Distribution of pH during galvanic corrosion of a Zn/steel couple

The spatial distribution of pH during galvanic corrosion of a model Zn/steel couple immersed in 0.01 M NaCl was investigated using a scanning tungsten pH electrode. The couple consisted of a steel substrate with a 3 μm-thick layer of zinc electroplated at its center. During galvanic corrosion, marked changes in pH occurred, which were confined to a thin layer of solution ca. 1.5 mm over the couple surface. As the surface was approached, the pH over the zinc layer decreased slightly from ca. 5.6 in the original bulk solution, while the pH over the steel surface distant from the zinc layer increased to ca. 11.5. The area of low pH extended not only over the zinc layer, but also over the steel surface adjacent to the zinc. Zinc corrosion products precipitated in the region of high pH. The pH distribution arising from the galvanic corrosion of the couple is discussed in terms of the potential distribution and the morphology of the precipitated zinc corrosion products.