A number of manufacturers have come up with a new type of higher strength re-bars which are claimed to be corrosion resistant generally known as CRS.
These bars have dual micro structures i.e. the surface layer is tempered martensite while the inner shell is ductile ferrite-pearlite to increase corrosion resistance of these bars certain elements like copper , chromium, and phosphorus are also added. Copper plugs pores in the rust. Phosphorous in the form of P2O5 acts as an inhibitor and chromium helps in spinel oxide layer.These protective layers is formed on the surface when they come in contact with atmospheric oxygen and moisture. This layer is formed over a period of time. In concrete oxygen is not available. Hence, the protective coating may not form when such bars are used as reinforcement in concrete. Obviously, there appears to be no advantage in using such bars in place of mild steel. Even, when exposed to atmosphere in marine environment the chlorides present in the air break the protective layer thereby reducing corrosion resistance. Therefore, in marine atmosphere these bars do not give good resistance to corrosion as in normal atmosphere. Secondly, their chemical composition is such that it may lead to pitting corrosion which is highly undesirable. In mild steel bars conforming to IS:1786, although is less corrosion resistance, but corrosion occurs all over the surface without pitting.
Effects of alloying element
Phosphorus : Higher phosphorus content contributes to the increase in strength and Corrosion resistance properties but brings brittleness due to the formation of low euctoid phosphicles in the grain boundary. Also lowers the impact Value at sub zero temperature level (transition temperature) and tenacity to cracking during welding
Copper : Being a pearlite stabiliser, it increases the strength and corrosion resistance Property [only when wet and dry cycle present]
Chromium : Present as an impurity from the scrap and influences carbon equivalent; Weldability and increases corrosion resistance property. High chromium bars not suitable for welding.
Result of Laboratory test on FBEC and CRS bars:
Result of Applied voltage test:
- FBEC : after 30 days no sign of corrosion.
CRS : after 24 Hrs most of the bar gets dissolved in similar condition testing.
- What various standard recommends about CRS