Marine environments for buried steel structures consist of sea water and various soil zones (mud, silit, sand, etc.). At the boundary between different zones, macro-corrosion may occur on the anodic side of steel surface.
As it has been experimentally proved that for buried steel structures macro-corrosion may be more dangerous than micro-corrosion, so we studied at first the estimating method on macro-corrosion rate between two zones as described below. By analizing the data in the 1st report, we got cathodic polarization resistance, γ
c (Ω-cm
2), as a controlling factor for the corrosion rate of buried steel structures and obtained the following relation:
C=0.34
e-(γ
c/10
4) where,
C: micro-corrosion rate (mm/yr), γ
c=[1.2-12×2, 000×(
IKD×10
-6)]/(
IKD×10
-6) (Ω-cm
2),
IKD: galvanic current density (μA/cm
2).
But since it is considered difficult to determine macro-corrosion rate at the boundary by γ
c alone, we carried out some experiments to find other controlling factors. As the result, the ratio of anodic and cathodic surface areas of test pieces was found to be another controlling factor; namely, with decreasing the ratio from 1:1 to 1:4, the macro-corrosion rate doubled.
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