Do Corrosion Resistant Steel bars resist corrosion?

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:

appvolt

 

   Result  of Applied voltage test:

  1. FBEC : after 30 days no sign of corrosion.
applied voltage resistance test

 

  CRS :   after 24 Hrs most of the bar gets dissolved in similar condition testing.

CRS

 

  1. What various standard recommends about CRS

 

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