In any concrete structure, steel is used to add strength/reinforce the concrete. Better quality steel can withstand mechanical load, stress, and impact as well as corrosive attack of gases and liquids from the environment that come in contact with the structure. As such, we need to select the appropriate grade and type of steel depending on the environment to which the structure is exposed. 

The problem
When steel corrodes, it forms an oxide layer. These corrosion products-oxides-have a larger volume than the original steel. This expansion puts pressure on the concrete cover. Since the concrete is already set and hard, it causes cracks as it expands to accommodate the larger volume of steel inside. This is the basic phenomenon of cracking or spalling of any concrete structure. In an extreme case, the cracks may be large enough to cause collapse of the structure.

Why stainless steel
One of the products traditionally used to reinforce concrete is plain carbon steel. This carbon steel has low strength and poor resistance to corrosion. Hence, carbon steel corrodes fast and reduces the load bearing capacity of the structure resulting in reduced life and collapse of the structure in extreme case. This necessitates costly and time-consuming repairs and maintenance of the structure. The mechanism of this process is as shown earlier. However, carbon steel was predominantly used as rebar material due to its low cost.
    In order to overcome this problem, engineers have devised various means over a period of time to offer passivity or corrosion resistance to the steel rebars. The alternate products developed were mainly with coating the same rust prone carbon steel rebar such as galvanised, fusion bonded epoxy coated rebars or addition of 0.5 per cent to 0.75 per cent of chromium as corrosion resistant element.