Bimetallic (galvanic) corrosion between stainless and carbon steel reinforcement
Stainless steel reinforcement can be used to minimise the risk of corrosion in severe environmental conditions. Due to its cost compared to carbon steel (black) bar most applications will use stainless steel reinforcement in conjunction with carbon steel reinforcement and in many instances the two materials will be in contact. Therefore there is a theoretical risk of bimetallic corrosion causing accelerated damage to the carbon steel.
As long as both metals are in the passive state their potentials will be more or less the same when embedded in concrete. Even if there should be minor differences in potential, both black and stainless steels can be polarised significantly without serious risk of corrosion. In dry, chloride-free situations, stainless and carbon steel exist in harmony. When both bars are cast in new concrete it is therefore unnecessary to provide electrical isolation between the different steels.
However, with chloride penetration into the concrete there is a risk that the carbon steel will be sacrificially consumed. This has particular relevance in repair situations, where (for example) chloride contaminated but undamaged concrete is left in place and new stainless bars are added to repair to replace the corroded bars. In this case corrosion of the carbon steel will be accelerated by the electrical contact with the new stainless bar in the new (chloride-free) concrete. It is therefore advisable to avoid mixed mats of steel if chloride penetration is expected at the level of both steel types.
Thus, assuming the ´correct´ use of the stainless steel, i.e., stainless steel is used at all positions where chloride ingress and subsequent corrosion might occur, the two metals can be coupled without problems.