ASTM A516 Gr. 60:
Min Yield = 32000 psi
Tensile Strength = 60000 to 80000 psi
% Carbon (Max) = 0.21 to 0.27
ASTM A516 Gr. 70:
Min Yield = 38000 psi
Tensile Strength = 70000 to 90000 psi
% Carbon (Max) = 0.27 to 0.31
Will the slight increase in % Carbon lead to a problem with the caustic soda? I wouldn't think so because you're going to be surface protecting the steel to begin with.