Galvanic corrosion in marine environments: Effects associated with the inversion of polarity of Zn/carbon steel couples

Abstract

This article focuses on the corrosion processes resulting from the galvanic coupling between an unprotected carbon steel coupon and a “passivated” Zn‐coated carbon steel coupon. Monitoring of the galvanic current showed that the galvanized steel coupon acted as cathode, which confirmed the inversion of polarity due to Zn passivation. However, it remained cathode for only 6 months and behave as anode for the rest of the experiment (i.e., 5 years). At the end of the experiment, both coupons presented localized degradation. The local corrosion rates were estimated by residual thickness measurements. The highest (200 μm/year) was measured on the more corroded parts of the galvanized steel coupon, while the lowest (13 μm/year) corresponded to the cathodic zones of the same coupon, covered by passivated zinc. The phenomena could be interpreted via the thorough analysis of the corrosion product layers by X‐ray diffraction and μ‐Raman spectroscopy as the composition of these layers depended on the anodic/cathodic character and intensity of the metal surface underneath.

Corrosion of Carbon Steel in Marine Environments: Role of the Corrosion Product Layer

“…However, the results obtained in each site proved comparable so that general trends could be deduced. More detailed information on the exposure conditions can be found in the original articles [26,30,36,37,[40][41][42][43][44].…”

“…It is also worthy to recall the average composition of seawater. The study of the water flowing through the Naval Group Research lab in Cherbourg led to the following composition (main ionic species only): It clearly shows that seawater is not, in any case, a simple NaCl solution, and the role of various other species, e.g., SO 4 2− , Ca 2+ , Mg 2+ , and HCO 3 − , is clearly illustrated by the composition of the mineral layers covering carbon steel in seawater [26,30,36,41,42]. Note also that the alkalinity of seawater (pH measured between 8.0 and 8.2 at Naval Group Research lab), is mainly associated with the presence of hydrogencarbonate ions.…”

“…Thirdly, some phases are electronic conductors, e.g., magnetite [32][33][34] and iron sulfides [16,23], which can favor galvanic cells. The composition of the corrosion product layer also varies with the exposure zone [26,[35][36][37][38][39][40][41][42] and may change in the long term [43].…”

“…This article reports recent advances in the understanding of localized corrosion processes of carbon steel permanently immersed in natural seawater, obtained through a detailed analysis of the corrosion product layers [26,30,36,41,42,44]. These layers, formed on steel surfaces in laboratory experiments or seaport exposure sites, reflect the complexity of iron chemistry in natural seawater.…”

“…These layers, formed on steel surfaces in laboratory experiments or seaport exposure sites, reflect the complexity of iron chemistry in natural seawater. Their composition differs in anodic and cathodic zones [9,41,42] so that they participate actively in the persistence of corrosion cells and favor localized corrosion processes [41].…”