Dr. S: Thanks for your interest. My knowledge of chemistry is only basic, however what seems to be driving all of this damage on all of the bronze cannons displayed outside in Washington DC is acid rain, bird droppings, and with sunlight possibly acting as a catylist, since cannons kept in the shade are in better shape than those in direct sunlight.
What I see is acid etching and pitting of the bronze. Whatever the reactions are, what is happening is that chemicals in the rain and bird droppings act to dissolve the copper out of the bronze, the greatest part of the product being soluble. The blue-green stains from the runoff are clearly visible on the white concrete mountings. The white or yellowish-white powdery residue left on the cannon is an insoluble tin compound or possibly just tin. This only runs off when it is undermined by runoff. The "whiter" the cannon appears, the more highly etched the surface is.
The bronze cannons were once painted black, which protected the bronze from attack by the acid rain. Once the paint wears off, etching sets in. Since etching increases the surface area of the bronze, the process may be progressive, unfortunately.
I've made one slideshow of one of the cannons which was professionally preserved ending with application of hot microcrystalline wax. You can clearly see the results of the etching and pitting of the bronze, now that the corrosion products have been removed. I'll supply a link to that slideshow shortly.
I appreciate your interest and expertise.