Tight, hard, corroded material on a reinforcing bar is not a cause for rejection. Even bars with loose, flaky, rust may be used, but the rust must be brushed off with a hard metal wire brush and thoroughly cleaned. As long as you have not lost significant cross-sectional area, you're fine. Concrete is very alkaline and offers a passivating environment to the steel; Carbonation will eventually rob you of this, but it takes time. So long as your design was duly considerate of durability, rust on the bar at the time of the pour is not going to affect your long term performance.
Keep in mind that rust is nine to twelve times thicker than the original base material. It takes a great deal of rust to affect a bar, however this is moderately less true for quenched and tempered where the outer perimeter of the bar is your strongest steel. Even then, I will only reject a bar when it has lost a significant amount of material, often then they appear quite bad with pot marks and a "smoothing" of the mechanical bonding deformations along the length.
I know it seems counter-intuitive, but rust on bars is normal, often typical, and perfectly fine. If it is really bad, or the owner is upset/nervous, I have in (one case only) advised that a galvanic anti-corrosion system be installed, partly at the Contractor's expense. That was in a case where it was quite egregious field practices that led to the corrosion, and it would not be possible to remove all the rust to our satisfaction (Owner and Engineer). Still wasn't necessary, but made good contractual sense.
Quick Tip: If the foundry grade marks, incidental groove and ridges, etc, are perfectly legible and visible, you're okay.
