Harold,
Much of the severe pitting on items such as your waffle iron occurred in the early 20th century and was caused by sulfur in the natural gas. This gas was often called "town gas". In those days the naturally occurring sulfur compounds were not removed from natural gas as they are today. Direct impingement of the flame containing sulfur compounds on the cast iron would result in the reaction of the iron and sulfur to form iron sulfide. This loss of iron would form deep pits in the cast iron. This is a simplistic explanation; the actual corrosion mechanism is more complicated.
This is not to infer that rust, accelerated by the presence of moisture or water or p___, did not cause corrosion. However, it is usually more general and less pitting in nature and occurs where moisture collected.
Roy Meadows