value after corrosion begins for 20~40min, and the low pH region exists on the zinc surface and the region adjacent to border area of the steel surface, whereas the pH value of the steel surface away from the border area is relative high. It is demonstrated that the IrO_x electrode could detect the pH distribution effectively in surface region during metal corrosion, which could help the understanding of corrosion process under different situations.