Steel galvanic coupling, effects

Contact of brass, bronze, copper or the more resistant stainless steels with the 13% Cr steels in sea-water can lead to accelerated corrosion of the latter. Galvanic contact effects on metals coupled to the austenitic types are only slight with brass, bronze and copper, but with cadmium, zinc, aluminium and magnesium alloys, insulation or protective measures are necessary to avoid serious attack on the non-ferrous material. Mild steel and the 13% chromium types are also liable to accelerated attack from contact with the chromium-nickel grades. The austenitic materials do not themselves suffer anodic attack in sea-water from contact with any of the usual materials of construction. 

Finally, the increase in the corrosion rate of the steel and the reduction of the corrosion rate of the copper upon coupling can be easily seen in the results they demonstrate the usefulness of cathodic protection and the detrimental effect of improper control of galvanic couple corrosion. 

L. Bertolini, M. Gastaldi, T. Pastore, MP. Pedeferri, P. Pedeferri, Effects of galvanic coupling between carbon steel and stainless-sted reinforcement in concrete , Int. Conf. on Corrosion and Rehabilitation of Reinforced Concrete Structures, Federal Highway Administration, Orlando, 7-11 December 1998 (CD-ROM). 

AUoy galvanic corrosion has been measured in alcohol fuel [27]. The corrosive effects of the alcohol fuels on different galvanic couples Zamak 5, low-carbon steel AISI 1010, stainless steel ABNT 420, and Al-Si alloy-4000 series were studied. Zamak was used as a permanent anode for these tests. Samples were immersion tested for three weeks at 50 °C. The alcohol fuel-water content was varied in this study and the corrosion product effect was investigated. Results indicated that higher ethanol fuel-water content was more corrosive regardless of the galvanic couples. 

Figure 7.8 Effect of the area ratio between the cathode and the anode in a galvanic couple of carbon steel and stainless steel in two different environments.

Steel reinforcements in concrete, being passive, are noble in potential with respect to steel outside the concrete that is galvanically coupled to the reinforcements. The measured potential difference is in the order of 0.5 V [62]. The effect of large cathode area and small anode area has caused premature failures of buried steel pipe entering a concrete building [63]. In this situation, use of epoxy-coated reinforcements (to coat the cathode) or insulated couplings should prove beneficial. 

An interesting variation of the effect of galvanic coupling occurs with metals that exhibit active-passive transitions. When noble metals such as platinum, which are good catalysts for hydrogen reduction, are coupled to a metal with an active-passive transition below the reversible proton-hydrogen potential, spontaneous passivation may ensue (Fig. 7). Thus, a porous coating of noble metal on titanium, chromium, or stainless steels will result in anodic protection of the substrate. 

This area effect in terms of current density is illustrated by combinations of steel and copper as either plates or the fasteners used to join them and immersed in a corrosive solution. If steel rivets are used to join copper plates, the current density on the relatively large cathodic copper plates will be low, cathodic polarization of the copper will be slight, and the voltage of the galvanic couple will maintain a value close to the open circuit potential. At the same time, the current density on the small anodic steel rivets will be high and the consequent corrosion quite severe, giving rise to a particularly vicious form of corrosion called galvanic corrosion [Fig. 3.8(fl)]. 

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