the most common corrosion issues with concrete that is contact with steel is chloride ingress. This can be a problem if you use water or sand that has high chlorine levels. In the past the sand that was used to make concrete was sourced from beaches and not cleaned and so had high chlorine content.
If you use sand and water, or concrete, make sure the sand and aggregate as well as the water used is filtered and clean. Even stainless steel is susceptible to Chloride ingress type corrosion.
Its a slow corrosion process but when it takes hold in the steel reinforcement of concrete, structural failure is inevitable. The corrosion products around the steel take up a larger volume and therefore create a pressure within the concrete. This causes the concrete to crack, crumble and lose its structural integrity.
...just for those who are plan to go to their local beach with a bucket or wheel barrow to pick up some sand 